ac corrosion of carbon steel in cathodic protection ... · corrosion rate vs 𝑖 at different cp...

POLITECNICO DI MILANO

School of Industrial and Information Engineering

Department of Chemistry, Materials and Chemical Engineering “Giulio Natta”

Master of Science in Materials Engineering and Nanotechnology

AC CORROSION OF CARBON STEEL IN

CATHODIC PROTECTION CONDITION: EFFECT

ON POTENTIAL AND CONFIRMATION OF

PROTECTION CRITERIA

Supervisor: Prof. Marco ORMELLESE

Co-Supervisor: Ing. Andrea BRENNA

Master Thesis of:

Francesco LIPARI

Matr.: 854228

Academic Year: 2016 - 2017

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Content

CHAPTER 1 - AC INTERFERENCE CORROSION OF CARBON STEEL .............. 1

1.1 CATHODIC PROTECTION: GENERAL [4] ......................................................... 1

1.1.1 Cathodic protection systems ............................................................................ 2

1.1.2 Protection potential .......................................................................................... 3

1.1.3 Protection current density ................................................................................ 6

1.1.4 Cathodic protection criteria ............................................................................. 7

1.2 AC INTERFERENCE ............................................................................................. 8

1.2.1 Stationary and non-stationary interference ...................................................... 9

1.2.2 AC interference sources................................................................................. 11

1.2.2 Capacitive coupling ....................................................................................... 12

1.2.3 Resistive coupling ......................................................................................... 12

1.2.4 Inductive coupling ......................................................................................... 13

1.3 CHARACTERISTICS OF AC CORROSION ...................................................... 14

1.3.1 AC voltage on the structure ........................................................................... 14

1.3.2 AC density ..................................................................................................... 15

1.3.3 AC density/DC density ratio ......................................................................... 16

1.3.4 Effect of polarization potential ...................................................................... 16

1.3.5 Soil characteristics ......................................................................................... 17

1.3.6 Frequency effect ............................................................................................ 19

1.3.7 Corrosion Rate ............................................................................................... 20

1.3.8 Morphology of AC corrosion ........................................................................ 21

1.4 AC CORROSION MONITORING....................................................................... 22

1.4.1 Weight loss measurements ............................................................................ 24

1.4.2 Perforation measurements ............................................................................. 25

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1.4.3 Electrical resistance (ER) measurements ...................................................... 25

1.4.4 Coulometric oxidation of corrosion product measurements ......................... 25

1.5 AC MITIGATION ................................................................................................ 26

1.5.1 Construction measures .......................................................................................... 26

1.5.2 Operation measures ....................................................................................... 27

CHAPTER 2 - AC CORROSION: PROPOSED MECHANISMS AND PROTECTION

CRITERIA ................................................................................................ 28

2.1 AC CORROSION MECHANISMS ...................................................................... 28

2.1.1 The mechanism reported on ISO 18086:2015 ............................................... 28

2.1.2 Analysis of equivalent electric circuits .......................................................... 30

2.1.3 Earth-alkaline vs. alkaline cations effect ....................................................... 35

2.1.4 A conventional electrochemical approach in the absence of CP ................... 36

2.1.5 The alkalization mechanism .......................................................................... 38

2.1.6 Theoretical corrosion models ........................................................................ 41

2.1.7 AC effect on overvoltages ............................................................................. 45

2.1.8 A two-steps mechanism ................................................................................. 47

2.2 CATHODIC PROTECTION CRITERIA ............................................................. 50

2.2.1 Cathodic protection criteria reported on ISO 18086:2015 ............................ 50

2.2.2 Cathodic protection criteria proposed by other authors................................. 52

2.2.3 A new proposal of CP criteria in the presence of AC interference ............... 54

CHAPTER 3 - MATERIALS AND METHODS ............................................................ 58

3.1 ELECTRICAL CIRCUIT...................................................................................... 58

3.2 MATERIALS ........................................................................................................ 60

3.3 GALVANOSTATIC TEST: AC EFFECT ON DC POTENTIAL ....................... 61

3.3.1 Aim of the test ............................................................................................... 61

3.3.2 Electrical circuit and test cell......................................................................... 63

3.4 LONG-TERM EXPOSURE TEST ....................................................................... 64

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3.4.1 Aim of the test ............................................................................................... 64

3.4.2 Electrical circuit and test cell......................................................................... 65

3.4.3 Protection potential and current density monitoring ..................................... 67

3.4.4 Mass loss measurement ................................................................................. 68

CHAPTER 4 - RESULTS AND DISCUSSION ............................................................. 70

4.1 PART 1: GALVANOTATIC TESTS: EFFECT OF AC ON IR-FREE

POTENTIAL ......................................................................................................... 70

4.2 PART 2: LONG-TERM EXPOSURE TEST: CURRENT AND POTENTIAL

MONITORING ..................................................................................................... 81

4.3 PART 3: LONG-TERM EXPOSURE TEST: CORROSION RATE AND

CATHODIC PROTECTION CRITERIA ............................................................. 88

4.3.1 Corrosion rate in the presence of AC interference ........................................ 88

4.3.2 Cathodic protection criterion in the presence of AC interference ................. 93

CONCLUSIONS .............................................................................................................. 101

1 GALVANOTATIC TESTS: EFFECT OF AC ON IR-FREE POTENTIAL ..... 101

2 LONG-TERM EXPOSURE TESTS FOR MASS LOSS MEASUREMENT .... 102

REFERENCES ................................................................................................................ 104

iv

List of figures

Figure 1.1

Types of cathodic protection: a) by galvanic anodes b) by impressed

current system [4] ........................................................................................ 2

Figure 1.2 Schematic illustration of the electrochemical mechanism [5] ..................... 5

Figure 1.3

a) A generic Evans diagram and b) Evans diagram for an active metal

in aerated environment, as carbon steel in soil [4] ...................................... 5

Figure 1.4

General scheme of electrical interference between two electrodes on

a body: a) conductor and b) insulator [4] .................................................... 9

Figure 1.5

Scheme of stationary interference between: a) two crossing pipelines

and b) two almost parallel pipelines [4] ...................................................... 9

Figure 1.6

Scheme of non-stationary interference caused by stray current

dispersed by a DC transit system [4] ......................................................... 10

Figure 1.7 An example of HVTL .............................................................................. 12

Figure 1.8 A Frecciarossa 1000 (ETR 1000) on an Italian high-speed railway [12] ... 12

Figure 1.9 Inductive coupling between an AC conductor and a buried pipeline [15] . 13

Figure 1.10

Inductive coupling between three-phases HVTL and a buried

pipeline [15] ............................................................................................... 13

Figure 1.11

AV needed to have a 𝑖𝐴𝐶 of 100 A/m2, in function of the defect

diameter and the soil resistivity [20] .......................................................... 17

Figure 1.12 Electric equivalent circuit [33] ................................................................... 19

Figure 1.13

Electrical response of the circuit in Figure 1.12: the red line is the

total AC, the green line is the AC passing through 𝑅𝑒𝑓𝑓 [33] .................... 20

Figure 1.14

Schematic illustration of the tubercle of “stone hard soil” that grows

at the coating defect in connection with AC corrosion [36] ....................... 22

Figure 1.15 Measurement of the AC gradient and localising remote earth [16] ........... 24

Figure 2.1

Schematic description of the AC corrosion process with cathodic

protection according to ISO 18086, where: 1) AC current on a

coating defect, 2) metal, 3) passive film and 4) iron hydroxide [16] ......... 29

Figure 2.2 A schematic illustration of the electrical equivalent circuit [36] ............... 31

Figure 2.3 Geometrical effects on pipe-to-soil resistance [36] .................................... 31

Figure 2.4

Illustration of the anodic- and cathodic branches of the

Volmer-Butler equation and the summarised total current [36] ................. 33

Figure 2.5

Schematic illustration of the steel-water interface acting as a

capacitor [36] .............................................................................................. 34

v

Figure 2.6 An electrochemical description of AC corrosion [33] ............................... 37

Figure 2.7 An electrochemical description of AC corrosion [33] ............................... 38

Figure 2.8

Mass balance schematics for 𝑂𝐻− ions produced by CP at a coating

defect [42] ................................................................................................... 39

Figure 2.9

Pourbaix diagram: the hatched area indicates the critical AC

corrosion zone [42] ..................................................................................... 39

Figure 2.10

DC on-potential (𝑈𝑂𝑁) and corrosion rate measured with ER

coupon [42] ................................................................................................. 40

Figure 2.11

DC on-potential (𝑈𝑂𝑁) and spread resistance (𝑅𝑆) measured with

ER coupon [42] .......................................................................................... 41

Figure 2.12 Potential shift vs. 𝐸𝑝 for a) 𝑟 ≤ 1 and b) 𝑟 > 1 [46] ................................. 43

Figure 2.13 Corrosion current vs. 𝐸𝑝 for a) 𝑟 ≤ 1 and b) 𝑟 > 1 [46] ........................... 43

Figure 2.14

DC potential vs the root-mean-square current for a) 𝑟 < 1 and

b) 𝑟 > 1 [47] ............................................................................................... 43

Figure 2.15 𝐸𝑟.𝑚.𝑠.,min vs r [47] ..................................................................................... 44

Figure 2.16 𝑖𝑟.𝑚.𝑠.,min vs r [47] ...................................................................................... 44

Figure 2.17 Electrical equivalent circuit proposed by Lalvani and Xiao [48]............. 45

Figure 2.18

Dimensionless corrosion current vs a) peak potential and

b) frequency [48] ........................................................................................ 45

Figure 2.19 ∆𝐸𝑐𝑜𝑟𝑟 vs 𝐸𝑝 [48] ....................................................................................... 45

Figure 2.20 Dimensionless corrosion current vs 𝐸𝑐𝑜𝑟𝑟 [48] .......................................... 45

Figure 2.21

Effect of AC on polarisation curves of carbon steel in 4 g/L 𝑁𝑎2𝑆𝑂4

solution [49] ............................................................................................... 46

Figure 2.22

Effect of AC on corrosion current and potential for carbon steel in 4

g/L 𝑁𝑎2𝑆𝑂4 solution [49] .......................................................................... 47

Figure 2.23

Relationship between DC on-potential, AC voltage and likelihood of

AC corrosion, where: 1) less negative cathodic protection level;

2) more negative cathodic protection level; 3) AC corrosion [16] ............. 51

Figure 2.24

Relationship between DC and AC current densities and likelihood of

AC corrosion, where: 1) less negative cathodic protection level;

2) more negative cathodic protection level; 3) AC corrosion [16] ............. 52

Figure 2.25 Effect of soil resistivity on the threshold 𝑈𝐴𝑉 value [54] ........................... 53

Figure 2.26

New CP criteria for mild pipeline steel in the present of AC

interference for a) Tang et al. [56] and b) A.Q. Fu [57] ............................... 54

Figure 2.27 Experimental corrosion rate in the 𝑖𝐴𝐶/𝑖𝐶𝑃 diagram. Safe and unsafe

regions refer to CP criterion as reported in ISO 18086:2015 [58] ............. 55

Figure 2.28 AC corrosion risk diagram: IR-free potential vs. 𝑖𝐴𝐶/𝑖𝐷𝐶 [58] .................. 56

vi

Figure 2.29 New CP criteria based on experimental corrosion rate data [58] ............... 57

Figure 3.1 Schematic view of the electrical circuit ................................................... 59

Figure 3.2 Electrical circuit (case and internal view) ................................................ 60

Figure 3.3 Carbon steel specimen in the sample holder ............................................ 61

Figure 3.4 Galvanostatic test – experimental set-up .................................................. 62

Figure 3.5 Galvanostatic test – electrochemical cell ................................................. 62

Figure 3.6

Long-term exposure tests – experimental conditions (red markers

refers to the first condition investigated; blue markers to the second

condition) ................................................................................................. 65

Figure 3.7 Long-term exposure tests – schematic view of the electrical circuit ....... 66

Figure 3.8 Long-term exposure tests – electrical circuit ........................................... 66

Figure 3.9

Long-term exposure tests – connection between the electrical circuit

and the corrosion cells .............................................................................. 67

Figure 3.10 Long-term exposure tests – test cells ....................................................... 67

Figure 3.11 IR-free potential monitoring .................................................................... 68

Figure 4.1 DC potential vs. AC density (𝑖𝐶𝑃 = 0 A/m2) ............................................ 72

Figure 4.2 DC potential vs. AC density (𝑖𝐶𝑃 = 0.15 A/m2) ....................................... 72

Figure 4.3 DC potential vs. AC density (𝑖𝐶𝑃 = 0.3 A/m2) ......................................... 72






Figure 4.9 DC potential vs. AC density (𝑖𝐶𝑃 = 10.0 A/m2) ....................................... 73

Figure 4.10

IR-free potential vs 𝑖𝐶𝑃 in absence of interference 𝑖𝐴𝐶 (𝑖𝐶𝑃 from 0 to

10 A/m2) ................................................................................................... 77

Figure 4.11

IR-free potential vs. AC density varying CP current density (𝑖𝐶𝑃

from 0.15 to 1 A/m2) ................................................................................ 77

Figure 4.12

IR-free potential vs. AC density varying CP current density (𝑖𝐶𝑃

from 2 to 10 A/m2) ................................................................................... 78

Figure 4.13 Protection potential shift vs AC density ................................................... 78

vii

Figure 4.14

Protection potential shift vs AC density (comparison between the

results obtained in this work and in [58]) ................................................... 80

Figure 4.15 IR-free potential monitoring in time (Series A and B) ............................ 83

Figure 4.16 IR-free potential monitoring in time (Series A and B) ............................ 83

Figure 4.17 Cathodic protection current density monitoring ...................................... 84

Figure 4.18 AC density monitoring ............................................................................. 84

Figure 4.19 𝑖𝐴𝐶/𝑖𝐶𝑃 ratio trend in time ......................................................................... 85

Figure 4.20 Potentials obtained during the two testes at 𝑖𝐶𝑃 = 0.3 A/m2 .................... 86




Figure 4.24 Potentials obtained during the two testes at 𝑖𝐶𝑃 = 10.0 A/m2 .................. 87

Figure 4.25

Corrosion rate vs 𝑖𝐴𝐶 at different CP levels. Blue markers refer to

the tests carried out during this thesis work. White markers refer

to results obtained from previous tests ..................................................... 90

Figure 4.26

Corrosion rate vs 𝑖𝐴𝐶/𝑖𝐶𝑃 at different AC and CP levels. Blue markers

refer to the tests carried out during this thesis work. White markers

refer to results obtained from previous tests ............................................ 91

Figure 4.27

IR-free potential with respect to the ratio between AC and CP current

density. Blue markers refer to the tests carried out during this thesis

work. White markers refer to results obtained from previous tests ......... 92

Figure 4.28

Relationship between DC and AC current densities and likelihood of AC

corrosion, where: 1) less negative cathodic protection level; 2) more

negative cathodic protection level; 3) AC corrosion [16] .......................... 94

Figure 4.29

Corrosion rates of carbon steel specimen under CP condition in the

presence of AC interference: 𝑖𝐴𝐶 vs 𝑖𝐶𝑃 graph ......................................... 95

Figure 4.30

Experimental corrosion rate in the 𝑖𝐴𝐶/𝑖𝐶𝑃 diagram. Safe and unsafe

regions refer to CP criterion as reported in ISO 18086:2015 ................... 96

Figure 4.31

Experimental corrosion rate in the 𝑖𝐴𝐶/𝑖𝐶𝑃 vs IR-free potential

diagram. Safe and unsafe regions refer to CP criterion as reported

in ISO 18086:2015 ................................................................................... 97

Figure 4.32

Experimental corrosion rate in the 𝑖𝐴𝐶 vs IR-free potential diagram.

Safe and unsafe regions refer to CP criterion as reported in ISO

18086:2015 ............................................................................................... 98

Figure 4.33

Experimental corrosion rate in the 𝑖𝐴𝐶 vs IR-free potential diagram.

Safe and unsafe regions refer to CP criterion as reported in [58] .............. 98

viii

List of tables

Table 1.1

Protection potentials for different metallic materials and environmental

conditions ................................................................................................. 8

Table 3.1 API 5L X52 – chemical composition by weight [59] .............................. 60

Table 3.2 Galvanostatic test – experimental conditions ........................................ 63

Table 3.3

Long-term exposure tests – experimental conditions (according to

Figure 3.6) .............................................................................................. 64

Table 4.1 (a)

IR-free potential shift in the presence of AC interference (𝑖𝐴𝐶 from

0 to 30 A/m2).......................................................................................... 74

Table 4.1 (b)

IR-free potential shift in the presence of AC interference (𝑖𝐴𝐶 from

50 to 500 A/m2)...................................................................................... 75

Table 4.1 (c)

IR-free potential shift in the presence of AC interference

(𝑖𝐴𝐶 = 1,000 A/m2) ................................................................................. 76

Table 4.2

IR-free potential of carbon steel in free corrosion condition in the

presence of AC interference .................................................................. 76

Table 4.3 IR-free potential after two weeks of cathodic protection applied .......... 81

Table 4.4

Mean values of IR-free potential and current densities in the first

tested conditions .................................................................................... 85

Table 4.5

Mean values of IR-free potential and current densities in the second

tested conditions .................................................................................... 85

Table 4.6

IR-free potential and potential shift in the first period of long-exposure

test (from AC application to current variations). IR- free potential

is expressed in V versus CSE ................................................................ 87

Table 4.7

IR-free potential and potential shift in the second period of long-

exposure test (from current variations to the end of the test).

IR- free potential is expressed in V versus CSE .................................... 88

Table 4.8

Corrosion rate due to AC interference on cathodically protected carbon

steel ........................................................................................................ 89

ix

Sommario

Le strutture metalliche interrate, come ad esempio le condotte d’acciaio per il trasporto di

idrocarburi, sono protette dalla corrosione esterna mediante un sistema di protezione

catodica combinato all’utilizzo di rivestimenti. La protezione catodica è una tecnica

consolidata che consente di annullare o minimizzare la corrosione del metallo mediante

l’applicazione di una corrente (catodica appunto) che polarizza il metallo al di sotto del

cosiddetto potenziale di protezione, -0.85 V CSE per l’acciaio al carbonio in ambiente

aerato. Tuttavia, la presenza di interferenza elettrica da corrente alternata non esclude la

corrosione in corrispondenza dei difetti dei rivestimenti, anche se il criterio di protezione è

correttamente rispettato. La corrosione da corrente alternata necessita di una sorgente di

alimentazione, tipicamente gli elettrodotti e le linee ferroviarie alta velocità/alta capacità

alimentate in corrente alternata a 50 Hz di frequenza. L’interferenza tra la linea interferente

e la tubazione avviene con un meccanismo induttivo o conduttivo. La pericolosità sta nel

fatto che la velocità di corrosione in corrispondenza dei difetti dei rivestimenti può essere

molto elevata, dell’ordine di qualche mm/anno.

Tuttavia, nonostante sia un argomento discusso da decenni, il meccanismo di corrosione non

è mai stato pienamente chiarito e, in secondo luogo, ci sono pareri discordanti sui criteri di

protezione da adottare in presenza di interferenza. A livello normativo, esiste uno standard

internazionale (ISO 18086) che definisce le soglie massima di accettabilità dell’interferenza

in presenza di protezione catodica ma su queste soglie non c’è pieno accordo in ambito

scientifico.

Il lavoro di tesi è parte di una ricerca in corso da oltre un decennio presso il Laboratorio di

Corrosione dei Materiali “Pietro Pedeferri” del Dipartimento di Chimica, Materiali e

Ingegneria Chimica "Giulio Natta" del Politecnico di Milano. Scopo della ricerca è studiare

gli effetti della corrente alternata sulla corrosione dei metalli, in particolare sull’acciaio al

carbonio in condizioni di protezione catodica. In questo contento, sono stati studiati gli effetti

della corrente alternata sulla cinetica di corrosione, i criteri di protezione catodica da adottare

in presenza di interferenza e il meccanismo di corrosione.

La tesi in particolare ha lo scopo di validare alcuni risultati ottenuti in passato in riferimento

a due aspetti: 1) effetto della corrente alternata sul potenziale di protezione; 2) studio e

Sommario

x

proposta di un criterio di protezione catodica in presenza di interferenza da corrente

alternata.

La prima parte della tesi (Capitolo 1 e Capitolo 2) è incentrata sugli aspetti generali del

fenomeno e sull’aggiornamento delle proposte riguardanti il meccanismo di corrosione

presenti in letteratura. In particolare, in questa sezione sono riportati e descritti in senso

critico i parametri considerati influenti per la corrosione da corrente alternata, così come

riportati sullo standard ISO 18086. I parametri più importanti sono la tensione alternata

indotta, la densità di CA, la densità di corrente di protezione, il potenziale di protezione, il

rapporto tra la densità di CA e di protezione, le caratteristiche del suolo, la frequenza del

segnale alternato. Ogni parametro è discusso in senso critico.

Nel Capitolo 2 sono descritti i principali modelli di meccanismo di corrosione dell’acciaio

in protezione catodica in presenza di CA. È stato effettuato uno studio bibliografico che ha

consentito di individuare i modelli più accreditati in ambito scientifico. In questa sezione è

anche descritto brevemente il meccanismo di corrosione proposto che tuttavia non è stato

oggetto della tesi.

Nel dettaglio, scopo delle prove effettuare è validare il criterio di protezione proposto in

passato all’interno del filone di ricerca e parallelamente confrontarlo con il criterio proposto

nello standard vigente ISO 18086. In breve, il criterio di protezione vigente limita il valore

massimo della tensione alternata (misurata in posizione remota) a 15 V. In aggiunta, il

criterio riporta il valore massimo di densità di corrente alternata accettabile in base al valore

di densità di corrente continua (di protezione) e del potenziale ON. Nello specifico, per livelli

di protezione catodica “più negativi” (𝐸𝑜𝑛 < −1.2 V CSE) si ha:

𝑉𝐶𝐴

|𝐸𝑂𝑁|−1.2< 3;

𝑖𝐶𝐴 < 30 𝐴/𝑚2;

𝑖𝐶𝐴

𝑖𝑃𝐶< 3 se 𝑖𝐶𝐴 > 30 𝐴/𝑚2;

mentre, per livelli di protezione catodica “meno negativi” (−1.2 < 𝐸𝑂𝑁 < −0.85 V CSE):

𝑉𝐶𝐴 < 15 𝑉;

𝑖𝐶𝐴 < 30 𝐴/𝑚2;

𝑖𝑃𝐶 < 1 𝐴/𝑚2 if 𝑖𝐶𝐴 > 30 𝐴/𝑚2.

Per validare questo criterio, fermo restando la validità dei 15 V alternati, sono state effettuate

prove galvanostatiche in soluzione simulante terreno su provini di acciaio al carbono esposti

a diversi valori di densità di corrente alternata e densità di corrente continua. Sono state

Sommario

xi

monitorate otto condizioni di protezione/interferenza: 1) 𝑖𝐶𝐴 = 10 A/m2, 𝑖𝐷𝐶 = 10 A/m2; 2)

𝑖𝐶𝐴 = 10 A/m2, 𝑖𝐷𝐶 = 1 A/m2; 3) 𝑖𝐶𝐴 = 30 A/m2, 𝑖𝐷𝐶 = 1 A/m2; 4) 𝑖𝐶𝐴 = 30 A/m2, 𝑖𝐷𝐶 = 0.2

A/m2; 5) 𝑖𝐶𝐴 = 20 A/m2, 𝑖𝐷𝐶 = 10 A/m2; 6) 𝑖𝐶𝐴 = 20 A/m2, 𝑖𝐷𝐶 = 2 A/m2; 7) 𝑖𝐶𝐴 = 50 A/m2,

𝑖𝐷𝐶 = 0.5 A/m2; 8) 𝑖𝐶𝐴= 50 A/m2, 𝑖𝐷𝐶 = 0.2 A/m2.

In sintesi, lo spettro di valori di densità di corrente alternata va da 10 a 50 A/m2 e quello

della densità di corrente continua da 0.2 a 10 A/m2 (sovra protezione catodica). Questi valori

sono stati scelti per completare le prove condotte in passato e per avere dati sensibili per il

confronto con il criterio da normativa.

Le prove, della durata di tre mesi, sono state effettuate su provini da 1 cm2 simulanti un

difetto del rivestimento di una tubazione protetta catodicamente e interferita. Le due correnti

sono state applicate mediante un apposito circuito elettrico messo a punto nelle precedenti

fasi della ricerca. Durante la prova, le densità di corrente alternata e continua e il potenziale

dei provini sono stati monitorati. Al termine della prova la corrosione è stata valutata

mediante misura di perdita di massa. A valle delle prove è proposto il seguente criterio di

protezione, più restrittivo di quello riportato sullo standard ISO 18086, basato sul valore

massimo accettabile di densità di corrente alternata:

𝑖𝐶𝐴 < 30 𝐴/𝑚2 se 𝑖𝑃𝐶 < 1 𝐴/𝑚2;

𝑖𝐶𝐴 < 10 𝐴/𝑚2 se 𝑖𝑃𝐶 > 1 𝐴/𝑚2.

In altre parole, sono state misurate velocità di corrosione non trascurabili, ossia maggiori di

10 µm/anno, su provini che non si sarebbero dovuti corrodere secondo il criterio di

protezione presente nella ISO 18086.

Un secondo set di prove è stato effettuato su provini della stessa tipologia applicando un

valore di corrente di protezione costante e aumentando nel tempo la densità di corrente

continua interferente. Scopo di queste prove non è misurare la velocità di corrosione ma

studiare gli effetti sul potenziale IR-free, che in campo è la grandezza più importante e facile

da misurare. Le prove sono state confrontate con alcuni risultati condotti in precedenti fasi

della ricerca e hanno buona riproducibilità. In particolare sono state studiate condizioni

molto diversificate: la densità di corrente di protezione varia da 0.15 A/m2 a 10 A/m2 e la

densità di corrente alternata varia da 1 A/m2 a 1,000 A/m2.

L’effetto della corrente alternata è quello di causare l’aumento del potenziale del metallo

proporzionalmente al valore di densità di corrente alternata applicato. È proposta

un’equazione empirica che correla il potenziale IR-free al valore di densità di corrente

alternata: 𝐸𝐼𝑅 𝑓𝑟𝑒𝑒(ln(𝑖𝐴𝐶)) = 𝐸𝑁𝑂 𝐴𝐶 + 5.5 × 10−2 ∙ 𝑙 𝑛(𝑖𝐴𝐶).

Sommario

xii

Come si evince, il potenziale IR-free cresce di 5.5 × 10−2 volt per decade di densità di

corrente alternata, e questa equazione empirica trova riscontro in tutte le prove

galvanostatiche effettuate.

xiii

Abstract

The external surface of carbon steel buried structure is protected from soil corrosiveness by

a cathodic protection (CP) system in combination with coating. CP reduces or stops

corrosion by means of an external DC current, which promotes the polarization of the

structure below the protection potential (-0.85 V CSE in aerated condition), where corrosion

rate is considered acceptable, i.e. lower that 10 μm/y. Nevertheless, in the presence of an

interference source, AC induced corrosion can take place, even if the protection criterion is

properly matched. Nowadays, AC induced corrosion still represents a controversial subject

and several aspects should be clarified, in particular regarding the corrosion mechanism and

the CP criterion to adopt in the presence of AC interference.

This thesis work is part of a research dealing with the study of the effects of AC interference

on carbon steel in free corrosion and CP condition. In this sense, some tailored tests,

galvanostatic and long-term exposure tests, were performed on cathodically protected carbon

steel specimens in soil-simulating solution in order to validate and confirm the preliminary

results obtained in the past. In particular, two aspects were studied: 1) the effect of AC

density on IR-free potential; 2) the CP criterion in the presence of AC interference. Results

show that IR-free potential is strongly affected by the presence of AC density, and it

increases as AC density increases. As regard the second aspect, a comparison between

experimental results and international standard is proposed.

The first part of this work (Chapter 1 and Chapter 2) summarizes and updates the information

extracted from literatures regarding the influencing factors of AC induced corrosion,

including the general aspects of the phenomenon and the corrosion mechanisms proposed

by authors. Moreover, the proposed CP criteria are reported, among which the CP criteria

present in the international standard in force (ISO 18086:2015). The purpose of the second

part of this work (Chapter 3 and Chapter 4) is to confirm, through experimental tests, the

validity of the CP criteria proposed on the ongoing research.

1

Chapter 1

AC interference corrosion of carbon steel

First discussions about the corrosion by alternating current (AC) of pipelines in cathodic

protection (CP) condition have to be dated back to the late 19th century. Nevertheless, only

in the past 30 years this phenomenon has been studied deeply, because [1]:

the growing number of high-voltage transmission lines;

the duty to place high-voltage transmission lines in proximity to pipelines and other

buried structures because of space limits imposed by private or government agencies

[2,3];

more applications using high-voltage power lines, as the high-speed railway in Europe;

the use of high-quality coatings that allows to increase the insulation conditions of the

metal but resulting in high AC densities at the coating defects along the pipeline;

poor or no awareness and knowledge of the phenomenon by pipeline operators.

Nowadays, it has stated that corrosion is possible at commercial AC frequencies (50 or 60

Hz), even if we are in presence of CP. Before going into AC corrosion characteristics, some

principles about cathodic protection are listed. This overview will be a sort of resume and

cannot be taken as literature reference, but it will set good bases in order to understand the

following discussion.

1.1 CATHODIC PROTECTION: GENERAL [4]

Cathodic protection is an electrochemical method applied to prevent or reduce corrosion in

metallic structures exposed to conductive environments. The aim is to supply a direct current

(DC) in the environment where the metal is located in order to lower its potential and reduce

or impede the corrosion. The mechanisms that rule this process and the systems through

which CP can be achieved will be explained in detail in the following paragraphs.

AC interference corrosion of carbon steel Chapter 1

2

1.1.1 Cathodic protection systems

As stated at the beginning of the Chapter 1, the cathodic protection is a technique that avails

itself of a continuous current that flows from an electrode (anode) to the metallic structure

to be protected (cathode), in the environment they’re placed in [4]. The cathodic current

induces a potential lowering in the cathode, and therefore the reducing or even setting to

zero the corrosion rate acting on the metal. The circulating current is obtained through two

different configurations: CP by galvanic anode (Figure 1.1a) or by impressed currents

(Figure 1.1b).

In the first case, current circulation, and CP, is obtained through the galvanic coupling of the

metallic structure with a less noble metal (Figure 1.1a).

Figure 1.1 - Types of cathodic protection: a) by galvanic anodes b) by impressed current system [4]

For this purpose, the selection of the anode material is made depending on the metal we want

to protect and also on the environment they’re placed in; some examples are listed:

steel protection is achieved through aluminium and zinc anodes in sea water while

magnesium is employed in soil and fresh water;

pure iron is usually used for stainless steel and copper alloys protection.

The galvanic anodes are subjected to corrosion, so their consumption is taken into account.

The second configuration, i.e. the impressed current system, involves a DC feeder (Figure

1.1b): the positive pole is connected to the anode and the negative one to the structure. In

this case, the anodes generally are consisted by an insoluble metal, such as activated

titanium. The choice between the two methods is made considering the nature of the


3

environment and the extension of the structures we want to protect. Galvanic anodes are

typically used in high conductivity environments (as sea water) and when a low protection

current is required; it’s preferred for the protection of small structures, valves or insulating

joints. Impressed current configuration is taken into account in presence of high resistivity

environments (as concrete or soil, when the resistance is usually higher than 50 Ω·m) and

when the protection of extended structures is required; it’s convenient for long pipelines

(>10 km) and complex networks, such as gas distribution systems.

1.1.2 Protection potential

Consider a metal (M) immersed and in equilibrium with an electrolyte containing its ions

(𝑀𝑧+). The equilibrium reaction is:

(Eq. 1.1) M = 𝑀𝑧+ + z𝑒−

In these conditions, the metal has an equilibrium potential (𝐸𝑒𝑞) defined by Nernst’s

equation:

(Eq. 1.2) 𝐸𝑒𝑞 = 𝐸0 + 𝐾 log𝑎

𝑀𝑧+

𝑎𝑀

where 𝐸0 is the metal standard potential, K is a temperature-dependent constant, 𝑎𝑀𝑧+ and

𝑎𝑀 are the metallic ions and metal activity in the electrolyte, respectively.

Depending on metal’s potential, in comparison with 𝐸𝑒𝑞, we can have:

if E>𝐸𝑒𝑞, the metal dissolves in the solution (anodic behaviour);

if E<𝐸𝑒𝑞, the metal deposits in form of metallic ions (cathodic behaviour).

The presence of an exchanging current between the metal and the electrolyte causes a change

in the potential: this is described by the Tafel equation, which relates the rate of an

electrochemical reaction to the overpotential, η. The dependence of the exchanging current,

i, on the difference between the actual potential and the equilibrium potential (Eq. 1.3) is:

(Eq. 1.3) 𝜂 = 𝐸 − 𝐸𝑒𝑞 = ±𝑅𝑇

𝛼𝑛𝐹ln (

𝑖

𝑖0)


4

where R is the universal gas constant, T is the absolute temperature, α is the so-called charge

transfer coefficient, n is the number of electrons involved in the reaction, F is the Faraday

constant and 𝑖0 is the exchange current density. The term ±𝑅𝑇

𝛼𝑛𝐹 is called Tafel slope: it

assumes positive or negative values for anodic or cathodic reactions, respectively.

The difference (𝐸 − 𝐸𝑒𝑞) that can be found in the previous equation (Eq. 1.3) is usually

defined as the driving voltage, indicated as ΔE. This difference describes the tendency of the

metal to corrode: a corrosive process may arise when ΔE is positive, i.e. when the metal

potential is higher than its equilibrium potential.

We find a positive driving voltage when a cathodic process has an equilibrium potential

greater than the metal equilibrium potential or involves an external current that takes

electrons away from the metal surface.

A corrosion reaction is the result of two semi-reactions: the oxidation reaction (anodic

process) that releases electrons and the reduction reaction (cathodic process) that consumes

electrons. For carbon steel in natural environment as soil, corrosion semi-reactions are:

(Eq. 1.4) 𝐹𝑒 = 𝐹𝑒2+ + 2𝑒− anodic process

(Eq. 1.5a) 2𝐻+ + 2𝑒− = 𝐻2 cathodic process

(Eq. 1.5b) 𝑂2 + 𝐻2𝑂 + 2𝑒− = 2𝑂𝐻− cathodic process

Depending on the environment conditions, the cathodic process is one between hydrogen

evolution (Eq. 1.5a) and oxygen reduction (Eq. 1.5b). The two semi-reactions (Eq. 1.4 and

Eq. 1.5a/b) are complementary, i.e. the number of e− released in the anodic process must be

the same of the number of e− consumed by the cathodic process, and they correspond to the

corrosion current (𝐼𝑐𝑜𝑟𝑟). 𝐼𝑐𝑜𝑟𝑟 is determined by the slowest process among the processes

depicted in Figure 1.2: this means that not only the driving voltage, but also kinetic factors

intervene in describing the corrosion phenomena. The determination of 𝐼𝑐𝑜𝑟𝑟 and of the

corresponding free corrosion potential (𝐸𝑐𝑜𝑟𝑟) can be evaluated at the intersection of the

anodic and cathodic curves in the Evans diagram, where E is the potential and i the current

density, expressed in a logarithmic scale.


5

Figure 1.2 - Schematic illustration of the electrochemical mechanism [5]

Figure 1.3 – a) A generic Evans diagram and b) Evans diagram for an active metal in aerated

environment, as carbon steel in soil [4]

Figure 1.3a depicts a generic Evans diagram: the intersection of the cathodic and anodic

curves determines 𝐸𝑐𝑜𝑟𝑟 and 𝑖𝑐𝑜𝑟𝑟 (in the logarithmic scale), while in Figure 1.3b a schematic

example of Evans diagram for an active metal in aerated environment, as carbon steel in soil,

is represented.

As we already stated at the beginning of Paragraph 1.1.2, below a certain potential value

(𝐸𝑒𝑞), the corrosion cannot start, because of thermodynamic reasons (the driving force for

corrosion is negative). In this case, we are in a thermodynamic immunity. If the potential of

the system overcomes 𝐸𝑒𝑞 corrosion has to be took into account. A condition in which


6

corrosion should be considered negligible or acceptable is reached when E is brought to

values close enough to the equilibrium potential: when 𝐸𝑐𝑜𝑟𝑟 > 𝐸 > 𝐸𝑒𝑞 the quasi-immunity

condition is established. Besides to thermodynamic reasons, also kinetic effects must be

considered in the potential lowering process, as in case of active-passive metals in the

presence of chlorides that can breaks the passive film (an example is stainless steel in

seawater). In this condition, the decrease in potential due to CP brings the metal to a passive

condition, reforming passivity. This condition is called protection by passivity.

1.1.3 Protection current density

In order to protect a metallic structure, a current 𝐼𝑒 must be supplied by an anode. When a

perfect protection level is achieved, the applied current is called protection current (𝐼𝑐𝑝).

Cathodic processes are typically oxygen reduction (Eq. 1.5b) and hydrogen evolution (Eq.

1.5a), depending on the environment and on 𝐸𝑐𝑜𝑟𝑟. For carbon steel in the presence of

oxygen (Figure 1.3b), approaching the protection conditions, the current is fixed to a limiting

value determined only by the quantity of oxygen that can reach the steel surface through

diffusion, i.e. the oxygen limiting diffusion current density (𝑖𝐿), that depends also on local

turbulence, temperature and on the presence of scaling (this last aspect will be investigated

later). Applying a 𝐼𝑐𝑝 equal to the cathodic current, the metal should be considered safe.

Protection current density in soil varies from about 1 mA/m2 in clayey soils, where oxygen

is almost absent, to 70 mA/m2 in sandy soils which are well aerated [4].

When potential is lower than hydrogen equilibrium potential, hydrogen evolution adds to

oxygen reduction and the cathodic current density increases by decreasing potential.

The value of 𝑖𝑝𝑐 depends also on the presence of an insulating coating on the metallic

surface: the current density needed to reach a non-corrosion condition decrease with the

coating efficiency ε:

(Eq. 1.6) 𝑖𝑐𝑝 = 𝑖𝐵(1 − 𝜀)

where 𝑖𝐵 is the protection current density of the bare metal structure. ε can vary in time

because of coating damaging or degradation, so after many years a higher 𝑖𝑐𝑝 can be

necessary to protect the metal.

Another process that can reduce the initial 𝑖𝑐𝑝, as mentioned before, is the scaling effect: a


7

calcareous deposit (composed by a mix of calcium carbonate and magnesium hydroxide

scale) can grow, if the environment allows it (sea water is one example) on the surface

because of the action of the cathodic current. This is helpful because the scale act as a barrier

that limits oxygen diffusion and maintains an alkaline environment on the surface.

The protective behaviour of deposits depends on sea water composition, current density and

mechanical action (abrasion and vibration) that determine thickness, porosity and adherence

of the scale. Once protection is interrupted, the calcareous deposit starts to dissolve.

1.1.4 Cathodic protection criteria

As mentioned in the Paragraph 1.1.2, the cathodic protection principle consists of lowering

the metal potential, in order to decrease the corrosion rate value. Depending on the metal has

to be protected and the environment it’s located in, different conditions must be taken into

account in order to achieve cathodic protection.

The immunity condition refers to metals having an active behaviour and it is achieved when

the potential is lowered below the equilibrium potential.

Usually the protection potential used in practical application is defined as quasi-immunity

potential: this potential is higher than the one applied in the immunity condition, but it

assures a corrosion rate that is acceptable from an engineering standpoint. The quasi-

immunity condition is preferred than the previous one because the corresponding potential

is easier to be achieved, also from a monetary and instrumental point of view; moreover,

decreasing the potential over a specific value, possible negative side effects have to be taken

into account, such as cathodic disbonding or hydrogen evolution. A corrosion rate of 10

µm/y is considered negligible [4]. Table 1.1 indicates the quasi-immunity protection

potentials used in soil and sea water.

For active-passive materials, such as stainless steel, aluminium steels and carbon steel in

concrete, it is not necessary to reach the immunity condition, because their anodic curve is

different from the active metal one: it is sufficient to induce a lower cathodic polarization,

which strengthens the passive film and gives rise to a better pitting corrosion resistance. This

is the protection by passivity.

In addition to these criteria, some practical approaches can be adopted in specific cases. For

example, when it is difficult to reach the immunity condition, the 100 mV depolarization

criterion can be adopted: after the current interruption, the off-potential of the metal must


8

increase of about 100 mV in a time frame that goes from 4 to 24 hours. If it happens, the

corrosion rate during the cathodic protection is supposed to be two orders of magnitude

lower than the one occurring in a non-protected structure [6].

Table 1.1 - Protection potentials for different metallic materials and environmental conditions [6]. The

protection potentials are expressed in V versus CSE.

Metallic Materials Soil Protection

potential

Carbon steels, low alloyed

steels and cast iron

Soils and waters in all conditions except those

hereunder described - 0.85

Soils and waters at 40 °C < T < 60 °C a

Soils and waters at T > 60 °C - 0.95

Soils and waters in aerobic conditions at T < 40

°C with 100 < ρ < 1 000 Ω·m - 0.75

Soils and waters in aerobic conditions at T < 40

°C with ρ > 1 000 Ω·m - 0.65

Soils and waters in anaerobic conditions and

with corrosion risks caused by Sulfate

Reducing Bacteria activity

- 0.95

Austenitic stainless

steels with PREN < 40

Neutral and alkaline soils and waters at

ambient temperatures

- 0.50

Austenitic stainless

steels with PREN > 40 - 0.30

Martensitic or

austenoferritic (duplex)

stainless steels

- 0.50

All stainless steels Acidic soils and waters at ambient

temperatures b

Copper Soils and waters at ambient temperatures

- 0.20

Galvanized steel - 1.20

a For temperatures 40 °C ≤ T ≤ 60 °C, the protection potential may be interpolated linearly

between the potential value determined for 40 °C and the potential value for 60 °C.

b Determination by documentation or experimentally.

1.2 AC INTERFERENCE

Interference corrosion can cause severe damages on buried structures. As a general

definition, interference is any alteration of the electric field caused by a foreign structure [7,8].

If the foreign body is a conductor, the current is intercepted; if it is an insulator, the current

is withdrawn. In both cases, there is a redistribution of current and potential lines within the

electrolyte. Figure 1.4 schematizes the electrical interference between two electrodes,

considering the two examples listed before.


9

Figure 1.4 - General scheme of electrical interference between two electrodes on a body: a) conductor and

b) insulator [4]

Figure 1.5 - Scheme of stationary interference between: a) two crossing pipelines and b) two almost

parallel pipelines [4]

1.2.1 Stationary and non-stationary interference

Interferences can be stationary or non-stationary. Stationary interference occurs when the

structure is immerged in a stationary electric field; this is the case, for example, of CP

systems. Figure 1.5 shows two possible cases in which we can have stationary interference.


10

In the first case (Figure 1.5a), the interfered current is collected from the pipelines where

they are nearer to the groundbed. Corrosion occurs at the crossing point, because here the

current encounter a lower soil resistance and it can be exchanged in an easier way between

the two pipelines. Figure 1.5b shows the same mechanism, but the pipelines are parallel.

Here the current is released more extensively, typically in zones in contact with low

resistivity soil. In both cases, if the interfered structure is provided with an integral coating,

interference can take place only in correspondence of coating faults and defect, and the

corrosion could be very severe since current concentrates in them. Interference effects can

be decreased if insulating coating, joints and drainages are adopted. Non-stationary

interference takes place when the electric field is variable, as in the typical case of stray

currents dispersed by traction systems. An example of interference from a DC traction

system is illustrated in Figure 1.6. We have interference only during the transit of the train,

and this leads to a corrosion in the anodic zone corresponding to the substation, that remains

fixed, while the cathodic zone follows the train: even if the time during which we have the

interference is small, the corrosive mechanism can be severe because of the high circulating

currents. This can be limited by lowering the electrical resistance of the rails and increasing

the resistivity of soil and pipelines and using drainage systems. Either DC or AC stray

currents can cause electric interference. For DC interference corrosion there is large

agreement on protection criteria for corrosion mitigation and international standards are

available for several years [4,9,10,11]. However, AC induced corrosion represents a

controversial subject and many aspects need to be clarified, especially with respect to the

mechanism by which AC causes corrosion of carbon steel in CP condition.

Figure 1.6 – Scheme of non-stationary interference caused by stray current dispersed by a DC transit

system [4]


11

1.2.2 AC interference sources

Generally, electric interference requires the existence of a source of disturbance, a coupling

mechanism and a receptor. In the case of AC interference, the source of disturbance is the

power line, the receptor is the metallic structure (as a pipeline) and the coupling between the

power line and the pipeline can occurs by different mechanisms: capacitive, resistive or

inductive mechanism [1,4]. These mechanisms are listed afterwards in this chapter.

In the practical case, high-voltage power lines and AC traction systems act as interference

sources. The reason why many cases of AC corrosion-related failures are reported is related

to the fact that buried pipelines and AC high voltage transmission lines use the same right

of way. The severity of interference is directly related to the pipeline’s electrically

continuous length that runs parallel to the source and to its external insulation from ground.

In the sections below the main sources of AC interference, i.e. high-voltage transmission

lines (HVTL) and AC traction systems, are described.

Electric power is not transported directly from the central stations to the users, but it has to

pass through substations. That’s why, to decrease energy losses during the long-distance

transmissions, electrical power is transmitted at high voltages, higher than the one needed

by the end-use costumers. So high-voltage transmission lines (HVTL) are required. HVTL

are made of high voltage (between 138 and 765 kV) overhead (Figure 1.7) or underground

conducting lines of aluminium alloy in most of the cases, because of weight and cost.

In order to be suitable to the users, the high voltage of the incoming electricity is reduced at

the substations by means of voltage transformers and then once again at the point of use, at

a final voltage that differs from country to country, depending on the local laws in force.

As far as high-speed rails lines are concerned, AC is preferred, with respect to the DC,

because AC power transmission system along the line is used mainly for long distance while

DC, on the other hand, is the preferred option for shorter lines, urban systems and tramways.

Commonly the choice of the voltage falls in the 25 kV AC voltage at a 16.7 or 50 Hz

frequency, because of the best efficiency of power transmission in terms of voltage and cost.

Nowadays the voltage of 25 kV has become an international standard.

The Italian high-speed railway (Rete Alta Velocità-Alta Capacità (AV/AC), RFI – Rete

Ferroviaria Italiana, Gruppo Ferrovie dello Stato Italiane Spa [12], Figure 1.8) uses in non-

urban sections a single-phase 25 kV AC electrification system at 50 Hz frequency.


12

Figure 1.7 – An example of HVTL Figure 1.8 – A Frecciarossa 1000 (ETR 1000) on

an Italian high-speed railway [12]

As mentioned at the beginning of Paragraph 1.2.2, AC interference, if present, causes the

coupling between the power line and the pipeline by different mechanisms: capacitive,

resistive and inductive coupling [1,4,13,14,15].

1.2.2 Capacitive coupling

The capacitive coupling is due to the influence of two or more circuits upon one another,

through a dielectric medium as air, by means of the electric field acting between them [13].

However, capacitive coupling is not very effective with buried pipelines, because the

capacitance between the pipelines and the earth is insignificant. For this reason, capacitive

coupling won’t be examined closely.

1.2.3 Resistive coupling

The resistive coupling is due to the influence of two or more circuits on one another by

means of conductive paths (metallic, semi-conductive, or electrolytic) between the circuits

[13]. This mechanism involves grounded structures of an AC power system that share the

earth with other buried structures. Coupling effects may transfer AC to a metallic buried

structure in the form of alternating current or voltage. The most common situation though

which we can have resistive coupling concerns grounded power systems affected by

unbalanced conditions, leading to a possible current flow to the earth. During a short-circuit

condition on an AC power system, a large part of the current in a power conductor flows to

the earth by means of foundations and grounding system of a tower or a substation. The


13

current flow induces a raise in the electric potential of the earth near the structure, often to

thousands of Volts with respect to remote earth, resulting in a considerable AC voltage

across the coating of a metallic structure. Lightning strikes to the power system can also

initiate fault current conditions [13]. Lightning strikes to a structure or to earth near a structure

can produce electrical effects similar to those caused by AC fault currents. These conditions

can lead to the damaging of the coating, or even of the structure itself.

1.2.4 Inductive coupling

The inductive coupling is due to the influence of two or more circuits upon one another by

means of the magnetic flux that links them [13]. This mechanism can be considered as the

main cause of AC interference on buried pipelines.

Figure 1.9 – Inductive coupling between an

AC conductor and a buried pipeline [15]

Figure 1.10 -Inductive coupling between three-phases

HVTL and a buried pipeline [15]

Indeed, inductive coupling is ever present when AC systems and buried pipelines share the

same path or when we have their crossing at some points.

AC flow in a power conductor produces an alternating magnetic field around it which

induces an AC in the coated pipeline. If a pipeline is close enough (usually some kilometres


14

[15]) and parallel to the electrical transmission line, the magnetic field will cross the pipeline

with the induction of an AC voltage on the pipeline (Figure 1.9). This is not the case of a

three-phases AC system: the current magnitudes in the three phases are equal and the three

overhead conductors are equally distant from the axis of the pipeline, no voltage will be

induced on the pipeline. However, the more frequently configuration (in which there is no

symmetry between the three-phases conductors and the pipeline) will result in a measurable

induced AC voltage [15] (Figure 1.10). In conclusion, in the case of a buried pipeline,

inductive and resistive coupling must be considered.

1.3 CHARACTERISTICS OF AC CORROSION

A buried pipeline, generally if it shares a common path with AC transmission lines, can be

affected by magnetic and electric fields generated by the power system (interference source).

In this situation, corrosion of the pipeline can occur if AC interference is present. The

evaluation of AC corrosion likelihood should be performed by considering the following

parameters [16]:

AC voltage on the structure;

alternating current density;

AC/DC density ratio;

polarization potential;

soil characteristics;

frequency of the signal;

morphology of AC corrosion.

These parameters are described in detail in the following paragraphs.

1.3.1 AC voltage on the structure

The acceptable AC voltage thresholds depend on the strategy adopted to prevent AC

corrosion; these strategies are listed in Paragraph 1.4. The ISO standard ISO 18086:2015 [16]

reports that the AC corrosion likelihood is achieved by reducing the AC voltage on the

pipeline and current densities, in a two steps procedure. As far as the AC voltage on the

pipeline is concerned, in the first step of this method it should be decreased to a value equal

or lower than 15 V rms (root mean square, in this case it is equal to the value of the direct


15

current that would produce the same average power dissipation in a resistive load [17]) over

an adequate period of time (for example 24 hours). Then, the second step consists of

achieving AC corrosion mitigation by reaching the cathodic protection potentials defined in

Table 1.1 (a more exhaustive table can be found in the standard ISO 15589-1:2015, Table 1

[6]) and maintaining iAC and iAC/iDC ratio under some specific values.

Moreover, when the system is subjected to a “more negative” cathodic protection level

(EON< -1.2 V CSE), the limiting AC voltage is set following the Eq. 1.7:

(Eq. 1.7) 𝐴𝑉

|𝐸𝑂𝑁|−1.2< 3

This criterion is reported in the Annex E (informative) of the standard mentioned above. As

it can be notice, when EON is lower than -1.2 V CSE, the highest AV accepted value is lower

than the 15 V CSE limit exposed before, i.e. when −1,2 < EON < −0,85 V CSE.

Nevertheless, the assessment of AC corrosion threat only on the basis of AV may be

misleading and different factors, as AC density, the ratio between AC and DC densities,

metal IR-free potential should take into account to define corrosion likelihood.

1.3.2 AC density

In the last 30 years many studies have been conducted on the effects of alternating currents

on metallic structures. In 1986, a corrosion failure on a high-pressure gas pipeline in

Germany was attributed to AC corrosion [18]. Analogous cases were found in Switzerland,

USA, Canada and France: the authors stated that the failures occurred although the cathodic

protection criteria were satisfied [19]. Wakelin et al. [20] summarized in an article some case

histories occurred in Canada before 1998, giving information about the conditions that

caused AC corrosion. In these studies, the authors tried to analyse the corrosion behaviour

with respect to AC density values. They reached the same conclusion; in the detail, AC

corrosion:

does not occur at AC densities lower than 20 A/m2;

is unpredictable at AC densities of 20 – 100 A/m2;

can be expected at AC densities greater than 100 A/m2.

These results are in agreement with some recent studies, such as the one reported by He et

al. [21], where the dependency of the corrosion rate of a X65 steel on the AC density is


16

reported. Goidanich et al. [22] declared that corrosion rate could be important when 𝑖𝐴𝐶 is

supposed to be higher than 30 A/m2 and a protection system should be evaluated in order to

reduce or halt AC corrosion.

The ISO standard ISO 18086:2015 [16] states that the AC density value, together with the

already discussed alternating voltage, is one of the most important parameters needed to

evaluate AC corrosion probability. A value of 30 A/m2 of AC density is shared to be critical.

Generally, an increasing AC density results in a larger amount of metal oxidation and higher

corrosion rates. As mentioned in the paragraph above, AC corrosion mitigation involves

modification in the AC density values only in a second moment. In addition to that, AC

density is not treated alone, but together with the cathodic current density. The criteria

adopted by the standard will be described in detail in Chapter 2.

1.3.3 AC density/DC density ratio

Not only the AC density, but also its ratio with the DC density, 𝑖𝐴𝐶/𝑖𝐷𝐶, is taken into account

by the ISO standard ISO 18086:2015 [16] as a parameter influencing the AC corrosion. It’s

reported that 𝑖𝐴𝐶/𝑖𝐷𝐶 < 3 when the AC density overcomes the 30 A/m2 rms value and an

“high” cathodic protection level is supplied, i.e. when 𝐸𝑂𝑁 < -1.2 V CSE.

It must be mentioned that 𝑖𝐴𝐶/𝑖𝐷𝐶 ratio doesn’t depend on the area of the metal exposed to

the electrolyte and that, firstly, this parameter should be not considered alone, but

additionally to the other parameters, such as the alternating voltage. For example, an 𝑖𝐴𝐶/𝑖𝐷𝐶

ratio of 10 can specify a condition in which we can have either a 𝑖𝐴𝐶 of 30 A/m2 and a 𝑖𝐷𝐶

of 3 A/m2 or a 𝑖𝐴𝐶 of 3 A/m2 and a 𝑖𝐷𝐶 of 0.3 A/m2. The ratios between these 𝑖𝐴𝐶 and 𝑖𝐷𝐶 are

the same, but the conditions operating in the two systems, and consequently the presumable

corrosion, are completely different.

1.3.4 Effect of polarization potential

The metallic structure potential is changed in presence of interference alternating current

densities. For carbon steel in free corrosion condition, the potential decreases with the AC

density [23,24,25,26], followed by an increase in the corrosion rate.

This trend is reported whenever only AC densities are taken into account. Actually, the

potential tends to increase with the alternating current, when a cathodic protection system is


17

present [27]. If 𝑖𝐴𝐶 is high enough to bring the potential to values greater than the protection

potential, i.e. - 0.850 V CSE for carbon steel, corrosion may start. The effect of AC on DC

potential is still controversial. In this study, a deep investigation on this effect has been done.

1.3.5 Soil characteristics

AC density (𝑖𝐴𝐶) at a coating defect depends on induced AV on the pipeline and on soil

resistivity by the following equation [28]:

(Eq. 1.8) 𝑖𝐴𝐶 =8 𝐴𝑉

𝜌𝜋𝑑

where ρ is soil resistivity and d the diameter of a circular defect having a surface area equal

to that of the real defect. As we can notice, 𝑖𝐴𝐶 is linearly proportional to the AV, while is

indirectly proportional to the soil resistivity and to the defect diameter, i.e. 𝑖𝐴𝐶 will be lower

in a soil having a higher electrical resistivity and diameter. Figure 1.11 reports the value of

the AV needed to have a 𝑖𝐴𝐶 of 100 A/m2, in function of the defect diameter and the soil

resistivity.

Figure 1.11 - AV needed to have a 𝑖𝐴𝐶 of 100 A/m2, in function of the defect diameter and the soil

resistivity [20]

The ISO standard ISO 18086:2015 [16] specify that the local soil resistivity is controlled by

the amount of soluble salts and by water content and is strongly influenced by the


18

electrochemical processes occurring on the metal surface in CP condition.

Really, the presence of a cathodic current causes the migration of cations towards the metal

affected by CP and consequently the pH electrolyte increases in the vicinity of the metal.

Depending on soil composition, the electrical resistance of the soil near the coating defect

can either increase or decrease according to the pH increase.

The Annex D (informative) of ISO standard ISO 18086:2015 [16] reports the effect of earth-

alkaline ions (as Ca2+ and Mg2+) and alkaline cations (as Na+, K+ or Li+) on the electrical

resistivity and on the formation of salts and deposits at the interface between metal and

environment. The former ones form hydroxides with relatively low solubility. The pH

increase shifts the carbonate-bicarbonate equilibrium towards the precipitation of

carbonates, with the formation of a calcareous deposit, leading to a coating defect resistance

increase up to a factor of 100. In addition to this, the presence of earth-alkaline ions extends

the passivity region expected from Pourbaix diagram of iron [5].

On the contrary, the latter ones form highly soluble hygroscopic hydroxides. Hence a low

electrical resistance due to the high ions concentration is observed, decreasing the electric

resistance at the coating defect up to a factor of 60. The current density on the metal at

coating fault of a given geometry is, therefore, dependent on the electrical conductivity and

the ratio of alkali and earth alkali ions. Moreover, the cathodic current density influences the

amount of hydroxide produced and affects, therefore, the local conductivity.

The soil composition, in relation with the corrosion behaviour of metallic structures, is

treated by other authors. For example, the ratio of alkali and earth alkali ions will be

discussed in the following chapter, being the basis of the corrosion mechanism proposed by

Voûte and Stalder [29]. Büchler et al. [30] accentuated the importance of soil composition in

the corrosion process: Ca2+ ions induce the formation of chalk layers on buried metal

surfaces, leading to a modification in the electrical conductivity, and hence in the corrosion

process, at the metallic-electrolyte interface.

In addition, the ISO standard ISO 18086:2015 [16] lists the soil resistivity parameters in

terms of AC corrosion risk:

below 25 Ω·m: very high risk;

between 25 Ω·m and 100 Ω.m: high risk;

between 100 Ω and 300 Ω·m: medium risk;

above 300 Ω·m: low risk.


19

As mentioned before, increasing the soil resistivity reduces the effects of the AC density on

the metallic structures.

1.3.6 Frequency effect

According to the studies [31,32,33], the frequency of the signal has an effect on AC corrosion:

corrosion rate decreases by increasing frequency. Especially, AC at power frequencies of 50

or 60 Hz, that are commonly used in commercial apparatuses, can cause corrosion.

Fernandes [32] discussed a kinetic effect of frequency on corrosion: with the increase of

frequency, the interval between successive anodic and cathodic half-cycles becomes shorter

and the metallic ions formed in the anodic cycle would be available for the immediate

re-deposition in the cathodic cycle. In addition, the author states that at high frequencies

hydrogen atoms formed during the cathodic cycle haven’t enough time to coalesce and form

hydrogen gas molecules. In this way, in the next anodic half-cycle, a layer of hydrogen atoms

covers the metal surface and prevents the metal dissolution reaction.

In another study by Yunovich and Thompson [33], the current flow (corrosion current)

through a steel specimen exposed to soil is calculated using an equivalent analog circuit

(Randle’s model, Figure 1.12). The circuit consists of a double layer capacitance (C1), the

solution resistance (𝑅𝑆) and the “effective resistance” (𝑅𝑒𝑓𝑓) that represents the combination

of the charge-transfer and Warburg (diffusion-related) impedances.

Figure 1.12 - Electric equivalent circuit [33]


20

Figure 1.13 - Electrical response of the circuit in Figure 1.12: the red line is the total AC, the green line is

the AC passing through 𝑅𝑒𝑓𝑓 [33]

The circuit also includes an AC power source (HVTL). The analysis allows to simulate the

electric behaviour of the metal and to calculate the current passing through each component

of an electric circuit varying the imposed AC frequency. AV is 24 V and AC density on the

specimen is approximately 400 A/m2.

The current through 𝑅𝑆 (related to corrosion process) and 𝑅𝑒𝑓𝑓 (the total current in the

circuit) is depicted in Figure 1.13, showing their dependence on the frequency of the AV

applied by the AC source. The impedance of the capacitor (C1) and the current crossing 𝑅𝑒𝑓𝑓

decreases with the frequency (going to zero when the frequency in infinite). Nevertheless,

although most of the current at 60 Hz passes through the capacitor (C1) and thus does not

affect corrosion reactions, there is an amount of AC (approximately 0.3% of the total current)

that flows through 𝑅𝑒𝑓𝑓 at 60 Hz frequency [33].

1.3.7 Corrosion Rate

The ISO standard ISO 18086:2015 [16] declares that evaluation of the AC corrosion

likelihood can be determined by the corrosion rate on a probe, following the mechanisms

described in Paragraph 1.4, above all the principle of the Electrical Resistance (ER) probe.

In the literature many articles can be found about AC corrosion, and a large corrosion rate

data variability can be collected. Nevertheless, only few essays contain information about

the time-dependence of AC corrosion of carbon steel in CP conditions. In addition to that,


21

the available data are characterized by a significantly dispersion, so the correlation of the

parameters associated to AC corrosion, such as AV and 𝑖𝐴𝐶, and the corrosion rate is a

difficult task.

One study was accomplished by Ragault [34] carrying out on-site experiments on a

polyethylene coated steel gas transmission buried pipeline cathodically protected and

parallel for 3 km with a 400 kV HVTL. The pipeline showed corrosion with corrosion depths

equal to 0.1 up to 0.8 mm after one year of installation. In addition to that, also on-site

experiments were carried out as close as possible to field conditions. 12 coupons were

installed for 18 months close to the test posts where the worst cases of corrosion were found.

Results showed that corrosion depth was comprised between 0.3 and 0.5 mm with AC

density from 30 and 4000 A/m2 and on-potential between -2.0 and -2.5 V CSE. The author

stated that there is no clear relation between AC density level and corrosion penetration

depth, but high level of AC density may be an indication of high AC corrosion risk.

1.3.8 Morphology of AC corrosion

AC corrosion morphology is localized. Camitz et al. [35], in order to study AC corrosion,

conducted a test consisting on cathodically protected steel coupons under different AC

densities; the conditions of the two sets of tests were 10 V AC for almost two years and 30

V AC for 18 months. The goal of these experiments was to record the IR-free potential of

the test coupons with an oscilloscope: the potential varied according to the AV signal and,

during the positive half cycle, off-potential shifted in the anodic direction to values less

negative than the limit value for CP, i.e. CP was periodically lost due to AC interference. In

several cases, the potential shifted to values even less negative than free corrosion potential.

Corrosion attacks could be classified into three groups:

small point-shaped attacks evenly distributed across the surface (uneven surface);

large point-shaped attacks evenly distributed across the surface (rough surface);

few large, deep local attacks on an un-corroded surface (“pocked” surface).

The type and composition of these attacks depend on the structure affected by AC corrosion

and also on its environment. Some example of studies based on the defect nature are listed

below. Nielsen and Cohn [36] described a corrosion tubercle of “stone hard soil” comprising

a mixture of corrosion products and soil often observed to grow on the coating defect surface

in the presence of AC interference, which is depicted in Figure 1.14. It was demonstrated


22

that the specific resistivity of the tubercle is significantly lower than the specific resistivity

of the surrounding soil. In addition, the effective area of the tubercle is considerably greater

than the original coating defect. The combination of these parameters causes a decrease in

the spreading resistance of the associated coating defect during the corrosion process,

making the corrosion process autocatalytic. The studies conducted by Ragault [34] and

Williams [37] are compliant on the fact that the main corrosion product on steel interfered by

AC is magnetite, sometimes combined with soil. Wakelin et al. [20] reported that the aspect

of the pit site could help to determine if AC corrosion is the primary cause of the failure.

Figure 1.14 - Schematic illustration of the tubercle of “stone hard soil” that grows at the coating defect in

connection with AC corrosion [36]

Ellis [38] reported that the AC corrosion occur forming hemispheric attacks (in which the pH

could be high) covered by hard corrosion products. Bolzoni et al. [39] proved again that AC

corrosion has a localized nature.

1.4 AC CORROSION MONITORING

The ISO standard ISO 18086:2015 [16] reports that the driving force of the AC corrosion

process is the alternating voltage (AV) occurring on the metallic structures. Additionally,

the corrosion damage induced on the pipeline by AV depends also on AC current density,

level of DC polarization, defect geometry, local soil composition and resistivity.

The standard states that there are three different approaches to prevent AC corrosion:

to limit the AC current flowing through a defect;

to control cathodic protection level;


23

to ensure that any coating remains defect free.

Depending on the chosen approach, the acceptable AC voltage thresholds may vary.

AC voltage measurements are made with reference to earth. Annex G (informative) of the

standard proposes the method to determine the reference electrode location to earth, using

the arrangement shown in Figure 1.15, where 1,2 and 3 are the reference electrode locations,

4 is the pipe and 5 is the soil.

AC voltage measurements should include the entire IR drop, so the reference electrode 1 is

not applicable for the procedure. The entire procedure is listed in the standard. Briefly, the

remote earth position can be found moving the reference electrode 2 (which is connected to

the structure by means of a voltmeter) and 3 (connected to the reference electrode 2 by means

of a second voltmeter) transversally with respect to the pipeline. When the second voltmeter

reading is close to zero, the position occupied by the reference electrode 3 is the remote earth

position, and the AC voltage measurement can be done by placing the first voltmeter (the

one connected to the structure) in the remote earth position.

The standard declares that AC measurements and controls should be integrated into the

routine monitoring of cathodic protection systems and included in the maintenance

procedures. They include:

measurements of AC voltage;

measurements of on-potential;

measurements of on-potential and/or off-potential and AC voltage on coupons or

probes;

AC and DC current densities on coupons or probes;

measurements of corrosion rate on probes;

measurements of DC and AC current on existing DC decoupling devices through all

earthing systems;

measurements of the electrical resistance of the earthing systems.

As far as the AC and DC current density measurements are concerned, Annex B

(informative) of the standard declares that they should be accomplished on coupons or

probes with 1 cm2 exposed area. The reason is to be addressed to the fact that AC corrosion

is observed on pipelines with an efficient coating with only small coating defects. At a given

potential, current density is higher on small coating defects. At a given potential, current

density is higher on small coating defects. So, for AC interference measurements, the 1 cm²

surface area has been adopted as a universal standard.


24

Figure 1.15 - Measurement of the AC gradient and localising remote earth [16]

Additional measurements shall be carried out during a representative time frame on sites

where AC interference is suspected to be more effective. Regarding the corrosion rate

monitoring, the ISO standard ISO 18086:2015 [16] asseverates that four general types of

corrosion rate measurements can be applied:

weight loss measurements;

perforation measurements;

electrical resistance (ER) measurements;

coulometric oxidation of corrosion product measurements.

1.4.1 Weight loss measurements

Weight loss measurements require installation of pre-weighed coupons in the vicinity of the

pipelines whose corrosion rate should be evaluated. In order to evaluate the occurred

corrosion rate in a reasonable time frame (months or even years), the coupon is excavated,

cleaned and weighted. The visual inspection provides detailed information of the corrosion

topography, maximum, as well as average corrosion rate but the coupon provides no

information until it is excavated.


25

1.4.2 Perforation measurements

Perforation measurements are made on special perforation probes, made of a thin steel plate,

whose thickness generally varies from 0.1 to 1 mm, and an internal electrode. The aim of

this type of measurement is to evaluate the corrosion rate of the localized attack calculating

the time needed to have the perforation of the whole plate thickness: as a matter of fact, once

it happens, the electrode gives a signal to the operator, and the corrosion rate can be assessed.

The advantages of this kind of measurements are that the probe has not to be excavated in

order to measure the corrosion depth and it’s very helpful when the localized corrosion attack

occurs with no or very low mass loss and in a very short time. The only drawback is that this

information is not available until the coupon is perforated.

1.4.3 Electrical resistance (ER) measurements

The technique involves the measuring the changes in the electrical resistance on a steel plate

integrated in a coupon because its thickness reduction by corrosion; analysing how the

electrical resistance vary in time, the corrosion rate can be assessed. In order to eliminate the

dependence of the electrical resistance on the temperature, the original coupon is connected

to another coupon, having the same characteristic but isolated from the corrosive

environment, through a protected coating.

Since a high level of AC current can pass through the coupon element, local heating of the

coupon element compared with the reference element could be expected. For this reason, ER

probes are disconnected from the pipeline and left in an open circuit condition for a short

period of time until thermal equilibrium is reached before the measurement. This will ensure

the best possible assessment of the element thickness.

1.4.4 Coulometric oxidation of corrosion product measurements

The cathodic protection current causes the increase of the pH value and the electrochemical

reduction of some of the corrosion products formed on the steel surface from Fe3+ to Fe2+.

The overall content of iron ions accumulated due to corrosion can be estimated by

electrochemical oxidation of Fe2+ to Fe3+ in the corrosion products.

Consequently, the amount of charge required for oxidation is proportional to the amount of


26

corrosion product formed over time. The coulometric oxidation can be performed with all

types of coupons or probes installed in the field and connected to a cathodically protected

pipeline. It is possible to determine the extent of corrosion that occurred in the past.

Moreover, repeating the coulometric oxidation allows to measure the further increase of the

corrosion. This technique can be considered valid if all the corrosion products are

electrochemically accessible and if the cathodic protection current is sufficiently high to

reduce the corrosion products.

1.5 AC MITIGATION

Some mitigation measures can be used in order to lower the AC corrosion degree. The ISO

standard ISO 18086:2015 [16] states that these measures can be divided up into construction

and operation measures.

1.5.1 Construction measures

Among the construction measures, the standard lists:

modification of bedding material: during the pipeline installation, sand can be used for

this purpose. However, it is useful to say that the bedding material behaviour can vary

in time, leading to a protection level decrease;

Installation of isolating joints: the AC voltage on a pipeline can be reduced by

installing isolating joints at certain positions in the pipeline in order to electrically

interrupt the longitudinal current path along the pipeline. The position of isolating

joints should consider the trend of AC and/or DC current distribution along the

pipeline;

optimization of pipeline and/or powerline route: modifying the path of the pipeline,

it’s possible to increase its distance with respect to the high voltage powerline routes,

decreasing the chances to have AC corrosion;

installation of mitigation wires: interference can be modified by the installation of an

insulated wire in close proximity but not connected to the pipeline and between

powerline and pipeline. Firstly, this is a measure to reduce or prevent short-term

interference.


27

1.5.2 Operation measures

In the standard, the following operation measures can be found:

earthing: direct or indirect earthing system installation is a method used to mitigate

the interference situation. The indirect earthing, which consists in decoupling devices

providing an electrical path for the AC current from the pipeline to earth, is preferred

in order to avoid the direct bonding disadvantages;

adjustment of cathodic protection level;

repair of coating defect.

28

Chapter 2

AC corrosion: proposed mechanisms and

protection criteria

2.1 AC CORROSION MECHANISMS

The mechanism causing AC corrosion of carbon steel (even in CP condition) is not yet fully

clear: many proposals can be found in literature, either referred to theoretical models or

based on empirical analyses. None of them is capable to explain the mechanism in its

completeness.

Some AC corrosion models proposed in literature will be discussed in the following

subchapters, referring to structures which are either cathodically protected or not (free

corrosion condition).

2.1.1 The mechanism reported on ISO 18086:2015

The ISO standard ISO 18086:2015 [16], in Annex A (informative), reports a simplified

description of the AC corrosion mechanism occurring on cathodically protected pipelines.

This proposal does not find full agreement and no experimental validation is provided. In

the presence of an alternating voltage (AV) induced on the pipeline in cathodic protection, a

current will flow through the metal surface corresponding to the coating defects.

During the cathodic half wave, the amount of current entering the steel surface and,

therefore, the rate of the cathodic reactions on the metal surface increases. During the anodic

half wave of the AC voltage, the current will leave the metal surface; the leaving of the

current takes place only if the AC voltage is sufficiently large, because other non-corrosive

processes consume part of the current.

The current leaving the metal surface can cause charging of the double layer capacitance and

oxidation of the pipeline steel. If the pH-value is sufficiently high (above 10), this oxidation

AC corrosion: proposed mechanisms and protection criteria Chapter 2

29

of the pipeline steel can result in the formation of an oxide film. This situation is found in

structures affected by CP because, as known, cathodic protection causes a pH increase at the

steel surface, up to values of 12-13 [40].

The electrochemical processes on the metal surface are schematically illustrated in Figure

2.1. During the positive half wave, the bare metal surface is oxidized resulting in the

formation of a passive film, due to the current that leaves the metal surface. During the

negative half wave, the passive film is reduced to iron hydroxide. These steps are repeated

on the following cycles, leading to the formation of a thicker iron hydroxide film and

consequently to a significant metal loss, because every AC cycle results in the metal

oxidation, i.e. the metal is consumed.

Figure 2.1 - Schematic description of the AC corrosion process with cathodic protection according to ISO

18086, where: 1) AC current on a coating defect, 2) metal, 3) passive film and 4) iron hydroxide [16].

The ISO standard, considering the description of the corrosion process, puts forward some

advices in order to reduce the AC corrosion rate, such as limiting the AC density, as first

obvious attempt.

If this is not possible, the formation and the following dissolution of the oxide film can be

avoided through sufficiently high DC current densities at sufficiently low AC current

densities. Similarly, AC corrosion can be stopped by preventing the dissolution of the oxide

film, limiting the cathodic current density. More information is provided in Paragraph 2.2.


30

2.1.2 Analysis of equivalent electric circuits

Nielsen and Cohn [36] proposed an electrical equivalent circuit analysis as a model that can

explain the AC corrosion process and mechanism, in which the impedances existing between

pipe and remote earth are well depicted in the circuit showed in the Figure 2.2.

This analysis allows to evaluate, through theoretical considerations, the influence of the

factors involved in the corrosion process and their importance. Nevertheless, it is

mentionable to declare that the equivalent circuit approximates the real system, and so the

results should be confirmed through experimental tests. In the circuit, some components can

be distinguished:

AC and DC sources, representing respectively the HVTL and the CP system, impose

a AC and DC voltage between the pipeline and remote earth at a specific location or

coating defect,

E01 and E02 represent the equilibrium potentials of the anodic and cathodic reactions

occurring at the metal interface,

Other elements represent impedances related to the physical and chemical factors that

the current encounters during its path from remote earth to the coating defect.

Authors divided these elements in static and dynamic elements, depending on their

dependency (double layer capacitance and diffusion elements) or independency (spread

resistance and charge transfer resistance) on the frequency, i.e. on time.

Among the static elements we can find:

Soil resistance or spread resistance (𝑅𝑆):

The current flows through the soil from remote earth to the coating defect; a resistance is

associated to this current flow, that depends on several factors, such as the soil solution

resistivity, soil porosity and geometrical factors existing at the interface between the soil and

the coating defect. This resistance is called spread resistance (𝑅𝑆). A great part of the ohmic

(IR) drop occurs at the coating defect. Actually, the current flux lines concentrate close to

the defect, causing a geometrical spread effect and an associated spread resistance (Figure

2.3). The spread resistance should be considered only in presence of an insulating coating

on the pipeline; when the pipeline is not coated, the resistance to be taken into account is

simply the soil resistance


31

Figure 2.2 - A schematic illustration of the electrical equivalent circuit [36].

Figure 2.3 - Geometrical effects on pipe-to-soil resistance [36].

From electrochemical impedance spectroscopy (EIS) measurements on steel electrodes

exposed in an artificial soil solution, authors found a relation between spread resistance 𝑅𝑆

[Ω·m2] and electrode area:

(Eq. 2.1) 𝑅𝑆 = 𝐾 · 𝑑 · 𝜌𝑆


32

where K is a constant depending on the geometry of the defect, d is a measure of the

extension of the defect, and 𝜌𝑆 is the soil specific resistivity. They established that small

defects have smaller spread resistances and are more susceptible to AC corrosion

Charge transfer diode analogy (VB-Elements):

A corrosion reaction consists of two semi-reactions: iron oxidation (Eq. 1.1, where Fe takes

place of M) and the cathodic process of oxygen reduction and/or hydrogen evolution (Eq.

1.4, 1.5a and 1.5b). E01 and E02 represents respectively as the equilibrium potentials of iron

oxidation and of the cathodic process. Considering the general equilibrium:

(Eq. 2.2) 𝑎𝐴 = 𝑏𝐵 + 𝑛𝑒−

at potentials different from the equilibrium potential, the process will proceed with a velocity

that can be described by the faradaic current according to the Volmer-Butler equation (Eq.

2.3):

(Eq. 2.3) 𝐼𝐹 = 𝐼𝐹,𝑎 + 𝐼𝐹,𝑐 = 𝐼0 [𝐶𝐴,𝑠

𝐶𝐴,𝑏exp (

𝐸−𝐸0

𝛽𝑎) −

𝐶𝐵,𝑠

𝐶𝐵,𝑏exp (

−(𝐸−𝐸0)

𝛽𝐶)]

where 𝐼𝐹 is the Faradaic current related to the anodic or cathodic process, 𝐼0 the exchange

current related to the process, 𝐶𝑖,𝑠 the surface concentration of species i, 𝐶𝑖,𝑏 the bulk

concentration of species i and β indicates the Tafel slope related to the anodic and cathodic

reactions. This equation consists of an anodic and a cathodic branch having the individual

current-potential characteristics (Figure 2.4):

(Eq. 2.4) 𝐼𝐹,𝑎 = 𝐼0 [𝐶𝐴,𝑠

𝐶𝐴,𝑏exp (

𝐸−𝐸0

𝛽𝑎)]

(Eq. 2.5) 𝐼𝐹,𝑐 = 𝐼0 [−𝐶𝐵,𝑠

𝐶𝐵,𝑏exp (

𝐸−𝐸0

𝛽𝐶)].

These branches can be described as exponential equations having a conducting voltage

direction and an insulating voltage direction, analogous to diodes. These directions oppose

each other in the anode and the cathode branch. Therefore, each electrochemical equilibrium

process involved in the AC corrosion event is represented in the electrical equivalent circuit

diagram as two opposites diodes (VB1 and VB2 in Figure 2.2).


33

Figure 2.4 - Illustration of the anodic- and cathodic branches of the Volmer-Butler equation and the

summarised total current [36].

The dynamic elements are characterized by:

Diffusion (W-Elements):

Diffusion is defined as the migration of species due to a concentration gradient. At the

coating defect, species are consumed or produced and can diffuse in the direction of the

concentration gradient. This takes place at the steel surface, because the cathodic protection

reactions cause an increase of the pH of the electrolyte, leading to a subsequent diffusion

process; the rate of this phenomenon can be described by Fick’s law. The diffusion element

is represented as a Warburg impedance element (W) which impedance 𝑍𝐷 is given by:

(Eq. 2.6) 𝑍𝐷 =𝜎

√𝜋𝑓=

𝑅𝑇

𝑍2𝐹2𝐴√2(

1

𝐶𝑂,𝑏√𝐷𝑂+

1

𝐶𝑅,𝑏√𝐷𝑅) ·

1

√𝜋𝑓

where σ is the Warburg coefficient, R the gas constant, T the temperature, z the number of

electrons involved in the electrochemical process, F the Faraday’s constant, A the area of the

coating defect, 𝐶𝑂,𝑏 the bulk concentration of oxidant, 𝐶𝑅,𝑏 the bulk concentration of

reductant and 𝐷𝑂 and 𝐷𝑅 are the related diffusion coefficients.


34

Interfacial capacitance (C):

The corrosion process leads to a dissolution of positively charged ions in solution, whereas

the excess electrons are “accumulated” in the steel-lattice. This process attracts the positively

charged ions from the electrolyte, resulting in the formation of an electronegative front on

the steel surface and on an electropositive one in the electrolyte (Figure 2.5).

Figure 2.5 - Schematic illustration of the steel-water interface acting as a capacitor [36].

At the interface, this charge separation can be seen as a capacitor, characterized by a

capacitance that is called, due to its nature, double layer capacitance. Looking at the equation

defining the impedance of a capacitor:

(Eq. 2.7) 𝑍𝐶 =1

2𝜋𝑓𝐶

where f is the applied frequency and C is the capacitance: impedance depends on the applied

frequency (i.e. time). Regarding to the corrosion processes, this kind of capacitor can be

associated to any interface existing at the corroding interface, such as a double layer or any

films covering the surface.

Authors state that the VB1 element is the key element in order to evaluate the corrosion

process. VB1 is related to the metal dissolution and re-deposition. Corrosion can occur only

if the anodic charge released due to iron dissolution exceeds the cathodic charge released

due to re-deposition of dissolved iron. In other words, being ∆𝑄𝑎 and ∆𝑄𝑐 the anodic and

cathodic charges, respectively, released in the time interval ∆𝑡, corrosion occurs if:

(Eq. 2.8) ∆𝑄𝑎

∆𝑡>

∆𝑄𝑏

∆𝑡.


35

Authors considered the influence of the circuit elements in the evaluation of the corrosion

itself. They assessed that the spread resistance 𝑅𝑆 acts the main role in controlling the AC

corrosion magnitude, at least at frequency of 50-60 Hz. Indeed, the impedance 𝑍𝐶 is much

lower that 𝑅𝑆 at these frequencies. The spread resistance determines the amount of AC-

voltage that is lost across the soil resistance: greater is the former, greater is the latter.

Besides, the size of the coating defect is one parameter that has a major influence on 𝑅𝑆, as

suggested in Eq. 2.1: larger defects are more helpful is reducing AC corrosion, going to

increase the 𝑅𝑆 value.

Another aspect to be examined is the soil composition, and more specifically the ratio

between earth alkaline and alkaline cations present in the soil. As mentioned in Paragraph

1.3.5, earth alkaline cations form hydroxides in high-pH environments, as the one due to the

CP. These hydroxides can be converted into carbonates and both hydroxides and carbonates

of earth alkaline cations are known to be solids and characterized by a low solubility. Solid

precipitates go to increase 𝑅𝑆 and therefore they reduce the AC corrosion likelihood.

Nevertheless, hydroxides of alkaline cations are quite soluble, and they do not form solid

precipitates. In conclusion, the presence of earth alkaline cations in combination with a pH

increase caused by cathodic protection is expected to increase 𝑅𝑆.

Spread resistance is influenced also by the presence of tubercles of “stone hard soil”, already

described in Paragraph 1.3.8. They are distinguished by a lower specific resistivity with

respect to the one of the surrounding soil. In addition, the effective area of the tubercle is

considerably greater than the original coating defect: the current flux to the pipe at the

coating defect can spread out using the entire area of the tubercle before entering the pipe.

The combination of these parameters causes a decrease in the spreading resistance of the

associated coating defect during the corrosion process, making the corrosion process

autocatalytic.

In conclusion, the authors stated that the spread resistance 𝑅𝑆 is the main parameter that

influences the AC corrosion of buried metallic structures at frequency of 50-60 Hz.

Consequently, every factor that influences 𝑅𝑆 is involved in the AC corrosion assessment.

2.1.3 Earth-alkaline vs. alkaline cations effect

As mentioned before (Paragraph 2.1.2), the chemical composition of the environment at the

steel-soil interface has a role in the assessment and in the controlling of AC corrosion, having

an influence on the spread resistance [29].


36

In the study conducted by Voûte and Stalder, the parameter considered is the ratio between

earth-alkaline cations (as Ca2+ and Mg2+) and alkaline cations (as Na+, K+, or Li+). Earth-

alkaline cations form hydroxides, i.e. Ca(OH)2 and Mg(OH)2, because of the alkalinity

conditions present at the metal surface in CP condition (CP causes an increase of pH in the

vicinity of the protected pipeline). The pH increase shifts the carbonate-bicarbonate

equilibrium towards the precipitation of carbonates (CaCO3, MgCO3), causing the formation

of a calcareous deposit (discussed also in Paragraph 1.3.5).

Authors stated that solid deposits with low solubility, as the ones formed by the presence of

earth-alkaline cations in soil, act to increase the spread resistance associated with the coating

holidays, going to lower the AC magnitude at the coating defects.

In addition, earth alkaline cations have been indicated to passivate the anodic branch of the

metal dissolution (VB1) process at pH values as low as 6, leading to the decrease of the AC

corrosion caused by a Volmer-Butler anodic dissolution mechanism. So, Stalder proposed

that the ratio of earth alkaline cations to alkaline cations is crucial to assess areas where AC

corrosion is most probable.

2.1.4 A conventional electrochemical approach in the absence of CP

Yunovich and Thompson [33] proposed a conventional electrochemical approach in order to

describe an AC corrosion model for carbon steel in free corrosion condition, i.e. a structure

not in cathodic protection. In this analysis, some assumptions are made:

metal loss reactions are non-reversible;

the cathodic reaction is oxygen reduction;

metal loss is the only available oxidation reaction;

each electrochemical reaction has a specific time constant.

The frequency of the AC signal is 60 Hz. Figure 2.6 shows how an AC signal interferes a

corrosion process. The values for potential and current are realistic values: a 𝑖𝑐𝑜𝑟𝑟 of 4.7 mA

and a 𝐸𝑐𝑜𝑟𝑟 of -0.7 V CSE leads to a corrosion rate of 0.08 mm/y for a 4,580 mm2 specimen

exposed surface.

The AC signal shifts the corrosion potential the anodic and cathodic direction, causing a

potential shift of 150 mV. For an active metal, the potential-current relationship is defined

by the Tafel’s law:

(Eq. 2.9) 𝐸 = 𝑏 + 𝛽𝑎 log(𝑖)


37

where 𝛽𝑎 is the anodic Tafel slope of the metal in soil (assumed 0.150 V per decade of

current). The potential of the metal (E) is the sum of the corrosion potential (𝐸𝑐𝑜𝑟𝑟) and the

alternating potential due to the presence of the AC interference (𝐸𝐴𝐶):

(Eq. 2.10) 𝐸 = 𝐸𝑐𝑜𝑟𝑟 + 𝐸𝐴𝐶 = 𝐸𝑐𝑜𝑟𝑟 + 𝐸𝐴 sin(2𝜋𝑓𝑡)

where 𝐸𝐴 is the amplitude of the potential shift and 𝑓 is the frequency (60 Hz). From Eq. 2.9

and Eq. 2.10, the relationship between current and potential becomes:

(Eq. 2.11) 𝑖 = 10𝐸𝑐𝑜𝑟𝑟+𝐸𝐴𝐶−𝑏

𝛽𝑎 = 10𝐸𝑐𝑜𝑟𝑟+𝐸𝐴 sin(2𝜋𝑓𝑡)−𝑏

𝛽𝑎 .

Figure 2.6 - An electrochemical description of AC corrosion [33].

The potential sinusoidal shift does not correspond to a sinusoidal shift of the current, due to

the non-linear relationship between potential and current (E is linearly proportional to log 𝑖).

The current increase during the anodic half cycle (A→B→0) is greater that its decrease in

the cathodic half cycle (0→C→A), due to the logarithmic dependence of the potential to the

current (Figure 2.7). Because of that, AC polarization of the metal produces a net anodic

(oxidation) current greater than the free-corrosion current.

It is only right to say that hydrogen evolution is depicted as cathodic reaction in Figure 2.6,

instead of the oxygen reduction suggested by the assumptions listed before. This seems

contradictory. Figure 2.6 is for illustrative purposes only, being the current oscillation easier

to understand on a straight line (hydrogen evolution curve) rather than on a curve line

(oxygen reduction curve).


38

Figure 2.7 - Potential and current shifts for a single period at 60 Hz AC [33].

2.1.5 The alkalization mechanism

The coexistence of high pH, because of the CP, and potential oscillations caused by AC

interference can lead to corrosion attacks: this theory, named alkalization theory, was

proposed by Nielsen et al. [41,42,43].

The effects, associated to AC interference, that characterize this mechanism are:

the alkalization of the environment close to the coating defect in the presence of high

protection current density;

potential oscillations between the passive, the immunity and the high-pH corrosion

region of steel potential-pH diagram in the presence of AC (Figure 2.9).

Alkalization of the environment at the coating fault arises from the cathodic protection

current: the cathodic process involved, the oxygen reduction (Eq. 1.5b), electrochemically

reduces water into hydroxides (𝑂𝐻−). Figure 2.8 represent the hydroxide (𝑂𝐻−) mass

balance in a volume element at the coating fault. The CP current density determines the

influx of 𝑂𝐻−, the neutralization of the produced 𝑂𝐻− is given by the chemistry at the soil-

metal interface expressed as the base (𝑂𝐻−) neutralizing effect (BNE value) and the outflux

of the hydroxides depends on the outward diffusion from the defect area into the bulk.


39

Figure 2.8 – Mass balance schematics for 𝑂𝐻− ions produced by CP at a coating defect [42].

Figure 2.9 – Pourbaix diagram: the hatched area indicates the critical AC corrosion zone [42].

This mass balance determines whether hydroxides accumulate at the surface, leading to a

local pH increase. Authors stated that there is an incubation time needed to reach a pH critical

value in the electrolyte near the metal surface.

Corrosion occurs because of potential oscillations: the time constant for metal dissolution

and passive film formation are different, being the first process faster than the second one.


40

In addition, when the pH is close to 14, high corrosion rates are possible due to the formation

of 𝐻𝐹𝑒𝑂2− [43]. This high pH value can be reached by imposing high CP current, because it

increases the pH locally at the coating defect and, in combination with potential oscillations,

could lead to the periodic entry in the high-pH corrosion domain in the Pourbaix diagram

(Figure 2.9). Nevertheless, the corrosion occurs only if the electrochemical reactions are fast

enough within the time during which the potential crosses the corrosion area, i.e. where

𝐻𝐹𝑒𝑂2− is formed [43]. The AC corrosion likelihood can be decreased avoiding the pH

increase: this occurs when the production rate of the hydroxyl ions is lower than its diffusion

rate, leading to a depletion of OH- on the metallic surface.

Some testes where carried by Nielsen in order to analyse the influence of the DC level on

AC corrosion [42,43]. Carbon steel coupons were subjected to a fixed 15 V AC, in a controlled

soil box experiment. The DC conditions of the coupon were periodically changed between

excessive CP (-2.25 V CSE) and mild CP (-0.85 V CSE) to study the changes in corrosion

conditions. High CP densities lead to high corrosion rates after the incubation time;

decreasing the CP level, the AC density dropped, and the corrosion rate decayed (Figure

2.10). A parallel discussion was made about the spread resistance. Being 𝑅𝑆 inversely

proportional to DC density, when the latter is increased, the former decreased (Figure 2.11).

This trend is supported by the fact that a high CP density produces more hydroxides at the

soil-metal interface, going to lower the local soil specific resistivity, and hence the spread

resistance (Eq. 2.1.).

Figure 2.10 - DC on-potential (𝑈𝑂𝑁) and corrosion rate measured with ER coupon [42].


41

Figure 2.11 - DC on-potential (𝑈𝑂𝑁) and spread resistance (𝑅𝑆) measured with ER coupon [42].

The authors concluded that high CP level has a dramatic influence on the AC corrosion

process. Excessive CP increases the AC corrosion rate and should therefore be avoided.

2.1.6 Theoretical corrosion models

In some articles [44,45], it’s proposed that the corrosion rate of a metal, due to an induced AV

on the pipeline, can be estimated knowing the absolute ratio of the anodic and cathodic Tafel

slopes (indicated as 𝑟 = |𝛽𝑎/𝛽𝑐|) of the processes occurring on the metal surface. r is

reported to be a parameter to determine the sensitivity with respect to the AC caused

polarization: when the characteristic curves of the processes are asymmetric, i.e. when

𝑟 = |𝛽𝑎/𝛽𝑐| ≠ 1, a shift of the corrosion potential is expected, due to the superimposition

of an external sinusoidal voltage on the metallic structure.

An analytical solution was advanced by Lalvani and Lin [46]: firstly, they studied the

relationship between corrosion rate and AV amplitude. They related the metal potential (E)

with the current density related to the anodic (a, dissolution of the metal) and cathodic

process (b, hydrogen evolution), though the Tafel equation:

(Eq. 2.12) 𝐸 = 𝛽𝑖 ∗ ln(𝑖𝑖) + 𝑏𝑖

where the subscript i stands for cathodic (c) or anodic (a), 𝛽𝑖 is the Tafel slope (expressed in

mV/decade), 𝑖𝑖 is the current density and 𝑏𝑖 is the vertical intercept of the Tafel lines related

to the anodic and cathodic processes.


42

Only hydrogen evolution is taken into account as cathodic process, because oxygen

reduction does not follow the Tafel law. The corrosion potential, 𝐸𝑐𝑜𝑟𝑟,𝐷𝐶, is the steady state

DC potential at which the anodic and cathodic current densities are equal to one another.

In presence of an alternate voltage (AV) interference, the potential E can be written as the

sum of a DC potential, 𝐸𝐷𝐶, and the AV signal:

(Eq. 2.13) 𝐸 = 𝐸𝐷𝐶 + 𝐸𝑝 sin(𝑤 ∗ 𝑡)

where 𝐸𝑝 and w are the peak potential and the frequency of the sinusoidal signal,

respectively. It can be shown that 𝐸𝐷𝐶 is equal to 𝐸𝑐𝑜𝑟𝑟,𝐷𝐶, when 𝐸𝑝 is equal to zero.

Integrating over a period of the sinusoidal signal, it is possible to derive an equation where

the corrosion potential assumed by the metal in presence of the AV interference (Ecorr,AV) is

related to the potential shift (-α) from 𝐸𝑐𝑜𝑟𝑟,𝐷𝐶:

(Eq. 2.14) 𝐸𝑐𝑜𝑟𝑟,𝐴𝑉 = 𝐸𝑐𝑜𝑟𝑟,𝐷𝐶 − 𝛼.

The potential shift (-α) is a function of 𝐸𝑝 and it depends on the Tafel slopes (𝛽𝑎 and 𝛽𝑐) and

hence on their absolute ratio r (𝑟 = |𝛽𝑎/𝛽𝑐|). When the two slopes are symmetric, i.e.

|𝛽𝑎| = |𝛽𝑐|, 𝛼 = 0, and hence 𝐸𝑐𝑜𝑟𝑟,𝐴𝑉 = 𝐸𝑐𝑜𝑟𝑟,𝐷𝐶: no shifts from the DC corrosion potential

is expected when 𝑟 = 1. Knowing 𝐸𝑝 and r is possible to calculate (-α), and so 𝐸𝑐𝑜𝑟𝑟,𝐴𝑉:

if 𝑟 = 1, α is always equal to zero:

if 𝑟 < 1, (-α) is negative, and 𝐸𝑐𝑜𝑟𝑟,𝐴𝑉 is lower than the initial corrosion potential and

it decreases with the 𝐸𝑝 increase (Figure 2.12a);

if 𝑟 > 1, (-α) is positive, and 𝐸𝑐𝑜𝑟𝑟,𝐴𝑉 is higher than the initial corrosion potential and

it increases with the 𝐸𝑝 increase (Figure 2.12b).

The dimensionless corrosion current (𝑖𝑐𝑜𝑟𝑟,𝐴𝑉/𝑖𝑐𝑜𝑟𝑟,𝐷𝐶) increases rapidly with 𝐸𝑝 for all

values of r (Figure 2.13a and Figure 2.13b).

In a revised work, the authors introduced the effect of the double-layer capacitance [47]. They

studied the dependence of the DC corrosion potential (𝐸𝐷𝐶) on the root-mean-square current

(𝑖𝑟.𝑚.𝑠.) and on the peak potential 𝐸𝑝, for different r values (𝑟 < 1, 𝑟 = 1 and 𝑟 > 1).

A shift from 𝐸𝐷𝐶 and an increase of 𝐸𝑝 causes an increase of 𝑖𝑟.𝑚.𝑠. for every r value. The

discordance about these three situations lies on the potential (𝐸𝑟.𝑚.𝑠.,min) at which the root-

mean-square current is a minimum (𝑖𝑟.𝑚.𝑠.,min) as a function of 𝐸𝑝. For the case 𝑟 = 1, no

shift in 𝐸𝑟.𝑚.𝑠.,min is observed.


43

Figure 2.12 - Potential shift vs. 𝐸𝑝 for a) 𝑟 ≤ 1 and b) 𝑟 > 1 [46].

Figure 2.13 – Corrosion current vs. 𝐸𝑝 for a) 𝑟 ≤ 1 and b) 𝑟 > 1 [46].

Figure 2.14 – DC potential vs the root-mean-square current for a) 𝑟 < 1 and b) 𝑟 > 1 [47].


44

Figure 2.15 - 𝐸𝑟.𝑚.𝑠.,min vs r [47]. Figure 2.16 - 𝑖𝑟.𝑚.𝑠.,min vs r [47].

For 𝑟 < 1 (Figure 2.14a) and 𝑟 > 1 (Figure 2.14b), an increase of 𝐸𝑝 results in a

corresponding shift potential to more active and more noble direction, respectively.

Moreover, the dependence of 𝐸𝑟.𝑚.𝑠.,min (Figure 2.15) and 𝑖𝑟.𝑚.𝑠.,min (Figure 2.16) on r and

𝐸𝑝 was considered. 𝐸𝑟.𝑚.𝑠.,min decreases for 𝑟 < 1 and increases for 𝑟 > 1. The absolute

shift from 𝐸𝑟.𝑚.𝑠.,min assumed at 𝑟 = 1 depends on 𝐸𝑝: greater is 𝐸𝑝, greater is the absolute

shift. 𝑖𝑟.𝑚.𝑠.,min increases for every r and 𝐸𝑝 value. It was found out that 𝑖𝑟.𝑚.𝑠.,min increases

with frequency and r.

Lalvani proposed another model, in collaboration with Xiao [48], where the corrosion process

characterizing a metallic structured subjected to an AV interference is represented by three

elements in an equivalent electric circuit (Figure 2.17): the double-layer capacitance 𝐶𝑑𝑙, the

solution resistance 𝑅𝑆 and the polarization impedance 𝑍𝑝.

Some results are in agreement with other studies, such as the dependence of the

dimensionless corrosion current on the peak potential 𝐸𝑝 (Figure 2.18a) and on the frequency

(Figure 2.18b), and the dependence of the potential shift, here labelled as ∆𝐸𝑐𝑜𝑟𝑟, on 𝐸𝑝

(Figure 2.19). The authors pointed out that the dependence of the dimensionless corrosion

current, i.e. of the corrosion rate, is directly proportional to r for any given signal frequency

(Figure 2.18b). Furthermore, this dependency is much more pronounced at relatively low

frequency. They ascribed this trend to the double-layer impendence: it decreases

significantly at high frequencies, leading to an increase in the no faradaic current and a

following decrease in the current generated through faradaic processes.

Nevertheless, some conclusions contrast with the results obtained by other authors and

discussed before (Paragraph 2.1.5): they declare, from experimental analyses, that AC

corrosion is independent from the DC corrosion potential (Figure 2.20).


45

Figure 2.18 – Dimensionless corrosion current vs a) peak potential and b) frequency [48].

Figure 2.19 - ∆𝐸𝑐𝑜𝑟𝑟 vs 𝐸𝑝 [48].

Figure 2.20 - Dimensionless corrosion current

vs 𝐸𝑐𝑜𝑟𝑟 [48].

2.1.7 AC effect on overvoltages

Some authors reported that AC has effect on cathodic and anodic overvoltages. Goidanich

et al. [49] performed galvanostatic polarisation tests on different metallic materials (carbon

steel, galvanised steel, zinc and copper) under different experimental conditions, in order to

Figure 2.17 - Electrical equivalent circuit proposed by Lalvani and Xiao [48].


46

investigate the influence of AC on kinetics parameters, such as corrosion current density

(𝑖𝑐𝑜𝑟𝑟), corrosion (𝐸𝑐𝑜𝑟𝑟) and equilibrium potential (𝐸𝑒𝑞), anodic (𝛽𝑎) and cathodic (𝛽𝑐)

Tafel slopes. Below are reported the effect of AC on polarisation curves (Figure 2.21) and

on corrosion current and potential (Figure 2.22) for carbon steel in 4 g/L 𝑁𝑎2𝑆𝑂4 solution.

The authors confirmed that AC has a strong influence on these parameters; the effect of the

alternating current was found to be dependent on the system studied and on the supplied AC

density. Generally, overvoltages decreased and the exchange current densities increased in

all test conditions, as the AC signal increases. Corrosion or equilibrium potential decreased,

apart from the tests conducted in soil-simulating solution (1.77 g/L 𝑁𝑎2𝑆𝑂4 and 0.41 g/L

𝐶𝑎𝐶𝑙2 · 2𝐻2𝑂), where it increased with AC density: this trend is in contrast with the

mathematical models discussed before (Paragraph 2.1.6), because a decrease in 𝐸𝑐𝑜𝑟𝑟 is

expected for 𝑟 < 1. Other discrepancies were observed. The authors concluded that AC-

induced corrosion is a complex phenomenon and no model could describe it in an exhaustive

way at that time. Several factors should be considered giving rise to a mixed mechanism.

Nielsen [42] performed galvanostatic polarisation tests on a carbon steel coupon in presence

of different AC densities. The results proved that an increase in 𝑖𝐴𝐶 causes an increase in

𝑖𝐷𝐶, raising the rate by which hydroxyl ions are generated in CP condition; the reported

trends of 𝐸𝑐𝑜𝑟𝑟 with respect to the direct and alternating current densities are in accordance

to the results obtained by Goidanich.

Figure 2.21 - Effect of AC on polarisation curves of carbon steel in 4 g/L 𝑁𝑎2𝑆𝑂4 solution [49].


47

Figure 2.22 - Effect of AC on corrosion current and potential for carbon steel in 4 g/L 𝑁𝑎2𝑆𝑂4 solution [49].

2.1.8 A two-steps mechanism

A. Brenna et al. [50] proposed a two-step AC corrosion mechanism of carbon steel under

cathodic protection condition. The first step regards the electrochemical breakdown of the

passive film, formed on the carbon steel surface because of the cathodic protection, while

the second step concerns the high-pH chemical corrosion occurring after the passive film

breakdown.

Step 1: the passive film electromechanical breakdown mechanism

The authors stated that the theoretical models describing the initiation process of passive

film breakdown could be grouped into three classes:

adsorption-induced mechanism;

ion migration and penetration models;

mechanical film breakdown theories.

Both adsorption-induced mechanism and the penetration models involve the presence of

aggressive anions in the environment surrounding the carbon steel, i.e. the electrolyte. In the

former, the aggressive anions are related to the formation of surfaces complexes: being

transferred to the electrolyte much faster than uncomplexed iron ions, it results in a local

thinning of the passive layer and in its complete breakdown, followed by the formation of a

pit. The latter depicts the penetration of the aggressive anions to the metal-oxide interface

through the passive layers; once accumulated, they cause internal stresses and pit nucleation.

The third model reported, proposed by Vetter and Strehblow [51] and Sato [52], ascribes the

mechanical breakdown to a sudden change of the electrode potential, allowing the direct


48

access for the aggressive ions to the unprotected metal. Despite investigating three different

aspects, Strehblow suggested that these mechanisms should be considered together in the

description of AC corrosion. After having introduced them, Brenna et al. focused on the third

mechanism, i.e. film breaking mechanism, taking into account the description provided by

Sato [52]. He suggested that the mechanical failure can be mainly attributed to high

electrochemical stresses, i.e. the so-called electrostriction pressure, generated by the

presence of an electric field across the film and by the interfacial tension, that cannot be

neglected because of the thin thickness of the oxide layer. The electrostriction phenomenon

regards dielectric materials under an electric field: it polarizes the randomly aligned

electrical domains within the material, causing an opposite charging of the two sides of the

domains. Therefore, these sides attract each other, leading to a thinning of the oxide layer in

the direction of the electric field. Considering a dielectric field of thickness L mechanically

free to deform from the electrolyte side but constrained on the metal surface, it is subjected

to a film pressure σ (N/m2), acting perpendicular to it, which is the sum atmospheric pressure

(𝜎0), the electrostriction pressure (𝜎𝐸) and the interfacial pressure (𝜎𝛾):

(Eq. 2.15) 𝜎 = 𝜎0 + 𝜎𝐸 + 𝜎𝛾 = 𝜎0 +𝜀0(𝜀𝑅−1)𝐸2

8𝜋−

𝛾

𝐿

where 𝜀0 is the vacuum electric permittivity, 𝜀𝑅 is the relative permittivity of the oxide, E is

the electric field and γ is the interfacial tension. The electrostriction is dependent on the

electric properties of the material, i.e. 𝜀𝑅, and on the square of the electric field, while the

interfacial pressure depends on the oxide layer thickness (L): 𝜎𝛾 has a strong relevance at

low film thickness. The breakdown occurs when the film pressure σ reaches the mechanical

resistance 𝜎𝑅, corresponding to the breakdown electric field 𝐸𝐵𝐷. 𝐸𝐵𝐷 can be derived from

Eq. 2.15:

(Eq. 2.16) 𝐸𝐵𝐷 = √(𝜎𝑅−𝜎0+

𝛾

𝐿)×8𝜋

𝜀0(𝜀𝑅−1).

It is reported [51,52] that the breakdown electric field is in the order of 106 V/cm. Moreover,

the authors suggested that 𝐸𝐵𝐷 may be related to a breakdown alternating voltage (𝑉𝐵𝐷),

above which passive film breakdown occurs. The measured alternating voltages results to

be the sum of three contributions:

(Eq. 2.17) 𝑉𝐴𝐶 = 𝑉𝐴𝐶,𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 + 𝑉𝐴𝐶,𝑜𝑥𝑖𝑑𝑒 + 𝑉𝐴𝐶,𝑚𝑒𝑡𝑎𝑙


49

where 𝑉𝐴𝐶,𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 is the voltage drop in the solution between the tip of the Luggin capillary

and the specimen, while 𝑉𝐴𝐶,𝑜𝑥𝑖𝑑𝑒 and 𝑉𝐴𝐶,𝑚𝑒𝑡𝑎𝑙 are the voltage contribution across the

passive film and the metallic phase, respectively. The authors reported that the contribution

of 𝑉𝐴𝐶,𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 and 𝑉𝐴𝐶,𝑚𝑒𝑡𝑎𝑙 in the alternating voltage measurement may be neglected,

leading to the definition of the electric field across the passive film as the ratio of 𝑉𝐴𝐶,𝑜𝑥𝑖𝑑𝑒,

and hence 𝑉𝐴𝐶, and the thickness L of the oxide film:

(Eq. 2.18) 𝐸 =𝑉𝐴𝐶,𝑜𝑥𝑖𝑑𝑒

𝐿≅

𝑉𝐴𝐶

𝐿.

As consequence, 𝐸𝐵𝐷 in Eq. 2.16 can be described as the breakdown alternating voltage

measured experimentally (𝑉𝐵𝐷) per unit length of the passive film thickness.

The experimental tests conducted by the authors confirm the truthfulness of the mechanism

above mentioned, as far as the electrochemical breakdown of the passive film is concerned.

The measured breakdown electric fields resulted to be in the order of 106 V/cm, as was

indicated by Vetter and Strehblow [51] and Sato [52]. Furthermore, the results suggested that

𝐸𝐵𝐷 may be related to the corrosion resistance of the passive film, described by the PREN

(pitting resistance equivalent number) index, and to the IR-free potential of the specimens:

the breakdown electric field resulted to be higher in correspondence of higher PREN indexes

and more negative IR-free potentials.

Step 2: high-pH corrosion of overprotected carbon steel

As already described in Paragraph 2.1.5, it is reported by the authors that the imposition of

high CP currents could lead to a local increase in the pH at the passive film defect. When

the pH is close to 14, high corrosion rates are possible due to the formation of 𝐻𝐹𝑒𝑂2− (Figure

2.9). Therefore, after the electromechanical breakdown of the passive film, Brenna et al.

stated that corrosion could be expected if the potential-pH condition of the carbon steel under

cathodic protection condition crosses the high-pH corrosion domain in the Pourbaix

diagram. The authors investigated the pH trend in the solution in contact with the

cathodically protected carbon steel specimens though galvanostatic tests. It was found that

pH values in between 13 and 14 were measured at the metal surface when CP current

densities of 5 A/m2 were furnished. Brenna et al. asserted that the environment in direct

contact with the carbon steel specimens should be analysed separately from the bulk

environment, because only the solution in close proximity to the metal, and consequently

the solution inside the cracks generated by the electrochemical breakdown of the passive


50

film, experiences a so relevant pH increase. High CP currents lead to overprotection

conditions and to strong alkalization in the cracks, and hence corrosion because of the

periodic entry in the high-pH corrosion domain of the Pourbaix diagram. The authors

hypothesized that only chemical corrosion can occur in overprotection conditions, being any

oxidation reaction inhibited by the cathodic protection. The chemical corrosion is controlled

by a chemical equilibrium between species, instead of electrochemical, and hence the

process results to be potential independent. Moreover, an increase in the CP current would

be counterproductive, going to increase the pH at the metal surface. In conclusion,

overprotection, caused by high cathodic protection current densities, resulted to be the worst

condition in the presence of AC, because it causes a pH increase leading the system into the

high-pH corrosion domain in the Pourbaix diagram, and therefore it should be avoided.

2.2 CATHODIC PROTECTION CRITERIA

After the characterization of the AC corrosion mechanisms described before, in this part of

the chapter the cathodic protection criteria reported in ISO standard and proposed by some

authors are discussed. These criteria usually are not derived by theoretical models, but rather

they come from empirical analyses and field experiences; nevertheless, they can be

explained starting from theory and results are, in most of the cases, in accordance with it.

2.2.1 Cathodic protection criteria reported on ISO 18086:2015

The ISO standard ISO 18086:2015 [16] reports two different methods that should be satisfied

in order to not incur AC corrosion. They differ in the cathodic protection level chosen to

protect the metallic structure, suggesting different voltage and current density thresholds.

AC values are r.m.s. (root-mean-square) ones and current densities are measured on a 1 cm2

circular coupon or probe. The description below is reported in the Annex E (informative) of

the standard. The standard states that the criteria as defined in ISO 15589-1:2015 [6] and

reported in Table 1.1 should be respected as first point. The achievement of a potential equal

to or lower than the protection potential is necessary to avoid any corrosion likelihood. The

first scenario describes a “more negative” cathodic protection level, i.e. when 𝐸𝑜𝑛 < −1,2

V CSE. In this case, one of the three parameters below, in order of priority, can be applied:

𝑈𝐴𝐶

|𝐸𝑂𝑁|−1.2< 3, where 𝑈𝐴𝐶 is the AC voltage;


51

𝑖𝐴𝐶 < 30 𝐴/𝑚2;

𝑖𝐴𝐶

𝑖𝐷𝐶< 3 if 𝑖𝐴𝐶 > 30 𝐴/𝑚2.

In this case, it’s recommended to ensure that no corrosion risks due to cathodic disbondment

and no adverse effect from hydrogen evolution are taking place on the metallic structure to

be protected.

The second scenario depicts a “less negative” cathodic protection level, i.e. when −1,2 <

𝐸𝑂𝑁 < −0,85 V CSE. As before, one of the three parameters below, in order of priority, can

be applied:

𝑈𝐴𝐶 < 15 𝑉;

𝑖𝐴𝐶 < 30 𝐴/𝑚2;

𝑖𝐷𝐶 < 1 𝐴/𝑚2 if 𝑖𝐴𝐶 > 30 𝐴/𝑚2.

AC voltage should be determined with a reference electrode placed at remote position

(Paragraph 1.4). Figure 2.23 and Figure 2.24 represent graphically the cathodic protection

criteria, with respect to the likelihood of AC corrosion. The former shows the relationship

between DC on-potential and AC voltage, the latter the relationship between DC and AC

current densities.

Figure 2.23 - Relationship between DC on-potential, AC voltage and likelihood of AC corrosion, where:

1) less negative cathodic protection level; 2) more negative cathodic protection level; 3) AC corrosion [16].


52

Figure 2.24 - Relationship between DC and AC current densities and likelihood of AC corrosion, where:


2.2.2 Cathodic protection criteria proposed by other authors

In the last years, other authors tried to propose their own cathodic protection criteria,

supported by experimental analyses. Some criteria regard controlling current densities, i.e.

protection current and alternating current densities, others AC voltages or CP potentials.

X. He et al. [53] reported the values of 𝑖𝐷𝐶 necessary to avoid AC corrosion, depending on

the AC interferences on the metallic structures. In particular:

𝑖𝐷𝐶 = 0,01 𝐴/𝑚2 if 𝑖𝐴𝐶 < 10 𝐴/𝑚2;

𝑖𝐷𝐶 >𝑖𝐴𝐶−10

100 if 10 < 𝑖𝐴𝐶 < 90 𝐴/𝑚2;

𝑖𝐷𝐶 = 0,8 𝐴/𝑚2 if 𝑖𝐴𝐶 > 90 𝐴/𝑚2.

This study is more permissive with respect to the British standard in terms of AC

interference, allowing the 𝑖𝐴𝐶 to reach values up to 90 A/m2. From what described before, it

seems that 𝑖𝐴𝐶 values higher than 90 A/m2 could be permitted, imposing an adequate 𝑖𝐷𝐶,

but it should be considered that a so high direct current density could lead to an

overprotection situation, that is dangerous from a corrosion point of view. Büchler [54]

implemented his researches on cathodic protection criteria taking into account the soil


53

resistivity. In Figure 2.25 are represented the maximum acceptable 𝑈𝐴𝑉 values for the “more

negative” and “less negative” cathodic protection levels, i.e. when 𝐸𝑂𝑁 < −1,2 V CSE and

−1,2 < 𝐸𝑂𝑁 < −0,85 V CSE, respectively. As discussed in Paragraph 2.1.2, a higher soil

resistivity is beneficial with respect to corrosion protection, going to increase the spread

resistance. Increasing the soil resistivity, higher 𝑈𝐴𝑉 values are admitted. These results

contributed to the definition of cathodic protection criteria defined in the international

standard (compare Figure 2.25 with Figure 2.23).

L.Y. Xu et al. [55] related the protection potential and the minimum 𝑖𝐴𝐶 needed to have AC

corrosion. In their article, they reported that a 𝐸𝑂𝑁 of -1 V CSE could bring the system in a

situation in which an 𝑖𝐴𝐶 of 400 A/m2 is still considered not harmful from a corrosion point

of view, while a pipeline characterized by a potential of -0.85 V CSE can bear an 𝑖𝐴𝐶 of 100

A/m2 before presenting the first corrosion phenomena. Doubts are revealed because the on

potential measurements is not reliable, containing the ohmic drop contribution.

Tang et al. [56] and A.Q. Fu [57], considering the effect of 𝑖𝐴𝐶 on the shift of protection

potential, state that the protection potential decreases as the AC density increases. Moreover,

they state that the -0.85 V CSE criterion is no more effective, because that potential in

presence of a relatively high 𝑖𝐴𝐶 could lead to corrosion. In addition to that, they estimate

also the amount of the cathodic polarisation value relative to the CP potential that can bring

to overprotection, and hence accelerating corrosion instead of avoiding it. Their results are

depicted in Figure 2.26a and Figure 2.26b.

Figure 2.25 - Effect of soil resistivity on the threshold 𝑈𝐴𝑉 value [54].


54

Figure 2.26 - New CP criteria for mild pipeline steel in the present of AC interference for a) Tang et al.

[56] and b) A.Q. Fu [57].

2.2.3 A new proposal of CP criteria in the presence of AC interference

Ormellese et al. conducted a study [58] based on the CP criteria reported in ISO 18086:2015.

Their aim was to strengthen the CP criteria discussed in Paragraph 2.2.1. The authors

suggested that the ISO standard furnishes no practical restrictions on the maximum

acceptable value of AC density at “low” CP level, i.e. when the applied cathodic protection

current density is lower than 1 A/m2. As far as the ISO standard is concerned, it expects no

corrosion in this situation, regardless the value of the interference AC density. They

performed long-term exposition tests on carbon steel in order to assess the effect of AC

interference and DC polarization on corrosion rate, calculated by means of weight loss

measurement.

The results obtained from these tests (Figure 2.27) showed that corrosion occurred on

coupons subjected to AC densities higher than 30 A/m2 and 𝑖𝐶𝑃 higher than 1 A/m2, going

to confirm the ISO standard criteria for “high” CP level. Nevertheless, corrosion rates up to

0.2 mm/y were measured for 𝑖𝐴𝐶 higher than 30 A/m2 and 𝑖𝐶𝑃 lower than 1 A/m2, although

the ISO standard stated these conditions safe from a corrosion point of view. Moreover,

corrosion occurred for 𝑖𝐴𝐶 higher than 10 A/m2 and 𝑖𝐶𝑃 higher than 1 A/m2. In addition,

Ormellese et al. reported an AC corrosion risk diagram, where IR-free potential and current

densities ratio (𝑖𝐴𝐶/𝑖𝐷𝐶) are correlated (Figure 2.28).


55

Figure 2.27 - Experimental corrosion rate in the 𝑖𝐴𝐶/𝑖𝐶𝑃 diagram. Safe and unsafe regions refer to CP

criterion as reported in ISO 18086:2015 [58].

The results showed that this diagram should be divided into two corrosion risk regions: the

high AC corrosion risk region corresponds to corrosion rates higher than 10 μm/y, which is

the threshold value as far as the corrosion rate is concerned. It can be noticed that the 𝑖𝐴𝐶/𝑖𝐷𝐶

threshold value decreases, i.e. AC corrosion risk increases, as the protection potential

becomes more negative. Under overprotection conditions, i.e. when protection potential is

lower than -1.1 V CSE, corrosion rate is not negligible (CR is greater than 10 μm/y) if

𝑖𝐴𝐶/𝑖𝐷𝐶 ratio is higher than 10. Because of these results, the authors suggested a modification

of the CP criteria reported in the ISO standard ISO 18086:2015 [16].

They concluded that AC corrosion is expected when:

𝑖𝐶𝑃 < 1 𝐴/𝑚2 and 𝑖𝐴𝐶 > 30 𝐴/𝑚2;

𝑖𝐶𝑃 > 1 𝐴/𝑚2 and 𝑖𝐴𝐶 > 10 𝐴/𝑚2;

𝑖𝐴𝐶 𝑖𝐷𝐶⁄ < 10 under overprotection conditions (𝐸𝐼𝑅−𝑓𝑟𝑒𝑒 < − 1,1 𝑉 𝐶𝑆𝐸).

Figure 2.29 depicts the new CP criteria based on the experimental corrosion rate data.

Comparing it with the “old” CP criteria (Figure 2.27), it can be shown that the new one is

more restrictive, going to exclude from the “safe region” part of the table that was considered

from the ISO standard safe from the corrosion point of view.


56

Figure 2.28 - AC corrosion risk diagram: IR-free potential vs. 𝑖𝐴𝐶/𝑖𝐷𝐶 [58].

Figure 2.29 – New CP criteria based on experimental corrosion rate data [58].

The goal of this work is to prove the validity of this modification proposal. Long-term

exposure tests will be performed, applying different protection current and alternating

current densities on carbon steel specimens, in order to assess the effect of DC polarization

and AC interference on corrosion rate, calculated by means of weight loss measurement.


57

In addition, galvanostatic tests will be carried out in order to study the effect of AC density

on the potential at fixed cathodic protection current density.

Figure 2.29 – New CP criteria based on experimental corrosion rate data [58].

58

Chapter 3

Materials and methods

In this chapter, the laboratory tests carried out to simulate the corrosion behaviour of a buried

pipeline in cathodic protection condition and in the presence of AC interference will be

described. The aim of this study is to validate the proposed cathodic protection criteria

(Paragraph 2.2.3) and to study the effect of AC on the measured potential. In particular, two

families of test were carried out in the presence of both AC and DC signals:

Long-term exposure tests for mass loss measurement;

Galvanostatic tests to study the effect of the AC interference on DC potential.

Even if the two experimental tests were different as far as their procedures and conclusions

were concerned, they shared the configuration of the electrical circuit and the preparation of

the carbon steel specimens; in order to avoid a redundancy in this chapter, they will be

described as first point.

3.1 ELECTRICAL CIRCUIT

The tests briefly consist in applying in the same time a direct and alternating current on a

carbon steel sample, simulating the cathodic protection current and the interference current,

respectively. To accomplish that, the electrical circuit must have two meshes, the DC mesh

and the AC mesh and, more important, it has to separate DC and AC signals, because they

have not to disturb one with each other. The disconnection between the two meshes is

important in terms of accuracy in controlling and measuring DC and AC during the tests.

During previous phases of the research, a specific electrical circuit was therefore designed

to supply and measure AC and DC signals independently (Figure 3.1):

the AC mesh consists in a feeding system (variable AC transformer, variac) that

supplies an AC between the specimen (working electrode, W) and a counter electrode

(𝐶𝐸𝐴𝐶) through a shunt, 𝑅1; two electrolytic 1,000 μF capacitors in series (total

capacity of 500 μF and a capacitive reactance of about 6 Ω) prevent DC circulation;

Materials and methods Chapter 3

59

The DC mesh consists in a galvanostat that supplies a DC between the specimen

(working electrode, W) and a counter electrode (𝐶𝐸𝐷𝐶) through a shunt, 𝑅2; a 20 H

inductor reduces AC circulation to less than 1%. The inductive reactance of a 20 H

inductor is about 6.3 kΩ.

AC and DC meshes share a common branch, where AC and DC overlap and flow to the

working electrode (W) through a variable shunt, 𝑅3. The wiring is embedded in a case

(Figure 3.1 and Figure 3.2) and the other parts of the circuit, i.e. the variac, the galvanostat,

the counter electrodes and the working electrodes, are connected to it through some

connectors that can be seen in Figure 3.1.

Figure 3.1 - Schematic view of the electrical circuit.

In the tests performed, the currents were measured through additional resistors (𝑅𝐷𝐶 and 𝑅𝐴𝐶

in Figure 3.1) placed between the case and the counter electrodes. This change was thought

in order to simply the readings of the current and because, for the long-term exposure tests,

several current density values were needed at the same time. This could not be achieved

through one single shunt, so others were added in parallel to the circuit. The description of

the exact circuit configuration for every test will be given in the following paragraphs.


60

Figure 3.2 – Electrical circuit (case and internal view).

3.2 MATERIALS

Both tests were carried out on carbon steel specimens, grade API 5L X52 [59]; the chemical

composition is reported in Table 3.1. The samples were prepared and cleaned according to

ASTM G1-03 [60]: sample preparation consists in cleaning the samples through abrasive

papers having different grit sizes (up to 1,200 grit). Then, samples were dried in order to

avoid the oxidation of the surface, before their use.

The carbon steel specimens were placed in a PTFE cylindrical sample holder made of two

watertight caps (Figure 3.3). This configuration leaves an area of 1 cm2 exposed to the

electrolyte. The aim is to simulate a coating defect of 1 cm2 of a cathodically protected buried

pipeline subjected to AC interference. A stainless steel rod screwed in a hole on the top of

the sample holder assures the electrical connection to the specimen. The rod is isolated from

the surrounding environment by a glass tube, placed around the screw and pressed against

the sample holder by a nut. The water seal between the plastic tube and the sample holder is

obtained by interposing an O-ring joint between them.

Table 3.1 – API 5L X52 – chemical composition by weight [59]

Grade % C max % Mn max % P max % S max

API 5L X52 0,31 1,35 0,030 0,030


61

Figure 3.3 – Carbon steel specimen in the sample holder.

3.3 GALVANOSTATIC TEST: AC EFFECT ON DC POTENTIAL

3.3.1 Aim of the test

The previous chapters discussed the behaviour of cathodically protected structures in

presence of AC interference. Recent works obtained in a previous research shown that AC

density causes an increase of carbon steel potential, when the specimen is in cathodic

protection condition. The positive shift of the potential is proportional to the AC density

value and, when 𝑖𝐴𝐶 exceeds a critical value, the pipeline potential overshoots the protection

potential, i.e. -0.850 V CSE for carbon steel. This condition can lead to corrosion.

In order to confirm this behaviour, galvanostatic tests were carried on carbon steel specimens

on which a stationary AC density was overlapped stepwise to the DC signal. In a

galvanostatic test, a constant cathodic polarization current is supplied to the metal and

potential is measured. The cathodic current density was supplied by a galvanostat through a

MMO-Ti (mixed-metal oxides titanium) counter electrode. In the meantime, an AC density

was overlapped to the specimen through another activated titanium anode by means of a

variac (Figure 3.4). This test consists in applying a fixed cathodic protection current density

and in measuring the potential changes with a stepwise increasing AC density. As first point,

the cathodic current density was applied alone for a period necessary to reach a steady-state

potential (30 minutes).

Sample

Sample holder Glass tubeScrew


62

Figure 3.4 – Galvanostatic test – experimental set-up.

Figure 3.5 – Galvanostatic test – electrochemical cell.

Then, AC density was overlapped to the specimen by steps and the protection potential was

measured at every step, by means of an external Ag/AgCl/KClsat. (+0.197 V vs. SHE)

reference electrode and a high impedance voltmeter (10 M). In order to avoid ohmic drop

contribution in the measurement, Luggin capillary filled with the same solution of the test

cell (Figure 3.5) was used. The distance between the tip of the capillary and the surface of

the specimen is about 1 mm.


63

The experimental conditions are listed in Table 3.2. Cathodic protection current density

varies from 0.15 A/m2 and 10 A/m2, in order to simulate ordinary cathodic protection and

overprotection condition. AC density ranges from 1 to 1,000 A/m2. The effect of AC on free

corrosion condition was also studied.

Table 3.2 – Galvanostatic test – experimental conditions.

CP current density (A/m2) AC density (A/m2)

0 (free corrosion condition) 0; 1; 10; 20; 30; 50; 100; 200; 300; 500; 1,000

0.15 0; 1; 10; 20; 30; 50; 100; 200; 300; 500; 1,000

0.3 0; 1; 10; 20; 30; 50; 100; 200; 300; 500; 1,000

0.5 0; 1; 10; 20; 30; 50; 100; 200; 300; 500; 1,000

1 0; 1; 10; 20; 30; 50; 100; 200; 300; 500; 1,000

2 0; 1; 10; 20; 30; 50; 100; 200; 300; 500; 1,000

3 0; 1; 10; 20; 30; 50; 100; 200; 300; 500; 1,000

5 0; 1; 10; 20; 30; 50; 100; 200; 300; 500; 1,000

10 0; 1; 10; 20; 30; 50; 100; 200; 300; 500; 1,000

3.3.2 Electrical circuit and test cell

The electrical circuit involved in the galvanostatic test was the one described in Paragraph

3.1. As discussed, the circuit separates the DC and AC signals. Current densities were

measured in correspondence of two additional resistors, 𝑅𝐷𝐶 and 𝑅𝐴𝐶, located in the DC and

AC meshes, respectively (Figure 3.1).

Figure 3.5 shows in detail the four-electrode test cell adopted. The four electrodes are the

carbon steel specimen (working electrode), the reference electrode Ag/AgCl/KClsat. (+0.197

V vs. SHE), the DC counter electrode (MMO-Ti), the AC counter electrode (MMO-Ti). The

geometrical disposition of the two counter electrodes with respect to the working electrode

was symmetrical. The soil-simulating solution composition used consists of 200 mg/L of

chlorides (by adding NaCl) and 500 mg/L of sulphate ions (by adding Na2SO4); electrolyte

electrical resistivity is 5 Ω·m, measured using a Hanna Instruments Model HI 98311

electrical conductivity meter. The electrochemical cell was filled by the solution until the

sample holder was completely submerged.


64

3.4 LONG-TERM EXPOSURE TEST

3.4.1 Aim of the test

The aim of the long-term exposure tests is to verify the validity of the cathodic protection

criterion in the presence of AC interference reported on ISO 18086 [16] (Paragraph 2.2.1).

This test consists in applying a cathodic protection current and interference AC densities on

carbon steel specimens, in order to determine the corrosion rate by mass loss measurement.

As first point, only the cathodic protection current was applied, in order to allow the carbon

steel specimens to reach a steady-stable protection potential (two weeks). The cathodic

current density was supplied by a galvanostat through a MMO-Ti (mixed-metal oxides

titanium) counter electrode. Then, the AC interference current was overlapped through

another activated titanium anode by means of a variac.

Table 3.3 – Long-term exposure tests – experimental conditions (according to Figure 3.6).

Specimen First condition investigated Second condition investigated

𝑖𝐴𝐶,1 (A/m2) 𝑖𝐷𝐶,1 (A/m2) 𝑖𝐴𝐶,2 (A/m2) 𝑖𝐷𝐶,1 (A/m2)

A.1 10 10 20 10

A.2 10 10 20 10

B.1 10 1 20 2

B.2 10 1 20 2

C.1 30 1 50 0.5

C.2 30 1 50 0.5

D.1 30 0.2 50 0.2

D.2 30 0.2 50 0.2

During the experiment, DC potential and both current densities (AC and DC) were

monitored. Four conditions were studied (A, B, C and D) and, for every condition, two

specimens were prepared (Table 3.3). The selected values of DC and AC densities are

reported in Figure 3.6, which summarized all the results obtained in the previous activities

of this research. In other words, the aim of the tests performed during this thesis work is to

confirm the preliminary results obtained and to validate the criterion based on current

densities reported on ISO 18086 standard [16]. Initially, the conditions represented by red

markers in Figure 3.6 were considered. Then, AC and DC densities on both samples were


65

changed in order to investigate other interference and protection conditions, represented by

blue markers in Figure 3.6.

Figure 3.6 – Long-term exposure tests – experimental conditions (red markers refers to the first condition

investigated; blue markers to the second condition).

3.4.2 Electrical circuit and test cell

The electrical circuit used for these long-term exposure tests is described in Paragraph 3.1.

Notwithstanding, despite the configuration adopted for the galvanostatic tests, the section of

the circuit outside the case is more complicated. As described in Paragraph 3.4.1, the test

consisted in applying direct and alternating current densities on eight samples at the same

time. In order to accomplish that, the two meshes (DC and AC mesh) were connected to an

external electrical circuit in order to provide both signals to the eight specimens. Proper

resistors were inserted in the electrical circuit in parallel (Figure 3.7, Figure 3.8 and Figure

3.9). As far as the test cells were concerned, cylindrical cells in polypropylene (Figure 3.10)

were used (diameter 110 mm; height 130 mm), where the two counter electrodes (𝐶𝐸𝐷𝐶 and

𝐶𝐸𝐴𝐶) and the working electrode were placed in. The cell cap assured the geometrical

disposition of the three electrodes in the cell (Figure 3.10 and Figure 3.11). The potential of

the working electrode was measured by means of an external Ag/AgCl/KClsat. reference

1

10

100

1000

0 1 10

i AC

(A/m

2)

iCP (A/m2)

0.1

Safe

Unsafe

1

10

100

1000

0 1 10

i AC

(A/m

2)

iCP (A/m2)

10 mm/y

(A1; A2)(B1; B2)

(C1; C2)

(D1; D2)

(A1; A2)

(B1; B2)

(C1; C2)

(D1; D2)


66

electrode placed close to the sample (few millimetres) and a high impedance voltmeter

(Figure 3.11). The soil-simulating solution was characterized by 200 mg/L of chlorides and

500 mg/L of sulphate ions, having an electrical resistivity of 5 Ω·m; it was measured using

a Hanna Instruments Model HI 98311 electrical conductivity meter. The electrochemical cell

was filled by the solution until the sample holder was completely submerged.

Figure 3.7 – Long-term exposure tests – schematic view of the electrical circuit.

Figure 3.8– Long-term exposure tests – electrical circuit.


67

Figure 3.9 – Long-term exposure tests – connection between the electrical circuit and the corrosion cells.

Figure 3.10– Long-term exposure tests – test cells.

3.4.3 Protection potential and current density monitoring

The IR-free potential and current densities monitoring was performed twice a week:

The current densities (AC and DC) were calculated by the measurement of the

alternating and direct voltage drops in correspondence of the resistors. Indeed, the


68

current flowing in the cell is the ratio between the voltage drop on the resistor and its

resistance. Current density is obtained dividing the current for the surface of the

specimen (1 cm2);

The IR-free potential of the specimens was monitored by means of a high impedance

voltmeter and an Ag/AgCl/KClsat. reference electrode (Figure 3.11).

As far as the test cells are concerned, a weekly check on the level of the electrolyte was

performed. If it was lower than usual, due to the evaporation of the solution, distilled water

was added in order to re-establish the original solution. This operation was done once a week.

Figure 3.11 – IR-free potential monitoring.

3.4.4 Mass loss measurement

At the end of the tests (three months), mass loss measurements were carried out in order to

evaluate the corrosion rate, according to the following procedure:

Pickling treatment in accordance to the standard ASTM G1-03 [60] (section 7) through

ultrasonic and chemical cleaning. An acidic solution (1:1 HCl and 3.5 g of

hexamethylenetetramine every 1,000 mL of solution) was used to remove the corrosive

products from the sample surface. Hexamethylenetetramine is a corrosion inhibitor

necessary to avoid the carbon steel corrosion. Three pickling cycles (5 minutes each)

were done at room temperature;


69

Multiple rinses in distilled water in order to remove the acid from the samples;

Rinse in acetone in order to remove the water from the samples;

Drying in order to evaporate easily acetone;

Mass loss measurement by means of a digital balance (accuracy = ±0.1 mg).

The mass loss rate is calculated as follows:

(Eq. 3.1) 𝐶𝑅𝑚 =𝛥𝑀

𝑆·𝑡

where CRm is the mass loss rate, g/(m2·s), 𝛥𝑀 is the mass loss, S the metal surface and t is

the time (duration of the test). The penetration rate of corrosion is calculated as follows:

(Eq. 3.2) 𝐶𝑅𝑝 =𝐶𝑅𝑚

𝜌=

𝛥𝑀

𝜌·𝑆·𝑡

where 𝜌 is the metal density (7.8 g/cm3).

70

Chapter 4

Results and discussion

In this chapter, the results of the laboratory tests described in the previous chapter will be

reported and discussed. These tests consisted in simulating the corrosion behaviour of a

cathodically protected buried structure, as a pipeline, in the presence AC interference.

Laboratory tests carried out during this thesis work can by divided in two groups:

Galvanostatic tests, to study the effect of AC on protection potential;

Long-term exposure tests, to investigate the effect of AC on corrosion rate and on CP

criteria.

4.1 PART 1: GALVANOTATIC TESTS: EFFECT OF AC ON IR-FREE

POTENTIAL

The effects of AC density on cathodically protected metallic structures were discussed

preliminarily in Chapter 2. Previous tests [58] showed that AC density causes an increase in

the pipeline potential, when a cathodic protection system is taken into account. The positive

shift of the potential is proportional to the AC density value and when 𝑖𝐴𝐶 exceeds a critical

value, the pipeline potential overshoots the protection potential, i.e. -0.850 V CSE for carbon

steel in aerated condition. This condition is not considered safe for a pipe in CP condition,

leading to corrosion.

Galvanostatic tests (Paragraph 3.3) were carried on carbon steel specimens, grade API 5L

X52 [59] (Table 3.1). The goal of this test is to validate and confirm these preliminary tests

by applying a constant protection current density and increasing stepwise AC density.

In a galvanostatic test, a constant DC density is applied (protection current density), and the

potential is measured. The protection current density, 𝑖𝐶𝑃, was applied in the range between

0.15 and 10 A/m2, in order to consider different protection level. Then, AC density, 𝑖𝐴𝐶, was

overlapped to the carbon steel specimen by steps, increasing its value every 30 minutes from

1 to 1,000 A/m2. For every step, the protection potential was recorded. In order to measure

Results and discussion Chapter 4

71

the IR-free potential, a Luggin capillary was used in order to minimize the distance, i.e. the

ohmic drop, between the reference electrode and the sample. An external Ag/AgCl/KClsat.

(+0.197 V vs. SHE) reference electrode was placed in the Luggin probe. Measurements are

referred in the following to CSE reference electrode (Cu/CuSO4,sat., +0.318 V SHE). Table

3.2 summarize the experimental conditions. It can be noticed that a further test was

accomplished, where only 𝑖𝐴𝐶 was considered.

Figures 4.1 to 4.9 report IR-free protection potentials with respect to a stepwise increasing

AC density, at a fixed protection current density. In general, the superimposition of AC

interference on a cathodically protected metallic structure causes a potential increase, i.e. the

potential moves in the anodic direction. Moreover, the potential shift depends on the AC

density intensity. This behaviour is not respected for the carbon steel specimen in free

corrosion condition, without cathodic protection. As explained in Paragraph 1.3.4, the

potential follows a different trend in this sense, moving in the more negative (cathodic)

direction with increasing 𝑖𝐴𝐶.

Figure 4.1 refers to the galvanostatic test carried out without 𝑖𝐶𝑃. The free corrosion potential

was -0.743 V CSE. In correspondence to the AC density overlap, the potential decreased up

to -0.897 V CSE when 𝑖𝐴𝐶 reached 100 A/m2. Then, the potential approaches a stable value,

around -0.820 V CSE, when 𝑖𝐴𝐶 ranged from 300 to 1,000 A/m2.

In Figure 4.2, a protection current density of 0.15 A/m2 was applied to carbon steel specimen.

Before applying the AC density, the potential is -0.976 V CSE, i.e. the metal is in protection

condition. Protection potential started to move in the anodic direction with 𝑖𝐴𝐶: with an AC

density of 300 A/m2 the potential assumed a value of -0.839 V CSE and remained almost

stable when 𝑖𝐴𝐶 was increased up to 1,000 A/m2 (-0.837 V CSE). In this test, the potential

shift (calculated with respect to the potential without AC interference) was about 140 mV.

Figure 4.3 and Figure 4.4 report potential monitoring corresponding to 𝑖𝐶𝑃 of 0.3 and 0.5

A/m2, respectively. Without the application of the interference AC density, the protection

potential was -1.123 and -1.147 V CSE, respectively. In both cases, the potential reaches the

protection potential for carbon steel, i.e. -0.850 V CSE, at 𝑖𝐴𝐶 of 1,000 A/m2, assuming a

value of -0.834 and -0.848 V CSE, respectively; the potential shift from the no-interference

condition was 0.289 V for 𝑖𝐶𝑃 of 0.3 A/m2 and 0.299 V for a cathodic protection current of

0.5 A/m2. When 1.0 A/m2 of CP current density was applied, the potential was -1.200 V

CSE, without AC interference (Figure 4.5). Then, it increased up to -0.899 V CSE when 𝑖𝐴𝐶

reached 1,000 A/m2, with a net polarization of 0.294 V.


72

A protection potential of -1.233 V CSE (Figure 4.6) and -1.283 V CSE (Figure 4.7) was

assumed, in correspondence of a cathodic protection current density of 2.0 and 3.0 A/m2,

respectively. At the end of the tests, the potential shows a positive shift of 0.294 and 0.344

V, having reached the value of -0.939 V CSE in both cases. In Figure 4.8 and Figure 4.9,

potential profiles of carbon steel specimens subjected to an 𝑖𝐶𝑃 of 5.0 and 10.0 A/m2 are

reported, respectively. The protection potential is -1.318 V CSE for the first specimen and -

1.465 V CSE for the second one. The application of AC density causes a positive shift of the

potential. In the presence of 1,000 A/m2 AC density, the positive shift was 0.335 V and 0.400

V with respect to the no-interference condition, respectively. Potential shifts and

experimental conditions are summarized in Table 4.1 (a-c). Table 4.2 lists the measured

potentials in absence of cathodic protection, with respect to the applied 𝑖𝐴𝐶.

Figure 4.1 - DC potential vs. AC density

(𝑖𝐶𝑃 = 0 A/m2).


(𝑖𝐶𝑃 = 0.15 A/m2).


(𝑖𝐶𝑃 = 0.3 A/m2).


(𝑖𝐶𝑃 = 0.5 A/m2).


73


(𝑖𝐶𝑃 = 1.0 A/m2).


(𝑖𝐶𝑃 = 2.0 A/m2).


(𝑖𝐶𝑃 = 3.0 A/m2).


(𝑖𝐶𝑃 = 5.0 A/m2).


(𝑖𝐶𝑃 = 10.0 A/m2).


74

Table 4.1 (a) – IR-free potential shift in the presence of AC interference (𝑖𝐴𝐶 from 0 to 30 A/m2).

AC density

(A/m2)

CP current density

(A/m2)

IR-free potential

(V CSE)

Potential shift

(V)

0

0.15 -0.976 -

0.3 -1.123 -

0.5 -1.147 -

1.0 -1.200 -

2.0 -1.233 -

3.0 -1.283 -

5.0 -1.318 -

10.0 -1.465 -

1

0.15 -0.995 -0.019

0.3 -1.119 0.004

0.5 -1.137 0.010

1.0 -1.200 0

2.0 -1.233 0

3.0 -1.283 0

5.0 -1.315 0.003

10.0 -1.465 0

10

0.15 -0.996 -0.020

0.3 -1.093 0.030

0.5 -1.130 0.017

1.0 -1.170 0.030

2.0 -1.227 0.006

3.0 -1.271 0.012

5.0 -1.308 0.010

10.0 -1.453 0.012

20

0.15 -0.994 -0.018

0.3 -1.089 0.034

0.5 -1.113 0.034

1.0 -1.163 0.037

2.0 -1.212 0.021

3.0 -1.250 0.033

5.0 -1.286 0.032

10.0 -1.424 0.041

30

0.15 -1.015 -0.039

0.3 -1.082 0.041

0.5 -1.097 0.050

1.0 -1.124 0.076

2.0 -1.196 0.037

3.0 -1.228 0.055

5.0 -1.267 0.051

10.0 -1.391 0.074


75

Table 4.1 (b) – IR-free potential shift in the presence of AC interference (𝑖𝐴𝐶 from 50 to 500 A/m2).

AC density

(A/m2)

CP current density

(A/m2)

IR-free potential

(V CSE)

Potential shift

(V)

50

0.15 -0.852 0.124

0.3 -1.036 0.087

0.5 -1.077 0.070

1.0 -1.122 0.078

2.0 -1.180 0.053

3.0 -1.205 0.078

5.0 -1.235 0.083

10.0 -1.352 0.113

100

0.15 -0.890 0.086

0.3 -0.958 0.165

0.5 -1.022 0.125

1.0 -1.082 0.118

2.0 -1.145 0.088

3.0 -1.182 0.101

5.0 -1.196 0.122

10.0 -1.315 0.150

200

0.15 -0.857 0.119

0.3 -0.913 0.210

0.5 -0.915 0.232

1.0 -0.947 0.253

2.0 -1.020 0.213

3.0 -1.044 0.239

5.0 -1.107 0.211

10.0 -1.236 0.229

300

0.15 -0.839 0.137

0.3 -0.882 0.241

0.5 -0.875 0.272

1.0 -0.942 0.258

2.0 -0.963 0.270

3.0 -0.959 0.324

5.0 -1.033 0.285

10.0 -1.128 0.337

500

0.15 -0.816 0.160

0.3 -0.850 0.273

0.5 -0.855 0.292

1.0 -0.920 0.280

2.0 -0.956 0.277

3.0 -0.949 0.334

5.0 -0.996 0.322

10.0 -1.072 0.393


76

Table 4.1 (c) – IR-free potential shift in the presence of AC interference (𝑖𝐴𝐶 = 1,000 A/m2).

AC density

(A/m2)

CP current density

(A/m2)

IR-free potential

(V CSE)

Potential shift

(V)

1,000

0.15 -0.837 0.139

0.3 -0.834 0.289

0.5 -0.848 0.299

1.0 -0.899 0.301

2.0 -0.939 0.294

3.0 -0.939 0.344

5.0 -0.983 0.335

10.0 -1.065 0.400

Table 4.2 – IR-free potential of carbon steel in free corrosion condition in the presence of AC interference.

AC density (A/m2) IR-free potential

(V CSE)

Potential shift

(V)

0 -0.743 -

1 -0.744 -0.001

10 -0.747 -0.004

20 -0.753 -0.010

30 -0.795 -0.052

50 -0.884 -0.141

100 -0.897 -0.154

200 -0.837 -0.094

300 -0.819 -0.076

500 -0.811 -0.068

1,000 -0.825 -0.082

As expected, in the absence of AC interference, i.e. only with cathodic protection current

density, the IR-free potential decreases as the current density increases, as reported in Figure

4.10. The potential in the hydrogen evolution region decreases linearly with the logarithm

of current density, following Tafel law; the cathodic Tafel slope is about 200 mV for decade

of current, higher than the expected value from theory (120 mV/decade). This discrepancy

can be due to the low content of hydrogen ions in neutral solution with an increase of the

activation overvoltage.

Figure 4.11 and Figure 4.12 report IR-free protection potentials of carbon steel with respect

to AC density and cathodic protection current. Figure 4.11 is referred to the 𝑖𝐶𝑃 varying from

0.15 to 1 A/m2, while Figure 4.12 from 2 to 10 A/m2. This second condition is typical of

overprotection condition, namely IR-free potential lower than -1.2 V CSE. In Figure 4.13


77

the potential shifts are represented, calculated as the difference between the potential

measured in the presence of AC and the potential without interference.

Figure 4.10 – IR-free potential vs 𝑖𝐶𝑃 in absence of interference 𝑖𝐴𝐶 (𝑖𝐶𝑃 from 0 to 10 A/m2).

Figure 4.11 – IR-free potential vs. AC density varying CP current density (𝑖𝐶𝑃 from 0.15 to 1 A/m2).


78

Figure 4.12 – IR-free potential vs. AC density varying CP current density (𝑖𝐶𝑃 from 2 to 10 A/m2).

Figure 4.13 – Protection potential shift vs AC density.

In conclusion, in the presence of AC interference, the carbon steel IR-free potential

increases, if the metal is in cathodic protection condition. Considering cathodic current

densities ranging from 0.3 to 10 A/m2 (Figures 4.3 to 4.9), the potential increase was almost

linear for AC densities up to 300 A/m2. Then, it can be noticed that the slope of the curve

became approximately null for 𝑖𝐴𝐶 greater than 300 A/m2, i.e. the IR-free potential moved

in the more anodic direction of few millivolts only. For instance, regarding the carbon steel


79

specimen subjected to an 𝑖𝐶𝑃 of 10 A/m2 (Figure 4.9), potential shifts of 337 mV and 60 mV

were measured when the applied 𝑖𝐴𝐶 moves from 0 to 300 A/m2 and from 300 to 1,000 A/m2,

respectively. This trend suggests that IR-free potentials stabilize at high AC densities, and

this occurred for any cathodic protection level. Nevertheless, for the carbon steel specimen

in free corrosion conditions, the potential follows a different trend, moving in the more

negative (cathodic) direction with 𝑖𝐴𝐶, as reported in Figure 4.1. The free corrosion potential

was -0.743 V CSE. In correspondence to the AC density overlap, the potential decreased up

to -0.897 V CSE when 𝑖𝐴𝐶 reached 100 A/m2. Then, the potential increases with 𝑖𝐴𝐶, until

approaching a stable potential value, around -0.820 V CSE, when 𝑖𝐴𝐶 ranged from 300 to

1,000 A/m2.

In cathodic protection, the presence of high AC densities may bring the potential outside the

protection level defined by international standards, -0.85 V CSE. As far as the performed

galvanostatic tests, the IR-free potential overcame the protection potential only in three

situations: 1) 𝑖𝐶𝑃 0.15 A/m2 and AC densities higher than 300 A/m2; 2) 𝑖𝐶𝑃 0.3 A/m2 and AC

densities 1,000 A/m2; 3) 𝑖𝐶𝑃 0.5 A/m2 and AC densities 1,000 A/m2. In other words, the

potential is outside the protection level when high AC density is overlapped to low CP

current densities. Nevertheless, as discussed in the following, the potential reading is not the

only parameter to control in terms of AC corrosion likelihood assessment. Bringing the IR-

free potential under the protection potential is a necessary but not sufficient condition to

avoid corrosion in the presence of AC: ISO 18086 reports that also the 𝑖𝐴𝐶 and 𝑖𝐶𝑃 values,

their ratio and the AC voltage intervene in the AC corrosion evaluation. Figure 4.13 reports

the linear regression in a logarithmic scale current. Fitting was carried out considering all

experimental data, regardless protection conditions. Fitting curve indicates a protection

potential variation of 0.120 V per decade of current (slope of the curve) and the linear

regression coefficient, having the current density expressed in a logarithmic scale, is 0.83.

The results obtained in this thesis work are in good agreement with preliminary galvanostatic

tests [58] carried out applying different AC densities on cathodically protected carbon steel

specimens, grade API 5L X52. Differently to the tests of this thesis work, the

abovementioned tests were carried out in soil simulation condition, i.e. silica sand in a

saturated soil-simulating solution. Nevertheless, its electrical resistivity was 5 Ω·m,

comparable to the resistivity of the solution adopted in this work. In the past test, CP current

density varies from 0.1 to 10 A/m2 and AC density from 10 to 200 A/m2. All detailed

information can be found in the thesis [58].


80

The results comparison is proposed in Figure 4.14, which shows the potential shifts with

respect to the applied AC density, expressed in a logarithmic scale.

Figure 4.14 – Protection potential shift vs AC density (comparison between the results obtained in this

work and in [58]).

Tests results are in good agreement. A linear regression was carried out in order to obtain a

general experimental equation providing the IR-free potential variation with respect to AC

density. This experimental equation (Eq. 5.1) resulted to be valid for all galvanostatic tests,

conducted at different cathodic protection current densities:

(Eq. 5.1) 𝐸𝐼𝑅 𝑓𝑟𝑒𝑒(ln(𝑖𝐴𝐶)) = 𝐸𝑁𝑂 𝐴𝐶 + 5.5 × 10−2 ∙ 𝑙 𝑛(𝑖𝐴𝐶)

where 𝐸𝑁𝑂 𝐴𝐶 is the IR-free potential measured prior to the interference AC density and the

slope of the curve is expressed in volt per decade of current.

In conclusion, the purpose of the conducted galvanostatic tests was to study the effects of an

interference alternating current density on the IR-free potential of buried metallic structures,

such as pipes for the transport of hydrocarbons, in cathodic protection condition. The results

showed that IR-free potential is strongly affected by the presence of AC density, and it

increases as AC density increases. Only high AC densities were able to bring the IR-free

potential over the protection potential of carbon steel, i.e. -0.850 V CSE in aerated

conditions, in correspondence to very low cathodic protection current densities, and hence

to cause AC corrosion on carbon steel. Nevertheless, IR-free potential monitoring is a

necessary but not sufficient condition to avoid any corrosion likelihood. Indeed, as the


81

standard in force state, other parameters should be checked, such as the cathodic protection

current and alternating current densities and their ratio.

4.2 PART 2: LONG-TERM EXPOSURE TEST: CURRENT AND POTENTIAL

MONITORING


criterion in the presence of AC interference reported on ISO 18086 (Paragraph 2.2.1).

Long-term exposure tests consist in applying a CP current and interference AC densities on

carbon steel specimens, in order to determine the corrosion rate by mass loss measurement.

As first point, only the cathodic protection current was applied, in order to allow the carbon

steel specimens to reach a steady-stable protection potential (two weeks). The cathodic

current density was supplied by a galvanostat through a MMO-Ti (mixed-metal oxides

titanium) counter electrode. Then, the AC interference current was overlapped through a

second activated titanium anode by means of an AC generator.

During the experiment, DC potential and both current densities (AC and DC) were

monitored. Four conditions were studied (A, B, C and D) and, for every condition, two

specimens were prepared (Table 3.3). The selected values of DC and AC densities are

reported in Figure 3.6, which summarized all the results obtained in the previous activities

of this research. Initially, the conditions represented by red markers in Figure 3.6 were

considered. Then, AC and DC densities were changed in order to investigate other

interference and protection conditions, represented by blue markers in Figure 3.6.

The measured IR-free potential average values for Series A, B, C and D referring to the first

two weeks (only CP applied) are summarized in Table 4.3. As expected, higher the CP

current density, lower the IR-free potential.

Table 4.3 – IR-free potential after two weeks of cathodic protection applied.

Specimen A.1 A.2 B.1 B.2 C.1 C.2 D.1 D.2

CP current density

(A/m2) 10.0 10.0 1.0 1.0 1.0 1.0 0.2 0.2

IR-free potential

(V CSE) -1.780 -1.758 -1.348 -1.358 -1.278 -1.338 -1.108 -1.118

The IR-free potential monitoring for the 100 days of testing is reported in Figure 4.15 (Series

A and B) and Figure 4.16 (Series C and D). Initially the tested conditions were:


82

Series A (𝑖𝐶𝑃 = 10.0 A/m2; 𝑖𝐴𝐶 = 10.0 A/m2)

Series B (𝑖𝐶𝑃 = 1.0 A/m2; 𝑖𝐴𝐶 = 10.0 A/m2)

Series C (𝑖𝐶𝑃 = 1.0 A/m2; 𝑖𝐴𝐶 = 30.0 A/m2)

Series D (𝑖𝐶𝑃 = 0.2 A/m2; 𝑖𝐴𝐶 = 30.0 A/m2)

Corresponding to the AC density superimposition (dotted line after 16 days of testing), the

IR-free potentials starts to increase, i.e. to move in the more anodic (positive) direction. The

potential shifts are about 80 mV for Series A, 50 mV for Series B, 77 mV for Series C and

139 mV for Series D. The specimens characterized by a 𝑖𝐶𝑃 of 10 A/m2 experienced the

lowest shift of potential.

After about 60 days of testing, no visible corrosion occurred on the samples (no corrosion

product in solution). Then, AC and CP current densities were changed in order to investigate

new experimental conditions:

Series A (𝑖𝐶𝑃 = 10 A/m2; 𝑖𝐴𝐶 = 20.0 A/m2)

Series B (𝑖𝐶𝑃 = 2.0 A/m2; 𝑖𝐴𝐶 = 20.0 A/m2)

Series C (𝑖𝐶𝑃 = 0.5 A/m2; 𝑖𝐴𝐶 = 50.0 A/m2)

Series D (𝑖𝐶𝑃 = 0.2 A/m2; 𝑖𝐴𝐶 = 50.0 A/m2)

These conditions are represented in Chapter 3, Figure 3.6, in the diagram reporting AC

density with respect to CP current density. The mean values of IR-free potentials and

currents during the two considered experimental conditions are reported in Table 4.4 and

Table 4.5 for each specimen. In the meantime, AC and CP current densities were monitored,

through the measurement of the alternating and direct voltages in correspondence to the

shunts of the electrical circuit. The monitoring graphs of these parameters and the calculated

ratio between them, 𝑖𝐴𝐶/𝑖𝐶𝑃, are reported in Figure 4.17, 4.18, 4.19. The measured values

are close to the nominal values designed at the beginning of the experiment.


83

Figure 4.15 – IR-free potential monitoring in time (Series A and B).

Figure 4.16 – IR-free potential monitoring in time (Series A and B).


84

Figure 4.17 – Cathodic protection current density monitoring.

Figure 4.18 – AC density monitoring.


85

Figure 4.19 – 𝑖𝐴𝐶/𝑖𝐶𝑃 ratio trend in time.

Table 4.4 – Mean values of IR-free potential and current densities in the first tested conditions.

Specimen

IR-free

potential 𝑖𝐴𝐶 𝑖𝐶𝑃 𝑖𝐴𝐶/𝑖𝐶𝑃

V CSE A/m2 A/m2 Dimensionless

A.1 -1.706 9.0 6.5 1.4

A.2 -1.662 9.0 8.7 1.0

B.1 -1.305 9.2 1.0 9.6

B.2 -1.295 9.2 1.0 9.1

C.1 -1.222 31.5 1.1 28.2

C.2 -1.239 31.5 1.1 28.8

D.1 -0.969 29.4 0.2 174.4

D.2 -0.979 30.3 0.2 182.3

Table 4.5 – Mean values of IR-free potential and current densities in the second tested conditions.

Specimen IR-free potential 𝑖𝐴𝐶 𝑖𝐶𝑃 𝑖𝐴𝐶/𝑖𝐶𝑃

V CSE A/m2 A/m2 Dimensionless

A.1 -1.630 18.1 6.1 3.0

A.2 -1.574 18.3 8.9 2.0

B.1 -1.363 18.6 2.3 8.2

B.2 -1.359 18.6 2.4 7.8

C.1 -1.101 52.4 0.6 90.7

C.2 -1.128 51.4 0.6 93.5

D.1 -0.922 48.9 0.2 259.4

D.2 -0.971 49.9 0.2 280.8


86

It’s interesting to compare the IR-free potentials measured in the long-term exposure tests

with respect to the potentials obtained in the galvanostatic tests described in Part 1. IR-free

potentials listed in Table 4.4 and 4.5 are indicated with red markers in Figure 4.20 to 4.24.

The two families of test have in common the CP current densities of 0.5, 1, 2 and 10 A/m2.

Only for comparison purpose, the long-term exposure tests carried out at 𝑖𝐶𝑃 of 0.2 A/m2 is

compared with the galvanostatic test conducted at CP current density of 0.3 A/m2. In all

conditions, there is no perfect match between the potential obtained in the two described

tests. Except for the tests carried out at 0.3 A/m2, the IR-free potentials of the long-term

exposure tests are lower than the potentials measured during the galvanostatic tests.

Nevertheless, long exposure tests aim to simulate the long-term behaviour in which oxygen

in contact is continuously consumed by the cathodic current. It follows that the oxygen

content, due to the long-term CP current density, is lower with respect galvanostatic test

where oxygen is in equilibrium with atmosphere and test duration is shorter. In conclusion,

the difference of about 100 mV can be related to the different oxygen content.

Figure 4.20 – Potentials obtained during the two

testes at 𝑖𝐶𝑃 = 0.3 A/m2.

Figure 4.21 - Potentials obtained during the two



testes at 𝑖𝐶𝑃 = 1.0 A/m2. Figure 4.23 - Potentials obtained during the two



87



Moreover, in the long-term exposure tests constant current densities were applied for several

weeks, as opposed to the galvanostatic tests, where the current densities were modified every

30 minutes, at least as far the AC densities were concerned. In the first case, the IR-free

potentials were let to reach a steady-stable value for a much longer time with respect to the

second test described. Nevertheless, the results obtained in the above-mentioned tests are in

agreement: IR-free potentials depend on the overlapped AC density and it increases as the

AC density increases. The potential shifts are summarized in Table 4.6 and Table 4.7.

Table 4.6 – IR-free potential and potential shift in the first period of long-exposure test (from AC

application to current variations). IR- free potential is expressed in V versus CSE.


𝑖𝐶𝑃 (A/m2) 6.5 8.7 1 1 1.1 1.1 0.2 0.2

𝑖𝐴𝐶 (A/m2) 9.0 9.0 9.2 9.2 31.5 31.5 29.4 30.3

IR-free potential

without AC -1.780 -1.758 -1.348 -1.358 -1.278 -1.338 -1.108 -1.118

IR-free potential

with AC -1.706 -1.662 -1.305 -1.295 -1.222 -1.239 -0.969 -0.979

Potential shift

(mV) +74 +96 +43 +63 +56 +99 +139 +139


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Table 4.7 – IR-free potential and potential shift in the second period of long-exposure test (from current

variations to the end of the test). IR- free potential is expressed in V versus CSE.


𝑖𝐶𝑃 (A/m2) 6.1 839 2.3 2.4 0.6 0.6 0.2 0.2

𝑖𝐴𝐶 (A/m2) 18.1 18.3 18.6 18.6 52.4 51.4 48.9 49.9

IR-free potential

without AC -1.780 -1.758 -1.348 -1.358 -1.278 -1.338 -1.108 -1.118

IR-free potential

with AC -1.630 -1.574 -1.363 -1.359 -1.101 -1.128 -0.922 -0.971

Potential shift

(mV) 150 184 -15 -1 177 210 186 147

The greatest potential shifts were found in the specimens subjected to 𝑖𝐴𝐶 of 50 A/m2, i.e.

the highest AC densities took into account in these long-term exposure tests. For every

specimen the IR-free potential remained below the protection potential, i.e. -0.850 V CSE

for carbon steel. Nevertheless, corrosion products were found on the samples, suggesting

that AC corrosion occurred. This aspect is investigated in Part 3.

4.3 PART 3: LONG-TERM EXPOSURE TEST: CORROSION RATE AND

CATHODIC PROTECTION CRITERIA

4.3.1 Corrosion rate in the presence of AC interference

In this section, results of mass-loss rate tests and CP criteria in the presence of AC will be

discussed. In particular, the proposed criterion will be compared to that reported on ISO

18086. As anticipated, the evaluation of corrosion rate (CR in the following) was made by

means of mass loss measurements. All specimens of long-term exposure test were weighted

before and after through a digital balance (accuracy = ±0.1 mg). The procedure described in

Paragraph 3.4.4 was necessary in order to remove the eventual corrosion products present

on the sample surface. Table 4.8 summarizes the measurements of the mass loss and the

subsequently corrosion rate occurred on all specimens.

The corrosion rate (mass-loss rate) is defined as the ratio between mass variation due to

corrosion, Δm, and the product (S·t), where S is specimen surface, 1 cm2, and t is exposure

time. Corrosion rate is expressed in m.d.d. (𝑚𝑔/𝑑𝑚2 · 𝑑𝑎𝑦). The penetration rate, CR, is

the ratio between the mass-loss rate and the mass density of the metal. For carbon steel, 1


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m.d.d. corresponds to about 5 μm/y. Despite being subjected to different alternating and

protection current densities, all specimens were characterized by roughly the same corrosion

rate: Series A and D experienced an average corrosion rate of 31 μm/y, Series C 34 μm/y,

and Series B 53 μm/y. Nevertheless, corrosion rates were higher than the greatest acceptable

value according to international standards, i.e. 10 μm/y, but they have not reached harmful

values. Before going into detail, it is necessary to point out that the corrosion rate is evaluated

starting from the current densities modification, i.e. from the 63th day of the test. Therefore,

the duration of the test is considered equal to 36 days, from day 63 to 99. This is because no

corrosion products were visible on all specimens before the 63th day, i.e. no corrosion

occurred. Being the specimens surfaces not corroded, it was thought that those 𝑖𝐴𝐶 values,

in correspondence with the respective CP levels, were not harmful from a corrosion point of

view.

Table 4.8 - Corrosion rate due to AC interference on cathodically protected carbon steel.

Specimen EIR-free 𝑖𝐴𝐶 𝑖𝐶𝑃 𝑖𝐴𝐶/𝑖𝐶𝑃 CR CR CRAVG

V CSE A/m2 A/m2 Dimensionless m.d.d. μm/y μm/y

A.1 -1.630 18.1 6.1 3.0 6.7 33 31

A.2 -1.574 18.3 8.9 2.0 5.8 29

B.1 -1.363 18.6 2.3 8.2 5.6 28 53

B.2 -1.359 18.6 2.4 7.8 15.6 78

C.1 -1.101 52.4 0.6 90.7 2.5 13 34

C.2 -1.128 51.4 0.6 93.5 11.1 56

D.1 -0.922 48.9 0.2 259.4 7.5 37 31

D.2 -0.971 49.9 0.2 280.8 5.0 25

The following figures contains all data collected from the beginning of the research project

carried out in the research group. Initially, only the results obtained from the above-described

tests will be treated, labelled with blue markers. Later on, they will be discussed in relation

to the other results (white markers).

Figure 4.25 shows the relationship between corrosion rate and the interference AC density.

The specimens represented in Figure 4.25 are distinguished according to the applied cathodic

protection level, 𝑖𝐶𝑃. Three ranges of CP current density were identified:

lower than 0.5 A/m2;

between 0.5 and 1 A/m2;


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higher than 1 A/m2.

Protection current densities higher than 1 A/m2 lead to AC corrosion, up to 100 μm/y, even

at lower interference AC densities: an 𝑖𝐴𝐶 of 20 A/m2 was enough to cause AC corrosion.

Contrarily, 𝑖𝐴𝐶 of at least 50 A/m2 were necessary to have a corrosion higher than 10 μm/y

at lower CP current density.

Figure 4.25 – Corrosion rate vs 𝑖𝐴𝐶 at different CP levels. Blue markers refer to the tests carried out

during this thesis work. White markers refer to results obtained from previous tests.

These results are in agreement with the previous data (white markers) provided by the

research group PoliLaPP. They confirm that a significant AC corrosion occurred for lower

𝑖𝐴𝐶 when the carbon steel specimens were subjected to higher CP current densities. For

instance, a corrosion rate of 50 μm/y was found in correspondence of an 𝑖𝐴𝐶 of 10 A/m2, i.e.

a low current density, and an 𝑖𝐶𝑃 greater than 1 A/m2: a so high cathodic protection current

density was able to bring the system to overprotection conditions, allowing AC corrosion to

occur at very low AC densities.

As far as the international standard is involved, it states that no AC corrosion should occur

in correspondence of AC densities lower than 30 A/m2 (labelled with a dashed line in Figure

4.25). Nevertheless, corrosion rates up to 100 µm/y were measured for interference AC

densities lower than 30 A/m2, in contrast to the CP criteria.


91

The corrosion rate likelihood should be also evaluated with respect to the ratio between the

AC density and the protection current density, i.e. the 𝑖𝐴𝐶/𝑖𝐶𝑃 ratio. In this sense, the

correlation between these two aspects was made taking into consideration the IR-free

potential (Figure 4.26). When the potential was lower than -1.2 V CSE, i.e. when the system

was in overprotection conditions, corrosion occurred at very low 𝑖𝐴𝐶/𝑖𝐶𝑃 ratios. This diagram

confirms that the most severe conditions in the presence of AC interference are characterized

by high AC density and CP current density. In other words, overprotection conditions (high

CP current density, E < -1.2 V CSE) seems to be the most dangerous condition.

Figure 4.26 – Corrosion rate vs 𝑖𝐴𝐶/𝑖𝐶𝑃 at different AC and CP levels. Blue markers refer to the tests

carried out during this thesis work. White markers refer to results obtained from previous tests.

White markers in Figure 4.26 refer to the results obtained from previous tests. These results

strengthen the hypothesis that higher 𝑖𝐶𝑃 could lead to AC corrosion, in presence of low 𝑖𝐴𝐶

values, that could not be able to cause any corrosion phenomena otherwise. 𝑖𝐴𝐶/𝑖𝐶𝑃 ratio is

considered by ISO 18086 in the description of the cathodic protection criteria, as far as “more

negative” cathodic protection levels are involved, i.e. in correspondence to IR-free potentials

lower than -1.2 V CSE. The standard states that the ratio between interference AC and

cathodic protection current densities should be lower than 3, in order to not incur AC

corrosion. Nevertheless, corrosion rates of 29 and 33 µm/y were found in some specimens

in overprotection condition in correspondence to 𝑖𝐴𝐶/𝑖𝐶𝑃 ratios lower than 3. Although these


92

CR values are not considered acceptable, i.e. higher that 10 μm/y, from the standard, they

are not so high to be harmful, from an AC corrosion point of view. Higher corrosion rates

were measured only in correspondence to 𝑖𝐴𝐶/𝑖𝐶𝑃 ratios higher than 10. In Figures 4.25 and

4.26, the points lying on the horizontal axis represent the results obtained during the first

step of the long-term exposure test. Although AC density and 𝑖𝐴𝐶/𝑖𝐶𝑃 ratios were measured,

having assumed that the corrosion rates were null for all specimens caused their

representation in the x-axis of these charts.

The ratio between AC density and the protection current density can be adopted to obtain

the so-called AC corrosion diagram, which reports IR-free potential with respect to the

logarithm of the ratio between AC and DC density (Figure 4.27). Data are compared with

obtained previous results. Corrosion rates are grouped in three categories:

lower than 10 mm/y;

between 10 and 50 mm/y;

higher than 50 mm/y.

This diagram confirms the previous observations: AC interference corrosion seems higher

in overprotection conditions, i.e. at lower IR-potential and high CP current density.

Figure 4.27 – IR-free potential with respect to the ratio between AC and CP current density. Blue markers

refer to the tests carried out during this thesis work. White markers refer to results obtained from previous

tests.


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Corrosion rates higher than 50 mm/y can be measured corresponding to a few A/m2 of AC

density in overprotection condition. For example, at IR-free potential of -1.2 V CSE, the

critical current ratio is between 10 and 100 A/m2 (Figure 4.27). Being the CP current density

high at this potential, generally between 1 and 10 A/m2, the maximum acceptable AC density

is between 1 and 10 A/m2 (as shown in Figure 4.25). As far as the results obtained in previous

tests are concerned, the white markers reinforce the assumption that a cathodic

overprotection condition, i.e. IR-free potentials lower than -1.2 V CSE, could cause AC

corrosion when 𝑖𝐴𝐶/𝑖𝐶𝑃 ratio is low: a corrosion rate greater than 50 μm/y was measured for

a 𝑖𝐴𝐶/𝑖𝐶𝑃 ratio of 10.

4.3.2 Cathodic protection criterion in the presence of AC interference

As first point, it should be meaningful to report again the cathodic protection criteria reported

in the international standard in force. The ISO standard ISO 18086:2015 [16] reports two

different methods that should be satisfied in order to not incur AC corrosion. They differ in

the cathodic protection level chosen to protect the metallic structure, suggesting different

voltage and current density thresholds. The standard states that the criteria as defined in ISO

15589-1:2015 [6] and reported in Table 1.1 should be respected as first point. The

achievement of a potential equal to or lower than the protection potential is necessary to

avoid any corrosion likelihood. The first scenario describes a “more negative” cathodic

protection level, i.e. when 𝐸𝑜𝑛 < −1,2 V CSE. In this case, one of the three parameters

below, in order of priority, can be applied:

𝑈𝐴𝐶

|𝐸𝑂𝑁|−1.2< 3, where 𝑈𝐴𝐶 is the AC voltage;

𝑖𝐴𝐶 < 30 𝐴/𝑚2;

𝑖𝐴𝐶

𝑖𝐷𝐶< 3 if 𝑖𝐴𝐶 > 30 𝐴/𝑚2.

The second scenario depicts a “less negative” cathodic protection level, i.e. when −1,2 <

𝐸𝑂𝑁 < −0,85 V CSE. As before, one of the three parameters below, in order of priority, can

be applied:

𝑈𝐴𝐶 < 15 𝑉;

𝑖𝐴𝐶 < 30 𝐴/𝑚2;

𝑖𝐷𝐶 < 1 𝐴/𝑚2 if 𝑖𝐴𝐶 > 30 𝐴/𝑚2.


94

Figure 4.28 represent graphically the cathodic protection criteria proposed by the standard

in force, with respect to the likelihood of AC corrosion, showing the relationship between

DC and AC current densities. The darker region, labelled with number 3, represents the

cathodic protection and alternating current densities for which AC corrosion is expected by

the standard.

Figure 4.28 - Relationship between DC and AC current densities and likelihood of AC corrosion, where:


Figure 4.29 reports corrosion rate in all the tested conditions in a graph reporting AC density

versus DC density. The black line separates the graph into two regions, according to the CP

criterion proposed by Ormellese et al. [58]:

Corrosion zone (or unsafe region), where AC corrosion is severe (conditions over the

line);

Protection zone (or safe region), where AC corrosion is negligible, and the metal is

protected from corrosion (conditions below the line).

In particular, the maximum allowed AC density is:

Corrosion zone, where AC corrosion is severe (conditions over the line);

𝑖𝐴𝐶 higher than 30 A/m2 when 𝑖𝐶𝑃 is lower than 1 A/m2;

𝑖𝐴𝐶 higher than 10 A/m2 when 𝑖𝐶𝑃 is higher than 1 A/m2.


95

Protection zone

𝑖𝐴𝐶 lower than 30 A/m2 when 𝑖𝐶𝑃 is lower than 1 A/m2;

𝑖𝐴𝐶 lower than 10 A/m2 when 𝑖𝐶𝑃 is higher than 1 A/m2.

Tests performed during this thesis work (blue indicators) are in good agreement with

previous tests obtained in the research (white indicators). The conditions studied during the

first phase of the work (before currents modification) correspond to the boundary condition

defined by the black line. In these conditions, corrosion rate is lower than 10 mm/y, i.e.

acceptable from an engineeristic point of view. In the second part of the work, after currents

modification, corrosion rate is not at all negligible, being higher than 10 mm/y.

According to the proposed criterion, overprotection condition seems to be the most severe

condition. Overprotection is reached at high CP current density (or low IR-free potential).

Accordingly, at high CP current density, the maximum acceptable AC density is lower (only

10 A/m2).

Figure 4.29 – Corrosion rates of carbon steel specimen under CP condition in the presence of AC

interference: 𝑖𝐴𝐶 vs 𝑖𝐶𝑃 graph.

The results obtained from the long-term exposure tests, i.e. the blue markers in Figure 4.29

representing the corrosion rates occurred, can be overlapped to Figure 4.28, representing

graphically the CP criterion proposed in the standard in force, in order to prove its validity

with respect to the abovementioned results.


96

Figure 4.30 – Experimental corrosion rate in the 𝑖𝐴𝐶 /𝑖𝐶𝑃 diagram. Safe and unsafe regions refer to CP

criterion as reported in ISO 18086:2015.

All markers lie in the safe regions of the CP criterion proposed in ISO 18086. Nevertheless,

some of these markers indicate that corrosion occurred for conditions, as far as AC and CP

current densities are involved, that are considered safe by the standard in force from the AC

corrosion point of view. Instead, all the indicators representing the corrosion rates higher

than 10 µm/y lie in the unsafe region in Figure 4.29, showing graphically the CP criterion

proposed by Ormellese et al. [58].

The effects of cathodic overprotection on the occurred corrosion rates with respect to AC

density and 𝑖𝐴𝐶/𝑖𝐶𝑃 ratio is represented in Figure 4.31 and Figure 4.32, respectively. Data

are compared with obtained previous results. Corrosion rates are grouped in three categories:

lower than 10 mm/y;

between 10 and 50 mm/y;

higher than 50 mm/y.

The cathodic overprotection condition is reached by imposing CP current densities higher

than 1 A/m2, with the IR-free potential that decreases with 𝑖𝐶𝑃. Figure 4.31 shows the

dependency on the cathodic overprotection of the measured CR, that assumed relevant

values, i.e. higher than 10 mm/y, for lower 𝑖𝐴𝐶/𝑖𝐶𝑃 ratios, in correspondence to

overprotection conditions, i.e. IR-free potential lower than -1.2 V CSE. Instead, Figure 4.32

represents the results with respect to the AC densities that caused them. Once again, cathodic


97

overprotection lead to AC corrosion for lower 𝑖𝐴𝐶: 18 A/m2 of 𝑖𝐴𝐶 were enough to cause a

corrosion rate higher than 50 µm/y when the measured IR-free potential was approximately

-1.35 V CSE. As mentioned before, the CP criterion present in ISO 18086 did not expect an

AC corrosion for AC current densities lower than 30 A/m2 and for IR-free potentials in

between -0.850 V CSE and -1.2 V CSE, i.e. for “less negative” cathodic protection levels,

corresponding to an applied CP current density lower than 1 A/m2. Figure 4.33 depict the

same graph and the same results as Figure 4.32, with the limitation imposed by Ormellese

et al. [58]. Differently from the previous figure, all corrosion phenomena described by a CR

higher than 10 µm/y lie in the unsafe region of the graph.

In conclusion, cathodic overprotection is found to be the worst condition, as far as AC

corrosion is concerned, because it occurs at lower AC densities and 𝑖𝐴𝐶/𝑖𝐶𝑃 ratios.

Figure 4.31 – Experimental corrosion rate in the 𝑖𝐴𝐶/𝑖𝐶𝑃 vs IR-free potential diagram. Safe and unsafe

regions refer to CP criterion as reported in ISO 18086:2015.


98

Figure 4.32 – Experimental corrosion rate in the 𝑖𝐴𝐶 vs IR-free potential diagram. Safe and unsafe regions

refer to CP criterion as reported in ISO 18086:2015.

Figure 4.33 – Experimental corrosion rate in the 𝑖𝐴𝐶 vs IR-free potential diagram. Safe and unsafe regions

refer to CP criterion as reported in [58].


99

The main purpose of the conducted long-term exposure tests was to verify the validity of the

cathodic protection criterion in the presence of AC interference reported on ISO 18086

(Paragraph 2.2.1). From the obtained results, it was demonstrated that the lowest 𝑖𝐴𝐶 value

needed to have any relevant AC corrosion phenomena cannot be given per se, but it depends

also on the applied cathodic protection current density. High 𝑖𝐶𝑃 values bring the system to

cathodic overprotection, and this situation is found to be the worst condition, as far as AC

corrosion is concerned. This aspect is confirmed not only by the results, but also by some

studies, reported in Chapter 2. For instance, in the alkalization theory proposed by Nielsen

et al. (Paragraph 2.1.5), it is reported that the CP current density is related to the production

of hydroxides (𝑂𝐻−) at the coating defect: if 𝑖𝐴𝐶 is high enough, a local accumulation of

hydroxides occurs, leading to a pH increase. If the pH approaches the value of 14, high

corrosion rates are possible due to the formation of 𝐻𝐹𝑒𝑂2− [7]. This high pH value, in

combination with potential oscillations, could lead to the periodic entry in the high-pH

corrosion domain in the Pourbaix diagram (Figure 2.9). The authors ended by saying that

high CP level has a dramatic influence on the AC corrosion process.

A discrepancy was found between the obtained results and the cathodic protection criterion

proposed by ISO 18086. The experimental conditions that caused AC corrosion lied on the

safe region of the 𝑖𝐴𝐶 vs. 𝑖𝐶𝑃 graph reported in the international standard in force: in this

sense, the results are in contrast with the cathodic protection criterion present in ISO 18086,

because AC corrosion occurred for those conditions that were considered safe from the

corrosion point of view. Contrarily, corrosion was expected for the second tested conditions

(Table 4.5), i.e. the only ones that caused AC corrosion, from the cathodic protection

criterion proposed by the research group PoliLaPP. Ormellese et al. [58], according from the

results obtained during several tests, published a proposal of modification of the CP criterion

present in the international standard in force. In this sense, the results published in this work

are in agreement with the cathodic protection criterion in the presence of AC interference

proposed by the research group PoliLaPP, strengthen their validity. It could be asserted that

high CP current densities should be avoided in the design of cathodic protection systems.

From the obtained results, the modification of the cathodic protection criterion proposed by

Ormellese et al. seems to be justified. 1 A/m2 resulted to be the highest 𝑖𝐶𝑃 value that can be

applied in order to reduce or halt any AC corrosion phenomena. In this sense, the interference

AC densities should be lowered under the value of 30 A/m2. When the ongoing CP current

densities results to be higher than 1 A/m2, it is recommended to further decrease 𝑖𝐴𝐶 to values


100

lower than 10 A/m2. No matter how, together with the current densities monitoring, the

criteria as defined in ISO 15589-1:2015 and reported in Table 1.1 should be respected as

well. The achievement of a potential equal to or lower than the protection potential is

necessary to avoid any corrosion likelihood.

101

Conclusions

The aim of this thesis work is to validate the proposed cathodic protection criteria in the

presence of AC interference and to study the effect of AC on the measured potential. In

particular, two families of test were carried out in the presence of both AC and DC signals:

Long-term exposure tests for mass loss measurements;

Galvanostatic tests to study the effect of the AC interference on DC potential.

The testes were performed on carbon steel specimens, having a surface of 1 cm2 and

simulating a coating defect of a cathodically protected buried pipeline, interfered by an

alternating current density, in soil simulating solution. For this purpose, a specific electrical

circuit was used, able to completely separate AC and DC signals.

The main conclusions are summarized in the following.

1 GALVANOTATIC TESTS: EFFECT OF AC ON IR-FREE POTENTIAL

Galvanostatic tests were performed on cathodically protected carbon steel specimens in soil-

simulating solution in the presence of AC stationary interference: this test consists in

applying a fixed cathodic protection current density and in measuring the potential changes

with a stepwise increasing AC density. In particular, the chosen cathodic protection current

densities ranged from 0.15 A/m2 to 10 A/m2, while the alternating current densities from 1

to 1,000 A/m2. The effects of the alternating current on the IR-free potential can be

summarized as follows:

IR-free potential is strongly affected by the presence of AC density;

AC shifts the IR-free potential in the more anodic (positive) direction, if the carbon

steel specimen is in cathodic protection condition;

the potential shift is proportional to the interference AC density;

the IR-free potential increase is almost linear for AC density up to 300 A/m2, then it

stabilizes at higher AC densities;

the IR-free potential - AC density curves are shifted downwards increasing the

cathodic protection current density;

Conclusions

102

the IR-free potential overcomes the protection potential in presence of very high AC

densities (greater than 300 A/m2) and low cathodic protection current densities (lower

than 0.3 A/m2);

AC shifts the IR-free potential in the more cathodic (negative) direction, if the carbon

steel specimen is in free corrosion condition, until reaching a stable value;

IR-free potential monitoring is a necessary but not sufficient condition to assess AC

corrosion likelihood.

The conducted tests were compared to the results previously obtained by the research group

PoliLaPP and their reproducibility was verified.

2 LONG-TERM EXPOSURE TESTS FOR MASS LOSS MEASUREMENT


criterion in the presence of AC interference reported on ISO 18086. Long-term exposure

tests consist in applying a CP current and interference AC densities on carbon steel

specimens, in order to determine the corrosion rate by mass loss measurement. Four

conditions were studied, at different AC and DC densities. Tests lasted three months. Results

showed that corrosion occurred only for specimens that experienced the following

conditions: 1) iAC = 10 A/m2, iDC = 10 A/m2; 2) iAC = 10 A/m2, iDC = 1 A/m2; 3) iAC = 30 A/m2,

iDC = 1 A/m2; 4) iAC = 30 A/m2, iDC = 0.2 A/m2; 5) iAC = 20 A/m2, iDC = 10 A/m2; 6) iAC = 20

A/m2, iDC = 2 A/m2; 7) iAC = 50 A/m2, iDC = 0.5 A/m2; 8) iAC = 50 A/m2, iDC = 0.2 A/m2. The

measured corrosion rates ranged from 31 to 53 µm/y. Cathodic overprotection, i.e. IR-free

potential lower than -1.2 V CSE, obtained from the application of cathodic protection current

densities higher than 1 A/m2, was found to be the worst condition, as far as AC corrosion is

concerned: in this sense, AC corrosion occurred for iAC lower than 20 A/m2. The

experimental conditions that caused AC corrosion lied on the safe region of the 𝑖𝐴𝐶 vs. 𝑖𝐶𝑃

graph reported in the international standard in force: in this sense, the results are in contrast

with the cathodic protection criterion present in ISO 18086, because AC corrosion occurred

for those conditions that were considered safe from the corrosion point of view. Contrarily,

corrosion was expected from the cathodic protection criterion proposed by the research

group PoliLaPP for the following conditions: 1) iAC = 20 A/m2, iDC = 10 A/m2; 2) iAC = 20

A/m2, iDC = 2 A/m2; 3) iAC = 50 A/m2, iDC = 0.5 A/m2; 4) iAC = 50 A/m2, iDC = 0.2 A/m2.

Ormellese et al. [58], according from the results obtained during several tests, published a

Conclusions

103

proposal of modification of the CP criterion present in the international standard in force. In

this sense, the results published in this work are in agreement with the cathodic protection

criterion in the presence of AC interference proposed by the research group PoliLaPP,

strengthening their validity.

104

References

[1] L. Di Biase, “Interaction and stray-current corrosion”, Shreir’s Corrosion, Vol. 4, Chapter 4.22, pp.

2833-2838, ISBN: 978-0-444-52787-5, Elsevier, 2010.

[2] R.W. Bonds, “The effect of overhead AC power lines paralleling ductile iron pipelines”, DIPRA

publication, DIPRA - Ductile Iron Pipe Research Association, 1997.

[3] Y. Hosokawa, F. Kajiyama, Y. Nakamura, “New cathodic protection criteria based on direct and

alternating current densities measured using coupons and their application to modern steel

pipelines”, Corrosion 60 (3), pp. 304-312, 2004.

[4] L. Lazzari, P. Pedeferri, “Cathodic protection”, pp. 370, ISBN: 8873980201, Polipress, 2006.

[5] P. Pedeferri, “Corrosione e protezione dei materiali metallici”, pp. 336, ISBN: 9788873980322,

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of pipeline systems. Part 1: On-land pipelines”, ISO – International standard Organization, 2015.

[7] EN 12954, Cathodic Protection of Buried or Immersed Metallic Structures – General Principles

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[8] A.W. Peabody, Control of Pipeline Corrosion, NACE Int. Publication, Houston, TX, 1967

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107

Ringraziamenti

Desidero ringraziare il Prof. Marco Ormellese per avermi dato la possibilità di scegliere

questo lavoro di tesi.

Ringrazio l’Ing. Andrea Brenna la cui assistenza e la grande pazienza nei miei confronti

sono state indispensabili per svolgere questo lavoro, e che si è reso sempre disponibile ogni

qualvolta io ne avessi bisogno.

Un grazie ai miei colleghi prima, e amici poi, che ho incontrato durante questi cinque anni

passati al Politecnico di Milano, e fatto compagnia durante le lezioni e le mille sessioni

d’esame.

Vorrei ringraziare i miei amici da una vita, Lorenzo, Loris e Benedetto, che da sempre mi

supportano (e sopportano) e che senza i quali la mia vita sarebbe sicuramente un po’ più

vuota.

Mi batte il cuore nel ringraziare il mio amore, Chiara, che più di tutti è riuscita, sia con le

buone che con le cattive, a spronarmi durante tutti questi anni. Insieme coglieremo i frutti

di questi anni di studio intenso, te lo prometto.

Ultimi ma non meno importati, anzi, vorrei ringraziare la mia famiglia, mio padre, mia

madre e le mie sorelle: senza di loro non sarei qua, e questa Laurea Magistrale è anche

vostra.

ac corrosion of carbon steel in cathodic protection ... · corrosion rate vs 𝑖 at different cp...

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