tutorial corrosion ii -...

Tutorial Corrosion II

Electrochemical characterization

with EC-Lab® techniques

1

1. Introduction

2. Types of corrosion

a) Uniform corrosion

b) Localized corrosion

3. Corrosion experiment

4. EC-Lab® corrosion techniques



5. Conclusions

2

OUTLINE

Corrosion is an electrochemical interaction between a material and its environment

which result in changes in the properties of the material

3

Two types of corrosion

Corrosion is commonly classified based on the appearance of the corroded material:

uniform and localized corrosion

1. Uniform Corrosion

Corrosion over the whole surface of the material

2. Localized Corrosion

Corrosion at small areas or local sites of the material surface in contact with the

corrosive solution

INTRODUCTION

Corrosion involves two electrochemical processes: Oxidation and reduction

1. Oxidation The oxidation process takes place at the anodic sites according to the reaction

M Mn+ + ne- (1)

2. Reduction

Reduction reaction takes place at the cathodic sites

The main reduction reactions encountered during the corrosion of a material is the

oxygen reduction (2,3) or the hydrogen ion reduction (4)

In acidic media O2 + 4H+ + 4 e- 2H2O (2)

In alkaline/Neutral media O2 + 2H2O + 4 e- 4OH- (3)

Hydrogen ion reduction 2H+ + 2 e- H2 (4)

4

INTRODUCTION

5

OUTLINE

1. Introduction








5. Conclusions

Also known as general corrosion, this kind of

corrosion corresponds to an uniform attack over

the entire surface of the material.

It is the most classical form of corrosion

characterized by a uniform reduction of

thickness of the surface of material.

It is relatively easy to measure, predict and

control this form of corrosion

6

UNIFORM CORROSION

Uniform Corrosion

7

Anodic domain Cathodic domain

Hydrogen reduction 2 H+ + 2 e- H2

Oxygen reduction

O2 + 2 H2O + 2 e- 4 OH-

M Mn+ + n e-

Polarization curve

Ecorr Corrosion potential

Reduction reactions Oxidation/dissolution reaction

UNIFORM CORROSION

1. Introduction








5. Conclusions

LOCALIZED CORROSION

8

In this section, six types of localized corrosion are described

1. Pitting corrosion

It is the most insidious form of corrosion. It is resulting in small

cavities or holes extending from the surface into the material

usually occurs in halide containing solutions (Cl-, Br-,..)

2. Crevice corrosion

Usually associated with small volumes of stagnant solution

leading to a limitation in the oxygen diffusion to a confined

space (under gasket, insulation material,..)

9

LOCALIZED CORROSION

Localized corrosion types

10

3. Selective corrosion

Preferential attack of a particular component of an alloy in

presence of electrolyte. The most well-known examples

are:

dezincification of brass (Cu-Zn)

destannification of tin bronze (Sn-Cu)

4. Intergranular corrosion

As the name suggests, this form of corrosion occurs along

the grain boundaries of metal and is usually results from

local differences in composition due to chemical

segregation effects or to specific phases precipitated on

the grain boundaries

LOCALIZED CORROSION

11

5. Galvanic corrosion

Corrosion resulting from contact between two different metals

or alloys in a conducting corrosive environment.

This corrosion occurs when three conditions are present:

a) Metals must be far apart on the galvanic series

b) These metals must be in electrical contact,

c) The metals must be exposed to a conductive electrolyte

6. Stress corrosion cracking (SCC)

the cracking is induced from the combined action of tensile

stress and a corrosive environment.

LOCALIZED CORROSION

12

LOCALIZED CORROSION

Activity Passivity Transpassivity


Dissolution

Passive film

M MxOy

Passivation

Etp Transpassive potential

Sharp increase of anodic current

(without Halide species)

Polarization curve of a passivable material

13

Polarization curve of a passivable material

LOCALIZED CORROSION

Passivity breakdown

Epit Pitting potential with Halide species

( Cl-, Br

-, I

-, F

- …)

Pitting corrosion

Pitting Passivity Activity


1. Introduction








5. Conclusions

14

OUTLINE

Potentiostat

Corrosion Cell

Experimental Set up

WE= Working electrode

RE= Reference electrode

CE= Counter electrode

(Pt, Graphite,..)

15

CORROSION EXPERIMENT

E

I


steady-state current-voltage curves

16

A. Controlled voltage techniques (Except ZRA)

1- Apply a voltage between RE and WE

2- Record the current flowing between the WE and the CE

B. Controlled current techniques

1- Apply a current between CE and WE

2- Record the voltage between the WE and the RE

Role of the potentiostat

17

Cell connection modes

Standard mode

VMP3 series SP300 series

CE to Ground mode (used for multielectrode

inverstigation)

Connection to the cell Each potentiostat cell cable has 6 terminal leads:


EWE= Ref1-Ref2 EWE= S1-S2

EWE= Ref3-Ref2 EWE= S1-S2

1. Introduction








5. Conclusions

18

OUTLINE

Uniform Corrosion Localized corrosion

• Ecorr vs time

• Cyclic Potentiodynamic Polarization (CPP)

• Multielectrode Potentiodynamic pitting (MPP)

• Multielectrode Potentiostatic pitting (MPSP)

• CPT

• Depassivation Potential: DP

• Corrosimetry (CM)

• ZRA

• Ecorr vs time

• Linear Polarization (LP)

• Corrosimetry (CM)

• Generalized Corrosion(GC)

• VASP

• CASP

• ZRA

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Corrosion techniques

EC-LAB CORROSION TECHNIQUES

1. Introduction








5. Conclusions

20

OUTLINE

Ecorr vs time set up

Activation/dissolution

Passivation / Inhibition

Time

E corr

It is a basic technique used to monitor the corrosion potential of the working electrode

versus time

Ecorr vs time

21


The LP technique consists of a potential scan around the Ecorr. It is used to determine

polarization resistance, corrosion current and Tafel parameters.

LP description

Linear polarization set up

Ecorr vs time

Potential scan

Linear Polarization

2 kinds of information

EL-Ei

22


I corr , βa and βc

Linear Polarization

Rp Fit

Ecorr ± 25 mV

Ecorr ± 250 mV

Tafel Fit

LP

LP

23

LP stainless steel in NaCl media (30 gL-1)

Rp


Polarization in

a narrow ΔE

Polarization in

a wide ΔE

The CM technique is used in corrosion for the determination of Rp versus time by a

repetition of the linear polarization around the corrosion potential at fixed time interval.

Corrosimetry: CM

Corrosimetry set up

Corrosimetry description

24

Polarization resistance

Corrosion rate


25

Corrosimetry (CM)

corrosion rate vs time

CM technique is advisable for long-time corrosion study


Technique dedicated to study the general corrosion. It consists of half a cycle or a cycle of

usual cyclic voltammetry with a digital potential sweep

Ecorr vs time

GC parameter

Reverse scan

Generalized corrosion

Ecorr

I corr

Generalized corrosion set up

26


Courbe à ajouter

VASP (Variable Amplitude Sinusoïdal micro-Polarization) is based on the EIS measurements.

It consists in the determination of the change of the polarization resistance in function of the

potential amplitude variation

)2sin()( tfVEtV acorr with maxmin aaa VVV

VASP

VASP set up

27


Electrochemical system

V(t) I(t)

Va

Ia

For each amplitude Va , Rp is determined and the Rp=f(Va) is plotted

28


The frequency f is determined by an EIS technique

and corresponds to the Rp parameter where Im(Z) =0

and Z(f)= Rp f= 0,23 Hz

VASP

The impedance response of a non-linear

electrochemical system depends on the

potential amplitude va

Impedance evolution of a non linear

system (Test box-3-2)

VASP on a Ni electrode in HCl media

VASP Fit I corr

Tafel

parameters

29


VASP

CASP

30

CASP set up

CASP (Constant Amplitude Sinusoidal microPolarization) technique is used to determine

the corrosion current and the Tafel parameters using Discrete Fourier Transform (DFT)

A voltage V(t) with a frequency f is applied around Ecorr

Three harmonics: , ,

Corrosion rate icorr

Tafel parameter ba and bc


CASP

31

harmonics

CASP Fit

Tafel parameters

I corr


32

Zero Resistance Ammeter (ZRA)


ZRA is corrosion technique used to measure the galvanic coupling current between two

dissimilar electrodes. It consists of applying zero volts between the WE and the CE and

then measuring the current and the potentials (Ewe, Ece) versus the reference electrode.

ZRA Set up

OCV

ZRA

OCV

Coupling current

Noise resistance

33

ZRA curves


Three methods are available in EC-Lab® to calculate the noise resistance Rn

1. Standard deviation (SD).

2. Power spectral density (PSD ) with DFT.

3. PSD with MEM (Maximum Entropy Method)

SD method

PSD with DFT method

PSD with MEM method

ENA Fit

1. Introduction








5. Conclusions

34

OUTLINE

CPP Set up

Cyclic Potentiodynamic Polarization (CPP)

CPP is a corrosion technique used to evaluate pitting susceptibility and repassivation

potential of a material. It is based on a potentiodynamic scan

35

Pitting potential

Corrosion potential

Repassivation potential

Corrosion rate


Ecorr

Cyclic Potentiodynamic Polarization (CPP)

36

The figure below give an example of the curve obtained by CPP technique

Reverse scan

ip

Erepa

The pitting potential is easily determined

in I-E presentation

Epit

ip

Pitting potential

Epit


This technique is designed to study pitting corrosion on one or several electrodes together in

the electrochemical cell. CE to ground connection is required in the case of MPP experiment

Multiple Potentiodynamic Pitting (MPP)

MPP set up

MPP description

Pitting potential

Corrosion potential

Corrosion rate

Corrosion resistance

37


Multielectrode Potentiodynamic Pitting (MPP)

MPP technique can be performed on up to 16 working electrodes simultaneously (with

VMP3 in Ce to ground mode)

“MultiPitting Statistics” allows the user to compute data simultaneously from several

data files.

38

Ecorr


MP statistics

MPP curves of the four steel electrodes

MPP investigations and Statistics

This technique is similar to the MPP technique with a potentiostatic control instead of a

potentiodynamic scan. It is designed to study pitting corrosion on one or several electrodes

together in the electrochemical cell

Multielectrode Potentiostatic Pitting (MPSP)

MPSP set up

MPSP description

Pitting potential

Metastable pits

39


40

MPSP technique allows the determination of the latency and growth phases for each

applied potential Ei.

Multielectrode Potentiostatic Pitting MPSP

40

ip

MPSP curve for a stainless steel in 30g.L-1 NaCl.

Growth phase

Latency phase


CONCLUSIONS

41

In the first part of this tutorial:

an overview of different corrosion forms was presented.

the main characteristics of each form of corrosion was described with photo

illustrations.

A corrosion experiment with a three electrodes system was presented.

The connection modes to Bio-logic potentiostats were also detailed.

In the second part of this tutorial:

the corrosion techniques available in EC-lab® software were detailed.

Experimental results were given here to illustrate the use of the corrosion

techniques.

Numerous corrosion analysis tools, available in EC-Lab® softawre were used.

tutorial corrosion ii -...

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