Understandin • •9 pipe corrosionCorrosion is a key cause 01 pipeline damage, with

sections having to be taken out 01 operation or replaced

due to corrosion damage every year. One 01 the principIe

reasons lor this damage is lailure 01 the protective

coating. This can result Irorn a number 01 lactors: the

type 01 coating used; the pipeline soil conditions; and,

where a pipe has undergone in-field rehabilitation, the

conditions pertaining to that rehabilitation.

Rehabilitation often involves application 01 the new

coating "over-the-trench" where it can be very difficult

to maintain the required application conditions or to

ensure that suitable suríace preparation is achieved.

Experience shows that many rehabilitated coating

lailures can be attributed to lailures during application

and improper surlace preparation (NACE study 1996L

Viscous elastic coatings (VECs) provide a means 01

overcoming these challenges, providing immediate

adhesion, impermeability to moisture and gases, good

cold flow behavior and eliminates surlace blasting.

Corrosion is a chemical-physical process during

which iron is converted into iron-oxide or iron-hydroxide

in the presence 01 water, which provides the required

oxygen (atmospheric oxygen is not the major cause 01

corrosion, as is clear Irom many desert areas where

steel requires almost no protectionL

In chemical terms, the oxidation 01 iron - rusting-

can be described by the lollowing tour steps:

• Fe [iron] --> Fe+++ lron ion) + 3 e- (three electrons)

• H,O (water) --> (H+) hydrogen ion + [OH-) hydroxyl ion.

• F+++ liron ion) with [OH-) hydroxyl ion --> Fe[OHJ, iron

hydroxide

• The H+ [hydrogen-ion) reacts with the electron e -

into H, (hydrogen)

While the H, could react with O, (oxygen) frorn the air

to Iorrn more H,O [water!. this reaction does not

typically playa role in the iron corrosion process.

In the case 01 a pipe, it is the role 01 the applied

coating to protect the steel substrate Irom water and so

to prevent the onset and progression 01 corrosion. A

variety 01 coating options are available, including:

lactory applied coatings such as FBE, HOPE PP or

urethane: or lield applied coatings such as bitumens,

PE/butyl tapes, waxes, or spray on epoxy, polyolelin or

zinc coatings. Selection will be based on íactors such as

estimated substrate liletime, operating environment,

nature 01 the substrate, and the cost 01 the materials,

application method and repair.

It is also important when selecting the coating to be

employed to consider some 01 the phenomena that play

a part in the corrosion process: salts & osmosis;

adhesion; microbiological induced corrosion [MIC);

surface preparation; and water permeability.

The lollowing discussion looks at some 01 the main

lactors contributing to corrosion 01 the pipe and how

viscous elastic coatings can be used to improve

outcomes in lield-based rehabilitation projects.

Salts & osmosisDissolved salts play an important part in the corrosion

mechanism and they are very difficult to remove. Salt's

role can be described as lollows:

• NaCl --> Na+ [sodium ion) + Cl- lchlorine ion)

• Fe+++ [iron ion) + 3 Cl- [chlorine ion) --> FeCl, [iron

chloride)

• FeCl, and H,O [water) give Fe(OH), [iron hydroxide)

and HCL [hydrochloric acidl

The hydrochloric acid accelerates the process by which

iron electrons are lost:

• Na+ and e- give Na

• Na + H,O --> NaOH and H, (hydrogenl

• NaOH + HCL --> NaCL + H,O.

Salts particles are widespread and difficult to

remove. Even rinsing a blasted pipeline coating with

clean water will not remove the salt particles and

contaminations in the voids of the blastéd pipe. As salts

attract water and as many pipeline coatings are not

100% water vapour or water impermeable the presence

of salt is always a risk in practice.

Further, if the coating is not completely impermeable

to water or water vapour, any salt present can result in

osmosis. This is the process where a concentration

gradient in a solution (in this case salt concentrationl

will drive the transfer of the solvent (water! through a

semi-permeable membrane [the coatingl until the

concentration imbalance is overcome. It can be a

significant problem wherever dissolved substances may

be present beneath a coating and can only be eliminated

by the use of a fully impermeable coating material.

As VECs are impervious to water, the risk of salt

particles combining with water at the substrate surface

is eliminated. Furthermore, it is no longer necessary to

wash the substrate with clean water prior to coating,

which is often a challenge in remote areas. The use of

VECs will also eliminate the phenomenon of osmosis.

And the high level of adhesion and wetting of VECs,

together with their self-healing characteristics, means

that minor pinholes and coating damage do not present

a corrosion risk.

AdhesionCoatings with poor adhesion to the pipe substrate will

certainly fail. However, achieving good adhesion is not

always easy as it is highly sensitive to application

circumstances and surface preparation. The differ-

ence between the surface tensions of the different

materials (surface and coating material] also plays an

important roleo

Table 1 (following pagel shows the different mecha-

nisms of adhesion between the coating and pipe surface.

In practice, coatings applied in the field usually adhere

to the surface by means of a physical or mechanical

bond. Primers are almost always required to create a

mechanical bond with hard or semi-hard coating materi-

als and there is typically a tendency to delamination.

In contrast, VECs exhibit excellent wetting character-

istics and display cohesive fracture in peel, meaning the

coating breaks away leaving a film in place on the pipe

surface. On many dry and clean surfaces, the material

shows cold flow and it will penetrate the pores on the

pipe surface over time. Pressure applied by an outer

wrap or earth loading will accelerate this process. Outer

wraps also provide additional protection to the VEC,

separating the main function of corrosion protection

from mechanical protection.

MicrobiologicaUy Influenced Corrosion (MIC)Microbiologically influenced corrosion - where corro-

sion is either initiated or accelerated by micro-

organisms - is responsible for almost 50% of corrosion

problems in the pipeline industry. It was first identified

as a mechanism in 1934, when sulphate reducing

bacteria (SRBI were found to be re spons ible for the

corrosion failure of cast iron pipes.

SRBs are anaerobic bacteria using sulphate as a

terminal electron acceptor and organic substances as

carbon sources. During the metabolic process, sulphate

is reduced to sulphide, which reacts with hydrogen

produced by metabolic activities or by cathodic reaction

of corrosion processes to form hydrogen sulphide.

Hydrogen sulphide is highly corrosive to ferrous metals

Above left: In

field applica-

tion of Kleiss's

Viscowrap

coating at Shell

Canada. Above:

This peel test

shows high

level surface

adhesion due

to the physical

bonding of the

coating to the

pipe

_ pretectien I corrosión mechanisms

Table 1: Different adhesion mechanisms between coating and substrate

Chemical bond

lonogenic bond

Two different or equivalent atoms form a bond. A covalent bond existing from one couple of electrons

Two different ions with opposite values form a bond.

Metal bond The atomic structure with free electrons

I Causes a cohesion: attraction between atoms of same substances.

Hydrogen bridges

"Van Der Waals" forces

1 With primer

I Causes adhesion: altraction between atoms of different substances.

Mechanical bond

Physical bond Viscotaq viscous elastic coating

Source: Kleiss & Co

and further reacts with dissolved iron to form an iron

sulphide film over the metal substrate. lron sulphides

have relatively low hydrogen evolution over-potential and

a galvanic coupling between the iron sulphide film and

the metal substrate is set up and corrosion accelerated.

Other significant micro-organisms in pipe corrosion

are lormative acid producing bacteria (APBl capable 01

forming organic acids, such as acetic, formic and lactic

acids. These acids have dual roles in MIC, causing acid

corrosion 01 many alloys as wells as supplying nutrients

lor the bacteria.

VECs tonta in no nitrogen nutrients to leed rnicro-

organisms, while their impermeability to water means

bacteriallile cannot be supported because no water is

present at the boundary 01 the metal and coating wrap.

In addition, because the VEC coatíng creates such an

intimate bond with the surface - penetrating pores-

there is no opportunity lor potentially harmlul substanc-

es to creep beneath the coating.

VECs also show no permeability lor ionic species

Irom soil, such as nitrates, nitrites and ammonium,

while their slightly basic PH8 lurther acts to inhibit any

potential bacterial growth.

Surface preparationNACE studies have previously established that poor

surface preparation is a main cause of corrosion

problems. Field-applied-coatings require an excellent

prepared substrate surface to get good adhesion and

sandblasting olten is required. While surface prepara-

tion techniques can be well controlled in-plant,

achieving similar high levels 01 control in the field is

more challenging. Any contaminants and salt particles

remaining in the voids of the blasted surface can lead to

rapid disbonding.

VECs only require substrate preparation to 5T-2,

which means removal of loose sand, grit and grease.

5and blasting is not necessary but the coating must be

applied at a temperature above the dew point and the

rnaterial's glass transition temperature (application

temperatures range Irom -300 C up to 700 C].

Water PermeabilityThe presence 01 water is extremely damaging lor the

substrate and no matter how well a coating has be en

applied in the factory, experience shows that disbonding

as a result of the presence of water occurs to some extent.

Corrosion is always the result of a combination of

causes. However, water permeability must be consid-

ered as a serious hazard as corrosion will inevitably

occur if water or water vapour is able to travel through

the coating, more so il sal! particles or contaminants

are present in voids in the blasted substrate.

VECs are made of amorphous a-polar polyolefins

with no reactive groups and Iree radicals. Permeability

to water is very low and VECs are virtually impermeable

to moisture vapour at ambient conditions. Due to the

absence of free radicals, VECs remain stable lor

decades and perform very well in aggressive soils, such

as those found in 5ubkha areas in the GCC countries.

ConclusionThe most important step in corrosion prevention is to

ensure that bare steel parts do not come into contact with

water. Protective coatings must be impermeable to water

and have to adhere to the surface as closely as possible.

Many established coatings cannot meet these require-

ments when applied during in-field rehabilitation

projects. Major factors contributing to corrosion failure in

over-the-trench rehabilitation coatings include: incorrect

material choice for the application; poor surface prepara-

tion; incorrect coating application; curing failure; liquid

absorption; soil stress; volume shrinkage; and osmosis.

VECs can overcome these challenges as they require little

surface preparation, aré easy to apply, do not cure, are

impermeable to water, and exhibit no shrinkage.

About the authorLeo van Beugen is Managing Director of Netherlands-

based Kleiss & Co, a manufacturer 01 pipeline enginee-

ring equipment and materials including the Vicotaq line

01 viscous elastic coatings.

I www.kleiss.nl