Khanna A.S. (Ed.) High-Performance Organic Coatings: Selection, application and evaluation

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Organic coatings for underground pipelines 327

• Minimum damage during transportation and handling

•

• Excellent corrosion resistance.

15.11.2 Limitations of three-layer PE/PP coatings

Three-layer PE/PP coatings also have some limitations. Some are as given

below:

• Shielding of cathodic protection current in case of coating

disbondment

• No fail-safe coating

• Prone to thinning across raised weld seams

• High cost compared to other coatings

•

• Sensitive to steel surface preparation and operating condition

15.11.3 Application of three-layer PE/PP coatings

The steel surface condition is one of the most critical areas of the coating,

option, but it must be applied immediately before primary heating. The

aqueous solution is evenly applied on the steel surface, which must be dry

before the FBE powder is applied. The advantages of chromate treatment

high temperature wet tests, such as the hot water soak and elevated tem-

perature cathodic disbondment test.

Epoxy powders used for single-layer FBE differ substantially when

employed as primers for three-layer PE/PP coatings. The chemical and

physical properties, as well as the powder ingredients themselves, are suited

to provide a desired combination of application temperature, gel time and

cure time. The powder application temperature and the distance to next

unit operation, combined with line speed, become the most critical factors

in producing a high quality three-layer PE/PP coating.

The thickness of the primer coating is normally 75 microns but many

to enhance the adhesion and cathodic disbondment properties. The purpose

of the adhesive is to provide an intermediate layer between the epoxy

primer and the polyethylene outer coating. The adhesive has a number of

polar groups which bond chemically to epoxy, provided that the epoxy is

Repair is costly and difficult

• Difficult to have meaningful quality test.

values. The steel surface should first be blast cleaned to a near white metal

surface with anchor profile of 50–75 microns. Pre-treatment is the preferred

are well known. The beneficial effect becomes especially evident during

specifications are now recommending a thickness from 100 to 250 microns

Excellent low temperature flexibility and impact resistance

having most influence on adhesion and cathodic disbondment resistance

328 High-performance organic coatings

not fully cured yet. At the same time, the adhesive layer provides a strong

sive and polyethylene.

Application of the adhesive must follow FBE primer very closely to

optimize the chemical bond between the primer and the adhesive. The time

lapse between the two should never exceed 15 seconds. There are several

layer PE/PP coating. Historically the oldest and still commonly used is low

density polyethylene, followed by medium and high density polyethylene.

Polypropylene can be used for applications in excess of 100°C. Polyethylene

is extruded on to the pipe in a co-extrusion die, together with the polymeric

adhesive. Co-extrusion offers the largest degree of adhesion between the

intermediate and outer layers, as it virtually squeezes both layers together

in a molten state, causing them to adhere under high pressure and high

temperature. At the same time, it does not allow air to penetrate between

the adhesive and polyethylene layers, which sometimes occur in non-

polyoleﬁ n powder applied during shop application has also been used to

weight coating. The three-layer PE/PP system provides extremely effective

defense against the hazards of transportation and installation, the stresses

of temperature change, and action by water, organic acids, alkalis and bac-

teria, as well as galvanic action. The chemical bonding between the layers

provides very high peel strength adhesion. Three-layer PE/PP coatings are

thick coatings, which not only provide the anti-corrosion properties of the

due to the thick layer of either polyethylene or polypropylene. A three-

layer polyethylene coating is rated for low temperature services, whereas

three-layer polypropylene coatings are rated for services up to 120°C.

15.12 Surface preparation and its importance in coating

The performance of the coating can be improved by proper surface cleaning

of line pipe. Coal-tar epoxy (CTE) coatings require less sophisticated

surface cleaning, while FBE and three-layer PE/PP require thorough surface

preparation. For CTE coating, the following procedure is adopted:

1. The pipe is heated to eliminate humidity.

2. The surface is shot-blasted to remove scale and rust.

3. Primer is applied to improve adhesion.

4. The coal-tar enamel is applied.

In the case of FBE coating, dual-layer FBE coating and three-layer PE/PP

bond to the polyethylene outer layer due to the chemical affinity of adhe-

different kinds of polyolefin which can be used as an outer coat for three-

extrusion processes. A polyolefin rough coat or rough finish consisting of

improve the friction between the polyolefin outer coating and the cement

FBE that is the first layer (primer), but also provide mechanical protection

coating, the first layer is always FBE, so surface preparations are more or

Organic coatings for underground pipelines 329

less the same for all three coatings. In coating application generally, the

following steps are performed:

1. Surface cleaning preparation

2. Abrasive cleaning

3. Final cleaning and inspection

4. Acid wash and chromate treatment of pipe

5. Preheating of pipe

6. FBE coating application and cooling for standalone FBE coated pipe

7. Application of adhesive and outer layer of polyethylene or

polypropylene

8. Final inspection.

15.12.1 Surface cleaning preparation

Pre-abrasive cleaning preparation includes removal of surface contamina-

tion and elimination of mill scale. Pipes are often contaminated with salt,

abrasive cleaning operation, otherwise there could be excessive contamina-

tion of the abrasive medium. Salts and soil can be removed effectively with

fresh water at high pressure, and organic contaminants by application of

hydrocarbon solvent. To remove moisture, it is necessary to preheat the

pipe in the temperature range 60–80°C. Electrical induction heating is a

good method of preheating.

15.12.2 Abrasive cleaning

The surface cleanliness should meet a minimum of near white surface as

per SSPC and NACE standards, which can be achieved by centrifugal-type

blast cleaning equipment using steel shot and steel grit as the abrasive

media. The use of various sizes of abrasive shot and grit will depend upon

the steel grade used for making the pipe. To obtain an adequate cleanliness

and anchor pattern, a second abrasive cleaning process is preferred.

15.12.3 Final cleaning and inspection

Final cleanliness of the pipe should be checked occasionally by tightly

pressing the surface with adhesive tape, and then examining the tape under-

white metal blast quality. This is essential to achieve the required anchor

soil, grease and mill scale. All these need to be removed before the first

Abrasive cleaning helps in obtaining a surface profile of 50 to 100 microns.

side for dirt particles. The specification for pipe coating requires a near

pattern. The profile should be angular with no undercuts that can result

from overblasting. The profile can be measured by Elcometer Surface

330 High-performance organic coatings

15.12.4 Acid wash and chromate treatment of pipe

Line pipe surface treatment is required for superior coating adhesion. Pipe

can be washed with weak phosphoric acid solution in water and further

with only treated water having hardness less than 200 ppm. This treatment

becomes essential in case an uncoated pipe has been undergoing corrosion

in the presence of chloride or sulfate ions.

15.12.5 Preheating of pipe

For successful application of coating on pipe, the steel temperature required

metal temperature should not exceed 275°C, as this may cause metallurgical

electrical induction. For best results, a uniform metal temperature should

be maintained. Tempsticks or infra-red pyrometers can be utilized for this

is shown as Fig. 15.6.

15.12.6 FBE coating application and cooling for

standalone FBE

The powder application is performed by electrostatic spraying. Spray guns

15.4 Round and dished surface profile.

15.5 Angular, dense surface profile.

Profile Gauge. Diagrams showing Round and Dished as well as Angular

and Dense surface profiles are shown as Figs. 15.4 and 15.5 respectively.

is based on a particular pipe size, line speed, and coating specifications. The

or surface defects. The heating source can be gas-fired radiant heat or

should be properly set up. Sufficient time should be given before quenching

purpose. A diagram showing the effect of low temperature on coating flow

Organic coatings for underground pipelines 331

properties. This minimum requirement depends on the preheating tem-

perature, the pipe size and the steel mass.

15.12.7 Application of adhesive and outer layer of

polyethylene/polypropylene

To obtain better peel strength, it is preferable to apply the adhesive before

between the chemical groups on the epoxy and the adhesive. If the adhesive

is applied too late, the peel strength will be very low and the peel will reveal

a smooth coat of epoxy with failure between the adhesive and epoxy. In

case of extruded adhesive, the epoxy needs to have achieved a level of

polyethylene or polypropylene is generally applied by side extrusion, cross-

head extrusion or according to client requirements.

15.12.8 Cooling of three-layer coated pipe

Cooling of the pipe should only be carried out after complete curing of the

coating has taken place. This helps in getting optimum resistance proper-

ties. This minimum time requirement depends on the preheating tempera-

insulation properties of the outer layers will enable the epoxy to continue

the curing process after the water quenching has started.

15.12.9 Final inspection

For achieving best quality coating, a thorough inspection and coordination

is required. Inspection should be considered as a means of process control

rather than of acceptance or rejection. If each processing stage was done

Coating material

Pipe metal surface

Voids

15.6 Improper coating application: effect of low temperature on

the pipe with water in order to achieve a film with optimum resistance

the epoxy coat gels. This is required for giving sufficient reaction time

mobility to support the extruded film without skidding. The outer layer of

ture, the pipe size, the steel mass and the three-layer system specification.

The specification of the three-layer film affects the cooling phase, since the

coating flow.

332 High-performance organic coatings

correctly, a high quality coating will be only a natural consequence. A

regular quality control test to be carried out during application includes

curing of the epoxy layer is usually assessed on a section of the pipe coated

without the adhesive layer. The peel test is important in assessing the cure

and the intercoat adhesion. Failure during the peel test should be within

attached to the outer coat, which is known as cohesive failure. A block

diagram showing the various steps involved in applying coatings to pipe-

lines is shown in Fig. 15.7.

15.13 Comparison of various coatings

As seen in earlier sections, no coating is perfect. Every coating has some

favorable characteristics and some inherent limitations. A comparison of

First stage

abrasive

cleaning by

using shots

Pre-heating of

pipe between

60–80°C

Surface

cleaning

Removal of •

contaminant

Acid wash

cleaning of pipe

followed by

water wash

Final inspection

of surface by

using adhesive

tapes

Second stage

abrasive cleaning

by using shots

and grits

• FBE application

• Adhesive

application

• PE/PP application

Pre-heating of

pipe as per

material supplier

Chromate

treatment of

pipe surface

Final inspection

of coated pipe

and defect

repair if

required

Quenching with

water

15.7 Surface preparation and coating sequence.

specifications

film thickness check, holiday detection, the peel test and cure test. The

the adhesive film, leaving some adhesive attached to the epoxy and some

Organic coatings for underground pipelines 333

different coating materials with respect to the required properties is given

in Table 15.1.

While FBE coatings have good corrosion and water resistance, oxygen

barrier properties, comparable cheap cost and compatibility with CP system,

and lack of suitability for directional drill pipes. Similarly, three-layer poly-

ethylene coatings have excellent physical and corrosion properties but limi-

tations in terms of temperature use, CP shielding and problems related to

quality tests, as well as high cost. Coal-tar coatings are cheap and absorb

less moisture but have less impact resistance.

Table 15.1 Comparison of coating coal-tar, FBE and three-layer PE/PP coatings

Property CTE FBE Dual FBE Three-layer

PE/PP

Electrical resistance Good Good Good Excellent

Water absorption

with time

Low High High Low

Maintenance of

dielectric strength

with time

Poor Fair Fair Excellent

Adhesion Poor Good in dry,

poor in wet

environments

Good in dry,

poor in wet

environments

Excellent

Water absorption Very low High High Very low

Impact resistance Good Good Excellent Excellent

Type of damage Large Pinpoint Pinpoint Pinpoint or

splitting

Abrasion resistance Poor Good Excellent Excellent

Cathodic

disbondment

Low Very low Very low Very low

Chemical

degradation

High Low Low Low

Retention of

physical

characteristics

Poor,

degrades

Good but

absorbs

water

Good but

absorbs

water

Excellent,

but PE

may

disbond

Field joints system Easy and

cheap costly costly and

costly

Integrity of joints Good Good Good Good

Repair system Easy Easy Easy

Integrity of repair Good Good Good Good

Testing Easy Easy

Difficult and Difficult and Difficult

Difficult

Difficult Difficult

they have limitations of handling on site, soft backfill material requirements

334 High-performance organic coatings

15.14 Coatings and cathodic protection

Corrosion protection with the help of coatings is known as a passive method

of corrosion protection. Although coatings are extremely successful as a

corrosion protection system for underground pipelines, there are some

corrosion protection. To overcome this problem, a supplementary system,

which is known as an active protection system, i.e. cathodic protection, is

used along with the coating system. The best way to prevent pipeline cor-

rosion is by a combination of coating and cathodic protection.

The ideal pipeline coating may well be perceived quite differently by

different agencies or persons depending upon one’s point of view. A perfect

meets the pipeline corrosion protection requirement. Cathodic protection

is an integral part of any pipeline corrosion protection system. However,

corrosion engineers often look at cathodic protection and coatings as sepa-

rate responsibilities. It is essential that both disciplines are done in concert.

A well-designed cathodic protection system in conjunction with the appro-

priate coating system ensures leak-free pipeline operation in the most

economical fashion. Desirable characteristics of coatings with respect to

cathodic protection are:

• Effective high quality insulator

• Effective moisture barrier

• Good adhesion to pipe surface receiving cathodic protection

• Ability to resist development of holidays

• Ability to maintain substantially constant electrical resistivity with

time.

By applying a high quality coating to a cathodically protected structure, the

required cathodic protection current density is substantially reduced. The

advantages of a high quality coating can be summarized as follows:

• Low required current density

• Low power consumption

• Low wear of anodes

• Larger spacing between CP stations and hence less CP cost

• Interference problems minimized.

The concept of shielding of cathodic protection by coating is a contro-

versial topic. If dielectrically strong coating disbonds from the pipe and

electrolyte enters this area, the cathodic protection current may be shielded

from reaching any active corrosion cell. This can be avoided by proper

coating selection and design. Cathodic protection for underground pipe-

inherent limitations in each coating that make it difficult to provide total

pipeline coating system can be defined as the system which economically

Organic coatings for underground pipelines 335

lines is normally designed for 20 to 30 years. The cathodic protection

current design requirement depends on the type of coating used over the

pipeline and the precautions taken during the laying of the pipeline to avoid

holidays in the coating.

15.15 Coating failures

A number of coating failures on underground cross-country pipelines have

East, Iran, India, Pakistan, etc. The main causes of coating failures have

quality of pipeline coating can be achieved by:

• Proper surface preparation

• Total control of coating application in plant

• Production of a meaningful quality and inspection, testing plans

• Independent, trained and knowledgeable coating inspectors

15.16 Future trends

Although the present coating systems are able to reduce corrosion to a

large extent on buried pipelines, each system has its own limitations. These

limitations need to be eliminated by making improvements in each coating

ers who can accept these challenges. The following are the areas that need

further improvement in coatings for underground pipelines:

• Shielding of cathodic protection current on three-layer polyethylene/

polypropylene coatings

• Disbondment of three-layer PE/PP coatings within a short span of

time

• Improvements in SCC prone coating

• Coatings for high temperature use

• High thickness coatings and suitability to bend

• Effectiveness of coating with less prepared surface

• Substituting for non-environmentally friendly coatings

• Standardization of coating standards.

been reported from many countries, that are neither area-specific nor

coating-specific. Coating failures have occurred in the USA, UK, Middle

been inadequate specifications, poorly prepared substrates, incorrect appli-

cation, use of unqualified personnel and bad construction practice. Good

• Adequate coating specifications

• Pre-qualification of all coating materials and coating processes

• Coating specifications inline with CP requirements.

system. An extremely promising field is open to those coating manufactur-

336 High-performance organic coatings

15.17 Coating standards

coatings for underground pipelines. Proper surface preparation is impor-

tant in pipeline coating applications. There are also many standards related

to surface preparation of line pipes. Universally accepted coating standards

and surface preparation standards are as given below:

• NACE Standards

• ISO Standards

• Canadian Standards

• American Standards

• European Standards

• AWWA Standards

• Indian Standards

• Australian Standards

• British Standards

• German Standards

15.18 Bibliography

protection of pipelines using the CSA/Canadian Standard Z 245.21’, by Jamie

W. Cox, Denis Grimshaw and Mike Jones, DuPont Canada, Inc., Global Pipe

Coating Systems

2. ‘Application and processing guidelines for FBE and three layer coatings’, by

Jotun Powder Coatings

3. ‘Relevance of coal tar coatings for pipelines in the present day scenario’, by

T.K. Roy, Director, STP Ltd

4. ‘Dual powder fusion bonded epoxies – ten years later’, by Kuruvila Varughese,

DuPont Powder Coatings, Houston, TX

5. ‘Internal and external coatings for water transportation pipelines’, by Kuruvila

Varughese, DuPont Powder Coatings, Houston, TX

6. ‘Improving adhesion properties of three layer polyethylene systems for under-

ground pipeline protection’, by Kuruvila Varughese, DuPont Powder Coatings,

Houston, TX

7. ‘Pipeline Protective Coatings – Failures and Solutions’, by David Norman, David

Norman Corrosion Control, Crantock, Cornwall TR8 5SA, UK

There are many standards for coal-tar, FBE and three-layer polyolefin

1. ‘Comparing benefits of high density polyethylene coatings for anti-corrosion