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

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Organic coatings for underground pipelines 317

• Easy to repair

• Resistant to soil chemicals, bacteria, root growths and marine

organisms

• No cathodic shielding

• Insoluble in hydrocarbons

• Low cathodic disbondment

• Long experience (track record).

15.8.2 Limitations of coal-tar coatings

Since no coating is perfect, coal-tar enamel coatings also have limitations.

Some of these are as follows:

• Comparatively poor mechanical strength

• Higher cathodic protection current requirement

• Increased cost of cathodic protection

• Require early rehabilitation of coating

• Health hazard for persons on the job

• Air-polluting fumes during coating application or rehabilitation.

15.8.3 Coal-tar coating application

Hot coal-tar enamel is poured over a rotating pipe that has been previously

pulled over the coating. A second layer of coal-tar enamel is then applied.

additional shear strength, providing resistance to soil stress, protection

whitewash or a single wrap of paper to prevent ultraviolet degradation of

the enamel during storage in direct sunlight. Coal-tar base coatings require

less surface preparation compared to other coatings. The following sequence

is generally followed during coating application in the plant:

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.

or coal-tar by a dipping or pouring process.

5. Whitewash is applied for protection from ultraviolet radiation.

grit blasted and primer coated with coal-tar enamel primer. A fiberglass

inner wrap, which becomes saturated with liquefied enamel, is immediately

A second fiberglass or mineral felt outer wrap, already impregnated with

coal-tar, is applied over the first wrap. Liquid coal-tar enamel spreads

through the wrap seams. The glass fiber gives the coal-tar enamel system

against rock damage during backfilling and improved resistance to sag at

elevated temperatures. The coating is then finished with either one coat of

The pipe external surface is wrapped in fiberglass embedded in bitumen

318 High-performance organic coatings

15.8.4 Health and environmental issues

The volatile components of coal-tar are known to be toxic when ingested,

absorbed through the skin or inhaled. However, when appropriate coating

operation procedures and safe handling practices are followed, ingestion of

coal-tar enamel can be reduced. Coal tar pitch is said to be carcinogenic. It

irritates the eyes on contact and causes brown staining in the eye on pro-

longed exposure, and it also irritates the nose and throat. Coal-tar pitch is

on the hazardous substances list because it is regulated by OSHA. It has

been shown to cause skin, lung and bladder cancers. In application, coal-tar

has to be applied hot at high temperature at which it produces noxious and

air-polluting fumes as well as leading to the possibility of severe burns to

personnel involved in the operation. Due to the health hazards of coal-tar,

its use in coating is slowly reducing in developed and developing counties.

The fundamental environmental issues for coatings are:

• Environmental acceptance in general

• Environmental friendliness of manufacturing process

• Environmental acceptance of application process

• Long-term effects on environment

• Whether it can be recycled or not.

15.9 Single-layer fusion bonded epoxy coatings

Fusion bonded epoxy (FBE) coatings were introduced around 50 years ago.

Among external coatings for steel pipelines, thermoplastic varieties had

been satisfactorily used with success for many years in a wide range of

conditions. However, the conditions reached a stage where coatings with

better performance as well as meeting increased environmental conditions

were required. In addition to its versatility in all aspects of corrosion resis-

tance, the high temperature usability of FBE without failure is its main

advantage.

Fusion bonded epoxy coating, also known as fusion bonded epoxy powder

coating, is an epoxy based powder coating that is widely used to protect

various sizes of steel pipes used in pipeline construction. FBE coatings are

thermoset polymer coatings. The name ‘fusion bonded epoxy’ is derived

from the means of resin crosslinking and method of application which is

different from that of a conventional liquid paint. FBE coatings are in the

form of dry powder at normal atmospheric temperatures. The resin and

hardener parts in the dry powder remain unreacted in normal storage con-

ditions. At typical coating application temperatures, usually in the range of

180–250°C, the contents of the powder melt and transform into a liquid

form. The liquid FBE film wets and flows on to the steel surface, on which

Organic coatings for underground pipelines 319

it is applied, and soon becomes a solid coating by chemical crosslinking,

assisted by heat. This process is known as fusion bonding. The chemical

crosslinking reaction taking place in this case is ‘irreversible’, which means

that once the curing takes place the coating cannot be converted back into

its original form by any means. Application of further heating does not melt

the coating and thus it is known as a thermoset coating.

Fusion bonded epoxies are available as standalone coatings as well as

part of multi-layers. FBE coatings with different chemical and physical

properties are available to suit application on the main body of the pipes,

also available to match different ranges of pipeline service conditions. All

the important and essential properties expected from a versatile and high

performance pipeline coating system are possessed by FBE systems. Some

are as follows:

• Ease of application

• Good adhesion to pipe

• Good resistance to impact

• Flexibility

• Resistance to soil stress

• Water resistance

• Electrical resistance

• Chemical and physical stability

• Resistance to soil bacteria

• Resistance to marine organisms

• Resistance to cathodic disbondment.

ing impact resistance, excellent adhesion and resistance to abrasion. They

are resistant to heat and cold and possess extremely low oxygen permeation

and hence provide extreme resistance to corrosion. The high temperature

performance, chemical resistance, resistance to soil stress and excellent

resistance to cathodic disbondment in comparison to traditional coatings

has resulted in the increased use of fusion bonded epoxy. FBE coatings

have become more attractive due to several advances both in the applica-

tion process and in the raw material. FBE coatings provide a more control-

lable application process and a product whose quality can be assured prior

to laying of the pipeline.

FBE coatings have very effective electrical insulation when originally

installed and continue to provide effective insulation even as the coating

begins to absorb water. FBE continues to have low cathodic protection

current requirements even after many years of installation if it remains well

on internal surfaces, on girth welds and on fittings. Similarly, variations are

bonded, but definitely more than three-layer polyethylene. Even if blister-

• Resistance to flow

Fusion bonded epoxy powder coatings have high flexibility and outstand-

320 High-performance organic coatings

ing and small areas of disbondment occur, the current requirement increases

marginally. However, the electrical resistance is low enough to allow

cathodic protection to prevent corrosion on the pipe with disbonded or

blistering coating. FBE is non-shielding. The amount of oxygen available

at the cathodic area will control the rate of the corrosion process under

disbonded coatings. FBE has an advantage over many other coatings

because it is a good barrier to oxygen. FBE is a fail-safe coating and experi-

ence has shown that if adequate cathodic protection is available and no

shielding is present, corrosion is minimal. Another interesting advantage of

FBE is that when FBE fails these areas can be found more easily with direct

current voltage gradient (DCVG) surveys.

15.9.1 Advantages of fusion bonded epoxy coatings

A single-layer FBE coating has been very common in North American

countries for many decades, due to their well-developed infrastructure and

the low cost of this type of coating compared to other coatings. Some of

the main advantages of this coating are:

• Resistance to biological attack

• Better overall corrosion resistance at low and high ambient

temperatures

• Excellent oxygen barrier

• Superior adhesion and hence excellent corrosion resistance

• Excellent chemical properties

• Non-shielding of cathodic protection current

• Impact damage limited to point of contact

• Easy repair in coating plant and at site

• Can be used up to high service temperature

• Excellent resistance to soil stresses

• Easy repair in coating plant and at site

• FBE is considered to be a ‘fail safe’ coating

• No stress corrosion cracking (SCC) problem

• Cheaper than three-layer polyethylene/polypropylene coating.

15.9.2 Limitations of fusion bonded epoxy coatings

As with other coatings, a single-layer FBE coating also has limitations:

• Susceptible to damage during handling, transportation and pipeline

construction

• Sensitive to steel surface preparation and condition

• Compatible field weld coating system

• Excellent flexibility

Organic coatings for underground pipelines 321

• High moisture absorption and permeation, especially at higher

temperatures

• Affected by UV during storage

• Hydroscopic material leading to increased cathodic protection demand

as pipeline ages

• Limited impact resistance.

15.9.3 Method of application

Application of coating to the pipe is as important as the material itself. The

application process for fusion bonded epoxy coatings can be summarized

as follows:

• Cleaning

• Heating

• Coating

• Curing

• Inspecting.

Cleaning is by far the most important step in the application of any

coating, since the performance of the coating is directly related to the

surface preparation. Blast cleaning is carried out to a near white surface

pattern. This is usually accomplished using one or more blasting machines

with steel grit as the cleaning medium. After cleaning, the interior of the

pipe is blown free of all residual grit and other contaminants. The next step

in the process is to uniformly heat the pipe to a temperature in the range

of 200–250°C or as per the recommendations of the coating powder supplier

using a non-contaminating heat source. Three heating methods are used,

required temperature, it passes through a powder coating machine where

the fusion bonded epoxy coating is uniformly applied at a thickness of

350–500 microns or as per client requirement using electrostatic deposition.

After application, the coating is allowed to cure using the residual heat in

the pipe. During the curing process, the coated pipe should be carefully

handled to avoid damage to the coating. After the coating has been cured,

the pipe can be force-cooled with air or water to facilitate inspection and

handling. Coated pipe is electrically inspected using conventional search

electrodes operating at 5 volts per micron of coating thickness. Small imper-

fections in the coating are repaired using hot melt patch sticks or two-part

ambient temperature curing liquid epoxy resins. FBE coatings are suitable

for underground offshore and onshore pipelines. For offshore applications,

in deep water laying, it is necessary to apply a weight coating of concrete

finish, to Swedish Standard SA 2 , with a nominal 40–70 micron anchor

i.e. electrical induction, gas-fired forced air and a combination of high

velocity direct flame impingement. After the pipe has been heated to the

322 High-performance organic coatings

to ensure that the pipe remains on the sea bed. The epoxy coating process

includes:

• Pre-abrasive cleaning

• Abrasive cleaning of pipe

• Acid wash and chromating

• Pipe heating to required temperature

• FBE coating application

• Cooling of pipe

• Testing of coating.

15.10 Dual-layer FBE coatings

The single-layer FBE system has been found to be excellent except for the

terrain. Many coating material suppliers have attempted to improve the

coating to give better performance, and eventually they came up with the

idea of a dual-layer FBE coating consisting of:

• An FBE primary corrosion coating

• An FBE abrasive-resistant outer coating.

The inner primary corrosion layer is generally of coating thickness around

250 microns and the outer layer has a coating thickness of 250 to 400

microns. The properties of the inner layer are the same as those of single-

layer FBE, while the properties of the outer layer FBE are changed to

accommodate high impact resistance. The higher coating thickness also

improves the cathodic disbondment properties.

This unique dual-layer fusion bonded epoxy system is applied as a powder

base coat and powder top coat simultaneously with the typical application

system for the conventional FBE. This coating is recommended for pipe-

lines operating in the toughest environments, including river and road

crossings and rocky, mountainous terrain. It has excellent impact resistance

good protection against possible damage to the coating during pipe trans-

portation and pipeline construction.

powder coatings or multiple layer FBE coatings. Dual powder coatings are

used to improve the gouge resistance and toughness of FBE during direc-

tion boring. A rough coat is frequently used to improve friction between

the FBE and a cement weight coating. Rough coats also improve traction

for barge laying operations and improve safety. Thicker dual powder coat-

ings can also enhance high temperature performance. The dual powder

coating system can be used at operating temperatures of 110°C or higher.

fear about damaging the coating during handling of the pipeline in difficult

Modified fusion bonded epoxy coatings used offshore include dual

and abrasion properties which, combined with its good flexibility, provide

Organic coatings for underground pipelines 323

The system was principally developed for road/river crossings and HDD

applications where resistance to abrasion and gouging are considered

important. It consists of:

• A primary layer of thermosetting epoxy resin powder as the corrosion

barrier applied by electrostatic charge to a preheated pipe

• A secondary layer of abrasion and impact resistant epoxy resin powder

to provide mechanical protection.

A diagram of a dual-layer FBE coating is shown as Fig. 15.2.

15.10.1 Advantages of dual FBE coatings

advantages:

• High operating temperature capabilities

• Excellent abrasion and impact resistance

• Improved handling characteristics

• Fully bendable to 1.5°/pipe O.D.

• Improved service temperature up to 110°C

• Excellent corrosion resistance

• Does not shield cathodic protection

• Excellent adhesion to steel and extremely resistant to soil stresses

• High UV resistance

• No SCC problems observed so far

• Improved cathodic disbondment.

15.10.2 Limitations of dual FBE coatings

In addition to a number of advantages, dual-layer FBE has the following

limitations:

• More expensive than most other coating systems

• Hydroscopic material leading to increased cathodic protection demand

as pipeline ages.

Pipe line

First layer of fusion

bonded epoxy

Second layer of fusion

bonded epoxy

15.2 Dual-layer FBE coating.

Dual FBE has all the benefits of FBE, with the following additional

• Poor flexibility at higher coating thicknesses

324 High-performance organic coatings

15.10.3 Coating application

Dual-layer FBE is applied almost in the same fashion as single-layer FBE.

After proper surface cleaning to SA 2

, FBE is applied through electro-

base coat and the rest of the guns spray FBE simultaneously during the

characteristics that synergistically combine to produce performance results

15.11 Three-layer polyethylene/polypropylene coatings

primer layer, which is usually a thin layer, and mechanical protection

to FBE, an adhesive middle layer is also used, making it a three-layer

as top coat. Three-layer coatings have been used both offshore and onshore

for the last 40 years. Special multilayer systems are available according to

requirements. These include systems with high glass transition tempera-

polyethylene coating systems consist of the following:

• First layer: FBE primer

• Second layer: adhesive primer (tie layer)

• Third layer: polyethylene/polypropylene.

A diagram of a three-layer polyethylene/polypropylene coating is shown as

Fig. 15.3.

A three-layer PE mainline coating consists of an FBE primer layer 75–

250 microns, and a polyethylene outer layer of approximately 2500 microns.

These are all applied in-shop. The FBE represents the foundation for the

system, providing a primary base, which when applied properly, adheres

well to the steel substrate. This is essential, because polyethylenes are non-

layer is its low oxygen permeation rate compared with polyethylene. The

and the FBE base layer. Middle layer adhesives are typically acid copoly-

static spray application in a booth where the first few guns spray the FBE

melt stage of the first layer. This results in an intimate chemical bond

between the two layers. A significant advantage of multilayer FBE technol-

ogy is that unique characteristics can be developed by selecting specific

properties for each coating layer. Each layer is designed to impart specific

that significantly exceed those of a single coating.

The three-layer polyolefin system offers corrosion protection with an epoxy

through a polyolefin top coat. Since unmodified polyolefins do not adhere

system. The polyolefin coating can be either polyethylene or polypropylene

tures, with modified polypropylene for high temperature operation, and

with increased polyolefin thickness for directional drilling. Three-layer

150 microns thick, an intermediate olefin copolymer adhesive of around

polar and do not readily bond to steel. The benefit of FBE as a primary

middle layer serves as a bridge between the low-surface-energy polyolefin

mers or polyethylene modified with polar end groups grafted on to the

Organic coatings for underground pipelines 325

polymer backbone. The polar groups in turn react with and chemically

resistance because of its thickness. In addition, polyethylene exhibits low

moisture permeability. For polyethylene (PE), as density increases so does

permeability resistance. With both polyethylene and polypropylene (PP)

increasing thickness improves permeability resistance. This trait is particu-

larly useful at elevated operating temperatures. Polypropylene, with its

high softening point, is especially suited for high-temperature pipeline

applications. Epoxy primer is the true anti-corrosion member, often mea-

sured by the cathodic disbondment resistance test. Cathodic disbondment

is a function of:

• Type of epoxy or FBE powder grade

•

the applicator

• Steel surface anchor pattern

• Pre-treatment used to ensure cleanliness.

FBE is the best material component for oxygen barrier properties.

Keeping both oxygen and moisture away from the steel surface, a three-

layer PE/PP coating will impede any corrosion over time. Thus the syner-

gistic effect of all three components plays a role in the performance of the

a passive anti-corrosion system for new pipelines typically designed for

greater than 30 years of service life for transporting oil or gas. Material

components play a role and each has a purpose here which, when com-

Pipe

Fusion bonded

epoxy

Co-polymer adhesive

Polyethylene/poly-

propylene coating

15.3 Three-layer PE/PP coating.

bond to the FBE. The polyolefin adhesive is also compatible with and fuses

with the unmodified polyolefin topcoat. The top layer of the structure is

the polyolefin, which provides a high degree of damage tolerance or impact

Thickness of film

Application temperature as defined by the supplier and controlled by

final coating. The exterior multilayer high density polyethylene coating is

326 High-performance organic coatings

bined, offer a synergistic approach to corrosion protection. Polyethylene is

one of the best moisture barrier resins and the best UV-resistant plastic

material. Copolymer adhesive is the tie layer between the PE top coat and

epoxy primer components. It must be fully compatible with these compo-

nents so as to chemically bond to the epoxy groups prior to crosslinking

and to melt blend with the PE topcoat while still molten. The adhesion level

must be greater than any soil stress or handling stress associated with the

coating system environment. The failure mode can be either ‘adhesive’ or

‘cohesive’ but failure should never occur between the steel and the epoxy

primer.

Based on the experience gained during the last 40 years, there is a shift

in industry to using high density polyethylene from low density polyethyl-

ene for the following reasons:

• It has high penetration, abrasion and impact resistance.

• There is less moisture permeation, which offers better disbondment

resistance.

• It results in using less top coat thickness.

• Pipelines can be easily operated at high temperature.

• There is less damage and hence less repair cost.

In the changing scenario of the present time, pipeline owners are giving

more emphasis to value-added coatings, low maintenance and low installa-

tion cost of pipelines, and demanding high performance that will ultimately

result in a long design life and a short payback period. This has led to

the use of powder material for adhesive systems and high density polyeth-

ylene top coat systems coupled with improved epoxy powder with higher

thickness. More emphasis is given to better adhesion, improved cathodic

disbondment, better water immersion values and higher service

temperature.

Compared to other coatings, three-layer polyethylene/polypropylene coat-

ings have a number of advantages and some are enumerated below:

• Excellent barrier to oxygen

• Superior adhesion

• Excellent chemical properties

• Moisture barrier

• Superior mechanical and thermal properties

• High dielectric resistance

• Good chemical resistance

• UV resistance

15.11.1 Benefits of three-layer PE/PP coatings