Heidersbach R. Metallurgy and Corrosion Control in Oil and Gas Production

CORROSION CONTROL 119

impermeable to moisture and oxygen migration. Zinc

pigments also provide a measure of cathodic protection

once the coating is breached. This cathodic protection

is greatly reduced if the primer is overcoated, but this is

often necessary for color coding or wear resistance

reasons. IOZ primers are not subject to ultraviolet (UV)

damage, so there is no need to overcoat them except for

the reasons above. Organic zinc primers are also widely

used, and they do beneﬁ t from overcoats.

6

Figure 6.3

illustrates the idea of using zinc - rich primers for cathodic

protection at coating holidays.

Desirable Properties of Protective Coating Systems

In addition to providing corrosion protection, coating

systems should also have:

•

Strong, durable bonding to the substrate

•

Flexibility, because organic coatings have different

coefﬁ cients of thermal expansion than metals, and

coating ﬂ exure is inevitable

•

Toughness, the ability to withstand mechanical

shock and loading

Figure 6.1 Protective coating system serving as a moisture and oxygen permeation barrier.

Drawing courtesy of NACE International, reproduced with permission .

Top Coat

Intermediate Coat

PRIMER

STEEL

Adhesion Strong

Coating Thoroughly

Wets Steel Surface

Physical as Well

as Chemical

Adhesion

No Voids at

Interface to

Accumulate Water

Surface Clean

No Salts to Create

Osmotic Blistering

Moisture Transmission Low

Impervious to Ions,

Oxygen, Carbon Dioxide

Moisture Absorption Static

and in Equilibrium

Figure 6.2 Protective coating system with slow - release corrosion inhibitors in the primer

coat. Drawing courtesy of NACE International, reproduced with permission.

Top Coat

Intermediate Coat

INHIBITIVE PRIMER

STEEL

Passive Layer

Ionization of Inhibitor

Reaction with Steel Surface

Moisture Absorption into Film

(Moisture Previous Coating)

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120 METALLURGY AND CORROSION CONTROL IN OIL AND GAS PRODUCTION

Figure 6.3 Inorganic zinc primer serving as the source of cathodic protection of the steel

substrate at a coating holiday. Drawing courtesy of NACE International, reproduced with

permission.

ORGANIC TOP COAT

ORGANIC INTERMEDIATE COAT

INORGANIC ZINC PERMANENT PRIMER

STEEL

Tight Adhesion Prevents Coating Undercut

Moisture Allows Zinc to Ionize

Cathodically Protecting Steel

Break in Coating to Steel Surface

Zn

++

The choice of coating systems often requires compro-

mises between these characteristics. As one example,

very hard coatings such as are often used on pipeline

exteriors are often difﬁ cult to repair if damaged in ship-

ping and construction. The necessary bonding to undam-

aged coatings is hard to achieve. This problem is also

apparent at ﬁ eld welds, where organic coatings must be

removed prior to welding.

Developments in Coatings Technology

Modern coating systems are generally longer lasting

than those that were available in the past. Worker safety

and environmental concerns have led to the develop-

ment of new coatings having higher solids and lower

VOC contents. The higher solids in modern coatings

frequently lead to the need for better surface prepara-

tion, as many of these coating systems are less tolerant

of surface contamination than the systems they are

replacing. Electrostatic spraying is an application that

was once conﬁ ned to manufacturing of relatively small

items, but recent developments allow for the use of

electrostatic spraying in major new construction and

rehabilitation. This technique is especially useful on

complicated geometries where it is difﬁ cult to apply

even coatings with other techniques.

Useful Publications

The following publications provide guidance on coat-

ings selection and on preparing contracts for protective

coatings:

7,8

•

NORSOK M - CR - 501, “ Surface Preparation and

Protective Coating ”

•

NACE Publication 6J162, “ Guide to the Preparation

of Contracts and Speciﬁ cations for the Application

of Protective Coatings ”

The Norwegian standard is directed toward offshore

platform construction, whereas the NACE standard also

covers less harsh environments.

Surface Preparation

Proper surface preparation is necessary for coatings to

bond properly to metallic and other substrates. Most

premature coatings failures can be attributed to

improper surface preparation or to allowing the prop-

erly prepared surface to degrade before coatings are

applied. Properly coated offshore platform deck coating

systems sometimes last 30 years before recoating

becomes necessary, but Figure 6.4 shows corrosion that

resulted from poor surface preparation on an offshore

platform deck after only 3 years. This failure could have

been easily avoided.

The principal surface condition factors that are

known to inﬂ uence this performance are the presence

of rust and mill scale, surface contaminants including

salts, dust, oils and greases, and the surface proﬁ le, which

must have enough roughness to allow mechanical adhe-

sion between the primer and the bare metal surface but

low enough so that paint covers the high spots with

adequate cover.

Abrasive blasting is usually the preferred means of

surface preparation, but high - pressure water jetting is

becoming more common due to environmental and

waste disposal concerns. Abrasive blasting leaves a

rough anchor pattern on the metal surface, while water

jetting usually does not. This has limited water jetting to

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CORROSION CONTROL 121

Figure 6.4 Poor surface preparation led to a lack of coating

adhesion and corrosion on this offshore platform deck in only

3 years.

cleaning metal surfaces for recoating, but modern devel-

opments with high - pressure water jetting are overcom-

ing this lack of anchor pattern, and it is likely that the

use of water jetting will supplant abrasive blasting for

many projects in the future. Abrasive blasting to provide

an anchor pattern is sometimes used on bare metal

surfaces after they have been cleaned by water jetting.

This blasting also removes any “ ﬂ ash rusting ” that may

have formed on the wet metal surface.

Neither of these techniques can remove grease and

other organic contaminants from the surface, and sol-

vents or other cleaning agents must be used prior to

either blasting or water jetting.

Surface preparation and other types of coatings

inspection are normally performed by third - party

inspection organizations. Surface preparation is the

most important part of any coatings application project,

and inspection by NACE or Norwegian FROSIO (the

Norwegian Professional Council for Education and

Certiﬁ cation of Inspectors for Surface Treatment) -

certiﬁ ed inspectors is often required. These third - party

inspectors are trained to insure that surface preparation

and coatings applications are conducted in accordance

with established international standards as speciﬁ ed by

the coatings inspection and application contract.

Inspectors with this training are also involved with eval-

uation of existing coating systems to determine the

extent of coating damage and recommend remedial

measures.

Surface Cleaning Neither abrasive blasting nor water

jetting can remove grease and other organic surface

contaminants. These problems must be removed with

various commercial products suited to this purpose.

Soluble salts may also be present on the metal surface.

They are not removed by abrasive blasting, and can lead

to osmotic blistering of newly applied coatings.

9

Chloride

salts are normally removed by water washing or water

jetting, often with proprietary chemical additions sold

for this purpose added to the water. It is common to

check for the presence of soluble chlorides by various

methods, and most coatings contracts will specify a

maximum level of acceptable chloride contamination

on the surface. The amount of chlorides detected will

vary depending on the detection method, and the

method should be speciﬁ ed in the coatings contract.

While chlorides are not the only soluble salt contami-

nation found on metal surfaces, washing that removes

chloride salts will normally remove any other salty con-

taminant that could cause coating degradation and lack

of adhesion.

Abrasive Blasting Abrasive blasting is the most com-

monly speciﬁ ed method of preparing steel surfaces for

protective coatings.

10

Tables 6.1 and 6.2 provide comparisons on the rela-

tive costs of abrasive blasting and other surface prepara-

tion methods. While the best surface preparation

produces the longest - lasting coatings, the reduction in

costs associated with less thorough surface preparation

techniques can be signiﬁ cant.

The choice of surface preparation techniques and

the necessary levels of cleanliness for new construction

depend on the generic type of primer (chemical nature

of the binder), severity of the environment, and the

desired coating service life. Coatings should be applied

in accordance with manufacturer ’ s recommendations,

and these recommendations should include surface

preparation guidelines listing minimally acceptable

surface preparation conditions. General guidelines for

selected generic coatings are shown in Table 6.3 . The

NACE/SSPC surface preparation standards are listed

in Table 6.4 . Note that while white metal blast cleaning,

NACE No. 1/SSPC - SP 5, is the cleanest possible sub-

strate for any of the coating systems, it is only required

for IOZ primers. Substantial cost savings can be

achieved if the surface preparation requirements for

these other coatings are relaxed, but these savings are

achieved at reduction in long - term performance of the

coating.

All of the NACE/SSPC standards contain the follow-

ing sections:

•

Procedure before blast cleaning

•

Blast cleaning methods and operations

•

Procedure following blast cleaning

•

Inspection

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122 METALLURGY AND CORROSION CONTROL IN OIL AND GAS PRODUCTION

TABLE 6.1 Comparative Costs of Different Surface Preparation Techniques

9

Surface Preparation Cost Performance Dust Debris

White Metal 100% 100% 100% 100%

Near White 80% 90 – 95% 100% 80%

LP WC 10% 70 – 80% 0% 3 – 5%

UHP WJ 25% 90% 0% 5 – 10%

Wet abrasive to white metal 120 – 150% 95% 0% 110 – 125%

Low Pressure Water Cleaning (LP WC) is cleaning performed at pressures less than 5000 psi (34 MPa). Minimum pressure is 3500 psi; the use of

a rotating tip is mandatory. Hand scraping of blisters and other defect may be required. Chemical decontamination is mandatory.

Ultrahigh Pressure Water Jetting (UHP WJ is cleaning performed above 25,000 psi (170 MPa); the use of a rotating tip is mandatory. Chemical

decontamination is mandatory.

Wet Abrasive Blast Cleaning to White Metal: Cleaning to SSPC SP 5, White Metal Blast Cleaning, use of water ring for dust control is mandatory.

Chemical decontamination is mandatory.

TABLE 6.2 Comparison of Abrasive Blasting Costs for

General Construction

Blast Method Relative Cost

NACE #1 White Metal Blast 100%

NACE #2 Near White Metal Blast 70%

Commercial Blast, SSPC#6/NACE#3 40%

Brush Blast — loose rough previous coat 20%

TABLE 6.3 Minimally Acceptable Surface Preparation

Levels for Selected Generic Coatings

Generic Coating Type

Recommended Minimum

Surface Preparation

Alkyds and oil - based

coatings

NACE No. 3/SSPC - SP 6

Commercial Blast Cleaning

Water - borne acrylics NACE No. 3/SSPC - SP 6

Commercial Blast Cleaning

Epoxy NACE No. 3/SSPC - SP 6

Commercial Blast Cleaning

Zinc - rich epoxy NACE No. 3/SSPC - SP 6

Commercial Blast Cleaning

Inorganic zinc NACE No. 1/SSPC - SP 5

White Metal Blast Cleaning

TABLE 6.4 NACE / SSPC Joint Surface Preparation

Standards

NACE No. 1/SSPC - SP 5 White Metal Blast Cleaning

NACE No. 2/SSPC - SP 10 Near - White Metal Blast

Cleaning

NACE No. 3/SSPC - SP 6 Commercial Blast Cleaning

NACE No. 4/SSPC - SP 7 Brush - Off Blast Cleaning

NACE No. 5/SSPC - SP 12 Surface Preparation and

Cleaning of Metals by

Water Jetting Prior to

Recoating

NACE No. 6/SSPC - SP 13 Surface Preparation of

Concrete

NACE No. 8/SSPC - SP 14 Industrial Blast Cleaning

NACE No. 10/SSPC - PA 6 Fiberglass - Reinforced Plastic

(FRP) Linings Applied to

Bottoms of Carbon Steel

Storage Tanks

NACE VIS 7/SSPC - VIS 4 Guide and Visual Reference

Photographs for Steel

Cleaned by Water Jetting

NACE VIS 9/SSPC - VIS 5 Guide and Reference

Photographs for Steel

Surfaces Prepared by Wet

Abrasive Blast Cleaning

Inspections of blast - cleaned surfaces include mea-

surements of the surface proﬁ le. If a surface is too

smooth, the primer will not develop adequate bonding

to the substrate. If it is too rough, the coating may be

too thin at high spots and pinpoint corrosion may occur.

Figure 6.5 shows the rough surface and anchor pattern

of a blast - cleaned pipeline prepared for recoating.

Water Jetting Water jetting is currently used primarily

for preparing surfaces for recoating. The reason for this

limitation is because most water jetting systems cannot

produce an adequate surface proﬁ le or anchor pattern.

This limitation has been largely overcome, but water

jetting is still used much less than abrasive blasting for

coatings surface preparation .

12

The NACE/SSPC surface

preparation standards listed in Table 6.4 describe water

jetting only for recoating purposes. This is shown in

Figure 6.6 , where water jetting is being used for surface

preparation on the outside of a large marine vessel prior

to recoating.

The advantages of water jetting, which uses high -

pressure water, are that it removes most contaminants,

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CORROSION CONTROL 123

Figure 6.5 The surface of a pipeline ready for recoating in

the ﬁ eld.

Figure 6.6 Water jetting surface preparation prior to recoat-

ing. Photo courtesy of Hammelmann Corp., Dayton, Ohio,

reproduced with permission.

TABLE 6.5 Comparison of Water Jetting and Abrasive

Blast Cleaning Standards and Surface Conditions

Surface Finish Grade ISO 8501 - 1 SSPC NACE

White metal Sa 3 Sp 5 No. 1

Near - white metal Sa 2½ SP 10 No. 2

Brush - off Sa 1 SP 7 No. 4

Solvent cleaning SP 1

Power tool cleaning St 2 or 3 SP 3

34

Power tool cleaning

to bare metal

S P 1 1

HPWJ and UHPWJ SP 12 No. 5

WJ - 1 clean to bare

substrate

WJ - 2 very thorough

or substantial

cleaning

WJ - 3 thorough

cleaning

WJ - 4 light cleaning

Wet abrasive blasting TR 2 6G198

Figure 6.7 Single and multicoat protective coating systems.

Photo courtesy of NACE International, reproduced with

permission.

Coating Systems

Single Coat

Multiple Coats

One Type

of Coating

Multiple Types

of Coatings

systems are intended to be single - layer coatings or mul-

tiple layers of the same type of coating. These choices

are shown in Figure 6.7 .

The various layers of a coating system must be com-

patible with each other, and this is normally done by

using the same coating manufacturer for all the layers,

typically primer, intermediate, and topcoat.

A single coat system is usually a relatively thin system

and usually provides minimal protection appropriate

for temporary protection during shipping and manufac-

turing. These single - ﬁ lm coatings are often referred to

as “ shop coats ” and should be replaced before service

in any aggressive environment. Some single - ﬁ lm systems

are much thicker, such as the trowel - applied coating

shown in Figure 6.8 , which is used for protection of girth

welds after welding in pipeline construction. Other

single - ﬁ lm coatings are sometimes applied as mainte-

nance coatings in the splash zone where surface prepa-

ration is difﬁ cult and thick ﬁ lms are necessary, and

interﬁ lm adhesion is difﬁ cult to achieve. Problems with

has no sparking or dust hazards, and removes soluble

salts. Corrosion inhibitors are sometimes added to the

water to prevent ﬂ ash rusting, although opinions differ

on how deleterious these thin rust areas are to primer -

to - metal adhesion.

Table 6.5 provides approximate comparisons between

water jetting and abrasive blast cleaning standards.

Purposes of Various Coatings

Protective coating systems usually consist of multiple

layers of paint having different purposes, but some

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124 METALLURGY AND CORROSION CONTROL IN OIL AND GAS PRODUCTION

single - coat systems include solvent entrapment in thick

systems, difﬁ culty maintaining desired coating thickness,

and potential holidays and misses.

Multiple - coat systems can be layers of the same

product or a variety of products that combine the

desired properties of each layer, for example, primer,

barrier intermediate coat, and topcoat.

Coatings are classiﬁ ed in two different ways. Some

classiﬁ cations specify coatings by binder chemistry, for

example, epoxy and polyurethane. The chemistry of

these binders is understood by coatings manufacturers

and many ﬁ eld - oriented coatings professionals. Most

ﬁ eld - oriented coatings professionals do not understand

the chemistry of coatings; they just know what works,

and what does not work. Coatings manufacturers list

their products by generic binder types or by appropriate

applications.

Generic Binder Classiﬁ cations

Alkyd Paints These paints provide minimal protec-

tion and are seldom used in oilﬁ eld applications. Note

that many manufacturers will call these systems paints

to distinguish them from the more protective chemis-

tries listed as “ coatings ” — implying protective coatings.

Bituminous Coatings These asphalt or coal - tar - based

coatings are seldom used in oilﬁ eld applications on

metal substrates with the exception of ﬁ eld - applied

repairs and girth - weld coatings on buried pipelines. At

one time, coal - tar coatings were standard coatings for

pipelines and other oilﬁ eld structures, but environmen-

tal and occupational health concerns have limited their

use in recent years.

Vinyl Coatings Vinyl coatings are widely used protec-

tive coatings for corrosion resistance in chemical service.

They are easy to apply and form tight homogeneous

ﬁ lms over the substrate. Intercoat adhesion is also excel-

lent. They do soften slightly when covered with some

crude oils. Their ﬂ exibility allows them to accommodate

the motion of the steel substrate, for example, during

ship or platform launching.

13

They are physically drying, one - component paints,

and usually cannot withstand temperatures greater than

75 – 80 ° C (165 – 175 ° F). They are relatively easy to apply

and quick drying, but they have low solids content and

have relatively high VOC contents.

Film thickness is typically only 50 μ m (2 mils), and

this, combined with their relative softness, means they

cannot withstand mechanical abuse. It also limits their

use on rough, previously coated surfaces.

Chlorinated Rubber Coatings Chlorinated rubbers

were once widely used for marine and other oilﬁ eld

applications,

13

but worker safety and environmental

concerns have limited their use in recent years.

Epoxy Coatings These coatings are the workhorses of

the marine coatings and oilﬁ eld industries. Most epoxies

require near - white blast coating surface preparation,

although some versions can be applied underwater as

repair coatings. With proper surface preparation, they

have excellent substrate adhesion combined with good

impact and abrasion resistance, and can achieve high

ﬁ lm thicknesses.

Epoxies chemically cure. They are available in

systems where the reactants are mixed during applica-

tion as well as single - component products, which require

more careful shipping and storage control and careful

consideration of application and curing temperatures.

These chemical - curing - related considerations mean

that the use of epoxy coatings requires highly trained

and experienced coatings contractors.

A wide variety of end - product properties can be

achieved by varying the components, and epoxies are

sold as pure epoxies, phenolic epoxies, coal tar epoxies,

solvent - free epoxies, water - borne epoxies, and so on.

All epoxies are subject to UV degradation. This

means they must be topcoated for most atmospheric

exposures.

Epoxy Mastic Coatings These coatings have been

marketed in recent years as easy - to - apply, inexpensive

thick - ﬁ lm coatings. They have relatively short service

performance compared with other coatings. Because of

this, and the fact that coating materials are minor costs

(approximately 10%) of coatings applications projects,

their reduced cost is not justiﬁ ed for many applications.

Figure 6.8 Trowel application of thick - ﬁ lm single coating of

coal - tar urethane for ﬁ eld application at pipeline girth welds.

Photo courtesy of NACE International, reproduced with

permission.

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CORROSION CONTROL 125

They are not generally recommended for atmospheric

or marine exposures, but they may be appropriate for

buried service, for example, as repair coatings on

pipelines.

Acrylic and Polyurethane Coatings These coatings

are usually used as colored topcoats over epoxy - based

primers and intermediate coatings. They have excellent

hardness and very good UV and chemical resistance.

They are two - component systems that require tempera-

ture and other application controls. The choice between

various topcoats is often determined by pot life, applica-

tion conditions, and curing time before they can be used.

Polyester Coatings Glass ﬂ ake - reinforced polyester

high - build coatings are used for high wear and abrasion

applications such as walkways and the tidal and splash

zones of steel structures.

Vinyl Ester Coatings These coatings ﬁ nd their best

uses as tank linings. They are often applied as thick

coatings (two coats of 750 μ m — 30 mils, total 1500 μ m –

60 mils ). Many linings are applied as sheet materials, but

these linings can be applied by airless spray as two -

component liquids that quickly cure. Glass ﬂ ake rein-

forcement improves abrasion resistance. They should

only be applied over blast - cleaned steel (NACE No. 2/

SSPC - SP 10).

IOZ Coatings IOZ coatings have metallic zinc pig-

ments. They are widely used as primers for long - lasting

atmospheric exposure protection. They are not gener-

ally recommended for buried or submerged service.

The zinc pigments make mechanical contact with

each other and with the steel substrate. This allows them

to provide galvanic protection of exposed steel at holi-

days, and they usually have at least 75% by weight

metallic zinc in the dry ﬁ lm.

5

The precise minimum zinc

content for effectiveness depends on which of several

commercially available inorganic binders is used.

IOZ primers will become dull gray upon weathering

and sometimes require topcoats for visibility or color

coding purposes. These topcoats reduce the effectiveness

of the cathodic protection provided by the underlying

IOZ primer. Unlike organic primers, IOZs are immune

to UV degradation, so sunlight exposure is never a

problem, and many IOZ primers with no topcoats have

been used for decades with minimal degradation.

Surface preparation for IOZ primers requires at least

a commercial blast cleaning (NACE No. 3/SSPC - SP5),

but white metal blasting (NACE No. 1/SSPC - SP 5)

is preferred and will result in better performance.

Topcoating of IOZ primers requires cleaning to remove

any reaction products that may have formed.

Table 6.6 compares the performance of IOZ and gal-

vanizing coatings.

For all but the most benign environments, for

example, onshore atmospheric exposures, galvanizing

is not practical, at least in part, because of the thin

coatings applied by standard commercial galvanizing

operations.

Zinc is an amphoteric metal, and it corrodes at unac-

ceptable rates in both acids and bases. This means that

no zinc - coating pigments can be used for direct expo-

sure to acids, for example, drilling muds and many other

completion and workover ﬂ uids, or to bases. Topcoats

are necessary in these environments.

Organic Zinc Primers Organic zinc primers are used

for many of the same applications as IOZ primers. Their

organic binders mean that, unlike IOZ coatings, organic

zinc primers must be topcoated to protect them from

UV degradation. The organic binder also means that the

cathodic protection provided by the metallic zinc pig-

ments is less effective, because the intermetallic con-

tacts between the pigment particles and between the

pigments and the steel substrate are less effective due

to the resistivity of the organic binder.

IOZ primers are fast drying and their overcoating

interval is relatively short. They are compatible with all

of the topcoat systems used in oilﬁ eld applications.

Problems occur with alkyds, but they are seldom used

in oilﬁ eld applications due to their limited environmen-

tal resistance.

Organic zinc primers are used to recoat and spot

paint over IOZ and galvanizing.

The surface preparation for organic zinc primers

requires at least a near - white blast cleaning (NACE No.

2/SSPC - SP 10), and they are less tolerant of surface salts

than IOZ primers.

Polyurea Coatings Polyurea coatings show much

promise and will become more prevalent in the future.

The advantages of polyureas include their quick setting,

TABLE 6.6 Comparison of Inorganic Zinc and Galvanized

Coatings

Inorganic Zinc Galvanizing

Not a metallic coating Metallic coating

Excellent corrosion

resistance

Excellent corrosion resistant

Chemically bonded to steel

substrate

Chemically bonded to steel

substrate

Individual particles Continuous zinc

Medium abrasion resistance Limited abrasion resistance

Slower reaction with acids Faster reaction with acids

Long life span Shorter life span

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126 METALLURGY AND CORROSION CONTROL IN OIL AND GAS PRODUCTION

TABLE 6.7 SSPC Environmental Zones

14

Zone Description

0 Dry interiors

1A Normally dry interiors

1B Normally dry exteriors

2A Frequent wetting by fresh water

2B Frequent wetting by salt water

2C Immersion in fresh water

2D Immersion in salt water

3A Acidic atmospheric exposure, pH 2 – 5

3B Neutral chemical atmospheric expsoreu, pH 5 – 10

3C Alkaline chemical atmospheric exposure, pH 10 – 12

3D Atmospheric exposure with solvents and

hydrocarbons

3E Severe chemical atmospheric exposure

TABLE 6.8 Selected Environmental Zones for which SSPC Systems Are Recommended

14

Painting System Environmental Zone

SSPC # Generic Type 0 1A 1B 2A 2B 2C 2D 3A 3B 3C 3D 3E

PS 11 Coal tar epoxy x x x x x x x

PS 12 Zinc rich (no topcoat) X X X X X X X X

PS 12 Zinc rich (with topcoat) X X X X X X X X X

PS 19 Ship bottoms

PS21 Ship topsides X X X X

CS 23 Metallic thermal spray X X X T X T T T T

T, recommended only with proper sealing or topcoating.

For zone 3E, use speciﬁ c exposure data to select a coating.

insensitivity to ambient temperature and weather

changes during application, and good mechanical prop-

erties. Unfortunately, their quick - setting properties limit

their adhesion to metal substrates, and they are not cur-

rently used as protective coatings for large metal struc-

tures. It is likely that these limitations will be overcome,

and polyureas will become standard protective coatings

for many oilﬁ eld applications when that happens.

Coatings Suitable for Various Service Environments

or Applications

Several organizations suggest appropriate coatings

systems for various environments and applications.

Tables 6.7 and 6.8 show the SSPC classiﬁ cation of

environments and suggested coatings systems for these

environments.

14

NORSOK, the Norwegian standards

organization, recommends the coating systems for off-

shore platform applications shown in Table 6.9 . The ref-

erenced publications also contain suggestions on surface

preparation methods and the necessary surface condi-

tions prior to coating application.

7,14

Many coatings manufacturers and suppliers list the

coatings by application instead of by generic type. These

listings generally follow terminology similar to that

shown in Tables 6.6 – 6.8 , and they often specify the

recommended operating temperatures (usually as

maximum acceptable sustained temperatures) that the

coating systems can withstand. Coating systems from

various manufacturers will perform differently, and it is

common to test new coatings systems for performance

in accordance with recommended accelerated exposure

testing procedures.

15

Coatings Inspection

Surface preparation and other types of coatings inspec-

tion are normally performed by third - party inspection

organizations. Surface preparation is the most impor-

tant part of any coatings application project, and inspec-

tion by NACE or Norwegian FROSIO (the Norwegian

Professional Council for Education and Certiﬁ cation of

Inspectors for Surface Treatment) - certiﬁ ed inspectors is

often required. These third - party inspectors are trained

to insure that surface preparation and coatings applica-

tions are conducted in accordance with established

international standards as speciﬁ ed by the coatings

inspection and application contract. Inspectors with this

training are also involved with evaluation of existing

coating systems to determine the extent of coating

damage and recommend remedial measures.

16

Documentation of coatings inspection is very impor-

tant, because the third - party inspector must convince all

interested parties that the surface preparation, coatings

application, and inspections have all been conducted in

accordance with contract requirements and industry

standards.

17

Owners want perfect coatings and contrac-

tors want payment; these conﬂ icting interests must be

resolved in accordance with pre - agreed procedures.

Written standards, for example, NACE, SSPC, and ISO

take preference.

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CORROSION CONTROL 127

TABLE 6.9 Recommended Coating Systems for Various Locations on Offshore Platforms

7

Application Coating System NDFT ( μ m)

Carbon steel 1 coat zinc - rich epoxy 60

Operating temperature < 120 ° C

1 coat two - component epoxy 200

Structural steel 1 coat topcoat

75

Exteriors of equipment, vessels, piping,

and valves (not insulated)

MDFT ( μ m)

335

All carbon steel surfaces in

noncorrosive areas (e.g., living

quarters)

Deck areas

Carbon Steel Thermally sprayed aluminum

or alloys of aluminum

200 min

Operating temperature > 120 ° C

All insulated surfaces of tanks,

vessels,and piping

Flare booms Sealer

Underside of bottom deck, jacket

above splash zone, crane booms,

lifeboat stations are optional stations

Walkways, escape routes, and other deck areas as speciﬁ ed Nonskid epoxy screed 3000

Under epoxy - based primer 1coat epoxy primer 50

o r

1 coat zinc - rich epoxy 60

1x epoxy tie coat

25

MDFT ( μ m)

85

Under cement - based ﬁ re protection 1 coat zinc rich epoxy 60

1 coat two - component epoxy

200

MDFT ( μ m)

260

Application (if not speciﬁ ed in others) 1 coat epoxy primer 50

Uninsulated stainless steel when

painting is required

1 coat two - component epoxy 100

1 coat topcoat

75

Aluminum when painting is required

MDFT ( μ m)

225

Galvanized steel

Insulated stainless steel piping and

vessels at temperatures < 120 ° C

2 coats immersion grade

expoxy phenolic

2 × 1 5 0

MDFT ( μ m)

300

Submerged carbon steel and carbon steel in the splash zone 1 coat epoxy primer

a

225

1 coat two component epoxy

225

Submerged stainless steel and stainless steel in the splash zone

MDFT ( μ m)

450

Internal seawater ﬁ lled compartments, e.g., ballast tanks

a

NORSOK states “ two component epoxy for both layers. ”

Common inspection hold points or checkpoints

include:

•

Pre - surface inspection

•

Post - surface preparation

•

Pre - painting

•

During and after application

The initial surface inspection determines the condition

of the steel surfaces before operations begin. The ISO

standards for degree of rusting serve to indicate the

relative initial surface condition before surface prepara-

tion and coatings begin:

18

•

A — Steel surface largely covered with adhering

mill scale but little, if any rust

•

B — Steel surface has begun to rust and from which

the mill scale has begun to ﬂ ake

•

C — Steel surface on which mill scale has rusted

away or from which it can be scraped, but with

slight pitting visible under normal vision

•

D — Steel surface on which mill scale has rusted

away and on which general pitting is visible under

normal vision

Each type of paint coating will have a speciﬁ c surface

preparation requirement that must be followed.

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128 METALLURGY AND CORROSION CONTROL IN OIL AND GAS PRODUCTION

Proper paint adhesion depends on the removal of all

greases and other organic contaminants prior to surface

preparation. Abrasive or water jetting cannot be relied

upon to do this, so solvent cleaning is usually necessary

to remove them. This contamination is normally detected

visually, but UV (black) lighting is sometimes used to

aid in organic contaminant detection.

12,19

The presence of soluble salts on metal surfaces has

been recognized for many years as leading to premature

coatings failures. These salts are normally chlorides and

sulfates, but other species may also be present. They are

usually removed by power washing. Proprietary com-

mercial products claim to aid in this salt removal.

Chloride contamination receives most of the soluble

salt attention, and any washing process that removes

chloride salts is likely to also remove other salts.

No internationally recognized standards on the

acceptable levels of salt contamination on a metal

surface are available, although efforts are underway

to develop them. Table 6.10 shows the limits suggested

by NACE for offshore structures. These levels are

to be measured using methods suggested by ISO

standards.

20,21

One of the problems with soluble salt detection and

measurement is the wide variability associated with

measuring the salt levels on rough surfaces. Replication

is difﬁ cult. One promising method of diminishing this

variability is the development of conductometric testing.

Instead of using colorimetric titrations or similar

methods of determining the salt content on a metal

surface, the surface can be exposed to a source of deion-

ized water, and the resulting conductivity can be mea-

sured as a direct indication of the soluble salts that are

present. This method has the added beneﬁ t of measur-

ing all salts, not just chloride ions.

20 – 24

Steel surfaces must

have a proﬁ le in order for the paint coatings to develop

adequate surface contact and adhesion. This is one of

the main results of abrasive blasting and is considered

to be a limitation to many water - jetting operations. If

the surface proﬁ le is too deep, paint ﬁ lms will not ade-

quately cover the highest points, and pinpoint rusting

will occur. This idea is shown in Figure 6.9 . Pinpoint

rusting is unsightly, but it can also lead to eventual

undercutting and wider corrosion problems. The surface

proﬁ le necessary to insure adequate bonding and to

avoid pinpoint rusting will depend on the type of coating

involved.

Anchor patterns and surface - proﬁ le determinations

are usually evaluated visually by placing laboratory -

prepared optical comparators on the surface and

comparing the adjacent abrasive - blasted surfaces to the

comparator. Figure 6.10 shows one version of commer-

TABLE 6.10 Recommended Maximum Allowable Total

Soluble Chloride Ion Contents for Offshore Structures

12

Coating Category New Construction Maintenance

Splash zone, exterior

submerged zone,

and ballast water

tank

20 mg/m

2a

20 mg/m

2a

Atmospheric zone 20 mg/m

2

50 mg/m

2

Stainless steels 20 mg/m

2

20 mg/m

2

Nace RP 0108 - 2008 table 15, reproduced with permission.

a

The level of residual salt contamination on the surface has a very

signiﬁ cant effect on the service life of the immersion - type coating

systems. A clean surface should be obtained prior to coating. However,

if the soluble chloride ion content is high and obtaining a clean surface

is too costly, the allowable soluble chloride ion content shall be agreed

to by the coating applicator and facility owner.

Figure 6.9 Protruding steel subject to pinpoint rusting.

Figure 6.10 Keane - Tator surface - proﬁ le comparator disc.

Photo courtesy of KTA - Tator, Inc., reproduced with permission.

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Heidersbach R. Metallurgy and Corrosion Control in Oil and Gas Production

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