shop coating failures. jun-1999

7/21/2019 Shop Coating Failures. JUN-1999

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Copyright ©1999, Technology Publishing Company

lthough corrosion protec-

tion has its complexities,

sometimes the most com-

mon kinds of problems are

the result of basic mistakes

and oversights. Renewed attention to the

fundamentals of sound application tech-

niques, careful monitoring during applica-

tion and cure, and well-thought-out coating

specifications can prevent many premature

coating failures associated with shop-

primed steel. This article reviews common

causes of failure and the types of failures

associated with them.

Pinpoint Rusting fromInsufficient Film Thicknesses

Although shop primers usually have a

recommended dry film thickness of 2 or

more mils (50 or more micrometers), they

are often applied at film thicknesses of

not much more than 1 mil (25 microme-

ters). When this occurs, premature failure

may result, manifested as pinpoint rust-

ing. Thinly applied coatings may barely

cover the peaks of the surface profile, es-

pecially if coarse blasting material is

used. The bare peaks will rust quickly,

thus accounting for the pinpoint rusting

(Fig. 1). It is our experience that when a

primer is applied at insufficient thickness-

es and exposed to the environment for

several months before topcoats are ap-

plied, the entire coating system will often

fail in a little over a year.

Mudcracking, Fracture, and Disbondment from ExcessivePrimer Film Thicknesses

It is common for applicators to avoid ap-

plying too little paint, but they should also

understand that more is not necessarily bet-

ter. This is true of most coatings, including

alkyds, epoxies, and urethanes, but it is es-

pecially true of a very common type of

by David Leyland and Rick Huntley KTA-Tator, Inc.

34 JUNE 1999 / JPCL –PMC

A

A Review of

Shop Coating FailuresSometimes big coating problems

can be prevented simply by paying attention to the

fundamentals of good practice.

Fig. 1 - Pinpoint rusting caused by insufficientfilm thicknessPhotos courtesy of KTA-Tator, Inc.




shop primer, inorganic zinc-rich. Most inor-

ganic zinc-rich primers are designed to beapplied at dry film thicknesses of 3 to 5

mils (75 to 125 micrometers). It is generally

accepted in the industry that film thickness-

es above 5 mils (125 micrometers) are

more susceptible to mudcracking (Fig. 2),

cohesive failure, and, ultimately, disbond-

ment. (It should be noted, of course, that

proprietary inorganic zinc-riches can vary

in their susceptibility to mudcracking. Some

products have shown signs of failure when

thicknesses are only slightly over the 5-mil

[125-micrometer] threshold, while otherproducts are formulated to resist cracking

at much higher thicknesses.)

In our experience, mudcracking and

the failures related to it occur mainly inareas where the coating is difficult to apply

and monitor, such as the inside angles of

complex structural steel configurations.

Because inorganic zinc primers cure

quickly, they help shops meet production

schedules. Unfortunately, a second applica-

tion of inorganic zinc does not adhere well

to a fully cured layer of inorganic zinc.

Shops may apply multiple coats of the

primer if the initial thickness is less than

specified. Just as excessive single layer

thicknesses of inorganic zinc can fail cohe-sively, so also can multiple coats of the

primer fail adhesively between the coats of

zinc and, if topcoated before the failure is

manifest, can lead to system disbondment.

Cohesive failure and disbondment

that result from excessive thicknesses of in-

organic zinc-rich primers and intercoat ad-

hesive failure between zinc coatings are

often manifested as a thin layer of primer

(often less than 1 mil [25 micrometers]) that

remains on the steel surface and a layer

that remains on the back of the disbondedpaint chip.

Because inorganic zinc-rich primers

dry quickly, they are also prone to dry

spray, which is a rough, non-cohesive, dis-

continuous film that results when an atom-

ized coating dries partially before it reaches

its intended substrate. Dry spray adheres

loosely at best to a surface, thus providing

a weak base for subsequent coating films.

Incomplete Cure of CoatingsSolvent Entrapment

Not only can excessive thickness make

coatings brittle, leading to mudcracking,

but also excessive thickness can make such

coatings too soft. Excessive thickness can

cause solvent entrapment in a primer,

which prevents the coating from curing

completely. Sometimes, when a primer is

applied too thickly, its surface dries first,

forming a hardened film that prevents therelease of the remainder of the solvent

from the underlying wet coating. Without

adequate cure, the coating can be easily

damaged when the steel is shipped or

erected. In extreme cases, spontaneous de-

lamination may occur.

Solvent entrapment may also occur

when a multicoat system is applied in the

shop before the primer has cured. There is

only a limited amount of floor space in a

Shop Coating Failures

JPCL –PMC

/ JUNE 1999 35

Fig. 2 - Mudcracking of a shop-applied inorganiczinc-rich primer (at center of photograph) occuredbecause of excessive film thickness at inside corner.




be aggravated by extreme environmental

conditions during coating application.In one failure that resulted from this

combination, an epoxy topcoat was speci-

fied to be applied in the shop over an

alkyd primer. Alkyd primers generally

have only fair to poor solvent resistance,

and epoxies are formulated with strong

solvents. In this instance, the initial appli-

cation worked well until the environmen-

tal conditions changed during the applica-

tion. Failure occurred because the

ambient conditions inside the shop soared

to more than 90 F (32 C), causing the sur-face of the epoxy to cross-link more

rapidly than normal. When the surface

hardened quickly, solvent from the bulk

of the underlying wet epoxy could not es-

cape and inhibited complete cure. The

strong solvent trapped in the epoxy film

readily attacked the alkyd primer. Because

the alkyd had poor solvent resistance, the

primer became soft, and its adhesion to

the steel was severely reduced.

Blistering and Corrosionfrom Application overSoluble Salts

If a surface is contaminated with water-sol-

uble salts and is then primed, the primer

may blister badly in the presence of ample

moisture. This phenomenon is referred to

as osmotic blistering.

We have seen coil-coated galvanized

steel fail by blistering within one year of

being placed in an environment with daily

condensation. Investigation showed that

the concentration of chlorides on the galva-

nized steel substrate beneath the film was

35 to 50 micrograms per sq cm. Salts can

be deposited on steel surfaces from a vari-

ety of sources, including the atmosphere in

industrial or coastal areas, highways where

de-icing salts are used, and salt-contaminat-

ed abrasives used to prepare the steel.

Premature Failures Caused by Exposure before Topcoating

Even if the coatings are properly applied in

the shop, problems can arise because of

unrealistic expectations for the primer be-

fore it is topcoated in the field. In some in-

stances, the purpose of the shop primer is

to provide corrosion protection to newly

fabricated steel for a limited time, until in-

termediate and finish coat layers can be ap-

plied over the primer. During this limited

time, the primed steel will be shipped to

the project site, perhaps stored, and even-tually erected and coated. Depending upon

the project, the interval between priming

and erection may be several months to a

year or more.

When a coating specification is being

prepared, it is extremely important to con-

sider the length of time before the primed

steel will be topcoated and the service en-

vironment during that interim period, not

just the exposure period after final erection.

The level of storage and protection should

be specified if the primed steel is to be

stored for long periods (e.g., blocked up

and covered under a roof). If the environ-

ment in which the primed steel is placed is

corrosive, then the length of time that the

primer can be expected to provide protec-

tion will be further reduced. While zinc-rich

primers will provide extended protection, a

single coat of paint, such as a standard

alkyd shop primer, cannot be expected to

protect steel during an extended construc-

tion schedule. So while an alkyd may be

specified because the service environment

of the final, completely coated steel is be-

nign, such as a building interior, the alkyd

will not be adequate if the primed steel is

to be stored outdoors for a long time be-

fore the building is constructed (Fig. 4).

In one example of this type of failure,

an operating cement plant erected a steel

structure that would ultimately have a be-

nign environment. Because of the ultimate


JPCL –PMC

/ JUNE 1999 37




service environment, the steel for the build-

ing was power tool cleaned, with intactmill scale remaining. The steel was then

shop-coated with a standard alkyd primer.

During the construction phase, which last-

ed about a year, the primed steel was

stored unprotected near the cement plant.

The steel became covered with an alkaline

cement dust fallout (pH 13) from adjacent

process operations. The alkyd saponified,

causing primer breakdown and fracture of

the mill scale.

One solution to premature primer fail-

ure due to severe or prolonged exposure

during the construction phase is to shop

apply more than just the primer. The origi-

nal concept behind the use of the shop

primer was to provide temporary corrosionprotection to the steel until construction

and subsequent field application of mid-

and topcoats. While this scenario still ex-

ists, some projects now require that all

coating layers be applied in the shop. A

typical three-coat, shop-applied system

consists of a zinc-rich primer, an epoxy in-

termediate, and a urethane finish coat. The

advantage of this system is that after the

steel is erected, only minor touch-up is re-

quired, rather than full multiple field coat

applications. However, the amount of touch-up will increase significantly if the

erector fails to use proper care in handling

pieces and if careful consideration is not

given to structural design to minimize the

amount of field welding or other practices

that could damage coated surfaces.

Conclusion

A fundamental understanding of the coat-

ing requirements by the individuals specify-ing, performing, and monitoring the work

can prevent most of the problems de-

scribed above.

Specifiers should be aware of the

characteristics of the coatings they call for,

including resistance properties, cure times,

and expected exposure of the primed steel

before final construction. Applicators

should have a copy of product specifica-

tions and product data sheets in order to

know dry film thickness requirements, ac-

ceptable ambient conditions for coating

and cure, and similar job needs. Applica-

tors must also have the training to fulfill the

specification.

Shop quality control personnel should

have a quality control/quality assurance

program in place and should have the

training as well as equipment to administer

the program. They should monitor every

facet of the surface preparation and coating

application to ensure compliance with the

written specification. As an integral part of this quality effort, thorough documentation

of each piece of steel coated should in-

clude information such as the date(s) of

surface preparation and coating, piece

number, degree of surface preparation,

type of paint, batch numbers of the paint,

ambient conditions during application and

cure, dry film thickness measurements for

each layer of coating, and cure time be-

tween coats. ❏


38 JUNE 1999 / JPCL –PMC

Fig. 4 - Cracking and rusting of mill scale onsteel intended for interior exposure but exposed

to the elements

shop coating failures. jun-1999

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