J P C L J u l y 2 0 0 7 49www.paintsquare.com

Maintenance Tips

The Importance of Sample Taking in Failure Analysisany expensive-sounding analytical tools helpan experienced investigator determine thecause of a coating failure. For example,

infrared spectroscopy can determine such things as coatingtype, contamination, and mix ratio; gas chromatography candetect residual solvents in paint chips and blister liquid; dif-ferential scanning calorimetry can measure relative degree ofcure; and scanning electron microscopy-energy dispersive x-ray spectroscopy (SEM-EDS) can detect and characterizeinclusions, debris, and failure planes. However, the best-equipped laboratory is still limited bythe type and quality of the samples thatit has to work with. This article will dis-cuss the type of samples needed for fail-ure analysis, and give suggestions onhow to obtain them.

Before SamplingBecause of the extreme variety of coat-ing types, substrates, and environments,there is no “cookbook” method for con-ducting a failure analysis. Consequently,there is no cookbook procedure for sam-ple taking. The investigator or sampletaker needs to be aware of the type ofproblem and the nuances of the specificjob in order to devise the best samplingscheme, and this information is notalways available at the time of sampling.However, there are some considerationsthat are almost always appropriate.Although not related to sample taking per se, if one is at a

jobsite, it is always good practice to look for patterns to thefailure. Perhaps the coating is peeling only from the south sideof the structure, or blistering only near joints, or rusting in apattern that might suggest the arcing of the spray gun. Suchobservations are important in determining the cause of thefailure, and not even the best laboratory analyst will be awareof them unless he is told.

Failing Versus Non-Failing SamplesOne of the most fundamental steps of failure analysis is tocompare failing samples with non-failing samples. Perhaps thecoating has failed because it is too thick or too thin, or full ofvoids, or because it was dry sprayed, or mixed wrong. In Continued

some cases, such as that of an inorganic zinc-rich primer hav-ing mudcracked because it was applied at 8 mils rather thanthe specified 2–3 mils, the cause of the failure may be soobvious that non-failing samples are not needed. However,causes of the failure are not often obvious, so having two orthree failing samples and at least two non-failing samples mayallow the investigator to discover trends that can explain thefailure.Sometimes labeling a sample as non-failing requires judg-

ment. If the problem is blistering, discoloration, or pinpoint

rusting, the failing areas are usually easy to distinguish fromareas that are not failing. However, when poor adhesion is theproblem, it should not always be assumed that just becausethe coating system is intact, with no peeling or missing paint,that it is a non-failing location. The author has received manysamples in the mail labeled “non-failing,” which contain paintchips as large as 1⁄4 in. x 1⁄2 in. While the “failing” samples mayindeed be much larger than this, it is questionable that suchrelatively large “non-failing” samples actually came fromareas of good adhesion. In this case, it is likely that both areasare bad, but the coating in the visually intact location justhasn’t gotten around to falling off yet. Indeed, when investi-gating an adhesion failure, it is sometimes found that the

By Dwight G. Weldon, Weldon Laboratories, Inc.

M

Peeling of a concrete floor coating from the underlying basecoat. When taking samples from a site likethe above, be aware that poor adhesion may extend beyond the area of the existing spontaneous failure.

Photos courtesy of the author

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Maintenance Tips

extent of poor adhesion is actuallymuch larger than the extent of existingspontaneous failure.

If the samples are poorly adherentpaint chips, obviously the failing sam-ples may be quite large and the non-fail-ing samples quite small. Nevertheless, itis still worth the effort to take bothtypes.

Problems InvolvingIntercoat Delamination

If the failure involves the peeling orblistering of one coat of paint fromanother, obtaining samples of both lay-ers is important. A fairly commonerror in sampling is to put only thepeeling topcoat in a sample bag andignore the coat beneath it. Sometimes,the source of the problem is indeed thetopcoat, but the problem is just as like-ly to involve the underlying coat.Perhaps it was applied at the wrongmix ratio, or developed a surfactantblush, or was even the wrong type ofpaint. Without sampling the underlyingcoating, these questions likely cannotbe answered. Therefore, the properprocedure is to peel off some of thepoorly adherent upper coat(s), put thismaterial in a labeled sample bag, and

then scrape or peel off some of theunderlying coat and put this material ina separate, properly labeled samplebag.

Avoiding Chips That Have Already Disbonded

Notice that the above procedure refersto peeling off a chip of poorly adherentpaint. Another common mistake in sam-ple taking is to simply pick up largepieces of failing paint that are lying onthe ground. While certain things can belearned from such samples, the back-sides of chips that have already fallenoff have been compromised by the envi-ronment. If dirt or some other type ofcontamination is found on them, deter-mining whether the contaminant waspainted over, or whether it got on theback of the coating chip after delamina-tion can be difficult. In the case of top-coat peeling from a primer, the sameconcerns are true of the exposedprimer. Usually, where the topcoat ismissing and the underlying primer isexposed, one can probe with a knife aninch or so away and find that the near-by coating adhesion is also poor. Fromthis nearby area, pristine samples canbe obtained.

Careful sampling was crucial to determining that the edge of this steel beam was ground, rather than blast cleaned, resulting in poor adhesion of the zinc-rich primer.

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Continued

Maintenance Tips

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Distinguish Front from BackAlthough the procedure for solving acoating failure in the laboratory can dif-fer substantially from project to project,the first step in nearly every case is acareful visual and microscopic examina-tion of the samples. Sometimes, coatingsfail for subtle or complicated reasons.Other times, they fail for very simple,obvious reasons that can be determinedby a careful microscopic examination.However, for an intercoat delaminationfailure, the significance of any micro-scopic observations, as well as manysubsequent analytical procedures,depends on one very important fact:which side of the chip is the back.Whoever removes the topcoat that ispeeling from a lower coat of paint oftenassumes that the analyst in the lab willbe able to tell the front from the back.Nine times out of ten this assumption iscorrect.

However, sometimes, even if the twosides of the chip look different, the frontis not easily distinguishable from theback. For example, some time ago, theauthor received samples of a clear var-nish that peeled from an underlyingcoat of varnish. The two sides lookedidentical. Although surface analysisusing a fourier transform infrared spec-trometer equipped with an attenuatedtotal reflectance (ATR) attachmentclearly showed that the two sides weredifferent compositionally, the signifi-cance of this finding was clouded by notbeing sure which was the front andwhich was the back.

When it is obvious upon taking sam-ples that the front and back can be easi-ly identified, there is no need to do any-thing further. However, if there is achance of some uncertainty, the samplercan make a small mark on the front sidewith an indelible marker. This simplestep eliminates the chance of confusion.Another way of identifying front fromback is to take at least one poorly adher-ent chip by attaching adhesive tape to it