The Case of…Improperly Cleaned Surfaces: Failures in ...025-31)FFiles8-14.pdf · metallic...

hat happens if coatingsare applied to an improper-

ly cleaned surface? The sim-ple answer might be to

expect premature failure, but whatexactly is an “improperly cleaned” sur-face? Was a contaminant left on thesurface? Was cleaning not otherwisesufficient to meet the specified stan-dard? Or was the specified coating notappropriate for the level of cleaning?

For example, if the specified surfacepreparation for a steel surface is“Power Tool Cleaning” (SSPC-SP 3) anda zinc-rich primer is applied and poorlyadheres, was the surface improperlycleaned? Most would recognize that thereferenced power tool cleaning stan-dard is not sufficient preparation for azinc-rich primer, as these primersrequire removal of all existing coatings,corrosion and mill scale and a rough-ened surface.

Proper surface preparation is alwaysone of the most critical steps in a suc-cessful coatings application. The purposeof surface preparation is generally two-fold: to clean and to roughen the sub-strate according to the specified require-

standards related to preparation ofmetallic substrates incorporate SSPC-SP 1, “Solvent Cleaning,” which isintended to clean the surface beforefurther preparation to remove rust,paint and other materials. When clean-ing allows surface contamination toremain, a problem is created. If any

By Jayson L. Helsel, P.E., KTA-Tator, Inc.Rich Burgess, PCS, KTA-Tator, Inc., Series Editor

paintsquare.com / JPCL August 2014 25

Cases fromthe F-Files

ments. The method or methods used toprepare surfaces for coating applicationmay clean and roughen simultaneously— as with abrasive blast cleaning.

With few exceptions, the surfacepreparation should reference recog-nized industry standards such as thosepublished by SSPC. All of the SSPC

The Case of…Improperly Cleaned Surfaces:Failures in Preparation or Specification?

Mechanismsof Failure

Fig. 1: Widespread rusting over a swimming pool bottom due to high surface profile and/or low coatingthickness. Photos courtesy of KTA-Tator, Inc.

W

26 JPCL August 2014 / paintsquare.com

F-Files: Mechanisms of Failure

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“interference” material is left on a sur-face, the coating may not properly con-tact and adhere to the substrate. Oil,grease, dirt and chalking are obviousinterference materials. Less obviouscontaminants may not even be visible,such as soluble salt contamination(e.g., chloride).

Coatings may actually attain goodadhesion if a relatively low level of saltcontamination is present. But whenenough moisture eventually permeatesthe coating and solubilizes the salt,water molecules begin to collectaround the salt. Free water and mois-ture in sufficient quantities will form agradient causing additional water tocollect and thereby cause coating tolose adhesion in the form of blisteringor delamination. The salt solution, anexcellent electrolyte, will also acceler-ate the corrosion of metal substrates.

Once a surface is cleaned of contam-inants, various methods to prepare sur-faces to the specified level of cleanli-ness are used. Surface preparationmethods include hand and power toolcleaning, abrasive blast cleaning andwaterjetting. The degree of cleaningrequired by a given project specifica-tion is dependent on the service envi-ronment (the environment that the coat-ing system must perform in), the coat-ing system and the intended service lifeof the coating system once installed.

In making selections for surfacepreparation and coatings, it is criticalthat these items are properly matched.When this is not the case, a coatingmay not achieve the degree of adhe-sion needed to perform as intended.When it comes to surface preparation,this “failure to perform” might be con-sidered a premature failure. But in reali-ty, accelerated degradation of the coat-ings could have been expected.

Generally, better surface preparation

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equals better performance. Althoughblast cleaning may not be required foralkyd coatings, they will have a longerservice life when applied over a blast-cleaned surface as compared to apower tool-cleaned surface (e.g., SSPC-SP 3). A review of manufacturers’ prod-uct data sheets (PDS) demonstrates thatalthough a coating might be suitable forone surface preparation method, the useof a second, more advanced degree ofsurface cleaning will provide better per-formance. Consider the implications ofspecification language that simply saysto prepare surfaces in accordance withthe manufacturer’s recommendation,particularly when applying “surface toler-ant” coating. Would it be unreasonableto argue that the failure was not prema-ture – the surface was properly cleaned,but the expectations were too great?

A short review of some examplesserves to bring this topic into focus.

Service or Storage?In one case where a failure occurred,alkyd coatings had been applied tostructural steel in the fabrication shop.The specified surface preparation was

“Hand Tool Cleaning” (SSPC-SP 2) and“Power Tool Cleaning” (SSPC-SP 3).After the steel was delivered to the jobsite and construction had begun, thecoating began to delaminate from thesteel. Investigation of the problemfound loose mill scale on the surface ofthe steel – and mill scale on the backof the failing coating. The conclusionwas that the surface had not beenproperly prepared, because that levelof power tool cleaning required removalof all loose material including mill scale.

The steel surfaces were adequatelycleaned for the service environmentthey were being placed in, but unfortu-nately not for the environment in whichthey were stored during construction.

Fitting the ProfileIn another instance, following comple-tion of a bridge painting project, rela-tively small areas of damaged coatingwere prepared by “Power Tool Cleaningto Bare Metal” (SSPC-SP 11) and theorganic zinc-epoxy-urethane system

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Fig. 2: Closer view of pinpoint rusting on pool bottom; more concentrated areas of rust typically had ahigher surface profile.

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for a moment, which include“Commercial Blast Cleaning” (SSPC-SP6/NACE No. 3), “Near White Metal BlastCleaning” (SSPC-SP 10/NACE No. 2)and “White Metal Blast Cleaning” (SSPC-SP 5/NACE No. 1). During inspection,they are differentiated only by theamount of “staining” allowed to remainon the surface. When any of these threecleanliness standards is specified, yetthe applied coating system delaminatesor exhibits pinpoint rusting, can it besimply argued that the degree of cleanli-ness was not achieved? The answer isno. The blast cleaning standards identifythe criteria for surface cleanliness ofthe steel but do not specify a surfaceprofile. The project specification shouldinclude an appropriate range for anangular surface profile. Too little of aprofile may not provide sufficient anchorfor higher build systems or for othercoating options such as metallizing. Toogreat of a profile, however, may alsolead to problems, which was the casewhen a steel swimming pool wasrecoated.

Swimming in FailureThe in-ground, steel swimming pool wasabrasive blast cleaned and painted inthe spring. There was not a coatingspecification for the work, but the sur-face preparation and system applicationwere to be completed in accordancewith the coating manufacturer’s PDSrequirements. The system applied con-sisted of two coats of epoxy coating ata DFT of 4 to 8 mils per coat. Shortlyafter the completion of painting duringthe summer, the coating began delami-nating and rust was evident on theunderlying surfaces. The delaminationwas reported to have occurred primarilybetween the two epoxy coats. Duringthe fall, the painting contractor repairedthe delaminating and rusted areas, and

the poor adhesion. The specified stan-dard, SSPC-SP 11, required a minimumprofile of 1 mil. The surface preparationstandard was achieved, but the appliedcoating system required a 2–3 milanchor profile.

Consider the blast cleaning standards

was reapplied. But after just a fewweeks, failures in repair areas wereobserved and testing found poor adhe-sion of the zinc primer to the steel. Thesurface appeared to have been cleanedto bare metal, but little to no profilehad been created, which likely caused

applied a full overcoat. Over the ensuingwinter, with the pool partially filled andout of service, delamination and rustingcontinued. The contractor again madespot repairs (not a full overcoat) the fol-lowing spring but failure continued tooccur.

An eventual investigation found thatthe pool had significant areas of rustingon the side walls, ledges and bottom(Fig. 1, p. 25). Rusting consisted ofspot and pinpoint rusting and was esti-mated to cover up to 20 percent of thepool’s surface area. At some of therusted areas, rust staining had spreadto adjacent areas, giving the appear-ance of an even greater degree of rust-ing. The most recently applied repairtopcoat (from the previous fall) wasdelaminating from the underlying origi-nal coatings on over 30 percent of thepool surfaces, including the ledges,walls and bottom. At areas where therepair coat was still attached, it couldtypically be easily removed. There werealso some areas where small blisterswere present but no rusting was appar-ent. When these areas were scrapedwith a putty knife, rusting was foundunderneath. Random adhesion testingfound that the repair coat had uniformlypoor adhesion (tape test rated as 0A)to the underlying original coatings. Theadhesion of the original coating layerswas also tested at areas where therepair coat had delaminated. The coat-ing adhesion was typically fair (rated as2A to 3A) in these areas. The dry filmthickness of the existing coatings var-ied widely and ranged from approxi-mately 3 to 18 mils at areas where allcoats appeared to be present. Areaswhere the repair coat had delaminatedwere lower in comparison ranging from2 to 7 mils in total thickness.

Because the appearance of pinpointrusting (Fig. 2, p. 27) suggested high


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profile might have been an issue, thecoatings were removed using a chemi-cal stripper in selected areas. Theseareas were primarily where the com-plete coating system was intact (includ-ing the most recent topcoat) and norusting was present. As the coating lay-

ers were progressively removed, rust-ing of the underlying steel was appar-ent at some locations. Profile measure-ment on the bare substrate found thatthe surface profile was consistentlyhigher than the 2 mils specified by thecoating product data sheets. When all


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been expected due to the coatingrepairs that were completed subse-quent to the original work. The combi-nation of a high surface profile, typical-ly 3 to 4 mils, and the application ofcoatings below the specified thicknessled to the pinpoint rusting observed onthe pool surfaces. The rusting likely

began after the first initial coat wasapplied, which would have createdadhesion problems for the second coat.

ConclusionThese coating failure examples showthe importance of specifying the appro-priate surface preparation – and profileif required. The failure does not impli-cate a failure to achieve the appropriatedegree of surface cleanliness. Rather,there was improper roughness andcoating system thickness.

Achieving a properly cleaned surfaceis no guarantee of success. Properlycleaned is not the same as adequatelycleaned. It is important to properlymatch the coating system (the level ofsurface preparation, coating types andcoats), roughness and expectations tothe service environment and likely ser-vice life of the coatings. To do other-wise invites failure – whether due to thecoatings actually failing or parties fail-ing to understand what should havebeen expected.

About the AuthorJayson Helsel, P.E., PCS, a senior coat-ings consultant with KTA-Tator, Inc.,manages failure investigations and coat-

ings projects and isinvolved with coatingssurveys and inspec-tion of industrialstructures. He holdsa Master of Sciencedegree in chemical

engineering from the University ofMichigan, is a registered professionalengineer and is a NACE-certifiedCoatings Inspector. He has been pub-lished in the past in JPCL, Durability +Design and in the Journal ofArchitectural Coatings, which featuredhis monthly column, “Getting It Right.”

coating layers were removed, the pro-file was measured and found to rangefrom 2.4 to 4.2 mils. Also, the coatingthickness varied significantly and wasfrequently below the specified thicknesspublished in the coating product datasheets of 8 to 16 mils for two coats.An even greater thickness should have

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