al in the clarifier content was attackingthe lining and decided to have an inde-pendent investigation performed to

determine thecause of thelining failure.

SiteInvestigationA site visitwas conduct-ed to examinethe lining inthe clarifier. Alight blue coat-ing had beenapplied to theinterior of theclarifier. Atthe time of theinvestigation,

the interior sidewalls had not been thor-oughly cleaned, and, in most areas, abrown residue was attached to the coat-ing to some degree. When the brownresidue was removed, the blue coatingexhibited gloss and showed no sign ofchemical attack.

The clarifier sidewall had two shellrings with many areas of coating delam-ination. The majority of the delamina-tion occurred on the second (top) shellring. Widespread pinpoint rusting wasmost severe on the second shell ring butalso appeared sporadically on the firstshell ring (Figs. 1 and 2).

In areas immediately adjacent tospots of delamination and in areas ofpinpoint rusting, a close examination of

with external leakage were noticed, andupon a short decommission of the tank,it was found that the lining had once

again failed. The food manufacturingcompany was certain that some materi-

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n this Case from the F-Files, afood manufacturing facilityowner insisted that, despite the

use of three different lining systems, theaggressiveness of the service environ-ment caused all three lining systems tofail in less than 10 years. Until a formalfailure investigation was performed, theowner was ready to pay for a fourth lin-ing system without knowing the realculprit.

Several years ago, the food manufac-turing company contracted to have theinterior of a clarifier relined. The clarifi-er was approximately 50 feet in diame-ter and 17 feet high. The original liningin the tank was an epoxy novolac andfailed approximately one year after theclarifier was placed into service. Thetank was relined with another type ofepoxy system, butthe new lining last-ed only aboutthree years beforesignificant rustingoccurred. The tankwas again relinedwith a 100% solidsepoxy system at ar e c o mm e n d e dtotal dry filmthickness of 16 to20 mils. The samecontractor per-formed the origi-nal application andboth relining oper-ations. About fiveyears after the third system wasapplied, multiple tank wall spot failures

C a s e s f r o m t h e F - F i l e s

The Case of Misplaced Blame…Lining a Clarifier Four Times to Get Acceptable Performance

By Rick A. Huntley, Manager, Consulting Services, KTA-Tator, Inc.Richard Burgess, KTA-Tator, Inc., Series Editor

I

Continued

Fig. 1: Area of corrosion where the coating was riddledwith pinholes and other voids. Photos courtesy of KTA-Tator.

Fig. 2: Rust scale is present in areas where the coating was full of holes.

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Continued

C a s e s f r o m t h e F - F i l e s

the coating revealed that it wasextremely rough. The lining had a dryspray appearance (Fig. 3), and globulesof coating were visible on the surface. Inthese rough areas, rust was bleedingfrom the coating in many spots. Wherethe coating was smooth, however, therewas no coating delamination and verylittle pinpoint rusting. There was anotable correlation between coating fail-ure and the smoothness of the coating:the rougher areas exhibited quitesevere failure while the smooth areasexhibited almost no failure.

The lining was removed, and the sub-strate surface was examined in severalareas. In areas with noticeable pinpointrusting, the surface of the steel was cov-ered with black corrosion spots, indicat-ing permeation through the coating inthose spots. Where the lining wassmooth, the surface of the steel wasdark gray, but free of corrosion spots.Both areas exhibited a sharp, angularsurface profile, and there was no evi-dence that the steel surface had notbeen properly abrasive blast cleaned.

The dry film thickness of the coatingwas measured using a calibrated elec-tronic dry film thickness gage. The dryfilm was found to vary considerably.Generally, the thickness measurementwas between 16 and 40 mils, althoughbecause of the roughness of the coating,the thickness gage measured the coatingfrom the tops of peaks of the surfaceprofile to the valleys of the profile, mak-ing the thickness appear greater than itactually was. Often, rust could be seenin depressions in the coating, and it wasvery likely that the coating thicknessapproached zero in small spots (Fig. 4).In fact, when pieces of coating wereforcibly removed and held up to thesunlight, the light could be seen throughholes in the coating.

Laboratory ExaminationSamples of the coating were taken to thelaboratory for analysis along with a wetsample of the lining most recently

applied. The samples were examinedwith a digital microscope with magnifica-tion ranging from 20X to 100X. In sum-mary, the examination revealed that thedry film thickness of most of the lining

samples varied considerably. Sampleshad a thickness range of 0 (voids) to 24mils, and there were numerous holesthrough several of the samples. The lin-

Fig. 3: Close-up of the surface of the coating. The coating had a rough, dry-sprayed appearance.

Fig. 4: Close–up of a sample showing the uneven, void-filled coating.Corrosion is starting on the steel surface.

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ing material was glossy on top, with novisible indication of chemical break-down. Additionally, the cross sectionalexamination revealed that there was nodiscoloration of the coating below thesurface, which would have indicated per-meation into the coating.

Infrared spectroscopic analysis ofseveral samples was performed with aFourier transform infrared spectrome-ter. Additionally, a wet sample of thelining material was properly mixed andallowed to thoroughly cure, then ana-lyzed. The spectra of all the samples,including the laboratory-prepared sam-ples, were nearly identical. There wasno noticeable evidence of chemicaldegradation in any of the lining samplesremoved from the clarifier interior.

Putting the Pieces TogetherThe field investigation and the laborato-ry analysis revealed that the cause ofcoating delamination, cracking, and pin-point rusting was poor application ofthe 100% solids epoxy lining. The coat-ing was applied inconsistently, and innumerous pinpoint spots, there was nocoating on the steel. The surface of thecoating was extremely rough in areas of

pinpoint rusting and coating delamina-tion.

The type of epoxy lining applied tothe clarifier interior prevents corrosionon the steel surface by forming a pro-tective barrier that greatly reduces theamount of moisture that comes in con-tact with the steel. Without this mois-ture, the steel does not corrode. To effec-tively protect the steel, the coating mustbe applied so that it covers all of thesteel at or greater than the minimumthickness. The minimum thicknessdepends on the particular coating andits moisture permeability characteris-tics.

In this case, the way the coating wasapplied created numerous holes in thecured system (Fig. 5). In many areas, adense pattern of pinpoint rustingoccurred where the holes in the coatingwere numerous and densely packed.Water penetrated the coating throughthe numerous holes and caused the steelto corrode. In some areas, the corrosionundercut the coating until the entiresurface was covered with rust. In areaswhere the pinpoint rusting was lessdense, only isolated spots of rust werepresent on the steel.

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Fig. 5: Voids in the coating created during the applicationare clearly visible near areas of coating delamination.

The laboratory microscope examina-tion confirmed that the coating was fullof pinholes. In many spots, there werevoids in the coating, even though theaverage thickness of the coating wasgreater than 20 mils.

The site examination revealed no evi-dence that the lining applied to the clar-ifier was subjected to any kind of chem-ical degradation. As noted above, whenthe residue from the tank contents wasremoved from the coating, the surfacewas glossy. Generally, when a coatingexperiences chemical degradation, thecoating loses its gloss. In areas wherethe coating was applied in a smooth andconsistent manner, it protected the steelsurface adequately. Additionally, nochemical degradation was detected dur-ing the laboratory microscopic exami-nation or the infrared spectroscopicanalysis of the samples. It is thoughtthat if this coating been applied in asmooth and consistent manner in allareas, no significant coating failurewould have occurred.

The failure of the lining system wasthe third failure in a relatively shortperiod; each of the three different liningsystems failed prematurely. The clarifi-er owner assumed that the culprit wasthe clarifier content. The failure investi-gation pointed to the application of thelining as the problem, not the liningitself. Interestingly, the same contractorapplied all three lining applications andmay have made the same applicationerrors in each instance.

RecommendationsThe extent of the cracking, delamina-tion, and rusting was too great toattempt a spot repair approach to reme-diate the failure. The failure investiga-tors recommended removing all of theinterior lining and reapplying the sameproduct or a different epoxy coatingsystem suitable for this environment.

They also recommended that a quali-fied inspector closely monitor the appli-

17

F - F i l e s

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Continued

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C a s e s f r o m t h e F - F i l e s

cation. In addition to monitoring envi-ronmental conditions, abrasive qualityand cleanliness, surface cleanliness androughness, mixing (as appropriate),coating thickness, and curing, theinspector should perform holiday test-ing to verify continuous lining applica-tion and to identify necessary repairsbefore putting the system in service.

Rick Huntley is the manager of Consulting Services for KTA-

Tator, Inc., where he has been employed for 18 years. He is cer-

tified by SSPC as a Protective Coatings Specialist and is a certi-

fied NACE International Coatings Inspector. His work with KTA

includes coating failure analysis, specification preparation, and

coating project management. He earned a Bachelor of Science

degree from Washington State University.

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Did you miss an article in the Cases fromthe F-Files series, which premiered ayear ago in JPCL? Here’s a list of the first12 articles in the series.• “Introducing a Series on AnalyzingCoating Failures,” March 2009• “The Case of the Yellowed Water TankLining,” April 2009• “The Case of the Perplexed PaintShop,” May 2009• “Accelerated Corrosion of aPedestrian Bridge,” June 2009• “The Case of Hurry Up and Wait”(Premature Rusting of Newly CoatedStructural Steel in a Chemical Plant),July 2009• “The Fix That Was Worse than theProblem” (Catastrophic Disbonding ofan Encapsulating Coating Applied to HotDip Galvanizing), August 2009• “The Case of the Job Shop” (CoatedEquipment Sent North but CoatingPerformance Went South), September2009• “The Cost of Going Green?” (FiliformCorrosion of Powder-Coated Aluminumon the Gulf Coat), October 2009• “Coating Failures on Galvanized MastArms,” November 2009• “Who Reads Instructions Anyway?”(The Case of the Failing Floor Coating),December 2009• “The Case of…Improper Selection ofa Maintenance Strategy Takes CorrosionProtection from Bad to Worse,” January2010• “The Case of a Failure toCommunicate,” February 2010Don’t have the issue? Find the article

in the JPCL archives on our electronichome, www.paintsquare.com.

Click our Reader e-Card at paintsquare.com/ric

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