chemical degradation, chalking and cracking. Substrate Consideration Several years ago, a large warehouse was newly constructed, and the ware- house floor was to consist of coated poured concrete. The original specifica- tion called for an epoxy floor coating to be applied, but the engineer asked for an alternate system based on cost. Ultimately, an equipment enamel was chosen to coat the floor. The floor was well-prepared using a portable centrifugal ypically when a premature coating failure is encountered, it is assumed that either there was a mistake made during the sur- face preparation and/or the application of the coating, or that there was some- thing seriously wrong with the formula- tion of the coating. In fact, in the major- ity of cases, one or more of these cir- cumstances is exactly what happened. Sometimes, though, the surface prepa- ration and the application are top-notch, and the coating is a quality product that was formulated and manufactured to exacting standards, and yet the coating still fails prematurely. In these cases one has to look elsewhere to determine the cause of the coating failure and ask the question, “Was this the right coat- ing for the job?” There are literally thousands of differ- ent coating products, each appropriate for some type of service environment. When a coating system is specified for a project, much effort is taken to assure that the surface preparation is adequate and the thickness of the coat- ing is appropriate. Yet, in some cases, not enough thought is given to whether the coating system itself is appropriate By Rick A. Huntley, PCS, Senior Coatings Consultant, KTA-Tator, Inc. Richard Burgess, PCS, KTA-Tator, Inc., Series Editor paintsquare.com / JPCL November 2014 17 Cases from the F-Files for the service environment to which it will be exposed and the surface to which it will be applied, and both must be considered when a coating system is chosen. Given the vast number of coating products available and the multitude of different service environments to which they can be exposed, there are proba- bly hundreds of different ways that a coating can fail if the wrong coating is selected for a particular environment. These failure mechanisms include, but are not limited to, rust-through, peeling, Oops, Wrong Paint! What Can Happen When the Service Environment Isn’t Characterized or the Wrong Coating is Selected Mechanisms of Failure T Fig. 1: This tank lining showed insufficient resistance to the acidic tank contents. Photos courtesy of KTA-Tator, Inc. Fig. 2: Peeling resulted after application of this white coating, which was incompatible with the factory-applied gray coat.
Transcript
chemical degradation, chalking andcracking.
Substrate ConsiderationSeveral years ago, a large warehousewas newly constructed, and the ware-house floor was to consist of coatedpoured concrete. The original specifica-tion called for an epoxy floor coating tobe applied, but the engineer asked foran alternate system based on cost.Ultimately, an equipment enamel waschosen to coat the floor. The floor waswell-prepared using a portable centrifugal
ypically when a prematurecoating failure is encountered, itis assumed that either there wasa mistake made during the sur-
face preparation and/or the applicationof the coating, or that there was some-thing seriously wrong with the formula-tion of the coating. In fact, in the major-ity of cases, one or more of these cir-cumstances is exactly what happened.Sometimes, though, the surface prepa-ration and the application are top-notch,and the coating is a quality product thatwas formulated and manufactured toexacting standards, and yet the coatingstill fails prematurely. In these casesone has to look elsewhere to determinethe cause of the coating failure and askthe question, “Was this the right coat-ing for the job?”
There are literally thousands of differ-ent coating products, each appropriatefor some type of service environment.When a coating system is specified fora project, much effort is taken toassure that the surface preparation isadequate and the thickness of the coat-ing is appropriate. Yet, in some cases,not enough thought is given to whetherthe coating system itself is appropriate
By Rick A. Huntley, PCS, Senior Coatings Consultant, KTA-Tator, Inc.Richard Burgess, PCS, KTA-Tator, Inc., Series Editor
paintsquare.com / JPCL November 2014 17
Cases fromthe F-Files
for the service environment to which itwill be exposed and the surface towhich it will be applied, and both mustbe considered when a coating systemis chosen.
Given the vast number of coatingproducts available and the multitude ofdifferent service environments to whichthey can be exposed, there are proba-bly hundreds of different ways that acoating can fail if the wrong coating isselected for a particular environment.These failure mechanisms include, butare not limited to, rust-through, peeling,
Oops, Wrong Paint!What Can Happen When the Service Environment Isn’t Characterized or the Wrong Coating is Selected
Mechanismsof FailureT
Fig. 1: This tank lining showed insufficient resistance to the acidic tank contents.Photos courtesy of KTA-Tator, Inc.
Fig. 2: Peeling resulted after application of thiswhite coating, which was incompatible with thefactory-applied gray coat.
18 JPCL November 2014 / paintsquare.com
the shop primer or theOEM coat of the wrongtype or color, or whensurfaces are beingrecoated. In any case, thenewly applied coatingmust be compatible withthe existing coating.Peeling is the most com-mon type of coating fail-ure that can occur whenpreviously applied coat-ings are overcoated. Thisproblem can be avoidedby choosing a compatiblecoating that is known toachieve sufficient adhe-sion to the previous coator sometimes by roughen-ing the surface of the pre-viously applied coat.Roughening the surfacemay also be necessarywhen the recoat windowhas been exceeded.
Another type of failurecan occur when a relative-ly hard, brittle coating isapplied over a soft, flexi-ble coating. One suchexample occurred at aniconic midwestern MajorLeague Baseball stadium.Structural steel columns
at the stadium had previously beencoated with a soft black bitumen coat-ing. As part of a renovation project, thecolumns were painted with a whiteepoxy topcoat. Although this coatinghad been used successfully on manytons of structural steel, it quicklycracked over the bitumen coating.Solvents from the epoxy had softenedthe bitumen coating, and when theepoxy cured, it developed cohesivestresses. Because the bitumen coatingunderneath the epoxy was soft enough
F-Files: Mechanisms of Failure
blast unit and the coating was appliedto the recommended thickness, yet onemonth later the applied coating beganto soften and deteriorate. The equip-ment enamel was based on alkyd resin,which deteriorated due to the presenceof moisture and alkalinity in the con-crete in a process known as saponifica-tion. This failure was described in fur-ther detail in a previous F-Files article,“Saponification: From Paint to theGrave.” (JPCL, May, 2014).
This alkyd coating, while perfectlysuitable for its intended purpose ofcoating metal surfaces of equipment,was clearly the wrong product for thisconcrete substrate. Similar problemscould arise if the same alkyd coatingwas applied to galvanized steel sur-faces (even if those surfaces were onequipment), because zinc corrosionproducts are alkaline and will similarlydeteriorate the alkyd paint in the pres-ence of moisture.
Other coating failures can occur if acoating is not well-suited for the sub-strate. For example, coatings for woodcan be problematic, especially on exte-
rior exposure environments. Woodexpands and contracts cyclically uponexposure to moisture. Coatings that areapplied to wood must have sufficientflexibility to resist cracking when thesubstrate underneath is expanding andcontracting. Applying a hard brittlecoating to wood in this exposure envi-ronment will ultimately lead to prema-ture cracking of the coating.
Surfaces that have been previouslycoated should also be considered,including when items are supplied with
Fig. 3: The coating used on this metal roof had poor UV resistance and degraded significantly.
Fig. 4: This failure was caused by an epoxy coating that was notintended for immersion service.
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to move slightly under the stress, theepoxy coating cracked, creating an alli-gator skin pattern also known as “alliga-toring.”
Service EnvironmentConsiderationA more common mistake associatedwith choosing the wrong coating sys-tem is the failure to properly character-ize the service environment to whichthe coating will be exposed, or simplychoosing the wrong coating system fora known environment. There are severalenvironmental factors that affect acoating’s ability to perform. These fac-tors include, but are not limited to, tem-perature extremes, pH, solar radiation(sunlight), chemicals, solvents, immer-sion or splash and physical damage.
Exposure to temperature extremescan cause a coating system to deterio-rate quickly. In chemical plants andother manufacturing facilities, the nor-mal operating temperature range of theequipment used in the process is typi-cally determined before a coating sys-tem is selected for that equipment.There have been many instances whereeither the temperature range was notwell understood during the designphase, or upset conditions occurredwhere the temperatures rose aboveexpected operating temperatures.Coating systems must be planned towithstand the highest temperatures towhich they may be exposed. It is notuncommon for an industrial coating tohave a temperature limitation of 250 For 300 F. Once the temperatureincreases above this level, the resin inthe coating will begin to char as chemi-cal bonds in the coating begin to break.Eventually the coating deteriorates,usually turning dark brown, and coatingfailures such as cracking and peelingoften follow.
Failure to choose a coating that ade-quately resists exposure to sunlight onexterior substrates can lead to prema-ture failure of the coating. This type offailure is commonly found even withconsumer paints. Different types ofcoatings have varying resistances to
solar radiation. Latex paints that arecommonly purchased by homeownersin paint stores or home remodelingstores generally come in interior andexterior versions. The exterior latexesare generally based on acrylic resintechnology and have good resistance
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F-Files: Mechanisms of Failure
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times the chalking is so severe that thecoating loses thickness in a processknown as erosion. The coating chalksand rain washes the chalk away, expos-ing more coating. The newly exposedcoating chalks and the deteriorationcontinues until most or all of the coat-ing is eroded away. The coating is thenno longer able to protect the substratefrom corrosion.
When a PVA block filler is used onexterior concrete block, the block fillerwill deteriorate in areas where moistureis prevalent. This is a common occur-rence especially in areas with signifi-cant amounts of rainfall. The PVA blockfiller is well suited for interior applica-tions but is prone to premature failureon exterior exposures.
Sometimes the severity of the envi-ronment is not understood well enoughor is underestimated. This can happenin coastal areas where metal structuresare frequently exposed to airborne saltmist, regardless of whether or not theyare located directly on the coastline.Coating systems that are designed toprotect inland metal structures areoften inadequate to protect the metalnear saltwater coasts. For example, analkyd system may be sufficient to pro-tect steel in inland areas as long as thesteel is not exposed to other corrosion-producing elements. On the seacoast,application of the same alkyd coatingas the primary barrier coat will likelylead to premature rusting and rapiddeterioration of the steel.
Coatings that are exposed to immer-sion in water or a water solution canfail catastrophically if the wrong coatingis selected for that application. If thecoating is generically unsuitable forconstant immersion, the failure can bealmost 100 percent. Even if the coatinghas a resin type that is generally con-sidered to be compatible with immer-
Epoxies have notoriously poor resis-tance to the UV spectrum in sunlightand chalk readily upon exterior expo-sure. The aesthetic issues associatedwith the chalking may not be a problemwhen the structure is not in an areawhere the public can see it, but some-
to the ultraviolet (UV) spectrum in sun-light while the interior latexes are basedon polyvinyl acetate (PVA) and havepoor resistance to both UV light andwater. The UV light will degrade thisresin system and the interior paint willchalk severely if exposed.
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sion service, it is important to choose acoating that is formulated specificallyfor immersion. For example, epoxycoatings are generally suitable forimmersion, but there are specific prod-ucts that are formulated with appropri-ate solvents and pigments to performwell in immersion. An epoxy coating notspecifically formulated for immersionmay absorb and transfer too muchwater to perform adequately and rust-ing and/or delamination may occur.
Condensation can cause similar fail-ures as well. This has occurred on tankexteriors where groundwater was beingtransferred to the tanks almost con-stantly. During the summer months thetemperature of the groundwater wasconsistently less than the exterior airtemperature resulting in continuouscondensation on the exterior of thetank. The exterior coating was in con-stant contact with this water, similar tothe immersion service on the inside ofthe tank. The exterior coating blisteredand delaminated as if it had been putdirectly into immersion service.
ConclusionThe list of failures that can occur if thewrong coating system is selected andinstalled is almost endless. Care shouldbe taken to choose a coating systemthat will perform in the most severescenario of the intended service envi-ronment and to clearly convey thatinformation to the coating manufacturerso that appropriate recommendationscan be made. There are plenty ofopportunities for something to gowrong on a coatings project withoutallowing coating selection to be amongthem. Spend adequate time evaluatingthe substrate and service environment.After all, you don’t want to be the oneto say, “Oops, wrong paint.”
About the AuthorRick Huntley is the technical manager of consulting services anda senior coatings consultant for KTA-Tator, Inc. He is a NACE-cer-tified Coating Inspector Level 3 (Peer Review) and an SSPC-certi-fied Protective Coatings Specialist. He is a primary instructor forvarious KTA training courses and holds a Bachelor of Sciencedegree in chemical engineering from Washington State University.
