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Performance or Preference?A Look at Selected Systems for Water Tank Interiors

hile water storage tanks may vary great-ly in size, style, and design, all share acommon need for maintenance or peri-

odic reconditioning. Concrete tank exteriors, for example, areregularly protected with acrylic or vinyl-acrylic systems in asmooth or textured finish. And for the interiors of concretetanks, polyurea or polyurethane elastomers are applied tomake interior concrete surfaces leak-proof.

Coating remediation in steel tanks is most common in areasof the tank that are difficult to access for painting: interiorsabove the waterline, for example. Unsealed roof lap-plateseams and intermittently welded roof support systems rep-resent common areas requiring coating maintenance. Theevaluation and study of the a Minnesota city’s 400,000-gal-lon steel water storage tank, completed by an engineeringfirm in 2002, revealed a need for renovation and a uniqueopportunity to conduct a test project with a new, NSF 61-approved, zinc-rich coating.

The Proposal Working with three partners–coatings manufacturerSherwin-Williams; the City of Anoka, MN; and coating con-tractor Classic Protective Coatings–engineering firm SEH,which had evaluated the tank, offered to conduct a side-by-side test of two interior paint systems with a single applica-tion of a moisture-cured urethane (MCU) organic zinc-richcoating. The coating had been recently approved by NSFInternational under standard ANSI/NSF 61 for use inpotable water tanks. Like traditional NSF 61-approvedepoxy coatings for water tank interiors, the MCU organiczinc-rich that would be tested is approved for applicationwithout a topcoat.

The goal was to differentiate between the coating systempreferences of the owner (by applying coatings frequentlyused) and the coating system performance (by comparing thefrequently used systems with the single application of zinc-

W

rich coating). According to the coating manufacturer, this testproject represented the first time a zinc-rich coating on itsown has been used in an actual in-service water tower. Forthe purpose of this study, performance was defined as thelack of blistering and peeling. Further, performance was tomean less than 10% corrosion—Rust Grade 4G—in accor-dance with SSPC-Vis 2, Standard Method of Evaluating

Editor’s Note: The field research project described here receivedan honorable mention award for Engineering Excellence at theMinnesota Section of the American Council of EngineeringCompanies. Continued

Completed test area: (left) epoxy/epoxy; (upper right) zinc-rich/epoxy/epoxy; (lower right) MCU zinc-rich

Photos courtesy of the authors

By Dan Zienty and Lee Dornbusch, Short Elliott Hendrickson Inc., St. Paul, MN, and Tony Ippoliti, The Sherwin-Williams Company, Cleveland, OH

trons that protect the steelsurface eventually exhauststhem. The anode provides theelectrons that passivate theprotected steel surfaces, mak-ing it a cathode. In the case ofa water storage tank, cathodicprotection may be providedby an impressed current sys-tem (see AWWA D104).Conceivably, lining the steelsurfaces with zinc-aluminummetalizing—a slow, veryexpensive, but long-lastingalternative—could also pro-vide passivation. One of theauthors is aware of one munic-ipality that has experienced

very good corrosion protection using such a method. The Anoka water tank evaluation revealed extensive rust

bleed and corrosion along the edge of the structural roof sup-port angles; around the compression ring; between intermit-tent welds, the upper shell stiffener ring, and roof; and atroof radial plate lap joints. These locations are notorious forpremature coating failures because it is difficult to applycoating systems in these nooks, crevices, and edges.

Condensation forms in these areas (hence, theyare called “vapor areas”) and causes prematurecoating failure, leading to rusting, flaking, orcoating delamination. Such corrosion can lead toserious structural problems and costly repairsif left unchecked. Depending on the design ofthe tank, seal welding of roof-supporting mem-bers in the vapor areas may offer a permanentyet costly solution. However, in other tanks,seal welding cannot be done because it wouldprevent the movement or expansion that hasbeen designed into such areas.

The evaluation allowed the following ques-tion to be raised: could an organic zinc coatingalone protect the roof and roof support systemfrom corrosion? Would it “sacrifice itself”—actas an anode—if not over-coated, or would ithave performance similar to other barrier-typeimmersion-grade epoxy coating systems?Barrier coatings protect against corrosion bypreventing an atmospheric or a submerged elec-trolyte—in this case water—from contactingthe substrate, thereby removing one of the fourcomponents necessary for corrosion.

Degree of Rusting on PaintedSteel Surfaces.

BackgroundUniversal corrosion theoryconfirms that for rust to formon a steel surface, an anode,cathode, electrolyte, andmetallic pathway must be pre-sent. If any of these elementsare missing, corrosion will notoccur. A steel constructionelement—column, girder,beam, or plate—is comprisedof countless grains or “cells”of steel. Some of the cells actas cathodes; others act asanodes. Their proximity toone another provides themetallic pathway needed for the transfer of electrons. Rain,snow, condensation, or potable water in a tank provides theelectrolytic component.

Corrosion is an electrochemical phenomenon. One methodto prevent corrosion is to prohibit the formation of rust byconnecting a more noble or “passive” metal (steel) to a lessnoble “active” metal. Less noble metals (zinc, in this example)act as sacrificial anodic materials because providing the elec-

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While this article addresses coatings for steel potable watertanks, it should be noted that in the late 1990’s, the EPAruled that concrete tank interiors must be constructed withNSF approved materials. Cements, aggregates, and admix-tures must all be NSF approved and, of course, clean,potable water must be used in the mixing and placing ofthese constituents. Suppliers of materials for concrete tanksthat have been tested and approved by the NSF can be foundon its website (www.nsf.org). Failure to use NSF-approvedmaterials of construction on concrete tank interiors mayresult in the concrete tank being unsuitable for the storageof potable water. Fortunately, coating interior concrete tanksurfaces with an NSF 61-approved coating or coating sys-tem can return concrete tanks to the level of safety demand-ed by the EPA (see AWWA OPFLOW).

Table 1: Systems TestedPrep

SSPC-SP 10,

Near-white

SSPC-SP 10,

Near-white

SSPC-SP 10,

Near-white

System

2-Coat Epoxy

(Meets AWWA

ICS No. 2)**

1-Coat

Organic Zinc /

2-Coat Epoxy

(Meets AWWA

ICS No. 3)

1-Coat

Organic Zinc

(Non-Standard

system)

First Coat

High-solids

epoxy (NSF

61-approved)

@4–6 mils

DFT

MCU organic

zinc-rich

coating (NSF

61-approved)

@3–5 mils

DFT

MCU organic

zinc-rich

coating (NSF

61-approved)

@3–5 mils

DFT

Intermediate

N/A

High-solids

epoxy (NSF

61-approval)

@ 4–6 mils

DFT

N/A

Topcoat

High-solids

epoxy (NSF

61-approved)

@4–6 mils

DFT

High-solids

epoxy (NSF

61-approved)

@4–6 mils

DFT

N/A

**Note: remaining interior areas of tank, not part of the test study region,protected with this coating system.

NSF and Concrete Tank Interiors

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For decades, barrier coatings containing rust-inhibitivepigments such as lead or zinc-chromate had been used asprimers under topcoat systems on the interiors of watertowers to protect steel from theeffects of corrosion; however,such pigments, which were basedon heavy metals, were laterdeemed detrimental to publichealth because of the heavy metalcontent. Rust-inhibitive coatingslimit corrosion by having theircorrosion-preventing pigmentssolubilize slightly under wet con-ditions. The solubilized pigmentsthen act to passivate the surfaceat the steel/coating interface.Manufacturers of coatings intend-ed for potable water contact havetheir coatings tested to determinewhether solvents or other toxinsleach from the coatings. No coatings with such leachates areapproved for use in today’s drinking water tanks. Further,coating system approval for each project in Minnesota is also

required by the Minnesota Department of Health.The engineering firm learned in 2002 that a new MCU

organic zinc-rich immersion-grade coating received NSFapproval. The manufacturer for-mulated the coating so that itwould receive NSF 61-approvalwith or without a topcoat.

There are practical reasons forthe manufacturer’s approach toformulation. One reason is that, ifthe interior topcoat were damagedin such a way to expose the NSF61-approved zinc-rich coating, nothreat to the public would bederived from drinking water thatwas exposed to the MCU zinc-richsystem. In the past, during thetransition to NSF 61-approvedcoatings, water tank interiors wereoccasionally Brush-Off Blast

Cleaned (SSPC-SP 7) prior to the application of “approved”coatings or coating systems. When these coatings were dam-

Continued

The Anoka water tank evaluationrevealed extensive rust bleed andcorrosion along the edge of thestructural support angles, the roofradial plate lap joints, and similarareas notorious for prematurecoating failures.

“

“

trolled test studies on the product, but had no “in-service”examples that would confirm its protective characteristicsin water towers without a protective topcoat. To produce“in-service” evidence of its product’s capabilities, and under-standing the risks to the City of Anoka, the manufactureragreed to assume full responsibility for paint, contractorrepairs, and inspection costs for coating failures (if any) inthe test areas. The inspection for failures would take place atthe end of the two-year warranty given to the owner. Theengineering firm also received approval from the MinnesotaDepartment of Health for this first-of-its-kind undertaking.

Given this win-win scenario, the Cityapproved the plan, and the engineeringfirm wrote the MCU organic zinc-richsystem and the guarantee into plansand specs that were sent to contractors.

The case study consisted of applyingthe three coating systems shown inTable 1 and Fig. 1 and comparing theirperformance. All three systems camefrom the manufacturer of the single-coat product in the test.

The City took the tank out of servicein June 2003. The specifications incor-porated a scope of work that includederecting a full-containment structureper SSPC-Guide 6, Class 2A, and com-pletely removing interior and exteriorcoatings. Though the project scheduleallowed seven weeks to complete thework, the contractor finished in fourweeks and at $7,000 under budget,allowing the tank to be returned to ser-vice earlier than anticipated.

Performance EvaluationThe engineering firm conducted a fol-low-up warranty inspection in June of2005—two years after testing thethree coating systems—including theinspection of the single-coat MCU zinc-rich. AWWA D102-03 Standard, inSection 5.2.1.General, states: “Whenspecified, the inside … surfaces of thetank shall be inspected within one yearafter coating work has been completed,to determine whether any repair workis necessary,” so this two-year warrant-ed test period provided more than suf-ficient time for any problems to surface.

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aged—or worse, if they blistered—the existing, non-NSF-approved coatings would be exposed to the stored drinkingwater, creating a potential health risk.

The Case StudyEnter the Anoka water tank repair project.

The engineering firm approached the manufacturer of theMCU zinc-rich and the City of Anoka with the idea of con-ducting a test project that included the application of thenewly NSF-approved coating on the City’s 400,000-gallontank. During development, the manufacturer conducted con-

Continued

Fig. 2: Condition of support angle at warranty inspection for MCU Zinc-rich

Fig. 3: Condition of MCU zinc-rich on roof plates at warranty inspection

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BenefitsThe initial success of the MCU zinc-richcoating, applied at 3 to 4 mils DFT, wasmeasured by the performance criteriaestablished for this test project casestudy (no blistering or peeling, less than10% corrosion per SSPC-Vis 4). Thissuccess indicated a method by whichthis water community could save thou-sands of dollars on labor, material, time,and repairs needed due to corrosion ofits storage tanks. The submerged area ofthe tank totaled approximately 9,000

square feet, a third ofwhich represented thevapor area that was usedfor the case study. A man-ufacturer’s recommenda-tions for conventionalcoatings, in vapor zones,for example, may requirea coating thicknessbetween 8 and 12 mils.The recommended appli-cation thickness for theMCU organic zinc-richtested is much less, how-ever, and there is no visi-ble evidence of unsuc-cessful coating perfor-

mance at this thickness. At anestimated cost average of$6.25 per square foot usingtraditional systems, recog-nized savings in the test areafor labor and materials wereestimated at $1.75 per squarefoot–significant savings, espe-cially if the single-coat systemperforms as long as or longerthan conventional systems.

Since the AWWA and manywater professionals recom-mend periodic inspection ofwater tanks every three tofive years, the engineeringfirm has another inspectiontentatively planned for 2009,at which time additional cor-rosion performance resultswill be measured.

The performance data discovered inthis two-year test project will be used toevaluate the longevity of the MCU zinc-rich coating as an alternative to conven-tional multi-coat coating systems forvapor zone renovation.

Future ValueAdopting a new coating technology oremploying an existing coating in a newenvironment may require owners orengineers to adjust their thinking, butnot necessarily to accept greater risks.The engineering firm successfully imple-mented this test project case studywithout any risk to the client or to thesafety of their drinking water supply byproviding the coating manufacturer theopportunity to assume responsibilityfor a product in which it had great con-fidence yet limited in-service statistics.

Overcoming ClientPreferences/Exceeding Owner

Performance“We had complete trust and confidencein SEH,” City of Anoka Public WorksDirector Craig Gray said. “Inspectorsfollowed the entire project, and safetytests were conducted before the tank

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At the engineering firm’s invitation, rep-resentatives of the coating manufactur-er participated in the inspection. Theinspection revealed no significant fail-ures of the single coat MCU organiczinc-rich system when compared withthe two more traditional systems (Figs.2 and 3). The only notable failuresoccurred over a manway cover,amounting to less than 1⁄10 of 1% of thetotal test area. The failure was traced toan application error, not to the productitself (Fig. 4).

Fig. 4: At the warranty inspection, the only notable failures were over a manway cover and were traced to an application error.

Adopting a new coating technologyor employing an existing coatingin a new environment may requireowners or engineers to adjusttheir thinking but not necessarilyto accept greater risks.

“ “

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was put back into service to ensure pub-lic safety. But most of all, we neededassurances that if something wentwrong I did not have to go in front ofour City Council and ask for another$100,000 to redo the job, and the agree-ment [the engineering firm] negotiatedwith the paint manufacturer to cover alllabor and materials gave us the securityand protection we needed to proceed.”

“Really, our only concern was meet-ing the project timetable, as we had tohave the tank back on line by August

when water demand is greatest. Thoughwe set a seven-week schedule, the pro-ject was done three weeks early, and thetank was back on line the first part ofJuly. The project just went very smooth-ly, and shows the potential that existsfor municipalities to substantially saveon labor and materials involved in

water tank painting and/or rehabilita-tion.”

The single-coat product SEH testedwas Sherwin-Williams’ Corothane IGalvapac, B65 Series. ClassicProtective Coatings applied the coatingsfor the testing.

Dan Zienty is a senior pro-fessional specialist forShort Elliott HendricksonInc. (SEH), a multidisci-pline, single-source con-

sulting firm of engineers, architects, plan-ners, and scientists with offices through-out the Upper Midwest and mountainregions (www.sehinc.com). He is basedin St. Paul, MN.

Tony Ippoliti of TheS h e r w i n - W i l l i a m sCompany, Industrial &Marine Coatings, is asenior corrosion specifi-

cation specialist based in Indianapolis,Indiana. He prepares protective coatingand lining specifications for engineeringfirms, steel fabricators, electric powergenerators, and water/wastewater facili-ties in the Midwestern U.S. He also per-forms corrosion surveys and assists inthe evaluation of existing protective coat-ing and lining systems. He has workedfor Sherwin-Williams since 1976. He isactive in SSPC, STI/SPFA, NACE, andAWWA.

Lee Dornbush is also asenior professional spe-cialist at Short ElliotHendrickson. He is anSSPC-certified ProtectiveCoating Specialist.

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