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coating had cracked and peeled. The corrosion productsunder the coating film were typically soft and pasty. Exposedcorrosion products easily crumbled and contained zinc andiron. Examples of the conditions, shown in Figs. 1, 2, and 4,demonstrate the degree of failure and that these failures wereclearly visible to the traveling public. Several surface condi-tions are shown in Figs. 3, 5 (p. 22), 6 (p. 22), and 7 (p. 23).

Factors in the AnalysisUnderstanding how hot-dip galvanizing corrodes and how toprevent corrosion was essential to the analysis.

Hot-dip galvanizing of steel results in a shiny metallic zincsurface and several alloyed layers with the lower layers hav-ing decreasing zinc content. The finish is often spangled as aresult of alloy and cooling properties, but spangling becomesless obvious as the surface weathers and changes color.1

From the time steel is removed from the molten zinc bath,the zinc surface can be classified into one of three agingstages: newly galvanized, partially weathered, or weathered.

Newly galvanized surfaces are essentially zinc metal.However, zinc is relatively reactive, and chemical changesoccur quickly at the surface. Zinc reacts with oxygen in theair, forming zinc oxide, ZnO. Thus, even newly galvanizedstructures will have some ZnO present on the surface. Overtime, a surface reaction between moisture and ZnO formszinc hydroxide, Zn(OH)2. Zn(OH)2 reacts with carbon diox-ide (CO2) in the air to form basic zinc carbonate (ZnCO3).

A partially weathered zinc surface consists primarily of

he next time you pull up to an intersection withtraffic signals look for the mast arms that supportthem. Are the arms galvanized, concrete, alu-

minum, or painted? Painted mast arms may have had thecoating system applied directly to blast cleaned steel or overhot-dip galvanized steel (a duplex system). This article dis-cusses the failure of coatings applied to galvanized mast armsfor the Florida Department of Transportation (FDOT); analy-sis of the failure, including key background information ongalvanizing; the findings; and the approach FDOT imple-mented to correct failing galvanized mast arms exhibitingcorrosion.

Florida ExperienceFDOT identified 60 coated and galvanized steel mast arms inthe Jacksonville, FL, area as having paint (coating) peelingfrom galvanized steel. Consultants were engaged to investi-gate the failures in 2004. Nearly two-thirds of the mast armsunder investigation were in service for only two years.Records indicated that six of the failed mast arms wereincluded in construction projects from 1997 through 2001.

The InvestigationClearly, the performance was significantly less than expect-ed. Initial examination of the mast arms in November 2004found that the failed coating systems had intact blisters filledwith corrosion products (without free moisture) and thesame type of corrosion products were present where the

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

Coating Failures on Galvanized Mast ArmsBy Paul Vinik, MSChE, P.E., Florida Department of Transportation

and Richard A. Burgess, B.S, M.S, KTA-Tator, Inc.

T

Fig 1: Coating peeling from the post and horizontal arm of a galvanized mastarm. The exposed surfaces contain white and red corrosion products.

Photos courtesy of the authorsFig 2: Peeling coating on the underside of the horizontal section of a

galvanized mast arm.

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Continued

zinc oxide and zinc hydroxide with lesser amounts of zinccarbonate. The zinc oxide and hydroxide yield porous filmsthat provide little protectionto the zinc surface. ZnO andZn(OH)2 continue to form aslong as oxygen and moistureare available. Zinc carbonatecontent in the film typicallyincreases until the film coversthe surface, and the surfacefilm takes on a dull gray color.

Depending on climate andexposure, the full weatheringprocess can take from sixmonths to two years. Zinc car-bonate, insoluble in water,forms a very protective film atthe surface. Even so, the galva-nizing can still corrode.Airborne pollutants, particularly sulfur dioxide (SO2), thatcontribute to acidic moisture can dissolve the ZnCO3 layer,exposing metallic zinc and allowing corrosion to continue.Consequently, galvanizing performs best under neutralatmospheric conditions. Even so, the zinc-rich surface willeventually be depleted in aggressive environments in as littleas a month but can last for decades in mild environments. Asthe zinc surface layer is lost, the zinc-iron alloy layers will beexposed and corrode. The corrosion products change color,from white-gray to a pale rust, and then to deeper shades ofrust, as the iron (steel) substrate is exposed.

Coating GalvanizingOrganic coatings are used to protect galvanizing and steelfrom aggressive environments and to provide color as wellas extend service life in milder environments. Coating man-ufacturers and trade associations such as the American

Galvanizers Association (AGA),2 SSPC: The Society forProtective Coatings (SSPC),3 and NACE International

(NACE)4 provide guidance forpreparing and coating galva-nized structures. ASTMInternational5 (ASTM) address-es the use of coating to repairnew galvanizing in ASTM A780, Standard Practice forRepair of Damaged andUncoated Areas of Hot-DipGalvanized Coatings, andASTM D 6386, StandardPractice for Preparation of Zinc(Hot-Dip Galvanized) CoatedIron and Steel Product andHardware Surfaces forPainting. The latter documentincludes recommended prac-

tices for preparing new galvanizing, partially weathered gal-vanizing, and weathered galvanizing.

Surface PreparationRemoving grease, oil, and dirt is necessary before paintingany surface, including galvanizing. Additional requirementsfor galvanizing include avoiding water quenching and chro-mate treatment to protect against white storage stain, whichfrequently follow the galvanizing process.6 If these practicescannot be avoided, the surface films they create must beremoved because they inhibit coating adhesion. Surfacesmoothing of the galvanizing is also necessary, particularlywhere liquid zinc run-off or other galvanizing byproducts arepresent. Otherwise, the rough surface texture, like roughwelds on steel, contributes to discontinuities in the subse-quently applied coating film. Further, unlike steel wheretightly adhered rust (iron oxides) may be allowed to remain,7

the oxide and hydroxide corrosion products of zinc are weakand loosely adherent. They must be removed physicallyand/or by surface treatments (chemical etching, conversioncoating) before applying organic coatings.

Newly galvanized steel (for painting purposes) is defined asnot having received a surface treatment after galvanizing andthe galvanizing occurred within 48 hours.8 Even so, steps arerecommended to ensure zinc corrosion products (ZnO andZn(OH)2) are removed from the surface. When abrasive blast

Fig 3: Appearance of failing coating on the side of a horizontal arm.

Fig 4: A close view of the surface beneath peeling paint on the underside ofthe mast arm in Fig. 2. The galvanizing is depleted and the steel substrate is

corroding.

Editor’s Note: This article is based on a paper the authors gaveat PACE 2008, January 27-28, Los Angeles, CA. PACE is thejoint conference of SSPC: The Society for Protective Coatingsand the Painting and Decorating Contractors of America.

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cleaning is used to texture the surfaceand remove the corrosion products,coating application should occur within60 minutes if conditions favor forma-tion of zinc salts. Partially weatheredgalvanizing should be evaluated for thepresence of a (chromate) surface treat-ment and wet storage stain (white zincsalts consisting primarily of Zn(OH)2).Once these issues are addressed, thepartially weathered steel can be pre-pared in the same manner as newly gal-

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

vanized steel—again with considerationof conditions favoring zinc corrosion.On the other hand, a weathered basicZnCO3 surface is suitable for paintingwithout extensive preparation provid-ed other interference materials areremoved.

Coatings and ApplicationThe most commonly used coating tech-niques for hot-dip galvanized mast armsare spray and electrostatic powder

coating or brush and roller applicationof liquid coating and fluidized bed pow-der coating may also be used. Manygeneric coating types can be successful-ly applied to properly prepared galva-nized steel.

Some resins, however, such as alkyds,oils and epoxy esters are not suitablebecause they may react with the alka-line salts that form on zinc surfaces.While often quite adequate in a dryenvironment, these coatings are quitesusceptible to failure in wet, damp, orhumid environments.

Polyamide epoxy is probably themost popular industrial coating applieddirectly to newly hot dip galvanizing.Once primed, the surface can be over-coated with a primer-compatible finishcoat, commonly polyurethane. Powdercoatings include epoxy, polyurethane,and polyesters. Use of surface treat-ments such as conversion coatings maybe recommended by the coating manu-facturers. The performance of a duplexsystem is generally understood to berelated to isolating the reactive zincmetal surface from the environment.Yet, the combination of coating systemdry film thickness (DFT), the specificproduct formulations selected, andoverall system permeability may notyet have had sufficient discussion andmay warrant further investigation.9

Weathered galvanizing is more easilycoated than new or partially weatheredgalvanizing. Some coating manufacturerdata sheets recommend allowing galva-nized steel to weather for several

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Fig 5: An intact coating blister on a galvanized mast arm. No free water waspresent.

Fig 6: Corrosion products found beneath the intact blister (Fig. 5) includedlight red rust stain.

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months before coating because at thisstage, application of coatings to galva-nizing carries the lowest risk, but thesubstrate does require some degree ofcleaning. Unfortunately, it is commonlynot the most practical time. Weatheredand aged galvanized steel exhibiting redrust is commonly coated with a zinc-richorganic coating.

Florida Requirements for the MastsFDOT 2004 Standard Specification forRoad and Bridge Construction(Standard Specification) Section 649—Steel Stain Poles, Steel Mast Arm andMonotube Assemblies—requires galva-nizing components in accordance withASTM A 123, Standard Specificationfor Zinc (Hot-Dip Galvanized) Coatingson Iron and Steel Products, using meth-ods suitable for painting. Surface prepa-ration of all galvanized surfaces is to bedone in accordance with ASTM D 6386followed by solvent wiping in accor-dance with SSPC-SP 1, SolventCleaning, and application of either atwo-coat liquid paint system or a pow-der coat system.

The specified liquid paint system is anepoxy primer (4.0 to 6.0 mils DFT) andaliphatic polyurethane finish (2.0 to 4.0mils DFT.) The electrostatically appliedurethane or triglycidyl isocyanurate(TGIC) polyester powder coat systemrequires a minimum DFT of 2.0 mils.The powder is to be cured by heating thecoated structure to 350 F to 400 F.

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Fig 7: Shaving through the corrosion product tothe substrate indicates rust stain originates in

spots at the steel substrate.

Thus, it appears the galvanizing processitself may have been contributory tothe exterior coating failure.

The findings in the Jacksonville mastpole coating failure investigation sug-gest a systematic, process problem wasthe root cause.

FDOT Approach to ResolutionIssues and Economic ConsiderationsFDOT inspected mast arm structures inJacksonville and Orlando, FL, inSeptember 2004 and reported an esti-mated failure rate of 15 to 20%. Basedon the September inspections andNovember 2004 investigations, it wasestimated that up to 3,000 of the rough-ly 15,000 coated galvanized mast armsin the state may have failed. A series ofimportant issues was identified whiledeveloping an approach to resolving theproblem with existing structures andfuture structures.

from aggressive blast cleaning. An addi-tional indication of inadequate surfacepreparation was high zinc edges fromdrip lines and zinc runoff.

The applied coatings consisted of apolyamidoamine epoxy primer andaliphatic acrylic polyurethane finishcoat. Intercoat adhesion was good andsystem thickness was generally withinthe specification requirements. The top-coat appeared to be free of defects.However, the texture of the back sur-face of the primer suggested that abra-sive blast cleaning may have been per-formed but was not uniform.

Quality control data from the galva-nizing process was not readily availablefor review so a retrospective evaluationof the exterior galvanized surfaces wasnot fruitful. However, boroscope exam-inations of the unpainted interior sur-faces found the galvanizing was not pre-venting corrosion of the steel substrate.

Findings of the InvestigationThe finding of zinc corrosion productsindicates the cause of failure was relat-ed to inadequate surface preparation,which left zinc oxide and hydroxides onthe galvanized surface. Zinc corrosionproducts are voluminous, as much as500 times that of the zinc metal con-sumed. Thus, even though other surfacecontaminants can also compromiseadhesion, the development of large andirregular blisters filled with zinc corro-sion products (Figs. 5 and 6, p. 22)showed an ongoing corrosion process,not an interfering bond-breaker.

Figure 7 (p. 23) shows the surfacebeneath coating removed adjacent topeeling paint. The spots of pinpoint redrust among the white corrosion prod-ucts indicate that lower, iron-containinglayers of galvanizing and/or the steelsubstrate were corroding. The patternwas consistent with overblast damage

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

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• Risk of catastrophic failure• Risk to the motoring public• Defining failure and degree of failure• Impact on mast arm service life• Resources required to identify failedunits• Remediation of failed units• Interfacing with vendors to eliminatefuture failures on new structures

The potential economic impact of gal-vanized mast arm coating failures is sig-nificant, not only to the State of Floridabut to the vendors as well. Each of themast arms represents an investment of$50,000—a potential collective finan-cial impact of up to $150 million.

Currently, any structures failing inless than 820 days fall under the latentdefect clause of the standard specifica-tions and must be repaired or replacedby the contractor. Structures older than820 days are being handled on a struc-ture-by-structure basis.

FDOT implemented a revised Section649 of the Standard Specifications in Juneof 2007 to address future structures. Thisrevised specification has no new directinstructions regarding materials or meth-ods to paint a structure. However, it doesrequire that the contractor designate a“responsible party” for the adhesion andcolor retention of the structures. Theresponsible partymust also provide a five-year warranty that starts at project finalacceptance and covers any repairsrequired for adhesion or color. The respon-sible party canbe the contractor or the fab-ricator. When the responsible party is thefabricator, it must be pre-approved by theDepartment and be listed on the Pre-quali-fied Fabricators of Painted GalvanizedSteel Strain Poles, Monotube, and MastArm Assemblies List. In addition, the fab-ricator must provide an annual bondbased on the number of structures provid-ed to FDOT the previous year.

Technical ConsiderationsThe immediate technical considerationsincluded identifying existing failed poles

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and the procedures/protocols for reme-diation and establishing performancecriteria to differentiate failing from non-failing poles.

Options for remediation of existingmast arms could range from in-situcleaning and repainting to replacement.A statewide repair procedure for cor-recting failing structures, summarizedbelow, was issued in April 2006.• Identify failed mast arms by the pres-ence of failed coating.• Inspect the interior surface for steelcorrosion.

• Replace the structure if interior cor-rosion is present.

• Repaint the structure if interior cor-rosion is not present.• Repaint the structure in the field or ina shop as follows:

• Pressure wash with 5000 psi• Abrasive blast SSPC-SP 10 (excep-

tion: leave good zinc)• Apply organic zinc primer and poly-

siloxane finish per manufacturer’srequirements

Changes made to prevent future fail-ures included implementing a revisedqualified product list (QPL) in April2007. Standard Specification Section649 revision established color and coat-ing adherence requirements and prepa-ration of coupon standards.

Briefly, a coating failure has occurredwhen color degradation is greater than8∆E’ or coating delamination is greaterthan 100 square inches.

ConclusionVendor resistance to the 5-year bondingrequirement was anticipated, but ven-dors have elected to participate. Thereare clear incentives for them to improvequality control to minimize the numberof mast arm coating failures that occur.This is expected to increase the unitcost to the state to some degree. The fullimpact of these changes is not fully

known at this time. However, it isexpected that the state will receive animproved product, and the industry ingeneral will likewise benefit.

Notes1. Surface treatments may be used to

prevent discoloration but are notrecommended when galvanizing willbe painted.

2. American Galvanizers Association6881 South Holly Circle, Suite 108,Centennial, CO 80112.

3. SSPC: The Society for ProtectiveCoatings 40 24th Street, 6th FloorPittsburgh, PA 15222-4656.

4. NACE International, 1440 SouthCreek Drive Houston, TX 77084-4906.

5. ASTM International, 100 BarrHarbor Drive, PO Box C700, WestConshohocken, PA 19428-2959.

6. The galvanizer should be advised thatthe pieces are to be painted.

7. SSPC Surface Preparation Standardssuch as SSPC-SP 2- Hand ToolCleaning, SSPC-SP 3-Power ToolCleaning, SSPC-SP 7- Brush Off BlastCleaning.

8. ASTM D 6386, Section 5.1.9. Mark B. Dromgool PCS Managing

Director, KTA-Tator Australia PtyLtd, Personal communication.

Richard Burgess, a

senior coatings con-

sultant with KTA-

Tator, Inc. (Pittsburgh,

PA), is the editor of

the F-Files series.

Paul Vinik, state chemical materials engi-

neer at FDOT’s State Materials Research

Park, works in Gainesville, FL. He is an

instructor for SSPC’s BCI, C1, and C2

courses.

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