Control and Treatment of Hot-Dip Galvanize SurfacesPresented at the 97th Meeting of theGalvanizers AssociationOctober 16-19, 2005Lexington, KY

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Galvanize SurfacesHot-dip galvanize vary in appearance, and therefore usability, because of:spangle sizezinc compositionmechanical treatmentchemical or oiling treatments.Marketplace problems develop because customers find the surface is:different in appearance than expectedaffected by darkening, water staining, or field handling markscovered with unwanted substances that affect the ability to pretreat, paint, or weld

Reasons forSurface TreatmentsImproved surface uniformityResistance to storage stainImproved adhesion/corrosion resistance of prepainted steelsSurface lubrication for formingResistance to handling marksPreparing for field painting

Improving Surface UniformityTemper passing gives a more uniform and duller surface topographyImproves painted appearanceTypical TM is 4 Hi using several hundred tons of rolling forcePercent extension can be up to 2%

Improving Surface UniformityTemper rolling hides and smoothes spangleImproves painted appearance by controlling surface roughness i.e., average roughness (peak height) versus peaks-per-inchcritical for exposed automotive surfaces needing a high DOI after painting.

Spangle - RegularDendrite growth dominates during solidificationSpangle or grain boundaries are depressedDifficult to smooth by temper passingForm due to impurities in the zinc, historically Pb content in the range of 0.05 to 0.1/0.15%Removal of Pb causes spangle to disappear

Spangle-freeAbsence of Pb results in grains growing by a cellular modeGrains are ~ 0.5 mm across - barely visible to unaided eyeGrain boundaries are very flatVery easy to smooth by temper passingSatisfies the need for a smooth surface and environmental concerns

GalvannealProduced by reheating to about 590C for 10-15 seconds to convert zinc to zinc-iron alloyConverts the appearance to matte grey and has a surface that results in very good paint adhesion needlelike crystals into which the pretreatment and/or paint can lockTemper passed to control the topographyExtensively used for high quality auto exposed finishes

Improving Resistance to Storage StainChemical Treatments Cr BasedPremature Spangle DarkeningTests for passivationChemical Treatments Non CrRemovable and PermanentRoHS issuesOils

Chemical TreatmentsFor decades these treatments have been based on chromium solutionsPrimary purpose is to reduce the susceptibility of metallic-coated sheet to storage stain (white rust)

Storage Stain White RustCorrosion stain typically white zinc hydroxide that forms between sheets in close contact that become wetZinc hydroxide forms in the absence of free air flowCan be grey, or black in color if enough zinc is consumed to allow iron to become involvedLight white will abate aver time if allowed to weather

Storage Stain on GalvannealStain that forms on water damaged galvanneal is grey or blackDark nature of the stain is the result of the iron in the coatingCan form easily on galvanneal as much of it is produced as unpassivated

Chromium BasedChemical TreatmentsUse chromic acid, chromium salts and mineral acidsDissolves some of the metal and forms a protective film of complex chromium and metal compoundsUsually thin (

Chromium BasedChemical TreatmentsGalvanize has surface layer of Al2O3 (tens of nanometers) that must be removedCT solution must dissolve Al2O3 layer with fluoride to allow deposition of Cr compoundsZinc protected via barrier and passivation effectsCr oxide acts a barrierCr+6 re-passivates exposed metalCr+6 is reason for the self-healing ability of chromate passivation films

Effectiveness of Cr Based PassivationResistance to staining varies as a function of the accelerated test usedIllustrates that oxygen may play a role in stain formation Condensation test open to airWater-film test isolated from airObvious that zinc has almost no resistance to staining in the absence of Cr on the surface

Premature Spangle DarkeningCan occur after a few days of exposureReported characteristicsOnly in rural environmentsOccurs within one week of installation and only on outsideSome sheets remain brightSome spangles darken more than othersAppears related to spangle forming elements (Pb & Sb)Not reported on spangle-free coatings

Some Disadvantages ofCr PassivationPaintabilityDecrease the adhesion of most paints to zincSeverely interfere with the deposition of iron and zinc phosphate treatmentsFor painting it is best to produce unpassivated sheetWeldabilityInterferes with spot weldability by poisoning copper alloy welding electrodes shortens electrode lifeUse only unpassivated sheet

Is It Passivated?Usually not possible to visually determineProducers use lab testing methods not available in fieldField tests:%5 HCL drop will fizz on unpassivatedDiphenylcarbohydrazide drops turns pink if Cr+6 present ASTM D 6492Use quick condensing humidity test 140F water in beaker test coupon as lid for 10-15 minutes

Condensing Humidity Test

Non-Chrome TreatmentsAlternatives being sought because of environmental concerns, e.g., RoHSRemovable non-chrome treatments available nowPermanent non-chrome treatments under very active development

RoHSArticle 4(1) of Directive 2002/95/c of the European Parliament on the Restriction of certain Hazardous Substances in electronic equipmentFrom July 1, 2006n new equipment cannot contain:LeadMercuryCadmiumHexavalent CrPBB and PBDE flame retardantsRequest to exempt Cr+6 not yet ruled on

OilsUsed sometimes as an alternative to passivationSpecially formulated contain polar products that adsorb onto metal surfaces Effective in protecting against humidity rust - prevent moisture condensing between contacting sheet surfacesNot effective in preventing penetration of bulk water staining will occur quickly if this happensUsed for prepaint products can be cleaned offProvide lubrication during forming

PretreatmentsUsed to obtain good bonding between the metal surface and paintPhosphate treatmentsZinc phosphateIron PhosphateChromate conversion treatments

Zinc PhosphateWidely usedFinal treatment on galvanize lines as base for field paintingPretreatment on coil prepainting linesPost fabrication factory painting linesAutomotive treating of entire body-in-whiteApplied via spray and dip methodZinc phosphate crystals provide an excellent surface for paint bonding and resist disbondment in corrosive atmospheres

Zinc PhosphateSeveral steps required including: cleaning, rinsing, surface activation, ZnP application, rinse, and often a sealing step (Cr or non Cr bearing)Key reaction involves an increase in the pH at the surface, resulting in precipitation and deposition of insoluble zinc phosphatePaint bonding is by:Mechanical keying similar to galvanneal- micro porousOxygen in film promotes chemical hydrogen bonding with the paint

Zinc PhosphateExperience has shown ZnP is effective in reducing paint undercutting corrosionParticularly effective with coatings containing high iron, i.e., galvanneal. May be a result of the superior bond formed. Automotive body panels made with ZnP treated 45A45A coatings have excellent corrosion resistance.

Bonderized SteelZinc phosphate treated on galvanize lineIntended to be field painted with good paint adhesionBeing used in some locales with the intent of being left unpainted as shown here

Bonderized SteelSome producers offer Bonderized sheet with a clear or tinted lacquer coating for added durabilityLow lustre appearance is an architectural look preferred in some areas

Chromate Conversion PretreatmentsYellow to brown contain complex oxidesThicker than passivation treatments 0.5-3 mmUsed on Zn and AlZn coatings to enhance the corrosion resistance of prepainted sheetApplied using tank/spray or roll coaters (DIP)Galvanize must be unpassivatedContain both Cr+3 and Cr+6, thus RoHS is a concernLess resistance to paint undercutting than ZnP

Surface LubricationProvides lubricity to forming and stamping operations prevents galling, scratching, fracturingTypically applied with electrostatic oilersTypes:Mineral slushing oils (most contain rust inhibitors)Vanishing oils (high volatile content)Dry lubricantsDry film lubricants (typically water-borne, applied on coating line)

Fingerprinting & Handling MarksSalt in perspiration causes permanent white stains on galvanize even if passivatedAlZn coatings subject to roll forming and handling marks appearing as black smudgesClear acrylic coatings applied to resist marking may also contain CrSome are paintable and if not painted will dissipate Others are not paintable and can last for years

Field PaintingDifficult to achieve adherence on passivated galvanizeOptions:Weather for 12 18 monthsConsider proprietary pretreatment solutionsLight sanding may be an optionEnsure surface is clean and dry (water break-free)Use paint designed for bonding to zinc

Dulling the SurfaceSome users desire or are mandated to have a dull surface (max reflectivity index of 0.35)If known beforehand, order temper passed galvanizeCommercial cleaning products containing small amounts of hydrochloric and/or phosphoric acid will remove the sheen

SummaryMany surface treatments in useBath chemistry influences appearance and performanceMechanical treatment aimed at appearanceMany treatments involve application of carefully formulated chemicals to:Protect from water damageImprove corrosion resistancePrepare for paintingAssist in metal formingAlter the appearance