Control and Treatment of Hot-Dip Galvanize Surfaces

1. Control and Treatment of Hot-Dip Galvanize Surfaces Presented at the 97th Meeting of the Galvanizers Association October 16-19, 2005 Lexington, KY

2. GalvInfo Center A zinc-coated steel sheet technical information center managed by ILZRO and cosponsored by the steel, paint and zinc industries.

3. Galvanize Surfaces Hot-dip galvanize vary in appearance, and therefore usability, because of: • spangle size • zinc composition • mechanical treatment • chemical or oiling treatments. Marketplace problems develop because customers find the surface is: • different in appearance than expected • affected by darkening, water staining, or field handling marks • covered with unwanted substances that affect the ability to pretreat, paint, or weld 

4. Reasons forSurface Treatments • Improved surface uniformity • Resistance to storage stain • Improved adhesion/corrosion resistance of prepainted steels • Surface lubrication for forming • Resistance to handling marks • Preparing for field painting

5. Improving Surface Uniformity • Temper passing gives a more uniform and duller surface topography • Improves painted appearance • Typical TM is 4 Hi using several hundred tons of rolling force • Percent extension can be up to 2%

6. Improving Surface Uniformity • Temper rolling hides and smoothes spangle • Improves painted appearance by controlling surface roughness – i.e., average roughness (peak height) versus peaks-per-inch • critical for exposed automotive surfaces needing a high DOI after painting.

7. Spangle - Regular • Dendrite growth dominates during solidification • Spangle or grain boundaries are “depressed” • Difficult to smooth by temper passing • Form 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

8. Spangle-free • Absence of Pb results in grains growing by a cellular mode • Grains are ~ 0.5 mm across - barely visible to unaided eye • Grain boundaries are very flat • Very easy to smooth by temper passing • Satisfies the need for a smooth surface and environmental concerns

9. Galvanneal • Produced by reheating to about 590°C for 10-15 seconds to convert zinc to zinc-iron alloy • Converts 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 “lock” • Temper passed to control the topography • Extensively used for high quality auto exposed finishes

10. Improving Resistance to Storage Stain • Chemical Treatments – Cr Based • Premature Spangle Darkening • Tests for passivation • Chemical Treatments – Non Cr • Removable and Permanent • RoHS issues • Oils

11. Chemical Treatments • For decades these treatments have been based on chromium solutions • Primary purpose is to reduce the susceptibility of metallic-coated sheet to storage stain (white rust)

12. Storage Stain – “White Rust” • Corrosion stain – typically white zinc hydroxide – that forms between sheets in close contact that become wet • Zinc hydroxide forms in the absence of free air flow • Can be grey, or black in color if enough zinc is consumed to allow iron to become involved • Light white will abate aver time if allowed to weather

13. Storage Stain on Galvanneal • Stain that forms on water damaged galvanneal is grey or black • Dark nature of the stain is the result of the iron in the coating • Can form easily on galvanneal as much of it is produced as unpassivated

14. Chromium BasedChemical Treatments • Use chromic acid, chromium salts and mineral acids • Dissolves some of the metal and forms a protective film of complex chromium and metal compounds • Usually thin (<0.1 mm) and invisible but have yellow or green tinge if applied heavier (0.1-0.6 mm) • Total Cr 1-2 mg/ft2, with < 50% Cr+6 in complex mixture of metal salts and oxides

15. Chromium BasedChemical Treatments • Galvanize has surface layer of Al2O3 (tens of nanometers) that must be removed • CT solution must dissolve Al2O3 layer with fluoride to allow deposition of Cr compounds • Zinc protected via barrier and passivation effects • Cr oxide acts a barrier • Cr+6 re-passivates exposed metal • Cr+6 is reason for the self-healing ability of chromate passivation films

16. Effectiveness of Cr Based Passivation • Resistance to staining varies as a function of the accelerated test used • Illustrates that oxygen may play a role in stain formation • Condensation test open to air • Water-film test isolated from air • Obvious that zinc has almost no resistance to staining in the absence of Cr on the surface

17. Premature Spangle Darkening • Can occur after a few days of exposure • Reported characteristics • Only in rural environments • Occurs within one week of installation and only on outside • Some sheets remain bright • Some spangles darken more than others • Appears related to spangle forming elements (Pb & Sb) • Not reported on spangle-free coatings

18. Some Disadvantages ofCr Passivation • Paintability • Decrease the adhesion of most paints to zinc • Severely interfere with the deposition of iron and zinc phosphate treatments • For painting it is best to produce unpassivated sheet • Weldability • Interferes with spot weldability by “poisoning” copper alloy welding electrodes – shortens electrode life • Use only unpassivated sheet

19. Is It Passivated? • Usually not possible to visually determine • Producers use lab testing methods not available in field • Field tests: • %5 HCL – drop will “fizz” on unpassivated • Diphenylcarbohydrazide – drops turns pink if Cr+6 present – ASTM D 6492 • Use quick condensing humidity test – 140°F water in beaker – test coupon as lid for 10-15 minutes

20. Condensing Humidity Test

21. Non-Chrome Treatments • Alternatives being sought because of environmental concerns, e.g., RoHS • Removable non-chrome treatments available now • Permanent non-chrome treatments under very active development

22. RoHS • Article 4(1) of Directive 2002/95/c of the European Parliament on the Restriction of certain Hazardous Substances in electronic equipment • From July 1, 2006n new equipment cannot contain: • Lead • Mercury • Cadmium • Hexavalent Cr • PBB and PBDE flame retardants • Request to exempt Cr+6 not yet ruled on

23. Oils • Used sometimes as an alternative to passivation • Specially formulated – contain polar products that adsorb onto metal surfaces • Effective in protecting against humidity rust - prevent moisture condensing between contacting sheet surfaces • Not effective in preventing penetration of bulk water – staining will occur quickly if this happens • Used for prepaint products – can be cleaned off • Provide lubrication during forming

24. Pretreatments • Used to obtain good bonding between the metal surface and paint • Phosphate treatments • Zinc phosphate • Iron Phosphate • Chromate conversion treatments

25. Zinc Phosphate • Widely used • Final treatment on galvanize lines as base for field painting • Pretreatment on coil prepainting lines • Post fabrication factory painting lines • Automotive – treating of entire body-in-white • Applied via spray and dip method • Zinc phosphate crystals provide an excellent surface for paint bonding and resist disbondment in corrosive atmospheres

26. Zinc Phosphate • Several 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 phosphate • Paint bonding is by: • Mechanical keying – similar to galvanneal- micro porous • Oxygen in film promotes chemical hydrogen bonding with the paint

27. Zinc Phosphate • Experience has shown ZnP is effective in reducing paint undercutting corrosion • Particularly 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.

28. “Bonderized” Steel • Zinc phosphate treated on galvanize line • Intended to be field painted with good paint adhesion • Being used in some locales with the intent of being left unpainted – as shown here

29. “Bonderized” Steel • Some producers offer Bonderized sheet with a clear or tinted lacquer coating for added durability • Low lustre appearance is an architectural look preferred in some areas

30. Chromate Conversion Pretreatments • Yellow to brown – contain complex oxides • Thicker than passivation treatments – 0.5-3 mm • Used on Zn and AlZn coatings to enhance the corrosion resistance of prepainted sheet • Applied using tank/spray or roll coaters (DIP) • Galvanize must be unpassivated • Contain both Cr+3 and Cr+6, thus RoHS is a concern • Less resistance to paint undercutting than ZnP

31. Surface Lubrication • Provides lubricity to forming and stamping operations – prevents galling, scratching, fracturing • Typically applied with electrostatic oilers • Types: • Mineral “slushing” oils (most contain rust inhibitors) • Vanishing oils (high volatile content) • Dry lubricants • Dry film lubricants (typically water-borne, applied on coating line)

32. Fingerprinting & Handling Marks • Salt in perspiration causes permanent white stains on galvanize – even if passivated • AlZn coatings subject to roll forming and handling marks appearing as black smudges • Clear acrylic coatings applied to resist marking – may also contain Cr • Some are paintable and if not painted will dissipate • Others are not paintable and can last for years

33. Field Painting • Difficult to achieve adherence on passivated galvanize • Options: • Weather for 12 – 18 months • Consider proprietary pretreatment solutions • Light sanding may be an option • Ensure surface is clean and dry (water break-free) • Use paint designed for bonding to zinc

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

35. Summary • Many surface treatments in use • Bath chemistry influences appearance and performance • Mechanical treatment aimed at appearance • Many treatments involve application of carefully formulated chemicals to: • Protect from water damage • Improve corrosion resistance • Prepare for painting • Assist in metal forming • Alter the appearance