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

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Presentation on theme: "Control and Treatment of Hot-Dip Galvanize Surfaces Presented at the 97 th Meeting of the Galvanizers Association October 16-19, 2005 Lexington, KY."— Presentation transcript:

1 Control and Treatment of Hot-Dip Galvanize Surfaces Presented at the 97 th 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 for Surface 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 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 Based Chemical 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  m) and invisible but have yellow or green tinge if applied heavier (  m)  Total Cr 1-2 mg/ft 2, with < 50% Cr +6 in complex mixture of metal salts and oxides

15 Chromium Based Chemical Treatments  Galvanize has surface layer of Al 2 O 3 (tens of nanometers) that must be removed  CT solution must dissolve Al 2 O 3 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 of Cr 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 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 –  m  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


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