Chromium problems

21 de Nov de 2008

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Chromium problems

  1. Ole Øystein Knudsen, Astrid Bjørgum SINTEF Materials Technology Materials Technology 1
  2. n Part of a wider ongoing research project financed by the Research Council and Norwegian light metals industry: Light Metal Surface Science n Participating companies n Hydro Aluminium n Elektro-Vakuum n Noral Lighting n Norsk Industrilakkering n Profil-Lakkering n DuPont Powder Coatings n Jotun Powder Coatings Materials Technology 2
  3. n Chromium - why …. and why not? n Properties and problems n Regulation - EU and Norwegian n Chromium free pre-treatments n Redox reactions with precipitation n pH controlled precipitation on the aluminium surface n Molecules that deposit on the aluminium surface n Strengthening the aluminium oxide Materials Technology 3
  4. n Focus in this presentation: n Treatments for aluminium n Commercially available processes n Emphasis for each process: n Basic principles for the formation of the coating n Process / production friendliness n Experiences Materials Technology 4
  5. Why …. and why not? Materials Technology 5
  6. n First patented in 1923 n Used extensively since for pre- treatment before coating, adhesive bonding and surface finishing n 43 000 tons of chromium was used in metal finishing operations in 1993 Materials Technology 6
  7. n The chemicals contain: n Hexavalent chromium (CrO3 or CrO42- or Cr2O72-) n Hydrofluoric acid (HF) n The hydrofluoric acid removes the oxide film on the surface n The hexavalent chromium reacts with the exposed aluminium metal and a trivalent chromium oxide precipitates Cr2O72- + 2 Al + 2 H+ ® Cr2O3·H2O + Al2O3 Materials Technology 7
  8. n Chromium oxide stabile in alkaline solutions up to pH 15 n Chromium oxide is water repellant (hydrophobic) and may act as a barrier coating towards water n Self healing effect: hexavalent chromium present in the conversion coating that may react at mechanical damages n Passivating both the aluminium matrix and the intermetallic particles Materials Technology 8
  9. n Highly effective n Preventing corrosion n Adhesion promoter for organic coatings and adhesives n Resilient: The process has low sensitivity towards variation in process conditions n Effective on most/all aluminium alloys n Quality control: Skilled workers can tell the amount of chromium on the surface by the color of the conversion coating Materials Technology 9
  10. n Toxic n Classified as human carcinogen n Workers at the production line are concerned about their health - liability for claims of workspace exposure n Consumers are concerned about hexavalent chromium present in products n Concern about hexavalent chromium in the environment, e.g. drinking water n Treatment of waste n Stringent disposal limits n Increased costs for tracking inventories, monitoring, reporting n Disposal of wastes containing chromium Materials Technology 10
  11. n Pacific Gas & Electric n Deposits of hexavalent chromium in the ground n Hexavalent chromium leached into the groundwater n Cancer and other diseases increased dramatically n In 1993 PG&E settled for $333 million, the largest settlement ever in a direct action lawsuit. n California 2001: Law that limits Julia Roberts as Erin Brockovich (2000) the level of hexavalent chromium in drinking water Materials Technology 11
  12. n EU directive 2000/53/EC: End-of life vehicle n Every year 8-9 million tons of waste are produced from end-of life vehicles n Aim: to harmonize the treatment of this waste in order to reduce environmental impact n Increase recycling and reuse of materials n Reduce and control the use of hazardous substances in vehicles n Vehicles put on the market after 1 July 2003 shall not contain: n lead, mercury, cadmium and hexavalent chromium n Norway: Chromium on the B-list of hazardous substances n Aim: Significant decrease in the use of chromium by 2010 Materials Technology 12
  13. What options do we have? Materials Technology 13
  14. n Redox reactions with precipitation n Molybdenum n Manganese n pH controlled precipitation n Phosphate n Titanium / zirconium based processes n Cerium n (Trivalent chromium) n Coupling agents between aluminium oxide and binder n Silanisation n Self Assembling Molecules (SAM) n Strengthening the aluminium oxide n DC and AC anodizing Materials Technology 14
  15. n Molybdates Basic prinsiple n Permanganates n Same formation mechanism as for chromium n Group 6B / 7B metal in high oxidation state reacts with aluminium in redox reaction: n The 6B / 7B metal is reduced and forms an insoluble oxide n Aluminium is oxidized n The insoluble oxide precipitate on the aluminium surface and forms a protective film Materials Technology 15
  16. Materials Technology 16
  17. n Molybdenium n Manganes n Mo6+ reacts with Al and forms n Mn7+ reacts with Al and forms Mon+, which precipitates as Mn4+, which precipitates as oxides on the Al surface MnO2 on the aluminium surface n Both processes gives a conversion coating that covers the aluminium matrix Materials Technology 17
  18. n Molybdenium n Manganese – Expensive + Good adhesion – Moderate corrosion protection + Yellow color – Poor adhesion properties – More hydrophilic than chromium - not as good barrier coating – Poor corrosion resistance Materials Technology 18
  19. Molybdenium Chromium Materials Technology 19
  20. Manganese Chromium Materials Technology 20
  21. n Phosphating n Titanium / Zirconium based processes n Cerium (and other rare earth metals) n Trivalent chromium Materials Technology 21
  22. n Hydrogen evolution at cathodic sites in the alloy (intermetallic particles) n pH increases near the intermetallics n The solubility of the oxide decreases when the pH increases n The film (oxide) presipitates n The conversion coating is therefore mainly formed on the intermetallic particles Materials Technology 22
  23. n Originally developed for steel and extensively used on steel and zinc n Available in a number of variations where zinc, iron, nickel or manganese are incorporated in the coating n A sealer or passivator may be applied on the coating, e.g. containing zirconium ions n Problem: Al3+ ions in the bath will inhibit the coating formation. By adding fluorides the Al3+ is bound in AlF63- and precipitates Materials Technology 23
  24. n Phosphoric acid exists in four levels of protonation: H3PO4 H2PO4- HPO42- PO43- n pH at cathodic sites on the surface increases due to hydrogen evolution: 2 H+ + 2 e- H2 n PO43- precipitates with aluminium or other metallic ions present in the solution when the pH increases Materials Technology 24
  25. n Anodic dissolution of aluminium: Al + 3 H2PO4- ® Al(H2PO4)3 n Secondary reactions take place: Al(H2PO4)3 ® Al2(H2PO4)3 + 3 H3PO4 Al2(HPO4)3 ® 2 AlPO4 + H3PO4 n Net reaction: 2 Al(H2PO4)3 ® 2 AlPO4 + 4 H3PO4 Materials Technology 25
  26. n Used in plants where both aluminium and steel components are treated, e.g. in the automotive industry n Perhaps the chromium free pre-treatment process that is most frequently used on aluminium today Materials Technology 26
  27. + Good adhesion + Coating formation rate comparable to chromium + Pre-treat Al in same process as steel and zinc – Less corrosion resistant than chromium – Colorless and invisible on aluminium Materials Technology 27
  28. n Titanium and zirconium (also hafnium) conversion coatings are formed the same way n The metal is exposed to a solution of H2ZrF6 , H2TiF6 or both n Hydrofluoric acid removes the aluminium oxide from the surface n Near intermetallic particles, where the pH is higher, the fluorides hydrolyse and TiO2 / ZrO2 precipitates n The oxidation number for Ti/Zr is +4 both in solution and coating - no redox reaction Materials Technology 28
  29. n The coating is not homogenus - precipitates on intermetallic particles n The amount of oxide precipitated depends on the composition of the alloy - the more intermetallic particles the more coating precipitates n Thin coatings - in the order of 10 nm or 10 mg/m2 n For some processes a polymer is included to seal the conversion coating n Ti and Zr oxides are stabile at pH 3 - 12 Materials Technology 29
  30. + The coating forms rapidly - seconds + Good adhesion and corrosion resistance has been found for some alloy/coating systems – In other systems not - the results varies with alloy, thermo- mechanical history of the alloy and organic coating – The coating is invisible Materials Technology 30
  31. n Solution containing trivalent cerium (Ce3+) and hydrogen peroxide (H2O2) n Film formation takes place by process similar to the Ti/Zr mechanism First: Later: n The hydrogen peroxide n Dissolution of aluminium oxide oxidizes the trivalent cerium on the rest of the surface n Small cathodic particles 2 Ce3+ + H2O2 2 Ce4+ appears where the same n Ce4+ precipitates at local precipitation process occurs cathodes when pH increases n The islands grow into a continuos film H2O2 + 2 e- 2 OH- Materials Technology 31
  32. n Trivalent cerium hydroxide is stabile at pH > 7 n Thickness n Matrix: 100 - 200 nm n Intermetallic particles: ~1 µm n Precipitation on intermetallic particles - decreases the cathodic reaction rate n Used in combination with manganese which mainly builds on the Al matrix n The coating is colored Materials Technology 32
  33. + Colored coating + Good corrosion resistance and adhesion properties have been reported, high Cu alloys in particular (2000) – In other alloy / coating systems poor corrosion and adhesion properties have been reported - The process has to be adapted to each specific alloy – Many process steps – Building of the conversion coating takes long time – Expensive Materials Technology 33
  34. n Self Assembling Molecules (SAM) n Silanes Materials Technology 34
  35. Basic principle: n Organic molecules with two functional groups n One binds to the surface oxide n The other to the organic coating Materials Technology 35
  36. n Relatively new process n few experiences available n few published results n Less corrosion resistant than Ti/Zr based processes and chromium n Sensitive to surface cleanliness prior to application Materials Technology 36
  37. Basic principle O- R O- H R Si O- R + 3 H2 O R Si O- H + 3 R-OH O- R O- H R: ethyl / methyl R: organic functional group R -C3H6-NH3 O Si -C3H6-O-CH-CH2 O O O -C2H4-Si (OR)3 Al Al Al Materials Technology 37
  38. n Applied by exposing the surface to a diluted solution of silanes in water n Homogenous n 50-100 nm thick n Cross linked via Si - O - Si bonds into a three dimensional network Joop Mulder, Corrosion Management, No 44, Nov./Dec 2001 Materials Technology 38
  39. + Good adhesion properties, also wet adhesion – Corrosion resistance not as good as chromium – Emission of alcohols in the process, which may need handling – Sensitive to surface cleanliness – Choice of organic functional group may be resin dependant Materials Technology 39
  40. n DC anodizing n Hot AC anodizing Materials Technology 40
  41. n The aluminium alloy is polarized anodically V n Electrolyte: Diluted acid, e.g. 15 % sulfuric acid e- n The aluminium oxidizes: 2 Al + 3 H2O ® Al2O3 + 6 H+ + 6 e- H2 n At the cathode hydrogen is formed: 6 H+ + 6 e- ® 3 H2 H+ Counter Substrate electrode Materials Technology 41
  42. n The oxide gets a hexagonal structure n A barrier layer in the bottom ~100 Å thick n A porous layer 1 - 30 µm (pre- treatment: ~1 µm) n The thickness of the oxide layer depends on: n Electrolyte n Temperature n Current density n Time of treatment n Alloy composition Materials Technology 42
  43. n Traditionally used for surface finishing: n Several µm thick oxide n Several minutes to build the oxide n Pre-treatment: n ~1 µm is sufficient n Faster 1 µm TEM image of DC anodized AA6060 Materials Technology 43
  44. n The aluminium is both anode and cathode n During cathodic hydrogen evolution, degreasing also takes place n No need for degreasing prior to AC anodizing n Process temperature: 80°C 1 µm TEM image of AC anodized AA6060 Materials Technology 44
  45. + Corrosion resistance – Need for special equipment: comparable to chromium power source + Easy to control metal removal – Not possible in spray lines and oxide thickness – Adhesion usually somewhat lower than for chromium Hot AC + Few process steps + Anodizing + rinsing + Rapid process - seconds Materials Technology 45
  46. n GSB: Gutegemeinschaft fuer die Stueckbeschichtung von Bauteilen, Germany n Qualicoat: The rest of Europe n Both have approved chromium free processes n Ti/Zr based pre-treatments: Both n Anodizing: Qualicoat n Phosphating: GSB Materials Technology 46
  47. n Many of them contain hydro fluoric acid n HF is also on the B-list of hazardous chemicals n Fluorides: 10 mg/l limit in waste water n No signals from Norwegian authorities or the EU regarding changes in the attitude to HF n New chemical compounds - do we know all their health and environmental effects yet? Materials Technology 47