Published on November 21, 2008
Ole Øystein Knudsen, Astrid Bjørgum SINTEF Materials Technology Materials Technology 1
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
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
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
Why …. and why not? Materials Technology 5
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
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
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
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
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
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
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
What options do we have? Materials Technology 13
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
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
Materials Technology 16
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
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
Molybdenium Chromium Materials Technology 19
Manganese Chromium Materials Technology 20
n Phosphating n Titanium / Zirconium based processes n Cerium (and other rare earth metals) n Trivalent chromium Materials Technology 21
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
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
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
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
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
+ 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
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
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
+ 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
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
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
+ 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
n Self Assembling Molecules (SAM) n Silanes Materials Technology 34
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
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
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
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
+ 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
n DC anodizing n Hot AC anodizing Materials Technology 40
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
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
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
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
+ 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
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
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
Presentación que realice en el Evento Nacional de Gobierno Abierto, realizado los ...
In this presentation we will describe our experience developing with a highly dyna...
Presentation to the LITA Forum 7th November 2014 Albuquerque, NM
Un recorrido por los cambios que nos generará el wearabletech en el futuro
Um paralelo entre as novidades & mercado em Wearable Computing e Tecnologias Assis...
The Chromium projects include Chromium and Chromium OS, the open-source projects behind the Google Chrome browser and Google Chrome OS, respectively.
Home of the Chromium Open Source Project ... Chrome user profiles are not backwards-compatible. If you try to use mismatched profiles and Chrome versions ...
Chromium is a mineral our bodies ... But taking excessive chromium supplements can lead to stomach problems and low ... WebMD does not provide medical ...
Chromium is a mineral that humans require in trace amounts, although its mechanisms of action in the body and the amounts needed for optimal health are not ...
Chromium-6 is a human carcinogen when inhaled, and there is growing concern about the possible health effects of chromium-6 in drinking water.
In animals chromium can cause respiratory problems, a lower ability to fight disease, birth defects, infertility and tumor formation.
Find patient medical information for CHROMIUM on WebMD including its uses, effectiveness, side effects and safety, interactions, user ratings and products ...
Chromium is the open-source web browser project from which Google Chrome draws its source code. The browsers share the majority of code and features ...
Download latest stable Chromium binaries for Windows, Mac, Linux, BSD, Android and iOS (64-bit and 32-bit)
Chromium Portable. Chromium Portable is a custom-built version of the Chromium web browser ... the easier it is to fix problems. Which Chromium Portable ...