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Oxidization protection qualities of chromating on aluminum

Hi

I am a consultant assisting the New Zealand Industry with compliance for the RoHS directive. My question is, is hexavalent chromium based chromating of aluminum used solely as a pre-treatment for powder coating or is it it of use in its own right as an oxidization protector? Anecdotally I have been told that that chomating on aluminum has a very restricted life span and is therefore not used as a long term oxidization protection, unlike the use on zinc plated steel.

Also does anyone know if there is much difference in corrosion protection between using trivalent and hexavalent chromating if a powdercoat is then applied over the top?

Finally does anyone know if thick film trivalent chromating affords any better protection to aluminum if used on its own without powdercoating?

Any help would be very much appreciated as I have been working on this question now for 2 weeks with no firm result.

Roland Sommer
- Christchurch, Canterbury, New Zealand
2004

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Hexavalent chromate conversion coating, properly applied, will withstand 168 hours 5% neutral salt spray, ASTM B117. In salt spray tests it is not unusual to withstand several hundred hours. In outdoor environments, unless very corrosive, years of protection from oxidation is possible. Trivalent chromates have little protection compared with Hex chrome. Products such as Iridite 14-2 and Alodine 1200s afford good oxidation protections. Don Baudrand Consultant - Poulsbo, Washington (Don is co-author of "Plating on Plastics" [on Amazon or AbeBooks affil links]            and "Plating ABS Plastics" [on Amazon or eBay or AbeBooks affil links]) 2004 Hex Chromate on aluminum is put on heavy and left bare for corrosion resistance and withstands 168 hours 5% salt spray. Hex chromate on aluminum is put on very very thin as a bonding base for powder coat. If you put it on too heavy the curing temperature will dehydrate the coating and explode it right through the powder coat. So far there is no trivalent chromate for aluminum. Robert H Probert Robert H Probert Technical Services Garner, North Carolina 2004

Is there any technical reason why trivalent chromating, especially thick film trivalent chromating cannot be used on aluminium?

Roland Sommer
- Christchurch, New Zealand
2005