Influence of copper on chromate conversion coating of aluminium alloys

The formation of chromate/fluoride conversion coatings is investigated for binary Al-2(.)3 at.-%Cu alloy and AA 2014-T6 aluminium alloy in order to determine the influences of copper on coating development. Copper is shown to cause detachment of the coating layer after a particular time of coating. The time of detachment is reduced with increase in the concentration of copper in solid solution in the various alloy regions. Therefore, a thicker layer of coating material can develop on the copper depleted matrix regions of AA 2014-T6 alloy compared with that on the copper rich intermetallic particles. The detachment occurs following enrichment of copper in the alloy during the initial stages of formation of the coating, with the main layer of hydrated chromia detaching from close to the alloy surface. Following detachment, subsequent thickening of the coating is restricted. It is suggested that this behaviour is related to the incorporation of nanoparticles of copper into the coating.