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Black anodising delaminates from heat of machining

My name is Tony Paterson. I work as an aluminium technical specialist in Johannesburg, South Africa. I have been trained up to an engineering doctorate and have been in the aluminium industry for some 30 years. A problem was given to me recently by a well established anodiser which left me perplexed. This is why I write. The problem concerns black architectural (not decorative) anodised 6063 aluminium (at this stage composition detail not established). Superficially the anodising appears of good quality and appears to have been correctly sealed. The samples has been machined with a router as is common in the architectural industry. Adjacent to the routed edge the anodising appears to be delaminating. It also delaminated on the outside side of that part that had been routed on the inside face (although not on the inside face adjacent to the machine markings). The cut sample also shows some delamination where it was clean cut square with a circular saw. Scratching the surface with a key does not scratch the surface or remove the anodising - so adhesion appears good. Looking superficially at the surface without magnification of any kind one is struck by the apparently superficial nature of the colour - which should be deposited at the base of pores and sealed. Questioning the anodiser it appears that this is a once off problem and that the bath compositions, voltages, amperages and processes show no anomalies. The reason I write is because it is all counter intuitive - it is all not what one would expect. Has anyone any similar experience or suggestions?

Tony Paterson
aluminium industry technical specialist - Johannesburg South Africa
June 21, 2011

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The router has work hardened the surface, the anodizing has converted the whole hardened layer to aluminum oxide, and because of the stress in the work hardened area, it flakes off. (1) Try a sharper router and more coolant (2) try to etch off the hardened area before anodizing

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

June 23, 2011