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  for Metal Finishing since 1989
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Anodising color loss and restoration

2002

My company manufacture an electronics product which has the exterior of the aluminium box machined to shape and anodized black the interior is then machined and the internal surfaces which need to be conductive for grounding purposes are chromate converted. We recently changed machinist and thus anodizer and chromater. The first batch went fine but the most recent batch has had all the black striped by the chromating process.

1. What caused the black coloring to be removed?

2. If it is from the pores not being sealed is it still possible to recolor and seal the pores?

Darren Wass
- Australia

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2002

I assume that you have a separate anodizer from your chromater.

Two possibilities came to my mind- first is the pre-cleaning processes and deox before chromate conversion or the stripping processes to rework an unsatisfactory chromating result. If your chromater does not mask all anodized surfaces, this may dissolve the anodic coating.

Secondly, chromating is acidic solution that could bleed the dye of the coating if it is not properly sealed.

You cannot re-dye them unless you have them stripped and reworked.

Dado Macapagal
- Toronto, Ont

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2002

I'd go with the poor or no seal theory. Typically this results in the black dye coming out upon first immersion in an aluminum soak clean as prep for chromating. Typical chromate and anodize is done with chromate all over, masking the surfaces to remain conductive, stripping the chromate from balance of part and anodizing those surfaces. In this way the paint or tape or plugs protect the chromated surface.

Either way, one finish or the other has to be protected from the aggressive and corrosive pretreatment chemicals needed to apply the subsequent finish, however, it's done all the time!

Milt Stevenson, Jr.
Syracuse, New York