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Anodize on top of a vacuum impregnated aluminum?

We manufacture a line of aircraft components and we use a lot of aluminum castings of various casting methods. For whatever the reason, a question was asked today: can anodizing be performed AFTER a vacuum impregnation? It seems to me that to an extent, the impregnation would mask the anodizing surface. What is the appropriate sequence? Alloys are A356 & A380 processed in Sand, Die, and Investment methods. Our castings need to be pressure tight. Our current sequence is (1) anodize the as-cast material, (2) machine the features, (3) vacuum impregnate, and (4) apply a chemical conversion coating to protect the machined surfaces (and allow electrical conductivity for bonding and grounding). Opinions and advisement will be greatly appreciated.

David Vranson
Manufacturing Engineer - Valencia, California
May 4, 2011

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First of two simultaneous responses -- May 5, 2011

I'll be the first to admit that I'm not real familiar with vacuum impregnation, but looking at it just from a common sense type aspect, if a vacuum impregnated part can adequately accept a conversion coating, then I would think it should be able to accept anodizing. Does the impregnated surface have any dialectic properties? If so, that would obviously present an issue.

Seems to me the best way to find out would be to give it a try on a scrap part, or a coupon.

Marc Green
anodizer - Boise, Idaho

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David,

Our company regularly impregnates our aluminum castings prior to anodising. The impregnant is designed to seal porosity. Dependent on the anodising being used the porosity can trap fluids and cause spotting out after the process has been completed and this can damage the anodising layer (very common to see it happening with sulfuric acid anodising and dyed coatings, possibly because the spotting out is white against a colored background).

Brian Terry
Aerospace - Yeovil, Somerset, UK
Second of two simultaneous responses -- May 6, 2011