Letter 20084

Anodizing Aluminum Castings [Kansas] 

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I work for a company that produces aircraft interior components, many of these components are made up of aluminum castings. Mainly Invewstment and Sand Casting process. We have different foundries producing these castings using the alloy 535. We have been having problems with the black anodizing on these parts. The parts for the most part take the black dye, but we get a speckling look where the anodizing didn't take. We have been fighting this problem for some time now. Any comments or suggestions are greatly appreciated!!

Nathaniel Yoder
- Lenexa, KS

The speckles are silicon and magnesium. You anodize only aluminum. The silicon and magnesium in the surface does not anodize and looks like speckles. There is a shop in the KC area that is expert in increasing the surface area of aluminum and decreasing the area of mag and silicon, but, depending on the quality of the casting, they can only go so far. Additionally, thin walls concentrate the mag and silicon so that the surface of the thin walls will be more speckled than the heavier sections.

Robert H Probert Robert H Probert Technical Services Garner, NC, USA Editor's note:        Mr. Probert is the    author of Aluminum How-To

The speckles could be casting porosity which retain acid from the anodizing process. Localized low pH would prevent proper dye absorption.

Chris Jurey Luke Engineering & Mfg. Co. Inc. Wadsworth, OH

Would vaccum impregnating these parts solve the speckling problem? If the porosity is the cause then the impregnation would seal the voids in the casting not allowing any acid to be trapped in the porosity. Does any one have any knowledge in vaccum impregnating aluminum castings and then anodizing them? Is the vaccum impreganation "resin" FDA approved?

Nathan Yoder
- Lenexa, KS

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