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How to achieve deep black on 356 castings?

Does anyone have any anodizing advice for producing a deep black finish on 356 castings? Any tips would be greatly appreciated!

Victor Waldman
job shop anodizer - Oxford, Connecticut, USA
2003

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Paint ! You only anodize aluminum. 356 is loaded with all sorts of stuff that is not aluminum and that stuff does not anodize and/or produce a pore that can absorb dye.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

2003

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Robert - Thanks for the feedback! Are there any special cleaning/pre-anodizing steps we can take to optimize our results? If the impurities are mostly silica based, would an HF step help obtain a deeper black, or is there something else that could help remove the impurities without pitting the part beyond recognition?

Thanks again,

Victor Waldman
Job shop anodizer - Oxford, Connecticut
2003

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First, stay out of the alkaline etch because it decreases the area of aluminum that is exposed and detrimentally INCREASES the area of silicon exposed. Treat in 100% Nitric Acid with 2 pounds per gallon ammonium bifluoride, about 20 seconds until the part turns "frothy white all over", then anodize, then rinse, then 5% nitric acid, then rinse, then black dye. You will be blacker because you stayed out of the alkali.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

2003