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Aluminum alloy etching process

I am Rameshkumsr K, Engineer. We are involved in manufacturing Automobile service equipments and accessories.
We like to inquire on the etching of casted aluminum components.
Normally, we etch it with caustic soda [affil links] and rinse with nitric acid. Is it necessary to anodize it, to be corrosion resistant?
The component has some critical machined dimensions(bore and threaded portions) which gets affected due to etching. So, would like to know the standard etching time, temperature and other process parameters for aluminum castings, if any.
Also, please clarify, whether the hardness of the casting would affect the etching process ?

K RAMESHKUMAR
- COMBATORE, TAMILNADU, INDIA
2006

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2006

You left out a critical piece of information--the purpose for the etch.
Is anodize necessary? That depends totally on the final use and what the purchaser desires.
Etch is a tradeoff of chemical strength, temperature and time. Straight lye solutions are cheaper than proprietary ones, but do not last as long. Cost of disposal is a huge factor.
If this is for cleaning, there are commercial non etch cleaners.

James Watts
- Navarre, Florida

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Typically in my experience etching aluminum castings is a no-no in the anodizing process.. The reason being the fact that etching the aluminum exposes the silicates that are cast along side of the aluminum in the casting process.. This will yield a poor anodized finish ultimately, splotchy dyed coatings.. etc..

Standard etching times vary from alloy to alloy.. if you are worried about critical dimensions, Plugging the holes may be a viable option. and is common in the anodizing business.. temperature of the etching solution will vary depending on how aggressive you would like your etch bath to be. higher temps at lower concentrations (Caustic) can yield the same aggressiveness of a bath at a higher Caustic concentration at lower temperatures.

The hardness of the casting shouldn't affect the etching process.. but remember Caustic attacks the aluminum in the casting not the other components. I personally do not recommend etching castings. Desmutting in either nitric or a proprietary desmutting blend is the best option.

Ryan Cook
Toccoa, Georgia
2006

adv.