Letter 34024

High temperature anodize with the Honda Look!  

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I am working with a local manufacturer who would like to have a "Matte" finish on aluminum castings that are used in high temperature areas of their final product (300 F). I have seen a part that has the desired appearance, best described as that seen on the crank case housing on a Honda motor cycle without the clear coat, very smooth and attractive. My question is how can I attain this smooth matte finish with a raw casting without using abrasive blasting that seems to cause pitting in the casting surface? I have made the assumption that the part was anodized without color and has a sealer coat but can I get the high temperature resistance with standard sealer coats? Would caustic etching provide the smooth almost machine turned appearance after the casting is deflashed in an abrasive tub? Can the anodized finish withstand the 300 degree heat and what final finish should I use? Again I assume the following: Deflash in vibratory bowl, caustic etch, anodize, seal, satisfy?

Battle Glascock
- Chattanooga, TN, USA

Sheasby and Pinner say, "fine cracks may be observed at 80 to 100 C" ---the low coefficient of expansion of the anodic film is approx 0.2 times that of the metal"------Salt spray tests on aluminum alloys which had been anodized and heated for 8 to 100 hours at 80 to 200 C showed that while the corrosion resistance of 1200 material (not a casting) was influenced very little in spite of some crazing, the anodized copper-containing 2024 alloy showed severe corrosion after heating". That said, however, all of the above applies to wrought alloys. I cannot find anywork on castings. I do not think the coating system will last very long at 300 F.

Do not ever alkali etch castings !!! Alkali etching removes aluminum. leaves silicon and copper, and decreases the amount of aluminum on the surface. You anodize only aluminum. Methods are available for processing castings whereby the silicon on the surface is decreased and the aluminum exposure is increased.

Vibratory finishing embeds foreign material into the soft (Kleenex-like) aluminum surface. Be dure to use designated media that has not been used on iron or brass. Prefer Aluminum Oxide grit.

Robert H Probert Robert H Probert Technical Services Garner, North Carolina Editor's note:        Mr. Probert is the    author of Aluminum How-To / Aluminio El Como

First, the easy part. Yes, clear anodize will withstand 300F. Now the hard part. In order to achieve that smooth surface finish as you see on a Honda, you're going to have to do extensive grinding and polishing of the alminum casting. Simply deburring and/or blasting will not get you anywhere near that smooth. Caustic etching will not smooth the surface, it will only reduce the gloss, in fact excessive etching will pit and roughen the surface. Sorry, but if you want to make a Honda, you're going to have to spend the money as they do.

Jeffrey Holmes, CEF - Spartanburg, SC, USA

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