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Crazing problem in anodizing
Q. I manufacture aluminum medical cases. Anodize process is to specification AMS A8625 [withdrawn / link is to spec at TechStreet], Material is 5052. After anodizing parts are silkscreened and baked at a temperature of 275-300 °F for 30 minutes. The result I am getting is what my supplier calls "crazing" and he does not know how to eliminate this problem can anyone help? Also, is the anodize seal intact or has the anodize process been compromised?
sheetmetal fabrication - Louisville, Ohio, US
A. 300 °F for 30 minutes can cause the anodize to craze or crack.
The anodize coating is a ceramic, thus has the high temperature resistance of most ceramics. The problem is the large differences in thermal expansion between the aluminum base metal (expands a lot) and the thermal expansion of the ceramic anodic layer (expands a little). This can put substantial mechanical stress on the anodize. The problem is worse when the anodize layer is thicker.
A. Dear Richard
Crazing is due to the thermal expansion and elasticity of aluminium is much greater that that of the anodic coating.
You could do the following thing to minimize the problem:
1. use higher anodising temperature
2. lower current density
3. a final short rinse in warm water (approx. 40 °C)
- San Diego, California, USA
A. Another option would be to silk screen it with dyes made for anodizing prior to sealing the part. You can find a method or two describing how to do this in "The Surface Treatment of Aluminum and Its Alloys" by Wernick Pinner and Sheasby, or you can contact anodizers that specialize in this practice and have them put on what ever design it is you are silk screening.
Accurate Anodizing Inc.
Compton, California, USA
March 6, 2015
A. The best practice for high temperature painting is insure the anodic layer is around 5 µm to 8 µm - this allows painting up to 200 °C with no issues. Standard practice at most exterior trim anodize aluminum suppliers.Art Delusky
Minth Automtive - Ningbo China