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topic 5950

Anodizing/Sealing *CAST* Alloys


I'm having a problem trying to anodize various *cast* aluminum alloys. Actually, they appear to anodize fine, but they are not sealing (DI water seal, seal quality determined by admittance). The alloys in question are A356 and 535 (Al-7Mg).

Scott Dickerson
- Fremont, California


The first thing I would recommend is to check your seal operation. One way to check this is to anodize and seal a piece of wrought aluminum (a panel), Then do your seal quality test on the wrought panel. If the panel passes, then your seal is working ok and the problem is with the material. Of course if the panel fails, then your DI seal is not within spec. Incidentally, you may see seal smut on the wrought panel - this is the formation of seal product (boehmite) on the surface of the part. It actually means that the seal is working ok. There are a couple of DI seal additives commercially available that work as anti-smutting agents.

The most common error in running a DI seal is that it is not hot enough - the water must be at or above 98 °C. The pH must be between 5.5 and 7. You can buffer to help keep the pH in balance. Immersion time may need to be 30 minutes or more. Many manufacturers in your geographic area specify seal times of 1 hour per mil of oxide for critical applications.

Sjon Westre
- Minden, Nevada

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