I just wanted to forward some comments regarding the question on Mg that appeared in the October issue. Your answer hit all of the key points, with e-coat/powder and anodizing being the most favorable. Depending on how critical the application is, a chromate conversion coating prior to e-coat and powder coating may be necessary to prevent excessive creeping under any scratches or defects. The former McDonnell-Douglas helicopter group (now part of Boeing) designed a Mg transmission housing and found that maximum corrosion performance after 1,000 hours of salt spray was produced by the following:
- Heavy coating of Tagnite (plasma anodizing technique)
- Three layers of Rockhard (baked phenolic sealer)
- Basecoat of epoxy/polyamide (containing strontium chromate)
- Topcoat of polyurethane (chromate free)
More information can be obtained by using www.tagnite.com/boeingreport.pdf
Feel free to pass this information along. I always enjoy reading your column, as well as the others that are featured in PF. Keep up the good work. Best regards. T.P.