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Aluminum Alloy B-535.2 Hard Anodize Surface Treatment (Seawater App)

COTS Cast aluminum structural members have been selected as fixtures for a sub-surface seawater test platform. I am curious about the compatibility of applying hard anodize to these components. The catalog states that they are composed of B-535.2 Aluminum Alloy. This alloy composition is approximately 92% Aluminum. Will this type of alloy gain any benefit from surface treatment with hard anodize?

James Mark Harper
Mechanical Engineer - Tucson, Arizona, USA
2006

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First of two simultaneous responses --

Some of the 535 I have seen did not have the magnesium alloyed, instead the magnesium was separated out in random hunks, some on the surface, and you do not "anodize" magnesium in 15%/wt sulfuric acid. Further the sodium chloride in the sea water will attack and dissolve hydrated aluminum oxide, aluminum oxide, and aluminum !

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

2006

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Second of two simultaneous responses -- 2006

This alloy has good corrosion resistance due to very low limits on alloying elements other than Mg; the nasty Cu is 0.05% max. For improved corrosion resistance, conventional anodizing may be better than hard, as the latter often contains microcracks (crazing). Due to the high Mg content, the anodizer should use a nitric acid deoxidizer prior to caustic etching (or even skip the caustic).

Hard anodizing would be beneficial in sand-washed areas. Sealing must be specified for hard anodize, as the default is unsealed.

Ken Vlach [deceased]
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.