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topic 287

The Chemical Mechanism of Sulfuric Acid Anodizing?


(1996)

I am trying to understand the chemical mechanism of sulfuric acid anodizing. Specifically, to determine the amount (if any) of aluminum sulfate produced. Is there any reference that discusses this issue. The MA State Form S requires the accounting of all listed chemicals and I do not have any information on the rate of formation. Any hints would be appreciated.

Art Sanders



Lowenheim's Electroplating, McGraw Hill, says that some part of the anodic coating is sulfate, and Langford and Parker's "Analysis of Electroplating and Related Solutions" [linked by editor to info about book at AbeBooks] gives an analysis for aluminum in anodizing solutions. But I believe that the amount of dissolved aluminum in these baths is insignificant when compared to the amounts found in other solutions such as in etching.

If you must get some value of the aluminum, I think the best way is by analysis, since you probably drag in more dissolved aluminum than you make in the bath. What is the threshold reporting value for aluminum sulfate in MA form S?

tom pullizzi monitor
Tom Pullizzi
Falls Township,
   Pennsylvania 


(1996)

The reporting threshold for MA Form S is 25,000 lbs of Alum produced. I would like to gain an understanding of the chemical mechanism of sulfuric acid anodizing. I have searched different material but can not seem to find one that specifically addresses the subject. Could you suggest any articles of reference books that would be helpful?

Art Sanders



November 15, 2008

Hi, Art. I'd be pretty confident that the reactions are explained in exhaustive detail in Surface Finishing and Treatment of Aluminum and its Alloys, by Wernick, Pinner & Sheasby because everything about anodizing is :-)

But the principle reaction is fairly simple. The aluminum is made anodic, which causes oxidation of the aluminum and generation of oxygen, and the aluminum is converted to Al2O3. There is some hydrolysis, so the reaction product also involves aluminum hydroxide rather than pure aluminum oxide, but I don't think sulfur plays a significant part in the reaction.

Regards,

Ted Mooney, finishing.com Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



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