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Letter 083
Air emissions from sulfuric acid
anodizing
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Does anyone have information concerning air emissions from
sulfuric acid anodizing operations? I've been told that in chromic
acid anodize, 80-90% of electrical current is wasted dissociating
water molecules creating the mist over the tanks. Is there a similar
problem with sulfuric acid anodizing? Thanks
Scott F 
Lockheed Martin
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Let me digress to plating first. Plating processes are not 100
percent "efficient". Some portion of the electrical input goes into
laying down metal per Faraday's law, but some percentage liberates
hydrogen and oxygen instead. Typical plating processes are maybe 70
to 95 percent efficient, with the notable exception of chrome plating
which is about 10-15 percent.
So, almost all plating processes "gas": some very little, some a
fair amount, and one (chrome) horribly. In anodizing, there is a
somewhat analogous situation, because you want and expect some
liberation of hydrogen and oxygen so that the aluminum gets oxidized.
The term "efficiency" becomes far less meaningful.
In any event, both chromic and sulfuric anodizing processes gas
significantly. The difference is that chromium is a known carcinogen
with a long history of causing nasal perforations and chrome
ulcerations. But the sulfuric acid is certainly irritating, and all
anodizing tanks require local ventilation.
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Ted Mooney, P.E.
finishing.com
Brick, New Jersey
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