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

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.

Ted Mooney, P.E.  finishing.com Brick, New Jersey