A bit of torch excitement!! Round 2....ding

" The nitric acid should work. The anodizing process uses acid, often chromic, phosphoric, boric or sulfuric acid. I once had some aluminum parts made for a project at work (I'm an engineer in the aerospace industry) and the mechanic who built them thought he could save some time by installing helicoils before he sent them to the anodize shop. When they came back, the helicoils were gone. The acid in the anodize tank ate them clean away. It might be interesting to try this with with muriatic acid, which is not as corrosive as nitric acid, and cheap and easy to find. Better protect any close tolerance areas, though, as the acid may have an effect on bare aluminum surfaces. Maybe you could just send your gear housing out to be anodized? "

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I personally like to attack the offending metal with galvanic or chemical corrosion. In aluminum, the galvanic approach won't do because it is a low nobility metal and we want to preserve the structure and not the fastener. So, you could use a mild acid to attack the carbon steel fastener. Citric acid is good but it takes a long time. Vinegar is good if you can get the strong stuff. Nitric acid is the best if you can get a 50% solution. Don't use hydrochloric acid, it loves to eat aluminum.
The procedure is to work from the bottom of the threaded hole or drill a small hole and inject the acid into the blind hole where the bolt is. You have to rinse it out well when you are done.

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