|MadSci Network: Chemistry|
There are two considerations in attempting to dissolve one metal in intimate contact with another. The first is their relatively solubility in different solvents or acids. The second is their relative position in the electromotive series.
Steel is iron with about 0.6% dispersed carbon. So for the sake of evaluating solubility, we can assume that it is iron. When I look in the Handbook of Chemistry and Physics (CRC Press), I see that iron is soluble in acids, insoluble in alkali, alcohol, and ether. Aluminum, according to the same reference, is soluble in alkali, HCl, H2SO4, but it is insoluble in concentrated nitric acid, HNO3. So it looks like concentrated nitric acid could dissolve the iron without dissolving the aluminum. If I look on the electromotive series, however, I see that aluminum is more reactive than iron, so there is a chance that the aluminum might attempt to dissolve to electrochemically protect the iron. Here is an example where you will need to run an experiment to determine if the iron can be dissolved, leaving the aluminum.
Another place to verify if nitric acid will selectively dissolve the iron is to talk to those who are in the business of chemical etching or anodizing. Aluminum parts which are in the process of being anodized are first etched in a basic solution of sodium hydroxide. After coming out of the etching solution, the surface of the aluminum is coated with a thin layer of iron and other alloying agents. This black film is called smut. And smut is removed by dipping the parts in a 50% solution of nitric acid in water. So give it a try and see if it works. Please be careful, however. Anything that can etch metal can do a real number on your skin!
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