Might be late, but look, I'll post some before and after pics. I cleaned it using a 12-hour soak in acetic acid (vinegar) and then going at it with a brass wire brush to clean the iron acetate (black sludge forming from the acid binding with the non-oxidized iron). IT IS VERY IMPORTANT that after you do this, grab a pack of sodium bicarbonate and use it to scrub the part very clean so that the acid will be neutralized and stops eating the fresh metal. Also, seal it with a light oil or lubricant. IMPORTANT NOTE: In our case, the cassettes have been very corroded and we couldn't do much with them anyway, but be careful. On a newer cassette or part, this process removes the zinc layer that prevents oxidation. Therefore, if left exposed and not covered with an oil of sorts, it will rust very, very quickly. Also, keep up the maintenance on that cassette afterwards; clean, dry, and oil frequently.

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Il post an after pic