I'm not sure of the pre-chrome acid, but for de-rusting of parts that are not subjected to much stress, I use vinegar. Cheap and safe, but it takes a few days for heavy rust. I've also used evapo-rust. It works great.
For parts that are subjected to a lot of stress (engine, driveline, steering, etc) acid should not be used. It can result in hydrogen embrittlement of the material and the part can fail under stress. Plating also can result in hydrogen embrittlement. For high stress parts platers should bake the part immediately after plating to drive the hydrogen out. (If I remember correctly, it's 400 degrees for several hours).
Evapo-rust is fairly neutral pH and is safe on your skin. It's my understanding that it doesn't produce hydrogen embrittlement.
Dave Evans
52_3100
