Your pan sounds like it is cadmium plated. The white powder is some sort of cadmium oxide and apparetnly is very dangerous, so wash your hands if you handle it.
To fix it, you need to have
oil pan re-plated. A chemical dip process I believe.
I wish I knew more about it, but I've told what I can. Perhaps you can find out more by Googling Cadmium Plating