I don't know about the corrosion becoming a powder, but I do know that I've had similar probs on my 51 plate. Has it bubbled where it has become white, as this is what happened to mine ?
I was told that it was water which got underneath the lacquer and created the white bubbling effect (corrosion).
Maybe the powder is where yours has bubbled, and then burst ??
I got mine refurbished a couple of months ago - had a metallic silver paint put on instead of the norm. They shine up nice in the sun. I rang Lexus and told them I had a couple of "small" questions to ask the wheel refurbishing guy, as I had my wheels refurbished and just wanted to find out about looking after them (which is a lie cos I ain't had the wheels done yet), and asked them if I could have his number - which they then gave me. I then rang the man's mobile and he quoted me £20 less than Lexus did. :D