Types of corrosion on computers

I've seen rust and dust, but there's an old vaxstation II at Goodwill Computers in Austin right now (very cheap, anyone welcome to buy it and get it off my mind) but as most things, I checked out the back and see some circles of white corrosion on the back where the cards? are contacting the case. I don't really have room or time but I don't own anything that uses QBUS or is almost related to some of the neat iron some of you all collect (I've mostly collected way too many home computer history items). Anyway, what is that type of white corrosion on metal? Would one probably assume this means any bus would be corroded and this wouldn't be a computer for the faint of heart hobbyist? I've only seen that maybe from batteries but it's in an unusual place and pattern (I think).