[eringobragh]

I keep on hearing about how the copper sulfate test or 'copper sulfate immersion' is a good way to test for "free iron" and show passivation. When I googled about it, most of the hits were about engineers and finishers questioning its effectiveness, and what I want to know is the chemistry behind it. When a metal is placed in water with copper sulfate, why does the metal start to deteriorate/rust?

[Borek]

It is not rust. Chemistry you are looking for is oxidation of iron by copper. That's activity series.