any metallurgists out there?

I want to know about corrosion of stainless steel in anaerobic conditions.

My submersible bore pump, which is barely two years old, is coming apart. Three stainless bolts that tighten the straps which hold the pump stages together are corroded. One bolt broke in service, another the head snapped of when I tried to undo it and the third came out OK. The surfaces at the break were very dark in colour.

I've bought new marine grade bolts but my son says that SS corrodes if there is no O2 to form an oxide coating. I've heard that's how zinc coatings work but it's new to me with SS. If this is true, will marine grade screws make a difference and how is a submersible pump supposed to last ten metres underwater which would clearly have no O2 dissolved? Would a coating of Fisholac be adequate protection?