No guarantee, but I used the black, zinc coated I thought them a good alternative to stainless. They are cheap and in my opinion, were better quality than those that came with the riser. I also use never seize or dielectic grease on all bolts I expect to remove again. I think this also prevents corrosion or electrolysis and prevents rust.
I have had mine in since the bike was new, with no rust. I live west of you on the river.
Last, I also remember reading reviews about another riser manufacturer's bolts failing, so I updated to these grade 5, black, zinc bolts. Very easy to change and available at an Ace near you. Very happy with the risers, otherwise.
Jim