Originally Posted by Wightdiver
Just removed the engine mounting bolts from the transom preparing to re flow coat everything and one of the bolts looked like this. Looks like someone has been chewing on it.
The other three are in perfect condition and the transom is totally dry, which is good news.
Some kind of electrolysis?
Anyone seen anything like this before?
It will be electrolysis, because I think that's how all corrosion works. Stainless steel relies on an impervious layer of oxide to counter corrosion. The oxide layer is only impervious if it forms on a very smooth surface, which is why stainless steel fittings are highly polished. Maybe that bolt was not polished enough, or was scratched or filed, or something?
If the oxide layer fails, or cannot be replaced, then stainless steel will rust very quickly because of electrolysis between the various alloys that make it up. Stainless steel propeller shafts enclosed in a shaft tube suffer this, and it's one reason why boats are not made of SS.
Though the transom holes are dry, if any dampness got between the sealant and the bolt, the oxide coat could be breached and with no air to renew the oxide layer, corrosion will set in and accelerate.
I think that may be a possible cause, but I'm dredging the depths of my head for old knowledge.