Hi there, Stew,
I must start by stressing that I am no metallurgist. However, I used to work with a team, some of whose members had to choose materials for long term immersion in sea water.
When they deserved it, I used to tease them by reminding them that aluminium is never found in nature in its metallic form.
I remember it being said that stainless steel can be either anodic or cathodic, depending on the environment. Also, stainless steel depends upon there being available oxygen to maintain its protective surface patina. Stainless screws in blind holes were treated with a yellow chromate gap-filler to 'fool' the stainless into thinking that it was in an oxygen-rich environment.
Maybe that's all been so scrambled in my memory (it's been a long time!) that it's garbage.
Still, maybe it will provoke some proper metallurgists to give you the real answer to your question.