Van #2 2021 EXT
They also sell zinc plateJust my 2 cents worth...
Originally from northern MN, so corrosion is always in the back of my mind. Several cars turned to dust. I keep (for sentimental reasons) a ziplock back containing talus from the frame of an old Chevy I used to own. So I am fond of stainless and do use it often on the exterior of the van, the ladder on the back door, the 1515 rack hardware, etc. Not really concerned with any galvanic corrosion where the threads on both ends of a fastener (nut and bolt) are the same metal, and the connection is made simply by squeezing the metal between them. Screws are another thing altogether, though. The connection requires the material the screw bites into to maintain structural integrity sufficient to hold the edges of the threads against the pull-out force, and we often do not have control over that. Seen a lot of screw failures where the screw itself corroded, or where the anchor material (usually wood) rots and gives up.
My belief is never rely on only glue to hold something together. Glue will eventually fail, so ALWAYS use mechanical fasteners. Glued and screwed subfloors, for example. IMHO thru-bolting will always be better than screws. The stainless screws holding the MaxxAir fan go into hardwood frames, hopefully long enough (2") and sealed sufficiently (DAP and butyl tape) to prevent the addition of moisture beyond what is inherently present in the wood. But it is a possible failure point down the road. In hindsight, it might have been better to put in at least a few thru-bolts, but then one has to weigh in the length of time something will last. Is a bridge that lasts twice as long worth four times as much money to build?
I just replaced my mailbox after the township knocked the old one over while re-blacktopping the road. (4"x6" post). Cannibalizing the old one for parts to use on the new, was surprised at the wide range of corrosion present... the deck screws I had added looked brand-new after 20 years in the environment (road salt, extreme temp cycles, being in treated lumber, etc). Not so good were the painted and plated hardware that came with the original post kit, most were completely corroded, many of which broke off during removal.
Sorry for rambling on. Was on a roll and too hot to go outside and work on the van.
It can be a bit if a rabbit hole, and as you stated the environment that the fasteners are in really matters;
wet (H2O), salt, chemicals, dis-similar metals, etc