I need to fab some stainless steel plates to mount to an aluminum mast on my catamaran that sits on the beach for half the year and is exposed to salt water. What variety of stainless steel is most suited for this application. I need to drill some holes (5/16") at the largest, and all I have access to is a drill press, so that’s also a criteria. The plates will be about 0.05" thick.
As The Flying Dutchman said, 316 Stainless (ASTM 240) is generally preferred in highly corrosive environments due to its corrosion resistance. However, one thing to be cautious about is creating a galvanic cell between the ‘stainless’ steel plate and the aluminium mast which will cause the latter to begin to corrode rapidly unless properly protected. I assume that the mast is anodized (i.e. it has a protective crystalline coating that inhibits oxidation) but a strong galvanic field will eat right through that in no time, and once it gets to the base metal aluminium starts to dissolve like Styrofoam in gasoline.
I’m not clear what you plan to do with this plate–0.05" is pretty thin, so I’m guessing that it is some kind of reinforcement rather than a bracket or primary load-bearing structure–but you definitely want to make sure that the mast and plate are separated and sealed by a non-conducting barrier–epoxy or protective tape–and use passivated rivets (do not use threated fasteners!) to connect the plate to the mast.
Personally, I would use aluminium plates; even if what they connect to creates a galvanic potential which makes them erode, you can easily toss the plates and make new ones. Not so much with a formed or extruded aluminium mast.
I agree on the choice of aluminum sheet rather than stainless, but I don’t understand your prohibition on stainless threaded fasteners. A 5/16 hole suggests to me (being in the marine industry) a stainless machine screw which can be adequately isolated from the aluminum with Never -Seez.
The thread in threaded fasteners is just perfect for setting up corrosion cells. I guess if you fill it in you can get a reasonable amount of protection, probably adequate for this application, but I would prefer passivated steel or aluminium rivets in this application, especially if this is a splice plate or reinforcement plate where the joint is in primary shear (i.e. the load is normal to the fastener axis).
But I admit a degree of professionally-based concern bordering on paranoia regarding corrosion in salt spray environments which may not be such a big deal in a casual application like this, so this may be overkill.
Thanks for the advice. Basically the mast has a hole where the diamond wire(?) is attached. The newer Hobies already have the retrofit kit installed, which is a stainless steel plate. Unfortunately, the hole is pretty big and I’d like to use a larger plate, so I thought I’d try to fab it myself. The mast is anodized, but the Hobie guy told me to coat everything in LanoCote, a corrosion inhibitor (made from sheep!).
The 5/16" hole is for the bolt that attaches to the two diamond wire anchors and the mast follower. There is a stainless steel tube that provides compression strength against the bolt.