Boat hardware and fittings
This category holds stainless hardware and fittings: eye plates, handles, snap fasteners, hinges and carabiners. Salt water calls for 316 stainless with molybdenum, fresh water gets by on 304, but mixing the two in one joint does not pay. Stainless rusts mainly in the crevice under the fitting, where it runs out of oxygen. And a fitting working in tension is through-bolted and backed from below with a plate, so that the load spreads.
Boat hardware and fittings
This category holds stainless hardware and small fittings, that is eye plates, eyes, handles, snap fasteners, hinges and carabiners. Most of it holds something else in place, so it is not picked on looks but on the material and on the way it is fastened. By material we mean here above all the stainless grade, because on the water the difference between the two common grades shows within a single season. Canvas, covers and their fastenings belong here too, while engines, lights and steering have categories of their own.
The first thing is the stainless grade, 304 or 316. The difference lies in molybdenum, of which 316 carries two to three per cent and 304 none at all. It is molybdenum that holds the steel against chlorides, so salt water calls for 316, while in fresh water 304 holds up. Mixing the two in one joint is the worst option, because the weaker piece corrodes first and the joint lasts only as long as that piece.
The second is crevice corrosion, and it is exactly why stainless rusts under a fitting. The steel protects itself with a thin oxide film that it renews on its own, only it needs oxygen to do so. In the narrow wet gap under a plate or under a bolt head the oxygen runs out, the film does not renew and the metal starts pitting precisely where you cannot see. That is why bedding compound goes under a fitting and the joint is made so that water runs out of it rather than standing in it.
The third is galvanic corrosion when metals are mixed. When a stainless bolt meets an aluminium part and salt water joins in, a cell forms in which the aluminium is the one sacrificed. The stainless stays, whereas the aluminium crumbles around the hole, and it does so even when the bolt looks new. It is solved by separating them, that is with a nylon washer or bush, or failing that with a thread compound.
The last is eye plates and spreading the load. A fitting on thin laminate does not tear out because it is weak but because the pull is held by only a few square centimetres around the bolt. So it is through-bolted and backed from below with a plate that reaches past the outline of the fitting, because that spreads the load over an area the laminate can carry.