Almost all metals used on boats are an alloy of two or more metals. Stainless steel, brass, bronze, and sometimes aluminum are all alloys. These metals can be susceptible to internal galvanic currents in the presence of seawater, which leads to galvanic corrosion of the least noble metal in the alloy.
Brass is a case in point. Brass is very “shippy.” You find nicely polished brass fittings all over the place onboard ships and yachts, so you might conclude that brass is a good metal for use in a marine environment. In fact, brass is copper alloyed with up to 40 percent zinc, so if brass fittings are used below the waterline the zinc is going to corrode away just like the sacrificial zinc anodes we just talked about in a process called dezincification. The remaining copper will crumble and fail and well, there goes your seacock, leaving a 2-inch hole in the bottom of your boat. Never use brass valves or fittings below the waterline. Always use marine grade bronze.
Another somewhat different example is stainless steel. Stainless steel is alloyed with metals such as nickel and chromium, and develops an oxidized layer that actually requires the presence of oxygen to stay intact. When stainless is deprived of oxygen, corrosion begins and it’s not so stainless anymore. Where this causes the most problem is in crevices such as the space around a stainless bolt in a hole, under deck fittings, or around a propeller shaft, where this is little or no oxygen available. When you see rust stains coming from stainless steel fittings like this, you are seeing evidence of crevice corrosion. When stainless steel experiencing crevice corrosion it is said to be active and is much lower in the galvanic series than stainless steel in a passive state.
Much the same process occurs with aluminum. Whereas aluminum will work fine as hull material, it suffers from crevice corrosion when used for fuel tanks with no breathing space around them or that sit on a damp surface.