Scale of Nobility
Metals are rated on which is called a Scale of Nobility. It simply means the materials ability to resist this kind of corrosion. There is also a chart called  the galvanic series, which shows the electrical potential of metals in seawater. A more noble metal is one that has a neutral or negative electrical potential. It will not generate a flow of positive ions, and is called noble. The reverse of this is the least noble metal, which has a high positive charge, and which will generate an electrical current. These include such metals as zinc, unalloyed aluminum and copper, iron and steel. Graphite and carbon bottom out the list, being the most highly charged metals.

Crevice Corrosion
This is the most common form of corrosion found on fiberglass boats, and is the least understood. Electrical currents are generated anytime there is a change in chemical composition. That's why powerful explosives can be made of such ordinary things like plastic. As its name implies, crevice corrosion involves water, metals and crevices. For our purpose, a crevice is any cavity that will trap and hold water, while at the same time reducing or eliminating air exposure to the water/metal interface. Crevice corrosion is the same thing as galvanism, only it occurs under different circumstances.

This is also called closed cell corrosion by virtue of the fact that little or no air is allowed to get to it. The water/metal interface results in oxidation of the metal which concentrates the hydrogen content of water, and turns the water into an acid. This changes the electrical make up of the affected materials, generating an electrical current that dissolves the metal involved. These crevices or closed cells can become dynamic, meaning that the process can perpetuate itself for a long time, either until the acidic water is exhausted or an oxygen source is created that lowers the acidity of the water and stops the corrosion. If no oxygen source is introduced, the corrosion process continues until the metal is completely gone.

To illustrate this phenomenon, consider a stainless steel bolt is hanged over the side on a string. It would hang there forever and nothing would happen to it. But put that faster into the bottom of a hull and watch what happens. Water gets into the screw or bolt hole where there is no free-flow of water, so that the small amount of water in the screw crevice turns acidic and creates a galvanic cell. This usually occurs right under the screw or bolt head, eroding the shank of the screw or bolt until it becomes loose. Once it does become loose, then a better flow of Ph balanced water is introduced, and the corrosion stops because the water is no longer acidic. Virtually the same thing will occur with stainless fasteners into an aluminum mast. But in this case, the corrosion stops as soon as the water evaporates from the crevice. In the case of an aluminum fuel tank, installed in such a way as that water gets trapped against the tank, like a foamed in place tank, or a tank sitting on a plywood deck, the very same thing happens. Which tends to leave us mystified why you could throw your aluminum parts over the side and they'd sit there forever without corroding, while the seeming protected parts on your boat corrode badly. Crevice corrosion always occurs in places you can't see, though it usually leaves telltale evidence.

Stress Corrosion
It is another form of corrosion and occurs when a metal is under heavy stress. This is a combination of crevice corrosion cells combined with heavy loading. It most often occurs on sailboat rigging and power boat propeller shafts. Old style swage fittings on sail boat rigging combines both stress and corrosion cells from entrapped water within the swaged cable. It also occurs at mast rigging attachments where water is entrapped between the mast and bolt-on parts, or even getting under welded parts.

Propeller shaft breakage has reach almost epidemic proportions these days. That's because builders are opting for low grade stainless shafts made of lesser alloys. All it takes is for a tiny pit to form on a shaft to initiate the crevice/stress corrosion cycle that will ultimately result in fatigue failure. This usually occurs at the stuffing box or keyway cuts, the natural weak points. Good propeller shafts don't break because they don't corrode. If you have this problem, it's ultimately a question of how many new shafts do you want to buy before you replace them with better quality.

Highly polished stainless steel is more corrosion resistant than those without a mirror finish. The reason is that unpolished metal has machine marks on it that serve as crevices for corrosion to start. Polishing smoothes these crevices over. However, high polishing won't help much for lesser grades of stainless.

From
A Boatman's Primer on the Essentials
by David Pascoe, Marine Surveyor