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.

Crevice Corrosion refers to the localized attack on a metal surface at, or immediately adjacent to, the gap or crevice between two joining surfaces. The gap or crevice can be formed between two metals or a metal and non-metallic material. Outside the gap or without the gap, both metals are resistant to corrosion. The damage is normally confined to one metal at localized area within or close to the joining surfaces.

This most common form of corrosion found on fiberglass boats and it 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. Crevice corrosion involves water, metals and crevices. 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, considered a hanging stainless steel bolt 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 which we can't see, though it usually leaves telltale evidence.

Some examples are given follows

1st (Left) picture of the upper pictures
This is is one of the telltales of crevice corrosion in through hull bolts. The fact that this is Taiwan stainless only makes the problem worse.

2nd picture of the upper pictures
Crevice corrosion of through hull bolts. In this case, the water got at the bolts from the inside. The use of nuts with nylon inserts (shakeproof type) accelerated the process. Notice that the acid etching leaves the metal  very bright. This is a total lack of oxygen to the cavity, whereas the exposed threads look rusty and had more oxygen available. That's because the nut completely wasted away.

3rd picture of the upper pictures
Classic crevice corrosion. In this case caused by a carbon rubber exhaust flapper clamped onto a stainless exhaust flange plate. Notice that the worst corrosion is where the clamp pressed the rubber the tightest.

4th  picture of the upper pictures (right picture- two pictures joint together)
Crevice corrosion of high-pressure pipe (left) and excessive stress cracking corrosion (right).  Conventional austenitic grades can be susceptible to stress corrosion at high temperatures. High alloy austenitic grades are extremely resistant to corrosion. Courtesy of Outokumpu.