A crevice is a small gap, crack or hole in a material. Crevice corrosion takes place in these areas. In seawater, a buildup of chloride, silt, sand or other material takes place in a crevice. These substances generally can't be washed out, and remain stuck in the crevice. A chemical reaction between the seawater and these substances takes place, leading to increasing acidity. As a result, corrosion begins and the metal becomes damaged. This can be identified by rusting or discolored patches on the metal's surface.
Galvanic corrosion takes place when two different metals are attached together and submersed in sea water. When water is surrounding the metals, some metal ions dissolve in the water. Seawater is an electrolyte, which means that it carries an electrical current through it. Consequently, as the current passes through the water it moves these particles from one metal to the other. The other metal acts as a magnet for these particles, which then attach to its surface. This is seen as a buildup of discolored residue, like rust, on its surface.
Stress corrosion cracking takes place in specific areas of materials in seawater. The corrosion targets surfaces that are under more pressure than others. The hydrogen, chloride or sulphides in the seawater together with the stress that the metal experiences cause small cracks on the surface of the material. For example, the hull of a ship faces aggressive bashing as it moves through seawater, and combined with the corrosive environment of the water, small cracks may appear.
Pitting is an isolated form of corrosion. Sea water enters small pits, dips or cells of a material and the corrosion takes place there instead of on its entire surface. Like galvanic corrosion, the dips in the material act as magnets. The water surrounds the material and ions from the material are dissolved into it. As the water moves around, it transfers an electrical current, which deposits the particles in the dips. The McNally Institute explains that this buildup accelerates aqueous corrosion and the dips get bigger.