Corrosion may be defined as the destruction of a metal by chemical or electro-chemical reaction with its environment. There is also another process by which metal is destroyed and this is known as erosion. Erosion may be defined as the destruction of a material by the abrasive active of a liquid or gas. The wastage or wearing away is purely mechanical with no electro-chemical reaction what so ever. This process usually occurs in pipelines etc. where the flow of liquid along with particles in suspension causes erosion of the pipeline.

Corrosion In The Marine Environment

In some parts of the ship, for example the ship side, there may be a combined effect of corrosion and erosion. In these places the erosion removes the outer surface and exposes fresh surfaces of corrosion.

Chemical Corrosion

The basic cause of erosion of metals is the spontaneous tendency to return their stable state,  that is the natural one, where this reaction takes place in the atmosphere, the metal involved beings steel or iron, it is thought of a as rusting  and as a chemical reaction explained by the simple formula:

Fe + O2 +H2O = Rust

This means that the rust will not form unless both oxygen and water are present. The rate of rusting will vary with humidity. Where the humidity is less than 65% rusting hardly takes place at all. For submerged portions of the ship, like the ship sides, the amount of oxygen present also affects the process and it is seen that rusting occurs much faster near the surface and hardly at great depths. Higher the air temperature, higher the rate of rusting. However, because solubility of oxygen reduces with temperature, the two counteract one another. The velocity of water passing along the steel surface is another factor, the more the velocity, more plentiful the oxygen supply.

Electro-Chemical Corrosion

Local corrosion occurs at a faster rate that general corrosion and is the result of and electro-chemical process. Here we have two reactions, namely the ANODIC reaction where dissolution of the Iron (Fe) occurs and CATHODIC reaction where the oxygen is consumed. This occurs on different parts of the surface and results in local corrosion or pitting. The relative rates of corrosion among different metals are directly related to the Galvanic Series.

In a marine environment, sea water acts as electrolyte and the more noble metal is catholically protected while the less noble metal become the anode and get corroded.

Areas of the ship where dissimilar metals are present and are most liable to corrosion:

1. Stern region due to the bronze propeller
2. In the vicinity of Engine room inlets and discharges.
3. Port holes.
4. Valve fittings in tanks.
5. Welded seams (joints)
6. Aluminum superstructures like mast houses etc.
7. Areas in which the metals may have a different  electrical potential
8. Areas where water accumulates like the deck stringer.