Corrosion

The following will treat corrosion as a process which cannot occur without the presence of water and therefore excludes other types of attack, such as those associated with high temperature oxidation or sulphidation.

Corrosion is an electrochemical process in which a metal reacts with its environment to form an oxide or other compound. The cell which causes this corrosion process has three essential constituents: an anode, a cathode and an electrolyte (electrically conducting solution). The anode is the site at which the metal is corroded; the electrolyte is the corrosive medium; and the cathode (part of the same metal surface, or of another metal surface in contact with it) forms the other electrode in the cell and is not consumed in the corrosion process. At the anode the corroding metal passes into the the electrolyte as positively charged ions, releasing electrons electrons which participate in the cathodic reaction. Hence the corrosion current between anode and the cathode consists of electrons flowing within the metal and ions flowing within the electrolyte.

The surface of one component may become the anode and the surface of another component in contact with it the cathode. Usually, corrosion cells will be much smaller and more numerous, occurring at different points on the surface of the same component. Anodes and cathodes may arise from differences in the constituent phases of the metal itself, from variations in surface deposits or coatings on the metal, or from variations in the electrolyte.

The metal may be immersed in an electrolyte or the electrolyte may be present only as a thin condensed or adsorbed film on the metal surface. The rate of corrosion is influenced considerably by the electrical conductivity of the electrolyte. Pure water has poor electrical conductivity and the corrosion rate will be much lower than say an acid solution of high conductivity.