Metal materials have become indispensable and important materials in economic construction, industrial production and daily life, especially the ferrous metal - steel, occupies a greater proportion. Its production has become one of the important symbols to measure a country's production level.

Although the metal is in continuous production, but at the same time there are a large number of metal ether under the influence of various external media and continuous corrosion. Corrosion is the chemical or electrochemical destruction of a metal in contact with an external medium, usually a liquid or a gas. According to the different reactions in the corrosion process, metal corrosion can be divided into two categories: chemical corrosion and electrochemical corrosion.

Corrosion occurs when no current is generated when the metal is in a dry gas (there is no moisture condensation on the surface, or when the surface chemically interacts with the medium in a non-dielectric solution) and is called chemical corrosion. Such as steel rolling metal oxidation at high temperature, in the organic liquid alcohol, oil and other corrosion.

When the metal material and the dielectric chemical action when the current is generated, then the corrosion is called electrochemical corrosion. This is mainly due to the metal material and dielectric action, in the metal interface electrochemical inhomogeneity, the formation of galvanic cell, so in the same metal surface or internal there is a different tendency to get electrons and lose electrons, produce micro current action and metal destruction.