A simple definition of corrosion is the deterioration of a material or its properties due to the reaction with the environment. Sometimes the deterioration is a weight gain; sometimes it is a weight reduction and sometimes the mechanical properties are affected. Most corrosion processes are electrochemical in nature.
TYPES OF CORROSION
The simplest form of corrosion is a uniform attack of all surfaces that are exposed to a corrodent. It can be electrochemical in nature or simply a direct attack.
Pitting is a local corrosion damage and caused is by the chemical nature of environment. Solutions that tend to produce pitting are brackish water, salt water, chloride bleaches, and reducing inorganic acids. Certain metals such as stainless steel are particularly prone to pitting attack.
Crevice corrosion is a local attack in a crevice between metal to metal surfaces or between metal to non metals surfaces. One side of the crevice must be exposed to the corrodent and the corrodent must be in the crevice. Crevice corrosion commonly occurs in poorly gasketed pipe flanges and under bolt heads and attachments immersed in liquids.
Dealloying is a process whereby one constituent of metal alloy is removed from the alloy, leaving an altered residual microstructure. The most common alloy susceptible to this process is yellow brass. The removal of zinc from brasses is called dezincification.
The use of existing corrosion data is the first step in solving corrosion problems or preventing potential corrosion problems. To simplify the subject of corrosion control, there are three factors that have equal weight in importance : material selection , environment control and design.