Most materials, particularly metals and alloys, when exposed to most environments, are affected on the surface by simple oxidation, or electrochemical (electrolytic) or chemical reactions with the acidic or alkaline agents which may be present. Corrosion may be either wet or dry according to whether the environment is primarily liquid or gaseous, and may take the forms of superficial scaling (in some cases resulting in the formation of a desirable protective layer) or pitting, galvanic attack between dissimilar metals, intergranular cracking due to localized attack at grain boundaries, stress corrosion, dezincification or selective leaching of alloys, erosion-corrosion in the presence of wear, high temperature corrosion, or microbial and bacterial corrosion. The rate of corrosion often depends upon atmospheric conditions: rainfall and relative humidity, as well as air pollution, all tend to accelerate the process. As a health factor, corrosion in water supply systems (pipes, tanks, wells, etc) causes impurities directly, and also indirectly by allowing foreign matter to enter into the system.
Estimates of corrosion costs per year based on a 1978 study for an industrialized nation found that the cost of corrosion was approximately 4.2% of the Gross National Product. Of this total, about 15% was estimated to be avoidable through the application of best practice corrosion control technology.