Loss Caused by Corrosion

CORROSION is a natural process. Just like water flows to the lowest level, all natural processes tend toward the lowest possible energy states. Thus, for example, iron and steel have a natural tendency to combine with other chemical elements to return to their lowest energy states. In order to return to lower energy states, iron and steel frequently combine with oxygen and water, both of which are present in most natural environments, to form hydrated iron oxides (rust), similar in chemical composition to the original iron.

Corrosion can be defined in many ways. Some definitions are very narrow and deal with a specific form of corrosion, while others are quite broad and cover many forms of deterioration. The word corrode is derived from the Latin corrodere, which means “to gnaw to pieces.” The general definition of corrode is to eat into or wear away gradually, as if by gnawing. For purposes here, corrosion can be defined as a chemical or electrochemical reaction between a material, usually a metal, and its environ.

The environment consists of the entire surrounding in contact with the material. The primary factors to describe the environment are the following: (a) physical state—gas, liquid, or solid; (b) chemical composition— constituents and concentrations; and (c) temperature. Other factors can be important in specific cases. Examples of these factors are the relative velocity of a solution (because of flow or agitation) and mechanical loads on the material, including residual stress within the material. The emphasis in this chapter, as well as in other chapters in this book, is on aqueous corrosion, or corrosion in environments where water is present. The deterioration of materials because of a reaction with hot gases, however, is included in the definition of corrosion given here.

To summarize, corrosion is the deterioration of a metal and is caused by the reaction of the metal with the environment. Reference to marine corrosion of a pier piling means that the steel piling corrodes because of its reaction with the marine environment. The environment is airsaturated seawater. The environment can be further described by specifying the chemical analysis of the seawater and the temperature and velocity of the seawater at the piling surface