Ulick R. Evans, the British scientist who is considered the "Father of Corrosion Science", has said that "Corrosion is largely an electrochemical phenomenon, [which] may be defined as destruction by electrochemical or chemical agencies...". Corrosion in an aqueous environment and in an atmospheric environment (which also involves thin aqueous layers) is an electrochemical process because corrosion involves the transfer of electrons between a metal surface and an aqueous electrolyte solution. It results from the overwhelming tendency of metals to react electrochemically with oxygen, water, and other substances in the aqueous environment. Fortunately, most useful metals react with the environment to form more or less protective films of corrosion reaction products that prevent the metals from going into solution as ions. While the term corrosion has in recent years been applied to all kinds of materials in all kinds of environments, this article will only consider the electrochemistry of corrosion of metals and alloys in aqueous solutions at ambient temperatures. Electrochemical corrosion occurring under such conditions is a major destructive process that results in such costly, unsightly, and destructive effects as the formation of rust and other corrosion products, the creation of the gaping holes or cracks in aircraft, automobiles, boats, gutters, screens, plumbing, and many other items constructed of every metal except gold. Systems such as boiling water nuclear reactors involving aqueous solutions are also examples of electrochemical corrosion but will not be covered. This article will also not cover the non-electrochemical process termed high temperature oxidation, a destructive process which is the exposure of a metal or alloy to high temperatures in a gaseous environment (usually including oxygen or gases with sulfur containing compounds) where much thicker layers of corrosion products are formed. However, it must be pointed out that if the high temperature oxidation process results in the formation of salt layers that melt at the high temperatures used, a difference in electrode potential between phases or heterogeneities in an alloy can lead to hot corrosion which has electrochemical features similar to that of the condensed aqueous films involved in atmospheric corrosion. This technologically important corrosion process leads to the failure of such applications as gas turbines, heat exchangers, and many others that operate at high temperatures. Consequences of corrosion
Corrosion has many serious economic, health, safety, technological, and cultural consequences to our society. Economic effects
Studies in a number of countries have attempted to determine the national cost of corrosion. The most extensive of these studies was the one carried out in the United States in 1976 which found that the overall annual cost of metallic corrosion to the U.S. economy was $70 billion, or 4.2% of the gross national product. To get a feeling for the seriousness of this loss, we may compare it to another economic impact everyone is worried about – the importation of foreign crude oil, which cost $45 billion in 1977. Health effects
Recent years have seen an increasing use of metal prosthetic devices in the body, such as pins, plates, hip joints, pacemakers, and other implants. New alloys and better techniques of implantation have been developed, but corrosion continues to create problems. Examples include failures through broken connections in pacemakers, inflammation caused by corrosion products in the tissue around implants, and fracture of weight-bearing prosthetic devices. An example of the latter is the use of metallic hip joints, which can alleviate some of the problems of arthritic hips. The situation has improved in recent years, so that hip joints which were was at first limited to persons over 60 are now being used in younger persons, because they will last longer. Safety effects
An even more significant...