Historical Theories on Corrosion by UR Evans

Corrosion in an aqueous environment and in an atmospheric environment 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. In this context, the term anode is used to describe that portion of the metal surface that is actually corroding while the term cathode is used to describe the metal surface that consumes the electrons produced by the corrosion reaction. (reference)

An electrochemical reaction is defined as a chemical reaction involving the transfer of electrons. It is also a chemical reaction which involves oxidation and reduction. Since metallic corrosion is almost always an electrochemical process, it is important to understand the basic nature of electrochemical reactions. The discoveries that gradually evolved in modern corrosion science have, in fact, played an important role in the development of a multitude of technologies we are enjoying today. These principles are illustrated with the use of a Daniell cell in which copper and zinc metals are immersed in solutions of their respective sulfates. The Daniell cell was the first truly practical and reliable electric battery that supported many nineteenth century electrical innovations such as the telegraph.

The fact that corrosion consists of at least one oxidation and one reduction reaction is not always as obvious as it is in batteries. The two reactions are often combined on a single piece of metal as illustrated schematically here for a piece of steel and in the following Figure for a piece of zinc immersed in an acidic solution.

Electrochemical reactions occurring during the corrosion of zinc in air-free hydrochloric acid

In this Figure, a piece of zinc immersed in hydrochloric acid solution is undergoing corrosion. At some point on the surface, zinc is transformed to zinc ions, according to equation. This reaction produces electrons and these pass through the solid conducting metal to other sites on the metal surface where hydrogen ions are reduced to hydrogen gas according to equation.

These equations illustrate the nature of an electrochemical reaction for zinc. During such a reaction, electrons are transferred, or, viewing it another way, an oxidation process occurs together with a reduction process. The overall corrosion processes are summarized in the following equation:

Briefly then, for corrosion to occur there must be a formation of ions and release of electrons at an anodic surface where oxidation or deterioration of the metal occurs. There must be a simultaneous acceptance at the cathodic surface of the electrons generated at the anode. This acceptance of electrons can take the form of neutralization of positive hydrogen ions, or the formation of negative ions. The anodic and cathodic reactions must go on at the same time and at equivalent rates. However, corrosion occurs only at the areas that serve as anodes.