chapter  2
9 Pages

The Corrosion of Steel

The aim here is to provide a general account in relation to the selection and performance of coatings and the operation of other control processes that may be used for structures and buildings.

2.1 CORROSION. THE BASIC PROCESS

The corrosion of steel arises from its unstable thermodynamic nature. Steel is manufactured from iron, which is made in a blast furnace by reducing ores such as haematite (Fe2O3) with carbon in the form of coke. This can be illustrated in simple chemical terms as follows:

This reaction occurs at a very high temperature but the final products, iron and eventually steel, are unstable, a great deal of energy having been supplied in the process. Consequently, when steel is exposed to moisture and oxygen it tends to revert to its original form. Again, in simple chemical terms,

Rust is a hydrated oxide, similar in composition to haematite. This explains why steel tends to rust in most situations and the process can be considered to be a natural reversion to the original ore from which it was formed. It does not, however, explain why steel corrodes more rapidly than most other constructional alloys. All of these, with the exception sometimes of copper, are found in nature in the form of minerals or ores, i.e. they are combined as oxides, sulphides, etc. Energy is expended in producing them either by heating, as with steel, or by some other method. As the natural mineral is more stable, all constructional metals have a tendency to revert back to their original form. However, this tendency, which can be calculated from the thermodynamics of corrosion processes, is concerned with the equilibrium state of a chemical system and the energy changes that occur. Although thermodynamics provides information on the tendency of a reaction to occur, it provides no data on the rate of reaction or, in chemical terminology, the reaction kinetics.