In any discussion of the mechanism of a chemical reaction, it is advisable to separate the factors which determine the tendency or driving force of the reaction to proceed from those which influence the rate of the reaction made possible by the existence of this tendency.

This tendency is an expression of the fact that the system is not in a state of equilibrium (or inherent stability), and it is measured by the difference in energy between the initial and final state of the system for any particular case. In the most cases the observed rate is determined not by the absolute magnitude of this tendency but by other factors, which depend primarily upon the environment.

In considering the group of three typical reactions involved in corrosion, primary factors will be denoted, which determine the tendency of the metal to corrode and thus influence its initial rate of solution and as secondary factors those which influence the rate of the subsequent reactions. This term in no wise implies that these secondary factors are of lesser importance; in fact, by influencing the nature and distribution of the final corrosion products, they usually determine the ultimate rate of corrosion, and the useful life of the metal, in each environment. Generally, one or two of the many factors involved exert outstanding influence upon the ultimate rate of corrosion; these we term controlling or dominant factors. In general, the primary factors have to do with the metal (or alloy) itself ; the secondary factors more with the specific environment. It is convenient to divide them in this way, although no sharp distinction can be made.

Accordingly on this basis some of the more important factors, discussing their general significance with respect to the mechanism of corrosion, are listed below:(Factors Associated Mainly with the Metal)

• Effective electrode potential of a metal in a solution
• Chemical and physical homogeneity of the metal surface
• Inherent ability to form an insoluble protective film
• Overvoltage of hydrogen on the metal

• Hydrogen-ion concentration (pH) in the solution
• Temperature
• Influence of oxygen in solution adjacent to the metal
• Specific nature and concentration of other ions in solution
• Rate of flow of the solution in contact with the metal
• Ability of environment to form a protective deposit on the metal
• Contact between dissimilar metals or other materials as affecting localized corrosion
• Cyclic stress (corrosion fatigue)