Corrosion Inhibition

Abstract

Changes in the composition of otherwise corrosive environments to reduce or prevent the corrosion of metals and alloys is widely practiced and has resulted in an extensive literature that will not be reviewed here. The inhibition of corrosion by chemical control of the environment is frequently defined in electrochemical terms,⁽¹⁾ since corrosion itself is a combination of at least two electrochemical electrode reactions, involving, respectively, electron sinks at which metal dissolution occurs and electron source areas at which an electronation reaction, such as the deposition of hydrogen, occurs. It follows that if the velocities of either or both of these electrode reactions, or any of the partial steps which they may involve, can be reduced, then some degree of inhibition of the corrosion will ensue. Such a simple definition of inhibition is preferable to a chemical one, in which substances that prevent the corrosion of a particular metal are defined in terms of their chemical constitution. Thus the species that promotes inhibition may be generated in situ,as opposed to being added to the environment from an external source, and moreover some of those substances which may be regarded as chemical inhibitors of a particular form of corrosion may indeed stimulate other forms. For example, those carbonates, hydroxides, nitrates, phosphates, etc. that are used to prevent the general corrosion of mild steels in some circumstances can themselves promote the stress corrosion of such steels. While then an electrochemically based definition of inhibition involving velocity reductions of the constitutive reactions is preferable, there are circumstances in which such a definition is somewhat deficient. Localized forms of corrosion (pitting, intergranular corrosion, stress corrosion, corrosion fatigue) by their very nature are frequently more objectionable in service situations than is general corrosion, in which a similar amount of metal dissolution is more widely dispersed over the exposed surfaces. Localized forms of corrosion involve a number of critically balanced features including, of course, the relative areas and sites for the dissolution reactions on the one hand and those for the electronation reactions on the other. While it is the case that reductions in the velocities of these reactions will promote inhibition to some degree, it is also the case that stimulation of corrosion can sometimes inhibit localized corrosion. Thus the intergranular stress corrosion cracking of mild steels in hot nitrate solutions can be inhibited by the addition of sufficient chloride to the solution, with an attendant change in the amount and distribution of dissolution. While it is preferable therefore to consider inhibition in electrochemical terms, it should be remembered that there are special circumstances in which stimulation can inhibit certain forms of corrosion, as well as a reduction in the velocities of any or all of the partial reactions.