Origins of pitting corrosion

Corrosion of metals and alloys by pitting constitutes one of the very major failure mechanisms. Pits cause failure through perforation and engender stress corrosion cracks. Pitting is a failure mode common to very many metals. It is generally associated with particular anions in solution, notably the chloride ion. The origin of pitting is small. Pits are nucleated at the microscopic scale and below. Detection of the earliest stages of pitting requires techniques that measure tiny events. This paper describes techniques designed to do this and discusses the measurements that result. Some metals show preferential sites of pit nucleation with metallurgical microstructural and microcompositional features defining the susceptibility. However, this is not the phenomenological origin of pitting per se, since site specificity is characteristic only of some metals. A discussion is presented of mechanisms of nucleation; it is shown that the events are microscopically violent. The ability of a nucleated event to survive a series of stages that it must go through in order to achieve stability is discussed. Nucleated pits that do not propagate must repassivate. However, there are several states of propagation, each with a finite survival probability. Several variables contribute to this survival probability. Examples are shown of several metals and some common features of their behaviour are discussed. It is shown that for some systems, the pit sites can be deactivated.