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 de ning the susceptibility. However, this is not the phenomenological origin of pitting per se, since site speci city 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 nite 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.CEST/2124

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A new method for the spatially-resolved measurement of pH during corrosion processes with the scanning electrochemical microscope (SECM) is presented. Antimony tips are employed because the dual-function characteristics of this material allow the combined amperometric/potentiometric operation of the SECM. The applicability of this technique is illustrated by considering the galvanic corrosion of a model zinc-iron pair immersed in 0.1 M NaCl aqueous solution. Spatially resolved images of pH and oxygen concentration above the metal specimens could be obtained in the same experiment.