Some recent experience with the scanning electron microscope in corrosion research

Corrosion and oxidation are the results of chemical reaction between a solid surface (such as of a metal or alloy) and its environment, to form a corrosion product. The product may adhere to the surface, thereby usually providing some protection from further attack, or may be removed from it (e.g. by volatilization or by dissolution in the environment). In either case this leads to changes in topography, morphology and, often, composition of the surface. Study of such changes can provide considerable information and assistance in the determination of the mechanisms of degradation. Scanning electron microscopy with associated microanalysis has thus enabled significant advances to be made in understanding many corrosion and oxidation processes. The progress attained in several areas where the technique has played a major part but has been extended to its maximum capabilities is discussed and related to the particular advantages and limitations of scanning electron microscopy in this field. One such area is the study of high-temperature corrosion processes. Here, the high resolution and depth of focus of the instrument have provided detailed information on the microstructure of high-temperature oxide and other corrosion-product scales and underlying alloy. For instance, the sizes and distributions of pores in the scales give important insight into the possibility of growth mechanisms involving gas-phase transport within the scales themselves. Associated microanalysis can assist in identification of rate-determining layers and of other localized diffusion paths which can short-circuit bulk lattice diffusion.