The use of polarization data in estimation of the corrosion rate of metals is discussed. The anodic polarization measured by applying a constant current for several seconds is found to be closely connected with the corrosion rate, and a special device was developed to obtain the relationship between the anodic polarization and the corrosion rate. It is shown that the established method can be applied with success to examining the corrosion behavior of given steels in neutral solutions containing inhibitors.

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Variations in the interfacial impedance of the electrode of AISI Type 304 stainless steel in electrolytic solutions of sodium sulphate have been measured as a function of polarizing voltage under anodic polarization. The capacitive component of the impedance exhibits three characteristic changes with potential and these changes correspond to the transitions from the passive to the transpassive state, from the transpassive to the secondary passive state and from the secondary passive to the oxygen evolution state which are observed in the current-potential curve of this steel. However, the changes observed in the capacitance-potential curve always appear at more negative potentials than the potentials at which the corresponding changes are observed in the polarization curve. These phenomena are discussed on the basis of compositional changes in the passive film.

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Corrosion behaviour of plastically deformed steel is very significant problem in process equipments, but there seems to be no established theory on the macroscopic behaviour as well as the microscopic corrosion mechanism of cold-worked steel. The present work has been carried out in order to study the relationships between micro-structure of deformed metal surfaces and the corrosion behaviour under the conditions as close as possible to those of the industrial plants. Detailed discussion is made on the effect of micro-structure on the corrosion rate under wide variety of environmental conditions with special attention to the relationship between the macroscopic and microscopic aspects. Results are summerized as follows: 1) Corrosion rate of steels cold-worked by uni-axial tension or cold-drawn is increased with plastic strain. Although the corrosion rate of cold-drawn steel is not simply increased with induced strain as in the case of deformed steel by uniaxial tension, such complicated corrosion behaviour can be understood by the analysis of mechanical conditions in the drawing process. 2) Micro-structure, such as dislocation, residual lattice strain and crystal orientation are made clear by using micro-focus X-ray diffraction and scanning electron microscope. It was found that the corrosion rates perpendicular to {110} and {112} planes, which are the principal slip planes of body-centered cubic metals, are greater than that perpendicular to {100} plane. 3) Effects of environmental conditions (pH, anion, temperature) on both the corrosion rate and surface geometry are studied. Corrosion behaviour in long term tests is also discussed based on the above mentioned observations.

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The influence of the ion permselective property of the asbestos gasket on the crevice corrosion of Type 316L stainless steel in 0.5N NaCl solution was investigated on comparing with the commercial ion exchange membranes through the polarization measurements and the chemical analysis of the solution within a crevice by using artificial crevice specimens. An anion exchange membrane attached to a stainless steel surface accelerates the crevice corrosion because of increasing the concentration of the dissolved metal ions and chloride ion in the solution within a crevice. The crevice corrosion characteristic of the stainless steel covered by the asbestos is nearly equal to that by the anion exchange membrane, and the electrochemical property of the asbestos is also similar to the anion exchange membrane rather than the cation one. The result of the transference number measurement by the membrane potential method also shows that the asbestos has an anion permselective property, i. e. t_-=0.75, t₊=0.25. These results show that the crevice corrosion is affected by not only the geometrical factor but also the ion permselective property of the insulating materials or the deposits covering the metal surface, and that such a material as the anion exchange membrane accelerates the crevice corrosion remarkably.

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