Processes of Electrochemical Corrosion★

Abstract

Abstract Theoretical considerations involved in determining the potential of hydrogen and oxygen electrodes are reviewed briefly. Overvoltage is discussed in detail. The various connected stages involved in the process of the passage of ions into solution are outlined. Use is made of the Evans diagram for real processes. In making the study of processes with hydrogen depolarization it was found convenient to use a model of a microelement. A comparison of experimental data obtained with models and from deductions from the Evans diagrams indicates that there was a complete agreement between the theory and experiment for processes with hydrogen depolarization. Considerable attention was given to the factors affecting the rate of oxygen diffusion to the metal in view of the fact that the rate of corrosion processes with oxygen depolarization is determined mostly by the rate of oxygen diffusion toward the metal surface. A model was used to examine the part played by the electrical resistance of a cell with oxygen depolarization. It was found that it was only when the resistance became very large and the potential difference was close to the initial value that the current started to decrease. Experiments showed also that changing the anodic material did not influence the rate of the cathodic process when it was controlled by the rate of oxygen diffusion to the cathode. The theory of microcells is examined in detail and a review made of some of the experimental data for processes with hydrogen depolarization. The influence of alloying elements on corrosion rates of metals in various media is discussed. A number of explanations are offered to show why the regularities observed on models are not applicable to corrosion of microelements. The “difference effect” is discussed in connection with corrosion processes having hydrogen depolarization. Other topics considered include electrochemical cells with differential aeration, corrosion processes in a moisture film, and atmospheric corrosion. Extensive data are reported in 54 figures and 6 tables. 3.6.8