A study of the electrochemical processes in limited microvolumes at the apices of sharp ended defects makes it possible to obtain data as ~ result of which certain aspects of crevice and pitting corrosion or corrosion cracking of metals may be explained. Access to a crevice defect by oxygen and the solution from the general volume is difficult and hydrolysis of metal ions under such conditions changes the pH of the solution and there is a shift in the electrode potential (Est) of the apex of a cracklike defect in comparison with the integral steady potential of the general surface of the investigated object. In this process a definite role must be assigned to the thermodynamic stability of the water since depending upon the pH of the solution and the value of the electrode potential either decomposition of the water with the liberation of hydrogen or oxygen or the formation of water may occur [i]. Despite attempts at direct measurement of some electrochemical characteristics (electrode potential, solution pH) in the microvolume at the apex of cracklike defects, there is still a lack of clarity with regards to this question. It has been clearly established only that under these conditions at the apex of a sharp-ended defect (or crack) in tests with low alloy and 18-8 chrome--nickel austenitic stainless steels and titanium alloys the pH of a chloride solution drops to 3.5-1 with a solution pH in the macrovolume of 6.3 [2-5]. The sharp change in the pH of the solution in the microvolume of a cracklike defect occurs as a result of separation of the anodic and cathodic elements when the products of the electrode reactions are not mixed. Depending upon the nature of the hydrating ions, there is a change in the concentration of hydrogen ions and, consequently, in the pH of the solution and the intensity of this change predetermines the course of the further local electrochemical process. To enlighten the role of the individual hydrating elements of alloys during local electrochemical corrosion, a series of experiments was made with separation of the anolyte and catholyte in corrosion in a 3% NaCI solution (pH = 6.5) with the application of anodic and cathodic polarization of 12KhI8NIOT steel, AT3 titanium alloy, DI6 aluminum alloy, and L-59 brass, and also of metals included in their composition either as bases or as alloying elements (Table I). The anodic polarization made it possible to judge the degree of change in the solution pH in the anolyte in intense corrosion of an alloy or metal. The cathodic polarization almost completely suppressed general corrosion and a change in the characteristics of the solution occurred as a result of decomposition of water: H20 + e- + 1/2 H2 + OH-. The tests were made in an electrochemical cell with volumes of anolyte and catholyte separated by a glass filter (Fig. I). The volume of anolyte solution in which the pH was determined and the chemical analysis of it for the presence and quantity of metal ions was made was 16 cm s. The working electrode was polarized with a P-5827M electronic potentiostat, a silver chloride electrode was used as the comparison electrode, and the auxiliary electrode was a platinum plate. In order to decrease the influence of oxygen on the electrochemical processes in the investigated material (working electrode) cleaned and dried argon was bubbled through the solution, which partially imitated the character of the solution in the microvolume of a developing microdefect or a narrow slit, access of oxygen to which from the macrovolume is difficult and in which convective solution exchange is absent and diffusion processes are weak. The lO-cm 2 working electrode was made of plates of the investigated materials rolled to a thickness of 0.i mm.