Methods of Physical Simulation of Electrochemical Processes at the Tip of a Crack

Abstract

We analyze stresses and the degree of plastic deformation at the contours of cracks of two types, namely, a sharp crack and a crack with slip lines. We show that at the tip of a crack of the second type, at the contour of the bottom–walls of the crack galvanic couple, there appear two symmetric microgalvanic couples at the slip lines. Microinhomogeneities of a metal (grain boundaries, nonmetallic inclusions, etc.) are submicrogalvanic couples whose activity increases due to inhomogeneity of microplastic deformations of their vicinities. We distinguished the mechanisms of corrosion-cyclic fracture and corrosion-static cracking. Under cyclic loading, breaking of grains occurs and grain boundaries and nonmetallic inclusions fail, which levels the influence of currents of submicrogalvanic couples on the growth rate of a crack. Under static loading, micro- and submicrogalvanic couples are permanently operating, the cold work of the surfaces of the metal of the walls of the crack is lower than that under cyclic loading, while the difference of potentials and the current of the bottom–walls of the crack galvanic couple are greater than those under cyclic loading. This explains both the phenomenon of branching and the higher growth rate of a crack of corrosion-static cracking with time. In addition to the electrode potential and pH of the medium, we propose to consider the following parameters as defining factors of the growth of a crack: currents of a galvanic couple, micro-, and submicrogalvanic couples and the time of repassivation of freshly formed metal surfaces appearing in the process of the growth of a crack. According to analysis of the literature data, these parameters cannot be experimentally determined. For this reason, we propose to use methods of simulation to investigate distributions of the potentials and currents of corrosion at the tip of a crack. We propose certain structures of physical and mechanoelectrochemical models of a crack and methods for measurement of distributions of the current density and the electrode potential at their surfaces.