Surface evolution of a corroding metal as a moving boundary problem by random assignment of anodic and cathodic sites

Abstract

During corrosion a metal surface dissolves when it is immersed in a corrosive electrolyte. The rate of loss of thickness of the metal known as corrosion rate is usually found based on weight loss data from immersion experiments. Similarly, polarization tests also provide an estimate of corrosion current which in turn gives corrosion rate. This type of quantification does not provide information on the unevenness of the corroded surface. In the present work, a simulation is developed based on random assignment of anode and cathode sites. Linearized polarization curves are used with slightly different Ecorr values for anode and cathode sites. A 2D Laplace equation is solved for the electric potential distribution in the electrolyte. Thus uniform corrosion is simulated as a micro-galvanic corrosion process by assuming the ionic conductivity in the electrolyte as the limiting step to determine the anodic current density on the anodic sites and the surface profile is updated as a 2D moving boundary problem. Interesting shapes of the corroding surface are obtained. The average depth of the surface is found to vary linearly with time indicating a constant corrosion rate. On the other hand the standard deviation of the surface profile also increased with time indicating a persistent unevenness of the corroding surface. Sensitivity studies for varying polarization slope and anode fraction are included.