A sequential non-iterative approach for modeling multi-ionic species reactive transport during localized corrosion

Multi-ionic reactive transport modeling can provide a better understanding of localized corrosion in iron and steel alloys. However, one-step numerical methods used to solve reactive transport equations in a fully-coupled manner may suffer from poor conditioning and numerical convergences issues. In this paper, a sequential noniterative approach (SNIA) is developed to enable robust numerical simulation of multi-ionic reactive transport models for localized corrosion. The electro-diffusive mass transport equations are decoupled from the homogeneous equilibrium chemical reactions equations, and are solved sequentially without iteration, leading to a two-step numerical method. The nonlinear mass transport equations are discretized using the standard finite element method, linearized and solved using the Newton-Raphson method. To ensure the non-negativity of ionic concentrations, the nonlinear chemical reactions equations are solved using the Newton-Raphson method together with the under-relaxation technique. Numerical studies, including model comparison, parametric and experimental validation studies, are conducted to simulate localized crevice/cavity corrosion of generic carbon steel and iron-chromium binary alloys and to demonstrate the efficacy of the proposed approach.