Finite Element Modelling of Microelectrode Arrays of Mg and AA2024 Galvanic Couple

Abstract

Aerospace aluminum alloys, such as AA2024-T3, are susceptible to microgalvanic corrosion and pitting due to its heterogeneous microstructure. Thus, they require additional corrosion protection methods. Magnesium rich primer coatings (MgRP) have emerged as an effective alternative to highly toxic chromate-based coatings. MgRP systems consist of an organic resin loaded with Mg pigment. The organic resin provides barrier protection, while metallic Mg pigment serves as a sacrificial anode by galvanic coupling with more noble Al alloy. The distance over which an active corrosion system can protect a defect exposing a metal surface is named galvanic throwing power and it is an important parameter used to evaluate the effectiveness of this type of protection method. Microelectrode array (MEA) is a technique commonly used to determine the galvanic throwing power. In this technique, wires of the metals of interest are mounted in a non-conductive matrix and the galvanic current distribution over the wires is measured. The wire diameter and the distance between the wires are important factors that affect the accuracy of MEA. While thin wires can provide finer spatial resolution, the experimental setup is challenging, and edge effects can decrease the representativeness of the measurement. Finite element analysis (FEA) is a tool used to predict both spatial and temporal potential and current distributions in electrochemical systems related to metal dissolution and cathodic reactions. FEA also can be used as an experimental design tool. In this work, FEA was used to assess important parameters and predict an optimal combination of wire diameters and distance between wires that result in higher accuracy of MEA. COMSOL Multiphysics (ver.5.4) was used to perform the finite element simulations. Potentiodynamic polarization scans of AA2024 and pure Mg immersed in 0.9 M NaCl solution were performed to generate E - i curves that were used as the electrochemical boundary conditions. Laplace equation was used to describe the current distribution that resulted from the galvanic coupling. The effect of the spacing between the wires and their diameters on the current distribution was studied. The results were compared to a reference solid plate configuration, where the two metals were in electrical contact.