Finite Element Analysis of the Galvanic Couple Current and Potential Distribution between Mg and 2024-T351 in a Mg Rich Primer Configuration

Abstract

The distance over which a Mg-Rich Primer (MgRP) coating galvanically protected exposed AA2024-T351 by sacrificial anode based cathodic protection (termed galvanic throwing power) was studied via finite element analysis (FEA) modeling. FEA enabled prediction of the spatial distribution of the galvanic current density over the surface of a simulated bare AA2024-T351 scratch. The galvanic current density in various full immersion and thin layer electrolyte geometries relevant to field service was investigated. Current and potential distributions extended across the simulated bare AA2024-T351 scratch when the electrolyte layer was thick, continuous and more conductive (higher salt concentration) and in the absence of a resistive coating separating the Mg from the electrolyte. Current and potential distributions did not extend across simulated defects when the electrolyte became discontinuous or the ionic path became tortuous due to drying or the addition of a resistive coating over the Mg. Additionally, galvanic protection intensified during a short period of time associated with drying and re-wetting cycles due to changing electrolyte conductivity and E-i behavior. These results qualitatively predict and agree with the experimental behavior of previously reported microelectrode investigations, of a similar Mg/AA2024-T351 couple configuration, to a first order approximation.