Experimental Considerations in in Predicting Damage during Galvanic Corrosion

Commercially available finite element software based on a solution of La Place's equation offers the opportunity for investigators to predict galvanic corrosion rates for engineering parts with complex geometries.   In these routines, potentiodynamic polarization data are used to determine the current density at a given mesh point based on the solution for the potential distribution.  However, stochastic processes, such as local initiation, can quickly complicate these models as they lead to deviations from the input polarization data.  In this presentation we discuss these experimental considerations for two systems: the galvanic couple between Monel 500 and low alloy steel 9310 and the couple between copper and aluminum.  For the case of Monel / steel in ASTM artificial sea-water, corrosion on the steel anode is relatively uniform and deviations from the polarization data are limited.  However, in the case of Cu/Al in chloride solution the initiation of galvanic corrosion is localized and, more importantly, the current density at the initiation site can be orders of magnitude greater than the current density one would predict from the polarization curve for Al at the couple potential.  We will present experimental data for both of these systems including measurements of damage evolution and local current density using several different techniques.  These results are then compared to model predictions.