Evaluation of Galvanic Corrosion Phenomenon between α-Mg Matrix and Metallic Dispersoids in Mg Alloys Using Surface Potential Difference

Abstract

Magnesium (Mg) is easy to be corroded in contact with other metals due to the formation of galvanic cell at the interface because it has the lowest negative standard electrode potential (SEP) in industrial metals. The traditional methods to evaluate corrosion resistance such as saltwater immersion test, salt spray test, and electrochemical corrosion test provide the macroscopic corrosion phenomenon, not microscopic information at the local interface. It is important and necessary to clarify galvanic corrosion mechanism at the interface between α-Mg matrix and metallic dispersoids of Mg alloys. The effect of dispersoids in Mg alloys on corrosion resistance hasn't yet evaluated quantitatively as the SEP of dispersoids wasn't almost investigated. In the present study, the surface potential difference (VSPD) at the interface between dispersoids and α-Mg of Mg-Fe cast material and AZ91D alloy was measured by using Scanning Kelvin Probe Force Microscope (SKPFM). Surface potential of pure metals measured by SKPFM had good correlation with SEP, and then VSPD values also corresponded with the theoretical values of the difference in SEP. Salt water immersion test of AZ91D alloy was conducted to clarify the relationship between VSPD measurements and corrosion resistance. Changes in topographic maps around the intermetallic dispersoids were investigated by using AFM before and after corrosion test, and resulted that corroded phases corresponded to the anodic phases indicated by surface potential.