Characterization of the Corrosion Behavior of a Mg Alloy in Chloride Solution

The influence of the microconstituent phases in the first stages of the corrosion process of the AZ91D Mg-Al alloy in a 3.5 wt% sodium chloride (NaCl) solution was studied at different immersion times. Scanning Kelvin probe force microscopy (SKPFM), scanning electron microscopy (SEM), and electrochemical corrosion techniques were used to track the corrosion process, and electrochemical tests also were used to evaluate the corrosion resistance of the alloy. In the first moments after immersion, as a result of the high activity of the AZ91D magnesium alloy in the electrolyte, the corrosion of the alloy corresponded to the uniform dissolution of the α-magnesium matrix. After 30 min of immersion, the corrosion mechanism evolved to the localized dissolution of the α-Mg matrix by microgalvanic coupling at the interfaces between the micro-constituents (the second phase of β-Mg₁₇Al₁₂ and Al-Mn precipitates), and pits in these zones were seen clearly with both SEM and SKPFM techniques. These microconstituents had a cathodic Volta potential with respect to the rest of the magnesium matrix, as detected by the SKPFM technique in the as-received magnesium alloy. These results confirmed that SKPFM is an adequate technique to evaluate the mechanisms that drive the early stages of corrosion activity.