Passivation and Potential Fluctuation of Mg-Alloy AZ31B in Alkaline Environments
Authors: Cong Hongbo , Li Shengxi , Birbilis Nick 
Published: February 2020

The pH effect on the passivation and corrosion behavior of Mg alloy AZ31B was investigated in chloride-free NaOH solutions in the range of 10 to 14. The electrochemical behavior of α-Mg matrix, secondary (β-phase) Mg17Al12, and constituent intermetallic particles (AlxMny) were compared in these solutions using various techniques such as open circuit potential (OCP) monitoring, potentiodynamic and potentiostatic polarization, scanning vibration electrode technique (SVET), microcapillary cell technique as well as electrochemical atomic force microscope (EC-AFM) technique. Optical microscope (both in-situ and ex-situ) was utilized to characterize corrosion morphology of AZ 31B alloy in different pH NaOH solutions and yield information on the cathodic and anodic behavior of different phases and particles. Microscopic examination of the passive film formation and composition on different phases/particles was carried out via field emission scanning electron microscope (FE-SEM) equipped with energy dispersive spectroscopy (EDS). Gallium focused ion beam (FIB) was used to examine the cross section of different phases after exposure. The results indicate that significant potential fluctuations (from -1.6 VSCE to -0.4 VSCE) occurred to AZ31B in high pH (≥ 13) NaOH solutions with the presence of oxygen. Removal of oxygen by argon sparging diminished the fluctuation range (from -1.6 VSCE to -1.3 VSCE). Oxide nodules and associated cracks were found at the interface between α-matrix and β-phase. Thick oxide film was found to form on the α-matrix and the thickness decreased as the distance increased from the α/β interface (~1500 nm) into the α-matrix (~700 nm). In contrast, oxide film thickness of <100 nm was determined on β-phase. AlxMny particles were found to preferentially dissolve in high pH NaOH solutions and crevice was identified at the interface between Mn oxide layer and α-matrix. In relatively low pH (≤ 11) NaOH solution, OCP remained constant at -1.4 VSCE during long term exposure with the presence of oxygen. No significant potential fluctuation was observed. SEM/FIB examination indicated that corrosion occurred on the α-matrix in these NaOH only environment. In comparison, β-phase remained intact with thin passive film. Mg(OH)2 dome formed on AlxMny particles suggesting cathodic nature of the intermetallic particles. This was also corroborated by the in-situ microscopic examination of the gas formation on the AlxMny particles. OCP and polarization experiments suggest that hydrogen evolution reaction is the dominant cathodic reaction at these pH levels and the bubbles were found to initiate and grow preferentially on the AlxMny particles. At pH 12, thin oxide film (<100 nm) formed across the entire sample surface without any preferential corrosion and/or passivation on different phases/particles. The OCP remained constant at -1.3 VSCE during long term exposure. This was approximately 0.1 V higher than the value observed in pH 11 solution and no significant potential rise and drop was observed. Both potentiodynamic and potentiostatic polarization indicated that anodic current density decreased with increasing pH from 10 to 12 but increased with increasing pH from 12 to 14. Lowest passivation current density was identified at pH 12, which agrees with SEM/FIB and optical microscopic examinations. It is hypothesized that below pH 12, microgalvanic effect exists between α-matrix (anode) and AlxMny particles (cathode), which leads to the preferential dissolution of α-matrix. This was verified by SVET technique and AlxMny particles were found to be the cathode compared to α-matrix. At pH 13 and above, better passive film formation on β-phase may lead to the significant increase in OCP where the dominant cathodic reaction switched to oxygen reduction reaction, which in turn caused significant oxidation of α-matrix by microgalvanic coupling. AlxMny particles were anodic compared to the α-matrix and preferential dissolution of Al is suggested by SEM/EDS.