Assessing the Corrosion of Multi-Phase Mg-Al Alloys with High Al Content by Electrochemical Impedance, Mass Loss, Hydrogen Collection, and Inductively Coupled Plasma Optical Emission Spectrometry Solution Analysis

The corrosion behavior for several die-cast Mg-Al alloys (AM50, AM50, and AZ91) was compared to commercial purity Mg and AZ31B-H24 utilizing simultaneous measurement of electrochemical impedance spectroscopy (EIS), hydrogen gas collection over a 24 h immersion period, gravimetric mass loss, and inductively coupled plasma optical emission spectrometry (ICP-OES) solution analysis of the total Mg concentration released. Tests were conducted in three electrolytes, unbuffered 0.6 M NaCl, 0.1 M tris(hydroxymethyl)aminomethane (TRIS), and 0.6 M NaCl buffered with TRIS to a pH of 7. EIS derived polarization resistance was monitored periodically, as determined from EIS circuit modeling using data collected to 0.001 Hz, and considering the pseudo-inductive low-frequency impedance time constant. EIS derived corrosion rates and oxidation charge density were similar to charge density determined from cumulative mass loss, ICP-OES solution analysis, and the volume of hydrogen collected for the die-cast AM50, AM60, and AZ91, as well as for Mg and AZ31B determined previously. The variation in the cathodic hydrogen evolution reaction kinetics for the die-cast alloys were also determined over 0, 3, 12, and 24 h immersion periods and compared to commercial purity Mg and AZ31B-H24. The global corrosion rate decreased with increasing Al content, even though Al wt% above the solubility limit (2 wt% at room temperature) resulted in increasing volume fractions of the Al₈Mn₅(Fe), Al₂Mn₃, and Al₃Fe intermetallic particles. Each of the alloys contained varying volume fractions of primary α, β-phase (Mg₁₇Al₁₂), and eutectic α+β depending on Al content and processing. Al in the solid solution α-Mg phase decreased the overall net anodic reaction rate for the Mg²⁺ half-cell reaction. The Mg₁₇Al₁₂ phase was reasoned to not function as a strong cathode as deduced from cathodic E-log(i) studies. Moreover, the extent of anodically-induced cathodic activation was speculated to decrease with increasing Al content, which was a factor in determining overall corrosion rate and accumulated damage. However, corrosion damage depth as determined from a pitting factor analysis increased with Al content.