Metallurgical and Electrochemical Characterization of the Corrosion of AZ31B-H24 Tungsten Inert Gas Weld: Isolated Weld Zones

The corrosion behavior of a magnesium (Mg) alloy, AZ31B-H24, joined by tungsten inert gas welding was investigated. The corrosion rates and morphology were characterized for each weld zone in isolation, and the microstructural and compositional factors that control corrosion were identified. Initially, the corrosion resistance of various isolated weld regions were determined utilizing electrochemical impedance spectroscopy (EIS). Major microstructural changes included grain growth and crystallographic orientation within the weld regions. In addition, solute solidification microstructures, composition in the alpha-Mg phase, and the area fraction of secondary phases changed significantly as a result of the formation of Al and Zn solidification structures within the weld fusion zone. Variations in corrosion rate by weld zone were rationalized in the context of microstructure attributes and anodically-induced cathodic activation. The polarization resistance, R-P, over time was evaluated by EIS, at the low-frequency limit, incorporating full consideration of the pseudo-inductive impedance behavior of Mg. Integrated, EIS-based corrosion rates showed a strong correlation to cumulative mass loss; inductively coupled plasma optical emission spectrometry solution analysis for Mg2+ concentration and hydrogen gas were collected for each isolated weld zone. The wrought base plate exhibited a greater corrosion rate than the isolated weld zones. However, the fusion zone possessed the fastest corrosion rate of the designated weld zones and the distinct heat-affected zones had the slowest corrosion rates. The metallurgical factors accounting for these distinct corrosion rates are discussed herein.