Metallurgical and Electrochemical Characterization of the Corrosion of a Mg-Al-Zn Alloy AZ31B-H24 Tungsten Inert Gas Weld: Galvanic Corrosion Between Weld Zones

Several techniques were used to investigate magnesium alloy, AZ31B-H24, joined by tungsten inert gas welding, primarily utilizing multichannel multi-electrode arrays in conjunction with in situ time lapse video, scanning electron microscope and Raman surface analysis, and mixed potential theory. Time lapse optical videos examined the dominant location of the hydrogen evolution reaction and darkening within the various weld zones, which is considered characteristic of anodic zones and cathodic activation in Mg alloys. The galvanic coupling between electrochemically connected weld zones was investigated in 0.6 M NaCl, 0.6 M NaCl saturated with Mg(OH)2, and tris(hydroxymethyl)aminomethane (TRIS). Anodically-induced cathodic activation was also investigated after 3 h, 24 h, and 48 h immersion times in 0.6 M NaCl and 0.1 M TRIS. The main goal was to characterize and explain the galvanic corrosion between weld zones with time. Both the heat affected zones and wrought base plate corrode initially relative to the fusion zone in chloride environments. The variation in the galvanically coupled anodic dissolution rates between various weld zones and subsequent cathodic activation also caused polarity reversal between electrochemically connected zones and was an important factor in weld corrosion. The variation in the extent of this phenomenon can be explained by the presence of Mg(OH)2 films. In welds, the switching of anodes and cathodes is an important effect during galvanic corrosion compared to fixed galvanic couple rankings based on initial electrochemical properties. It was observed that cathodic activation and polarity reversal occurred to a far greater extent in the chloride environments than in TRIS.