Corrosion of Fusion Welded Magnesium-Aluminum Alloy, AZ31: Effects of Microstructure and Composition in Isolated Weld Zones

Abstract

Executive SummaryThe corrosion behavior of tungsten inert gas (TIG) welded magnesium alloy AZ31B was explored using a variety of approaches. The main goals were to elucidate the microstructural and compositional factors that control corrosion and provide insight for corrosion mitigation strategies. In the initial work, the corrosion resistance of the as-received commercially pure Mg rod, AZ31B-H24 wrought base plate and AZ31B-H24 TIG welds were investigated by DC electrochemical methods as well as a 24 h open circuit potential (OCP) measurement complimented by electrochemical impedance spectroscopy (EIS) in 0.6 M NaCl solution. This approach provided a reliable indication of corrosion rate corroborated using four independent methods (1) mass loss, (2) EIS, (3) hydrogen gas collection and (4) Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) chemical analysis of exposure solutions. Analysis and comparison of corrosion rates of the wrought base and the isolated weld zones using these four parallel measurements enabled an unparalleled and reliable estimation of weld zone corrosion rate and provided a clear analysis of weld corrosion as a function of heat affected zone (HAZ), base and liquated base metal weld zones.Weld structure and composition were characterized with optical microscopy and SEM.Significant changes to the alloy microstructure and corrosion behavior were observed after welding. The differences in corrosion rate can be attributed to microstructure attributes in the weld fusion zone (FZ) and HAZ. These include constituent particles which grow by Ostwald ripening within the α-Mg matrix during processing, eutectic solidification structures as well as recrystallization to form various grain sizes seen throughout the various weld zones. This dissertation is focused on first identifying and isolating the microstructural variables within the weld and then assessing how these factors in isolation affect the corrosion rate. These include mainly grain size, intermetallic particle (IMP) formation and distribution, solute segregation and crystallographic orientation. Specifically, the FZ was characterized by a randomized texture, large grain size and Al-Zn rich solidification boundaries. The HAZs were characterized by a basal texture, large recrystallized grains and larger, more widely spaced IMPs (in comparison to the wrought base). Further mechanistic understanding of corrosion as a function of weld zone was investigated by attempting to isolation and examine the effect of specific microstructural details on the corrosion rate.ii The galvanic interactions between zones were also investigated using multichannel multielectrode arrays (MMAs) comprised of sectioned weld zones in conjunction with in-situ time lapse video, scanning electron microscope (SEM) surface analysis and mixed potential theory analysis. The controlling region in weld corrosion was determined from time lapse videos by examining the dominate location of the hydrogen evolution reaction (HER) and darkening within the various weld ...