Corrosion Mechanisms of Fusion Welded Magnesium Alloys As a Function of Microstructure

Abstract

Over the last 15 years, the naval and automotive communities are seeking to expand the use of magnesium (Mg) alloys in light-weighting applications [1]. Mg alloys are also potential alternative materials for ultra-lightweight cellular truss structures [2-5] due to their low density and relatively high specific strength [6]. These properties can be combined with novel sandwich configurations to make low cost, lightweight structures. However, joining strategies remain an important issue. Research has been conducted on the processing and development of aluminum truss structures joined by a combination of laser welding, inert gas welding, and brazing [4, 5, 7]. However, in comparison to Al, corrosion of welded Mg is a challenge, as corrosion mitigation in mildly aggressive and pH neutral conditions is difficult. The focus of this presentation is on tungsten inert gas (TIG) welding and its implications towards corrosion reviewed in isolation in each zone. Previously, there has been considerable focus on the characterization and optimization of weld parameters for Mg-Al alloys, such as AZ31. The addition of Al improves weldability in terms of melting, flow and solidification characteristics. As an alloying element, Al ennobles the corrosion potential and Mg-Al alloys have the lowest corrosion rates for current commercial Mg alloys up to ~3 wt% of Al [6]. Above this concentration of Al, the corrosion rate of the alloy begins to increase. However, the joining of Mg structures via conventional welding introduces additional corrosion concerns related to multi-phase formation, solidification structures, and recrystallization. The modifications can alter both intrinsic corrosion rates in each zone and galvanic coupling. To improve the understanding of the metallurgical factors controlling the corrosion behavior of TIG welded AZ31B, both individual and composite assessments of the weld regions were explored herein using a variety of approaches. The main goal is to elucidate the microstructural and compositional factors controlling corrosion in each zone and to provide insight for mitigation strategies. Initially, the corrosion resistance of various weld zones were investigated in isolation by DC electrochemical methods and 24 hr open circuit potential (OCP) measurements complimented by electrochemical impedance spectroscopy (EIS) in 0.6 M NaCl [8]. This approach was also applied to commercially pure Mg as a control material and provided a reliable indication of the instantaneous corrosion rate. The EIS method was corroborated using three other methods (1) mass loss, (2) hydrogen gas collection and (3) Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) chemical analysis. Analysis of corrosion rates in each weld zone using these four methods enabled an unparalleled and reliable estimation of Mg corrosion rate and provided a clear analysis of weld corrosion as a function of the isolated weld zones [8, 9] (Figure 1). The intrinsic corrosion resistance varied by zone. Additionally, different OCPs which can induce galvanic corrosion between zones were observed. Weld structure and composition were characterized by optical and scanning electron microscopy. Changes to the alloy microstructure were observed after TIG welding. The differences in corrosion rates of isolated zones can be attributed to microstructural attributes in the fusion (FZ) and HAZs including recrystallization of coarse grains, crystallographic orientation effects, constituent particles formed during welding and divorced eutectic solidification structures within the weld FZ. Additional work will characterize each of these weld induced variables in isolation and in combination. Acknowledgements This work was funded by the Office of Naval Research Grant N000141210967 with Dr. David A. Shifler as scientific officer.