The microstructure and mechanical properties of tgf beta receptor Mg–9Al–2Sn–xMn (x = 0, 0.1, 0.3 wt.%) alloys in as-cast, solution treated and aged conditions are investigated and the following conclusions can be drawn.
This research work is collaborated by General Motors Global Research and Development (GM R&D), Warren, MI, USA, and Shanghai Jiao Tong University (SJTU), Shanghai, China. Dr P. Fu would like to acknowledge the support of a Specialized Research Fund for the Doctoral Program of Higher Education (20110073120008) and a project from Shanghai Science and Technology Committee (12DZ0501700).
The desire to use lightweight metallic alloys in the automobile and aerospace industries has increased in recent years as the search for lightweight solutions has become amplified. Magnesium alloys are one of these lightweight metallic alloys currently being investigated, because of its low density, 1.74 g/cm3, and high mechanical stiffness. The mechanical benefits of magnesium, however, are contrasted by a high corrosion rate as compared to aluminium or steel. Because of magnesium\’s electrochemical potential, as illustrated in the galvanic series, it corrodes easily in the presence of seawater. The high corrosion of magnesium has relegated the alloy to use in areas unexposed to the atmosphere, including car seats and electronic boxes [1,2]. However, the corrosion resistance of the Mg-based alloys is generally inadequate due to the low standard electrochemical potential −2.37 V compared to the SHE (Standard Hydrogen Electrode) and this limits the range of applications for Mg and its alloys. Therefore, the study of corrosion behaviour of magnesium alloys in active media, especially those containing aggressive ions, is crucial to the understanding the corrosion mechanisms, and hence, to improving the corrosion resistance under various service conditions. The reason for the less corrosion resistance of magnesium and its alloys results primarily from two mechanisms: (i) oxide films forming on the surface is not perfect and protective; (ii) galvanic or bi-metallic corrosion can be caused by impurities and secondary phases .
This research focused on comparing immersion testing with potentiodynamic polarization testing, which are the two main techniques for corrosion studies, in an effort to expose the magnesium alloy to environments similar to those environments experienced by automotive engine blocks . It is well known that Mg alloys are susceptible to corrosion such as pitting and stress cracking corrosion (SCC). Major studies shows that the SCC susceptibility of Mg alloys is increased in solutions containing chloride .
The galvanic couples formed by the second phase particles and the matrix are the main source of the localized corrosion of magnesium alloys . The corrosion of AZ31 magnesium alloy in simulated acid rain solution is controlled by the rate of anodic dissolution and hydrogen evolution, and the corrosion rate of AZ31 increases with increasing concentration of Cl− ion . The corrosion attack of Mg and its alloy in dilute chloride solutions depends on both Al content and alloy microstructure . Yingwei song et al. , investigated the effect of second phases on the corrosion behaviour of wrought Mg–Zn–Y–Zr alloy and they found that the increase of exposure time, the second phases can promote the corrosion rate significantly and cause pitting corrosion. Rajan Ambat et al. , studied the evaluation of micro structural effects on corrosion behaviour of AZ91D magnesium alloy and they reported that size and morphology of β phase and coring were found to have significant influence on corrosion behaviour of AZ91D alloy. Pardo et al. , explored the influence of microstructure and composition on the corrosion behaviour of Mg/Al alloys in chloride media and it was found that the aluminium enrichment on the corroded surface for the magnesium alloy, and the β-phase (Mg17Al12), which acted as a barrier for the corrosion progress for the magnesium alloys. The corrosion product consisted of magnesium hydroxide, fallen β particles and magnesium–aluminium oxide; the amount of each component was found to be a function of chloride ion concentration and pH .