Development of Surface Finishing Method of Magnesium Alloys for Expansion in Their Application

镁合金具有重量轻、强度高等上级性能，而且镁资源丰富。尽管这些优异的性质，非常低的耐腐蚀性阻碍了它们在实际应用中的广泛使用。本研究的目的是找出表面处理方法，以提供足够的耐腐蚀性镁合金。我们开发了锡酸钠浴和恒电位阳极极化法化学转化技术。该技术利用镁合金在化学转化过程中的阳极极化，使镁合金表面连续均匀地溶解，从而提供足够的镁离子，形成致密均匀的转化膜。从优化工艺条件出发，采用不同的镀液成分、温度、阳极电压和浸泡时间对AD 91 D合金进行浸泡，并通过光学显微镜和扫描电镜观察镀层表面形貌，通过腐蚀试验对镀层进行评价。结果表明：镀液温度80℃，阳极电位-1.1V（Ag/AgCl），NaOH浓度0.125moldm ↑ [3]<-3>。通过SEM观察，在优化条件下沉积的转化膜由微米级的小颗粒紧密接触组成，颗粒间的差距也被更小的颗粒填充。这样的结构使得膜中的结构缺陷密度低，因此耐腐蚀性高。镁合金的强制溶解过程还抑制了取决于诸如富Mg α相和富Al β相的组成的表面的不均匀溶解，从而抑制了在这些不同相上的不均匀沉积。因此，该技术有望适用于各种形状镁合金的涂层处理。

Magnesium alloy has superior properties such as very light weight and high strength, and natural resource of magnesium is abound. Despite of these excellent nature, very low corrosion resistance hinders them from wide usage in practical applications. This study has aimed to find out the surface finishing method to provide enough corrosion resistivity to magnesium alloys. We developed chemical conversion technique using sodium stannate bath and potentiostatic anodic polarization method. In this technique anodic polarization of magnesium alloy during immersion in the chemical conversion bath causes dissolution of magnesium surface continuously and uniformly, and thus supplies magnesium ions enough to form dense and uniform conversion film. Form optimization of process condition, AD91D alloy was immersed in the bath with various bath composition, temperature, anodic voltage, and immersion time, and surface observation using optical microscope and scanning electron microscope, and corrosion test were applied for evaluation of the coating film. As a result, following optimized condition was obtained; bath temperature 80℃, anodic potential-1.1 V (Ag/AgCl), NaOH concentration 0.125 mol dm^<-3>. From SEM observation of the conversion film deposited in the optimized condition, the film was composed of small particles in μm scale in closely contact to each other, and the gap between these particles is also filled with smaller particles. Such structure enables low density of structural defects in the film and thus high corrosion resistivity. Forced dissolution process of magnesium alloy also suppresses the non-uniform dissolution of the surface depending on composition such as Mg-rich α phase and Al-rich β phases and thus non-uniform deposition on these different phases. This technique is, therefore, expected to be applicable to the coating treatment of magnesium alloy with various shape.