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

镁合金具有重量轻、强度高等优异性能，镁资源丰富。尽管具有这些优良的性质，但极低的耐腐蚀性阻碍了它们在实际应用中的广泛应用。本研究的目的是找到一种表面处理方法，使镁合金具有足够的耐腐蚀性。我们开发了锡酸钠电解槽和恒电位阳极化法的化学转化技术。在该技术中，镁合金在化学转化液中浸泡时的阳极极化使镁表面连续均匀地溶解，从而提供足够的镁离子以形成致密而均匀的转化膜。在工艺条件优化的基础上，将AD91D合金浸泡在不同镀液成分、温度、阳极电压和浸泡时间的镀液中，用光学显微镜和扫描电子显微镜对镀液表面进行观察，并用腐蚀试验对膜层进行评价。结果表明，最佳工艺条件为：镀液温度80℃，阳极电位-1.1V(Ag/AgCl2)，氢氧化钠浓度0.125 moldm~(-1)~(-3)。从优化条件下沉积的转化膜的扫描电子显微镜观察，薄膜是由μm级的小颗粒紧密接触组成的，这些颗粒之间的间隙也被更小的颗粒填充。这种结构使得薄膜中的结构缺陷密度低，从而具有高的耐腐蚀性。镁合金的强制溶解过程也抑制了表面的不均匀溶解，这取决于富镁的α相和富铝的β相等成分，从而抑制了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.