| Budget Amount *help |
¥3,810,000 (Direct Cost: ¥3,300,000、Indirect Cost: ¥510,000)
Fiscal Year 2007: ¥2,210,000 (Direct Cost: ¥1,700,000、Indirect Cost: ¥510,000)
Fiscal Year 2006: ¥1,600,000 (Direct Cost: ¥1,600,000)
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| Research Abstract |
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.
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