The corrosion behaviour of sputter-deposited amorphous Mn-Ta alloys in 0.5 M NaCl solution

Amorphous Mn-Ta alloys were successfully prepared by DC magnetron sputter deposition in a wide composition range. Their corrosion behaviour was investigated in 0.5 M NaCl solution of pH 11.5 at 30°C by electrochemical measurements and XPS analysis. The corrosion rates of these alloys were significantly lower than that of manganese metal, and decreased with increasing alloy tantalum content. The alloys containing 30 at.% or more tantalum were spontaneously passive and showed a wide passive region exceeding the potential of transpassive region of manganese. XPS analysis revealed that the spontaneously passivated films were not composed of a mixture of manganese and tantalum oxyhydroxide and consisted of a double oxyhydroxide containing Mn²⁺ and Ta⁵⁺ cations. The passive films are enriched in tantalum ions in almost the entire film except the top-most surface where manganese hydroxide is precipitated. The passive double oxyhydroxide film with an almost fixed composition grows with anodic polarization potential, revealing that both tantalum and manganese are effective in forming the barrier type oxyhydroxide film. The formation of tantalum-enriched double oxyhydroxide film is responsible for the high corrosion resistance of the alloys.