Stability of sputter-deposited amorphous Mn-Ta alloys in chloride-free andchloride-containing H2SO4 solutions

Amorphous alloys were prepared in a wide composition range by the addition of tantalum to manganese by the sputter deposition method. The effect of al loying tantalum on the corrosion behavior and the stability of sputter-depo sited amorphous Mn-Ta alloys was studied. Different techniques such as weig ht-loss measurements, electrochemical techniques, and X-ray photoelectron s pectroscopy (XPS) analysis were used. The addition of Ta remarkably reduces the rate of dissolution of the alloy and Ta passivates spontaneously. No s igns of pitting corrosion or transpassive dissolution of Mn were observed. The anodic current density decreases significantly with increasing the tant alum content. XPS studies show a preferential oxidation of Ta in the air-fo rmed (native) oxide. The preferential oxidation was remarkable after immers ion in 1 M H2SO4 solution for 30 min. The thickness of the passive film for med on the sputter-deposited Mn-30Ta alloy increases linearly with the appl ied potential as that observed in valve metals. The formed passive film con sists mainly of Ta2O5. At potentials higher than +0.7 V (standard calomel e lectrode) high oxidation states of Mn were found to be incorporated in the passive film. (C) 2001 The Electrochemical Society. All rights reserved.