Stability of sputter-deposited amorphous Mn-Ta alloys in chloride-free and chloride-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 alloying tantalum on the corrosion behavior and the stability of sputter-deposited amorphous Mn-Ta alloys was studied. Different techniques such as weight-loss measurements, electrochemical techniques, and X-ray photoelectron spectroscopy (XPS) analysis were used. The addition of Ta remarkably reduces the rate of dissolution of the alloy and Ta passivates spontaneously. No signs of pitting corrosion or transpassive dissolution of Mn were observed. The anodic current density decreases significantly with increasing the tantalum content. XPS studies show a preferential oxidation of Ta in the air-formed (native) oxide. The preferential oxidation was remarkable after immersion in 1 M H2SO4 solution for 30 min. The thickness of the passive film formed on the sputter-deposited Mn-30Ta alloy increases linearly with the applied potential as that observed in valve metals. The formed passive film consists mainly of Ta2O5. At potentials higher than +0.7 V (standard calomel electrode) high oxidation states of Mn were found to be incorporated in the passive film. (C) 2001 The Electrochemical Society. All rights reserved.