Oxidation of surface lanthanum precipitates in a free-machining titanium alloy investigated by in situ AFM and cyclic voltammetry

F. Depentori, J. Laukart, F. Brunke, S. Benfer, C. Siemers and W. Fürbeth

Mater. Corr. 65 (2014) 425-430; DOI: 10.1002/maco.201407607

Abstract

A new, free-machining titanium alloy of the composition Ti6Al2Fe1Mo0.9La0.5Cu0.3Si was investigated for its corrosion properties in 1.5 wt% NaCl solution. The alloy showed localized corrosion of metallic lanthanum precipitates. Localized corrosion kinetics were investigated via cyclic voltammetry and potentiodynamic polarization. Furthermore, the process could be visualized in situ via electrochemical atomic force microscopy (in situ AFM). Results show that before dissolution, the precipitates undergo a strong volume increase. This was attributed to the oxidation of lanthanum to La(OH)3. Investigations show that the oxidation process can be triggered by either applying electrochemical polarization to the alloy or by exposing the alloy for enough time in NaCl solution. The holes created after removal of the lanthanum precipitates can act as sites of further corrosion, but only at high anodic polarization of around 3 V versus normal hydrogen electrode.

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