Characterisation of Surface Oxidation of Nickel-Titanium Alloy by Ion-beam and Electrochemical Techniques

The passivating effects produced on Ni49Ti51 (at.%) shape memory alloy by electropolishing in methanol-sulphuric acid electrolyte or thermal treatment in air are investigated with ion-beam techniques (secondary ion mass spectrometry (SIMS) and mass resolved ion scattering spectrometry (MARISS)) and electrochemical techniques (polarisation curves, photocurrent spectroscopy and impedance). Electropolishing produces a limited passivation due to formation of a thin layer of amorphous TiO2, which causes only a slight hindrance to the oxygen evolution reaction. Thermal treatment in air at 450degreesC causes a more substantial passivation due to formation of a thick oxide layer, estimated at about 80 nm for a 16 h treatment on the basis of weight increase, disregarding roughness. Data indicate that the film produced by thermal oxidation has probably a duplex structure, with a porous outer layer with inclusions of Ni species and a compact inner layer, in contact with the substrate, consisting of (essentially) nickel free TiO2 rutile.