Selective de-alloying of NiTi by oxochloridation

The systematic modification of a nickel-titanium-alloy by annealing in a complex gas atmosphere was investigated. A mixture of HCl and H₂O in inert argon was chosen. The reaction kinetics was investigated at 600 deg. C, 700 deg. C and 800 deg. C. The reaction kinetics displayed a significant dependence on the temperature. It was monitored by means of a thermogravimetric balance that showed a quasi-parabolic scale growth at 600 deg. C, a paralinear or so called Tedmon kinetic at 700 deg. C with a distinct weight maximum after about 35 h, and finally a linear evaporation kinetic at 800 deg. C. This behaviour is attributed to the concurrent reactions of oxidation, chloridation and evaporation of corrosion products. The kinetics of these reactions is different for the two alloying elements and with respect to the equiatomic composition they are coupled to each other. Cross sections prove that a stochiometric titanium depletion is achieved leading to the formation of a Ni₃Ti layer (d = 50 μm) which is in turn covered by a pure titanium oxide layer (d = 40 μm). The applicability of this technique for tailored surfaces with a high degree of biocompatibility is discussed