Reactivity of nickel-titanium with methanolic bromine and iodine solutions. Etching characteristics of a shape-memory alloy

NiTi alloys having composition Ni sub x Ti sub 1--x (0.47 < = x < = 0.51) exhibit a martensitic transformation that gives rise to a shape-memory effect near room temperature. Normally chemically inert, these alloys react with wet methanolic Br sub 2 and I sub 2 solutions to yield crystalline nickel halides and uncharacterized Ti-containing products. The crystalline products are independent of whether NiTi is in its high-temperature (austenite) or low-temperature (martensite) phase. Reaction rates were measured for austenitic NiTi: concentrated Br sub 2 /MeOH solutions corrode the surface at room temperature at a rate of up to 2000 A/s; solutions of I sub 2 /MeOH reach a decomposition rate of only approx 5 A/s. For the Br sub 2 /MeOH reaction, kinetic studies were conducted with samples of pure martensite, pure austenite, or mixtures of these phases. An average apparent activation energy of 13.4 plus/minus 1.6 kJ/mol was obtained irrespective of the NiTi phase(s) present. At concentrations > approx 3M Br sub 2 in MeOH, the reaction with NiTi polishes the surface of the bulk alloy, yielding features on the order of approx 1 mu m; at low concentrations, visibly textured surfaces are created.