Formation of porous surface on NiTi and suppression of Ni ion release

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A dealloying Ni surface having a porous structure was prepared from NiTi alloys through immersion in molten metals. The concentration of Ni from the NiTi substrate to the surface was decreased in a stepwise manner forming an inner Ti2Ni layer and outer alpha-Ti layer on the substrate. The amounts of metallic ions eluted from these specimens to 1mass% lactic acid solution were evaluated. Despite the increased surface area of the dealloyed specimen, Ni ion elution was suppressed to the same level as that in the non-treated NiTi substrate. The anatase layer formed by two-step thermal oxidation suppressed the Ni ion release from the dealloyed substrate.