Effects of acid type on corrosion and fracture behavior of Ni-Ti superelastic alloy under sustained tensile load in physiological saline solution containing hydrogen peroxide

The effects of acid on the corrosion and fracture behavior of Ni-Ti superelastic alloy have been examined by sustained tensile-loading test in physiological saline solution containing hydrogen peroxide. For immersion test without applied stress at the same solution pH, although corrosion is enhanced markedly irrespective of the type of acid added, corrosion morphology depends on the type of acid added. The amount of dissolved nickel ions increases in the order of the solutions with lactic acid > phosphoric acid > acetic acid. The amount of dissolved nickel ions increases with decreasing solution pH. In contrast, under applied stress, the time to fracture in the solution with phosphoric acid becomes longer than that in the solution without acid, although the time to fracture does not increase in the solution with acetic acid or lactic acid. The time to fracture is only slightly affected by solution pH for the same type of acid added. Fractographic features change in the solution with phosphoric acid. The present study suggests that the fracture behavior of Ni-Ti superelastic alloy in physiological saline solution containing hydrogen peroxide depends on acid type rather than on solution pH; the fracture behavior is not necessarily consistent with the corrosion behavior in the case without applied stress.