Role of the Electrochemical Potential and Solution pH to Environment-Assisted Cracking of Super-Elastic TiNi Alloy

Abstract

The susceptibility to environment-assisted cracking (EAC) of super-elastic TiNi alloy was investigated as a function of the electrochemical potential and solution pH. The investigation was conducted using a slow-strain-rate tensile test apparatus with a potentiostat. The test solutions were sulfate solutions with various pH values adjusted by H₂SO₄ or NaOH. The alloy deforming under cathodic reaction fractured under the relatively small strain where the alloy was in the stress-induced martensitic phase. A larger EAC susceptibility was obtained at lower potential and lower pH, which indicates that this is a general feature of hydrogen embrittlement. The severe EAC region of TiNi alloy was different from that of TiAl alloy. The EAC susceptibility was strongly correlated with the cathodic charge density, irrespective of the pH or potential: a charge density below 0.025 MC m⁻² yielded almost no EAC; however, above 0.025 MC m⁻² EAC was induced, and the EAC susceptibility was independent of the charge density. Hydrogen in solid-solution state was detected in the alloy at a charge density below 0.025 MC m⁻², and hydride started to form at a density above 0.025 MC m⁻².