Influences of Potential and pH on Initiation of Environment-Assisted Cracking of Ti–6Al–4V

Abstract

We have investigated the influences of potential and pH on environment-assisted cracking (EAC) of Ti–6Al–4V in aqueous solutions. The EAC test was conducted by slow strain rate technique. Solution pH was controlled from 1 to 7, and applied potential was fixed between −1.8 and +0.2 VSHE. In the solutions of pHs 3 and 7, mechanical properties of the material was almost independent of applied potential. This means that the material shows almost no EAC susceptibility in the potential-pH region for the EAC test period. At pH 1, however, both maximum stress and fracture strain changed with applied potential, and showed minimum values at an applied potential of about −1.0 VSHE. According to surface analyses by scanning electron microscopy and X-ray diffraction technique, some cracks and Ti hydride were detected from only the specimens fractured at about −1.0 VSHE in the solution of pH 1. Therefore, it can be concluded that Ti–6Al–4V is most susceptible to the EAC at about −1.0 VSHE and a pH less than 1. The EAC region was in good agreement with Ti ion stable region in a potential-pH diagram of Ti/water system, and was quite similar to that on the EAC of TiAl.