Titanium depends upon the protective nature of a surface film, essentially an oxide, for its corrosion protection. Stress corrosion cracking results when the local breakdown of that film occurs causing a significant shift in the composition of the electrolyte and establishing electrochemical currents in the restrictive area which combine to prevent repassivation of the site.
Extensive testing has been conducted to define the threshold stress (KIscc) to sustain the electrochemical reaction. A large variation is seen between alloy, heat treatment, and mill conversion practice which can be related back to the notch and its strain conditions and the sensitivity of the alloy to the electrolyte present in the notch area. It has been demonstrated by rapid freezing of the reaction site that the electrolyte is a highly acidic reducing environment, and is in such a region of the Pourbaix diagram that the passive film will not reform. The electrochemist can further define the alloys susceptibility by evaluating the anodic current density in the pit area to sustain pitting and the cathodic current density to charge hydrogen. The pH of the electrolyte controls the hydrogen uptake efficiency at the cathodic site. Hydrides, formed at the local cell cathodic site under subsequent anodic control will react and pit at low anodic current density.
Stress corrosion cracking in the titanium alloys is the ability of the structure or test configuration to sustain by a moving crack front an actively corroding local site of a nonpassivating electrolyte. The classical argument as to whether the halogen or hydrogen are the cracking species is obscure, as both are interacting to sustain the reaction.