Identification of Crevice Corrosion in ASTM Grade-12 Titanium in Simulated Rock Salt Brine at 150°C Available to Purchase

ASTM Grade 12- titanium (Ti-0.3Mo-0.8Ni) is a candidate corrosion resistant material for high-level nuclear waste containers which will be emplaced in mined geologic repositories such as those in rock salt. The crevice corrosion behavior of this alloy was investigated in simulated rock salt brine solutions at a temperature of 150°C. A distinct corrosion product with a range of interference colors was observed in a mechanically simulated crevice after two- to four-weeks' exposure. Low pH (∼4) accelerated the reaction rate, and deaerated solutions gave less voluminous corrosion than aerated ones. Also, increasing specimen size, decreasing crevice gap, and preoxidation of the cathodic area gave more crevice attack. Higher temperatures (∼250°C) do not necessarily accelerate crevice corrosion. These results are consistent with those expected from macroscopic concentration cell formation accompanied by oxygen depletion, potential drop, and acidification inside the crevice. Transmission electron microscopy (TEM) and scanning electron icroscopy (SEM) techniques were extensively utilized to identify the film formed inside the crevice at each stage of the corrosion process. In the early stage of cell formation, anatase-type titanium dioxide TiO₂ was formed which acted as a barrier to further corrosion inside the crevice. Traces of titanium pentoxide (Ti₃O₅) were also identified. In the case of severe crevice corrosion, the corrosion product was identified as the rutile form of TiO₂ which is not an effective barrier to further corrosion. Measurements of the open circuit corrosion potential at 80°C implied that there is a breakdown of the passive film as the pH of the brine falls below unity, based on the mixed potential theory. The observation of pitting inside the crevice supports the depassivation hypothesis. Accelerated hydrogen uptake was observed in the crevice region. This was caused by breakdown of the passive film or by high acidity in the crevice. Based on these observations, and pH and potential measurements inside the crevice of commercially pure titanium by other workers, a mechanism for crevice corrosion in ASTM Grade-12 titanium has been developed. It involves the initial formation of compact anatase crystals inside the crevice. As the macroscopic cell develops further, it is postulated that either the anatase form of TiO₂ will transform to the lower oxide Ti₃O₅ or to the rutile form of TiO₂, or alternatively titanium dissolves into the solution after the breakdown of the protective film and subsequent hydrolysis takes place to form the lower oxide and the rutile form of TiO₂. The role of alloying elements (molybdenum and nickel) and dissolved species are discussed with respect to these postulations.