A technique was developed to monitor crevice corrosion of Grade-12 titanium in a high magnesium synthetic brine under hydrothermal conditions. The current between the cathode and anode and the potential of the whole crevice assembly provide information on the various stages of corrosion. Occasionally, the decoupled potential of the cathode and anode are measured to give supporting information about the crevice conditions. A parallel test is run on a specimen that has small wells to collect crevice solution whose pH is determined at room temperature.
The current/potential results show that the crevice corrosion incubation period for a Grade-12 titanium crevice formed between two Teflon® plates is approximately two days at 150°C. Optical and scanning electron microscopy (SEM) observations show that the corrosion starts as isolated pitting that spreads along the surface as shallow pits. The corrosion conditions change significantly as the titanium oxide corrosion product fills the crevice, and the rate of corrosion may be greatly reduced after several days. The rate of crevice corrosion of commercial purity (Grade-2) titanium under similar conditions is approximately three orders of magnitude higher. In this case, active dissolution of metal starts during the initial heating of the autoclave, and the incubation period is negligible.