Title: Effects of a Small Amount of Impurity Elements on Pitting Potential of Commercially Pure Titanium
Abstract: It has been well known that titanium has low susceptibility against pitting corrosion in halide solutions because of its highly stable surface film. However, we have encountered a pitting corrosion of commercially pure titanium in chemical equipment handling a bromide solution at high temperature. Some peculiarities were observed in the corrosion accident; Only JIS grade 1 of titanium was subject to pitting corrosion while grade 2 and grade 3 were not attacked. Furthermore, pit formation occurred only in polished zone of grade 1 surface.In order to interpret these corrosion behaviours, laboratory tests were conducted on the effects of a small amount of impurity elements, such as oxygen and iron, and surface treatments on the pitting potential of commercially pure titanium.The electrochemical test results revealed that pitting potential of titanium became higher in the following order, grade 1>grade 2>grade 3 at higher temperatures while the order was reversed at lower temperatures. In addition, surface oxidation treatment improved the resistance of titanium against the pitting corrosion.These phenomena seem to be related to the intermetallic compound TiFe precipitated in titanium and the stability of the passive film which is affected by oxygen in titanium substrate.