Pitting corrosion is a form of corrosion unique to passivated metals. As a high-performance metal, titanium has excellent resistance to pitting corrosion compared with stainless steel or aluminum alloys. However, with the increasing application of titanium in high-temperature concentrated chloride solutions, the number of pitting corrosion cases of titanium equipment is also increasing.
In general, pitting corrosion of titanium is more difficult to occur than crevice corrosion, which usually occurs in the form of pitting corrosion on the crevice surface. Electrochemical techniques can be used to determine the pitting potential of a metal to assess its tendency to pitting corrosion. Pitting corrosion occurs when the pitting potential exceeds the rupture potential of the surface oxide film.
Pitting corrosion of titanium has the following characteristics and patterns:
1. In chloride or bromide solutions, the pitting corrosion susceptibility of titanium increases as the temperature rises. The effect of pH on titanium's resistance to pitting corrosion is relatively small. Experimental data show that titanium is not susceptible to pitting corrosion because it has a very high rupture potential of about 100 V in sulfate or phosphate solutions. In contrast, the rupture potential of titanium in chloride solution is about 8~10V, and it may be even lower in bromide or iodide solution, only about 1V, so titanium is more likely to undergo pitting corrosion in halide solution.
2. When the iron content in titanium is high, its resistance to pitting corrosion decreases. The titanium-iron (Ti-Fe) phase is usually the nucleation site for pitting corrosion. For example, in ruthenium oxide-coated titanium anodes used in the chlor-alkali industry, pitting corrosion of the titanium itself can lead to the destruction of the ruthenium oxide coating if the impurity iron content is high.
3. The state of surface pretreatment has a significant effect on titanium pitting corrosion. After vacuum annealing and anodizing, titanium has the highest pitting corrosion potential and is not easy to corrode; while after wet sandpaper polishing, titanium is most prone to pitting corrosion. In addition, under the condition of the same surface pretreatment, grade 3 industrially pure titanium has the highest pitting corrosion potential and the least pitting corrosion susceptibility. The experimental data show that increasing the oxygen content of titanium can improve its pitting corrosion potential, thus improving the resistance to pitting corrosion.
