The corrosion resistance of titanium to moist chlorine gas and chloride containing solutions is the basis for the largest number of titanium applications. Titanium is widely used in chlor-alkali cells; dimensionally stable anodes; bleaching equipment for pulp and paper; heat exchangers, pumps, piping and vessels used in the production of organic intermediates; pollution control devices; and even for human body prosthetic devices.
The equipment manufacturer or user faced with a chlorine or chloride corrosion problem will find titanium’s resistance over a wide range of temperatures and concentrations particularly useful.
Chlorine Gas Corrosion
Titanium is widely used to handle moist chlorine gas and has earned a reputation for outstanding performance in this service. The strongly oxidizing nature of moist chlorine passivates titanium resulting in low corrosion rates in moist chlorine. Dry chlorine can cause rapid attack on titanium and may even cause ignition if moisture content is sufficiently low. However, one percent of water is generally sufficient for passivation or repassivation after mechanical damage to titanium in chlorine gas under static conditions at room temperature. Factors such as gas pressure, gas flow, and temperature as well as mechanical damage to the oxide film on the titanium, influence the actual amount of moisture required.
Approximately 1.5 percent moisture is apparently required for passivation at 390°F (199°C). Caution should be exercised when employing titanium in chlorine gas where moisture content is low.
Chlorine Chemicals Corrosion
Titanium is fully resistant to solutions of chlorites, hypochlorites, chlorates, perchlorates and chlorine dioxide. Titanium equipment has been used to handle these chemicals in the pulp and paper industry for many years with no evidence of corrosion. Titanium is used today in nearly every piece of equipment handling wet chlorine or chlorine chemicals in a modern bleach plant, such as chlorine dioxide mixers, piping, and washers. In the future it is expected that these applications will expand including use of titanium in equipment for ClO2 generators and waste water recovery.
Titanium has excellent resistance to corrosion by neutral chloride solutions even at relatively high temperatures. Titanium generally exhibits very low corrosion rates in chloride environments. The limiting factor for application of titanium and its alloys to aqueous chloride environments appears to be crevice corrosion.
When crevices are present, unalloyed titanium will sometimes corrode under conditions not predicted by general corrosion rates. Studies have shown that pH and temperature are important variables with regard to crevice corrosion in brines. Corrosion in sharp crevices in near neutral brine is possible with unalloyed titanium at about 200°F (93°C) and above. Lowering the pH of the brine lowers the temperature at which crevice corrosion is likely, whereas raising the pH reduces crevice corrosion susceptibility. However, crevice corrosion on titanium is not likely to occur below 158°F (70°C). The presence of high concentrations of cations other than sodium such as Ca2+ or Mg2+, can also alter this relationship and cause localised corrosion at lower temperatures.
Bromine and Iodine Corrosion
The resistance of titanium to bromine and iodines is similar to its resistance to chlorine. It is attacked by the dry gas but is passivated by the presence of moisture. Titanium is reported to be resistant to bromine water.
Titanium is not recommended for use in contact with fluorine gas. The possibility of formation of hydrofluoric acid even in minute quantities can lead to very high corrosion rates. Similarly, the presence of free fluorides in acid aqueous environments can lead to formation of hydrofluoric acid and, consequently, rapid attack on titanium. On the other hand, fluorides chemically bound or fully complexed by metal ions, or highly stable fluorine containing compounds (e.g., fluorocarbons), are generally noncorrosive to titanium.