CHLORINE, CHLORINE CHEMICALS AND CHLORIDES

Chlorine and chlorine compounds in aqueous solution are not corrosive toward titanium because of their strongly oxidizing natures. Titanium is unique among metals in handling these environments.

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
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 (Table 2)^(3). 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 (Figure 1)^(4). 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 degrees F (199 degrees C).^(3) Caution should be exercised when employing titanium in chlorine gas where moisture content is low.

CHLORINE CHEMICALS
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.^(5) 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 C1O2 generators and waste water recovery.

CHLORIDES
Titanium has excellent resistance to corrosion by neutral chloride solutions even at relatively high temperatures (Table 3). 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 (See Crevice Corrosion). TIMET studies have shown that pH and temperature are important variables with regard to crevice corrosion in brines.

The temperature-pH relationship defines crevice corrosion susceptibility for TIMETAL 50A, Code-12, and 50A Pd in saturated sodium chloride brines (Figures 2, 3, and 4). Corrosion in sharp crevices in near neutral brine is possible with unalloyed titanium at about 200 degrees F (93 degrees C) and above (Figure 2). 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 degrees F (70 degrees C). The presence of high concentrations of cations other than sodium such as Ca^+2 or Mg^+2, can also alter this relationship and cause localized corrosion at lower temperatures than those indicated in the diagrams.

TIMETAL Code-12 and 50A Pd offer considerably improved resistance to crevice corrosion compared to unalloyed titanium (Figures 3 and 4). These alloys have not shown any indication of any kind of corrosion in laboratory tests in neutral saturated brines to temperatures in excess of 600 degrees F (316 degrees C). TIMETAL Code-12 maintains excellent resistance to crevice corrosion down to pH values of about 3. Below pH 3, TIMETAL 50A Pd offers distinctly better resistance than TIMETAL Code-12. TIMETAL Code- 12 or 50A Pd will resist crevice corrosion in boiling, low pH salt solutions which corrode TIMETAL 50A (Table 4).