Corrosion resistance of titanium in chlorine, bromine, iodine and fluorine and their compounds

Among various metal materials, titanium has good corrosion resistance in wet chlorine, chloride solution (except high temperature and high concentration of ZnCl2, AlCls and CaCl2), chlorine-containing compounds (such as chlorate, chlorite, hypochlorite and perchlorate, etc.) solution, and has been successfully applied in bleaching plants, electrolytic chlorine plants and wastewater treatment plants. However, titanium in high temperature and high concentration chloride solution will occur crevice corrosion, especially in contact with PTFE and other organic compounds, crevice corrosion is more serious.

Titanium in dry chlorine gas produces violent corrosion, and even cause fire and spontaneous combustion. the reaction between Ti and Cl produces TiCl4, which is an exothermic reaction. The reaction of Ti with Cl to form TiCl4 is an exothermic reaction. As long as the water content in the medium is very low, the heat released can promote the combustion of titanium until the dry chlorine gas or titanium is exhausted. If water is present in the chlorine gas, titanium tetrachloride undergoes hydrolysis to form white titanium hydroxide. Titanium hydroxide is a stable solid compound, unlike titanium tetrachloride (which has a boiling point of 136°C), which is a highly volatile liquid. The boundary between "dry" and "wet" is related to factors such as ambient temperature and alloy composition. Reportedly, industrial pure titanium at about 200 ℃ in the chlorine gas to maintain the passive state of the minimum water content of about 1.5%; room temperature, as long as the minimum water content to maintain at 0.3% ~ 0.4% above will not catch fire. Titanium-palladium alloys and titanium-nickel-molybdenum alloys can maintain the passivity of the metal at even lower water content.

Titanium's corrosion resistance to bromine and iodine is similar to that of chlorine, and as long as a certain amount of water is maintained, titanium is guaranteed not to corrode. However, titanium will corrode in fluorine, hydrofluoric acid or acid fluoride solutions, even at very low concentrations, and there are virtually no corrosion inhibitors available, so titanium is not recommended for use in environments exposed to fluorine atmospheres. Acidic fluoride solutions corrode titanium rapidly due to the presence of hydrofluoric acid. However, certain fluorides that complex with metal ions, or extremely stable fluorides (e.g. fluorocarbons), generally do not corrode titanium.