Study of chemical stability of titanium in model hydrochloric acid solutions of refining production

Abstract

The report includes the results of the studies of titanium corrosion rate in hydrochloric solutions in open and closed systems as a function of various factors, including concentration of hydrochloric acid, temperature, concentration of oxidizer (sodium chlorate) and salts of precious metals, and excess oxygen pressure. The studies provided the stability limits of titanium VT1-0 in hydrochloric media at temperatures above 100 оС. It was found that the rate of titanium corrosion in the temperature range of up to 110 оС, concentration of hydrochloric acid up to 110 g/dm3, and under excess (more than 7 bar) oxygen pressure does not exceed 0.1 mm/year. Similar corrosion rate is observed in the temperature range of up to 160 оС with hydrochloric acid concentration less than 40 g/dm3. The rate of titanium corrosion in closed systems in refinery hydrochloric solutions, with the concentration of at least one of platinum, palladium, gold, or rhodium of over 5 g/dm3, decreases even in the absence of excess oxygen pressure. The presence of salts of precious metals in a closed system with excess oxygen pressure of more than 7 bar does not result in any significant change in titanium corrosion rate. It was found that gold and palladium may potentially deposit on titanium surface, primarily at defect sites, at temperatures above 110 оС upon contact with solutions of precious metal salts. No deposition of other precious metals was observed. In all cases, a continuous uniform corrosion is typical for titanium. The corrosion rate at 120–160 оC in hydrochloric solutions with added sodium chlorate is in the range of 0.3–0.5 mm/year, while it is already 10–12 mm/year at 180–200 оС. Thermodynamic calculations demonstrated potential modification of sodium chlorate decomposition at temperatures above 160 оC, which results in more than 30-fold acceleration of corrosion.