Failure Analysis of Process Equipment Used in the Production of Polyvinyl Chloride

Polyvinyl chloride (PVC) is produced in petrochemical plants by polymerization of vinyl chloride monomer prepared by cracking of ethylene dichloride, which takes place at 500 °C in furnace tubes made of 347 stainless steel. Hydrochloric acid resulting from the cracking process is recycled in the gaseous state and used in an oxychlorination process. However, after less than half the life expectancy, gas leakage has been detected due to a crack in a furnace tube. Another problem has been encountered in the dewatering and drying unit of PVC where the outlet gas at a temperature of 40-60 °C contains 110 ppm of hydrogen. In this case, a dryer support made of 304 stainless steel has fractured. Various electron-optical techniques and x-ray diffraction have been used to characterize the microstructure of damaged materials. The mechanical strength has been evaluated by microhardness measurements. It is concluded that the furnace tube has been damaged by chloride stress corrosion cracking, which is correlated with precipitation of Cr-rich phases including M₂₃C₆ carbide and sigma phase due to long-term exposure at elevated temperatures as well as the presence of chloride ions in the environment due to un-cycled HCl. However, the dryer support is found to be damaged by hydrogen-assisted quasi-cleavage.