Effect of Relative Humidity on Ammonium Chloride Corrosion in Refineries

Corrosion behavior and the hygroscopic properties of solid ammonium chloride (NH(4)Cl) salt were investigated. This study was conducted to determine the root cause of significant corrosion caused by NH(4)Cl salt deposition in reactor-effluent streams in hydroprocessing units. Corrosion and water absorption tests were performed under various relative humidity (RH) conditions with solid NH(4)Cl salts using a temperature and humidity control chamber. Eight types of materials commonly used in refineries were examined, including carbon steel (UNS K02702), Type 304 (UNS S30400) stainless steel, duplex (UNS S39274) stainless steel, Grade 2 titanium (UNS R50400), Alloy 400 (UNS N04400), Alloy C-276 (UNS N10276), aluminum brass (UNS C68700), and aluminized carbon steel (hot-dip aluminized UNS K02702). Significant corrosion was observed around 60% RH on all the alloys except Alloy C-276. Carbon steel corroded above 20% RH. Its highest corrosion rate was observed at 60% RH and 80 degrees C. Type 304 stainless steel and duplex stainless steel showed pitting at 50% and 60% RH. Although the metal surface on Alloy C-276 was tarnished at 50% RH, it had excellent corrosion resistance at all RH levels. It was suggested that the corrosion of NH(4)Cl is the most severe around a critical RH, above which water absorption becomes significant. In addition, based on the water absorption tests and simulation results, it was found that the critical RH of NH(4)Cl salt has a close relationship to temperature.