ABSTRACT
The corrosion behavior and the hygroscopic properties of solid ammonium chloride (NH4Cl) salt are investigated. This study was conducted to determine the root cause of aggressive corrosion due to wet NH4Cl salt deposition in reactoreffluent streams in hydroprocessing units. In addition, the corrosion resistance and behavior of carbon steel and alloys in high-concentration NH4Cl solutions were investigated.
Corrosion and water-absorption tests were performed under various relative humidity (RH) conditions with solid NH4Cl salts using a temperature-and humidity-controlledchamber. Eight types of materials commonly used in refineries were examined. UNS K02702 (carbon steel)was corroded above 20% RH. Its highest corrosion rate was observed at 60% RH and 80°C. It was suggested that the corrosion of NH4Cl is the most severe around a critical RH, above which water absorption becomes significant.
Immersion tests were performed inboiling 20 wt% and 40 wt% NH4Cl solutionswitheighteen materials.The corrosion rates of carbon steel were as high as 25.4 mm/y and 60.4 mm/y in 20 wt% and 40 wt% NH4Cl solutions, respectively. Pitting was observed in UNS S41000, UNS S40500, UNS S32100, UNS S30403, UNS S31603 and UNS N08800. The maximum pit depth had a close relationship with the Pitting Resistant Equivalent Number (PREN). Neither general corrosion nor pitting was observed in titanium alloys and alloys with a PREN of 40 and higher. The corrosion rate of UNS K02702increased with increasing temperature, and the increase was especially rapid in the higher temperature range.
