Effect of Temperature and Humidity on Corrosion in NH4Cl Environments

Abstract Ammonium chloride (NH4Cl) is recognized for its corrosiveness in refinery sector affecting metallic alloys by different under-deposit corrosion mechanisms. Corrosion is focused on where a local environment develops under the hygroscopic NH4Cl salt. In biofuel refining and co-processing units, NH4Cl-rich environments may form either through combination of HCl and NH3 present as vapors or via sublimation of the salt followed by its deposition on cooler parts. In the latter scenario, ambient humidity and temperature play critical role: deposited NH4Cl readily absorbs moisture, leading to deliquescence and the formation of a highly corrosive aqueous phase. Given the dual influence of temperature and humidity on corrosion behavior, it is essential to assess the compatibility of corrosion-resistant alloys (CRAs) under these conditions. This study investigates the corrosion performance of various CRAs in NH4Cl environment under controlled temperature and humidity. Experimental evaluations were conducted across a temperature range from 130°C (266°F) to 220°C (428°F), with susceptibility to stress corrosion cracking (SCC) assessed as key performance indicators. The findings highlight the synergistic effect of elevated temperature and humidity in accelerating corrosion processes, emphasizing the need for robust material selection strategies in moisture-sensitive NH4Cl-containing environments.