Study of Aqueous Ammonium Chloride Corrosion in Refinery Crude Distillation Unit Overhead Conditions

One of the main corrosion threats to the crude distillation unit overhead piping is related to the presence of HCl and NH₃. It is generally recognized that three forms of corrosion associated with the presence of HCl and NH₃ can potentially exist in the overhead conditions: namely under-deposit NH₄Cl salt corrosion, dew point HCl corrosion and aqueous HCl corrosion. The different forms of corrosion have been studied in a joint industry program for refinery crude distillation unit overhead conditions, which led to development of an extensive corrosion database and a corrosion prediction model. The study of the under-deposit NH₄Cl salt corrosion has been presented in an earlier paper, this paper focuses on the observations for dew point and aqueous phase ammonium chloride corrosion. By varying temperature, pH, ammonium chloride concentration and flow, corrosion rates were investigated at defined dew points and aqueous conditions for six commonly used materials (carbon steel, 410 stainless steel, Alloy 400, Titanium, Alloy 625 and Alloy C-276). This paper describes the test methods and testing conditions, and discusses the major data trends observed from the testing program.