A Study on the Corrosion Behavior of Carbon Steel Exposed to a H₂S-Containing NH₄Cl Medium

NH₄Cl corrosion failure often occurs in the overhead systems of hydrotreaters, and this failure is always accompanied by the appearance of H₂S. A combination of electrochemical and surface spectroscopic (SEM/EDS, AFM, XRD) techniques was used to investigate the effect of different factors, including the surface roughness, temperature, dissolved oxygen, pH and H₂S concentration, on the corrosion behavior of carbon steel in an NH₄Cl environment with the presence of H₂S. The effect of H₂S concentrations (at the ppm level) on the corrosion behavior of carbon steel was systematically revealed. The experimental results clearly indicated that the corrosion rate reached a minimum value at 10 ppm H₂S. The steel surface was covered by a uniform corrosion product film in a 10 ppm H₂S environment, and the corrosion product film was tight and protective. The ammonia from NH₄Cl helped maintaining the protectiveness of the corrosion films in this environment. Dissolved oxygen mainly accelerated the cathodic reaction. The cathodic limiting current density increased with increasing temperature, and the anodic branch polarization curves were similar at different temperatures. The anodic current density decreased as the pH decreased, and the cathodic current density increased as the pH decreased. The absolute surface roughness (R_a) of carbon steel increased from 132.856 nm at 72 h to 153.973 nm at 144 h, and the rougher surface resulted in a higher corrosion rate. The critical innovation in this research was that multiple influential factors were revealed in the NH₄Cl environment with the presence of H₂S.