Investigation of Unusual Black Film Formed on Stainless Steel Surfaces in Diluted Sulfuric Acid Service

Abstract An investigation was conducted to explore the root cause of the formation of a MoS2 rich black film on the surface of 316L stainless steel equipment exposed to diluted sulfuric acid (H2SO4). Traditional corrosion immersion tests were performed by exposing stainless steel coupons to diluted sulfuric acids of various concentrations at 20°C, with or without impurities such as chloride and sulfide. Scanning electron microscopy with energy-dispersive x-ray spectroscopy (SEM/EDS) analysis was employed to characterize coupon surface after exposure. Stable black films were developed on some stainless coupon surfaces when 20 ppm sulfide ions were added to test acids. EDS analysis confirmed that a sample of these films was rich in Molybdenum (Mo) and Sulphur (S) with an atom ratio of approximately 1:2, thus likely MoS2. In addition, thermodynamic modeling calculation predicted the formation of this Molybdenum disulfide (MoS2) phase when sulfide is present in the acids.