Effect of Fluoride Ion on the Corrosion of 304 Stainless Steel in (NH4)2SO4 Solution Containing Chloride Ions

The corrosion behavior of stainless steel in contact with an ammonia desulphurization slurry, which contains a high concentration of both fluoride ions (F-) and chloride ions (Cl-), affects the commercialization of the ammonia desulphurization technology. In this paper, 304 stainless steel was taken as a typical stainless steel sample to explore the influence of F- on its corrosion behavior in a simulated ammonia desulfurization slurry. The open circuit potential, polarization voltage, polarization current, cyclic voltammograms, electrochemical impedance spectroscopy, and Mott-Schottky plots, for different F- concentrations, have been studied using electrochemical methods. At the same time, the influence of different F. concentrations on the corrosion of stainless steel was studied by an immersion test, and the specimens were characterized by scanning electron microscopy and X-ray photoelectron spectroscopy. The experimental results show that in the simulated ammonium desulfurization slurry, at a given concentration of Cl-, low concentrations of F- aggravated the corrosion of stainless steel, while a high concentration of F- (the molar ratio of F- to Cl- is higher than 10) produced significant corrosion inhibition. The reason may be that F. can combine with H+, preventing the pH of the solution from becoming too low, and simultaneously, F- can also react with metal ions to form insoluble sediments, which prevent the aggravation of the corrosion process.