Controlling the thickness and composition of stainless steel surface oxide film for anodic sulfide removal

Hydrogen sulfide is hazardous and toxic, and is present at many waste streams of industry processes. Anodic sulfide oxidation is a potentially effective approach to remove sulfide in aqueous media. This study therefore prepared and optimized surface oxide film on AISI 304 stainless steel (SS) as active and stable anode for sulfide removal. Among stainless steel samples of different anodization pretreatment duration, the anode with 15 min pretreatment (SS-15) resulted in the best sulfide removal at the same testing conditions, and achieved 48% and 18% improvement in terms of sulfide removal efficiency compared with the untreated SS and graphite, respectively. ANOVA-based tests (overall and for pairwise comparisons) also suggest the data are indeed valid. When anode potential was set no more than 1.62 V vs. reversible hydrogen electrode (RHE), direct sulfide oxidation dominated; and further increase in anode potential would initiate oxygen evolution reaction (OER) which contributed to indirect sulfide oxidation. Surface characterization found that the varying corrosion pretreatment duration led to oxide films of different thickness and chemical composition. The pretreatment duration from 15 min to 90 min increased the oxide film thickness from 178 nm to 264 nm and decreased the composition of FeOOH and NiOOH, which were most likely the catalytic active species for sulfide oxidation revealed by high-resolution XPS. Electrochemical analysis confirmed that the smaller thickness and enriched FeOOH/NiOOH composition in SS-15 led to a lower charge transfer resistance, a lower mass transfer resistance, and meanwhile a greater material stability. Depth profiling via high-resolution XPS identified metallic Fe inside oxide films and its presence may also promote the conductivity of the oxide film. This study demonstrated that anodization pretreatment of stainless steel was able to prepare effective and stable anode for electrochemical sulfide removal by controlling the thickness and composition of oxide films.