Investigation Of The Corrosion Behavior And Passive Film Degradation For Austenitic Stainless Steel In H₂S-containing Environment

Austenitic stainless steels are widely used as structural materials in oil industry due to their good corrosion resistance.Accelerated energy demands have boosted the exploitation of much deeper and sourer oil-gas fields that bring plenty of challenges to the service of stainless steels.Therefore,it is of both theoretical and practical significance to investigate the corrosion behavior of stainless steel.In this paper,electrochemical tests,immersion tests,X-ray photoelectron spectroscopy(XPS),Auger electron spectroscopy(AES)and time of flight secondary ion mass spectrometry(ToF-SIMS)were used to investigate the corrosion behavior and passive film degradation of 316L stainless steel.The effects of H2S,temperature,Cl-concentration,applied potential and pH were discussed,respectively.According to the results,the corrosion mechanism of H2S was clarified.The relationship between the corrosion resistance and the passive film composition was built up.The specific conclusions were listed as follows:(1)In H2S-containing environment,the pitting potential was more negative and the passive current density was larger,both of which indicated an acceleration effect of H2S on the corrosion behavior of 316L stainless steel.The presence of H2S could inhibit the formation of oxides and promote the formation of sulfides.This resulted in the irreversible damage to the corrosion resistance of stainless steel.The presence of sulfides in the passive film increased the acceptor and donor densities and thus resulted in the degradation of the passive film.(2)The composition and the sulfuration mechanism of the passive film were investigated by anodic polarization.The main composition of the passive film formed at various potentials was similar,however,the fraction of the main elements was different.The concentration of Cr decreased with the potential.Besides,S species were significantly favored in the passive film formation at higher potentials.Due to the fact that sulfides have more defects than oxides,the passive film formed at higher potential was less protective.(3)The effect of pH on the passive film structure under H2S-containing conditions were investigated.The passive films showed a three-layer structure.The outer layer was mainly composed of hydroxides.The intermediate and inner layers,which were influenced by both pH and H2S,were dominated by oxides and sulfides.(4)The synergistic effect of H2S and Cl-on the passive film degradation of stainless steel was studied at high temperature.Both H2S and Cl-could accelerate the corrosion of 316L stainless steel.H2S could inhibite the formation of oxides and promote the formation of sulfides.The sulfides in the passive film presented n-type semiconductor and accordingly was beneficial for the adsorption and diffusion of Cl-into the film.Since Cl-had an affinity to metals,so it could accelerate active dissolution and promote passive film to be thinner and less protective.(5)The effect of pH on the corrosion behavior and passive film composition of 316L was investigated.As pH increased,the pitting potential increased while the oxygen evolution potential decreased.The conversion from pitting corrosion to oxygen evolution reaction occurred at pH 12.5.Oxygen evolution reaction was kinetically faster than pitting corrosion,regardless of the pH.Moreover,oxygen evolution reaction would postpone the occurrence of pitting corrosion.The at%of Cr and Mo in the passive film decreased with pH,while Fe content increased.Since chromium oxides were more stable than iron oxides,the lower Cr and Mo content at higher pH was responsible for the degradation of the passive film.