Study On Corrosion Behavior Of Low-Temperature Liquid Oxy-Nitrided 304 Stainless Steel In High H₂S-Containing Environment

The reserves of high-sulfur natural gas in the Sichuan basin account for more than 90%of the same kind of natural gas in China,and it has the characteristics of a complex medium（up to 17 vol%H₂S,up to 11 vol%CO₂,elemental sulfur,Cl^-,etc.）,diverse flow patterns,high temperature and pressure,high toxicity and strong corrosivity,so it is difficult to exploit safely.Austenitic stainless steel is widely used in oil and gas fields,but its corrosion and strength properties in the high sulfur environment are not ideal,which limits its application.In this paper,a composite modified layer was introduced on the austenitic stainless steel surface by low-temperature liquid oxy-nitriding（LTON）.The corrosion resistance of 304 austenitic stainless steel and LTON surface modification layer was compared by carrying out corrosion test in high H₂S-containing environment.The growth mechanism of the corrosion product layer and the strengthening mechanism of the S-phase layer are revealed,and the specific results are as follows:（1）The microstructure of the surface modification layer of 304 austenitic stainless steel by low-temperature liquid oxy-nitriding was clarified,and it was revealed that the LTON surface modification layer significantly improved the wear performance.The results show that the low-temperature liquid oxy-nitriding layer is composed of the outer dense Fe₃O₄ phase and the inner nitrogen-rich S-phase.The S-phase is a nitrogen-containing supersaturated solid solution containing a large number of dislocations,stacking faults,and twins,which lead to high hardness.The temperature and load during the wear test affect the wear behavior of 304 stainless steel.For 304 and LTON samples,the increase in temperature and load results in a slight decrease in the average friction coefficient,and an increase in wear volume loss.LTON treatment shortens the duration of the initial wear stage and reduces the maximum wear volume loss of 304 stainless steel from 1.086 mm³ to 0.144 mm³ under the condition of 10N/200℃.（2）Experiments on the effect of low-temperature liquid oxy-nitriding on the corrosion kinetics of 304 stainless steel in high H₂S-containing liquid and vapor phases were carried out.The difference in H₂S corrosion behavior between the liquid phase and vapor phase was analyzed,and the strengthening mechanism of LTON in H₂S environment was elucidated.In the H₂S/CO₂-saturated solution,the prolongation of immersion time makes the corrosion behavior of 304 stainless steel transition from local corrosion to uniform corrosion.The corrosion product formed in the vapor environment has a dense structure,where the pores between adjacent corrosion crystals are small;in the liquid environment,the corrosion product structure is looser and the pores are larger,resulting in a higher corrosion rate and a thicker corrosion product film.As a comparison,the corrosion products on the LTON surface are denser and less cracked.The active nitrogen atoms in the S-phase layer can consume the acidic medium on the near-surface,preventing the local p H from decreasing,and leading to the formation of NH₄⁺.The existence of the S-phase layer reduces the corrosion rate of 304 stainless steel in H₂S/CO₂/Cl^-by 89.8%.The reduction in the corrosion rate may be attributed to the barrier effect of the Fe₃O₄ layer on the corrosion medium and the lower ion/atom mobility due to the huge amount of interstitial atoms in the S-phase layer.（3）The influence of the H₂S/CO₂ partial pressure ratio on the corrosion behavior of 304 stainless steel was carried out.The influence of low-temperature liquid oxy-nitriding on the corrosion mechanism of 304 austenitic stainless steel in various H₂S/CO₂ partial pressure ratio media were revealed.The increase of the H₂S/CO₂ partial pressure ratio increases the particle size of corrosion products for the 304 stainless steel,which intensifies the local corrosion under the loose and cracked corrosion product layer,resulting in an increase of the corrosion rate from 0.52 mm/a to 0.68 mm/a.For the LTON sample,the change of the partial pressure ratio of H₂S/CO₂ has little effect on the corrosion behavior as well as the thickness of the corrosion products.The Fe₃O₄ layer can effectively block the acidic corrosion medium,and the sulfides and oxides formed by corrosion accumulate outside the original Fe₃O₄ oxide film.Low-temperature liquid oxy-nitriding reduces the corrosion rate of 304 stainless steel by more than 86%in various H₂S/CO₂ partial pressure ratio environments.（4）The erosion-corrosion and stress corrosion experiments in high H₂S-containing acidic medium were carried out,and the effect and corrosion mechanism of low-temperature liquid oxy-nitriding on 304 stainless steel in high H₂S-containing acidic medium were revealed.Low-temperature liquid oxy-nitriding enhances the erosion-corrosion performance of 304 stainless steel in SiO₂/H₂O,HCl/SiO₂/H₂O,H₂S/SiO₂/H₂O media,and reduces the diffusible hydrogen content in 304 stainless steel by 80%after electrochemical hydrogen charging.In the process of stress corrosion in saturated H₂S/CO₂ medium,the immersion time affects the corrosion behavior and the formation of surface corrosion products of 304 stainless steel,while the applied stress affects the cracking behavior.With the prolongation of immersion time,the local corrosion pit becomes wider and deeper,and the corrosion product layer becomes thicker;with the increase of applied stress,the deformation slip band on the outer surface of the C-ring becomes obvious,and severe cracks eventually developed along the bottom of the pit.LTON treatment can enhance the stress corrosion resistance of304 austenitic stainless steel and prevent the penetration of hydrogen atoms.The S-phase inhibits the lattice distortion and hydrogen-induced plastic deformation caused by hydrogen atoms,thus improving the hydrogen-induced cracking resistance of 304 stainless steel.