Study On The Corrosion Behavior Of Two High Nitrogen Austenitic Stainless Steels

Generally speaking,high nitrogen austenitic stainless steel（HNSS）has good corrosion resistance and excellent mechanical properties.Due to its stable and uniform austenitic structure,it has been widely used in the fields of pipeline transportation,construction and chemical engineering,shipbuilding and marine engineering,etc.Initially,in order to improve the corrosion resistance of austenitic stainless steel,8～10 wt.%Ni was mainly added.As the demand for austenitic stainless steel increases,the price of Ni increases.In order to save resources and reduce allergy to human body fluids（sweat,saliva,blood,etc.）,Ni in austenitic stainless steel should be minimized or removed as much as possible.Cheap austenitic stabilizing elements such as Mn and N should be used.Previous work has shown that adding alloying element N to stainless steel can reduce the tendency for pitting in chloride solutions.This is due to the improved protective ability of the passive film formed on the surface of austenitic stainless steel.The development of austenitic stainless steels with high corrosion resistance and low cost using N as an alloying element has received widespread attention.Therefore,the research on the corrosion resistance of high nitrogen austenitic stainless steel is still an important topic.In this study,Fe-20Cr-20Mn-0.64N（HNSS1）and Fe-20Cr-20Mn-0.75N（HNSS2）were selected as experimental materials to investigate the corrosion behavior of two high nitrogen steels in 0.5 mol/L Na Cl solution using electrochemical testing methods;The protective properties of two kinds of high nitrogen steel passive films on the substrate were studied by potentiostatic polarization method,passive film formation mechanism,and mott-schottky curve.The elemental content of the passive film was analyzed by XPS.The results show that HNSS2 with high nitrogen content has low activity,low corrosion current density,and good corrosion resistance.The passive films of both materials have p-n type semiconductor characteristics,where in the film formation rate of HNSS2 is higher than that of HNSS1,and the passive film of HNSS2 has a lower carrier density than that of HNSS1.Due to the high content of Cr₂O₃ in the passive film of HNSS2 and the strong stability of the passive film,it has better protection for the substrate.In 10%FeCl₃ solution,HNSS1 and HNSS2 were immersed for 24 h and 72h,respectively,to study the two-dimensional and three-dimensional macroscopic corrosion morphology.The corrosion rate under corresponding conditions was calculated by weight loss method.According to the research,the surface of HNSS1 is severely corroded and has obvious corrosion pits.Its corrosion rate is faster than that of HNSS2,its weight loss is greater than that of HNSS2 and its corrosion resistance is significantly lower than that of HNSS2.The corrosion behavior of Fe-20Cr-20Mn-0.75N with good corrosion resistance was studied by electrochemical testing in different Cl^-concentrations.The constant potential polarization and corresponding mott-schottky curves at different applied potentials were selected to calculate donor and acceptor densities,a point defect associated electric field model and oxygen vacancy diffusion coefficient were obtained.The passive film formed under constant potential of 0.2 V_SCE was tested by X-ray photoelectron spectroscopy to analyze the composition of the passive film and study its protective properties against the substrate.The research shows that Fe-20Cr-20Mn-0.75N high nitrogen steel exhibits re-passive behavior in different Cl^-concentrations,with a greater corrosion tendency and an increased corrosion rate.As the applied potential increases,the carrier density gradually increases,the oxygen defect diffusion coefficient D₀ increases,and the protective effect of film on the substrate decreases.In low Cl^-concentrations,the oxide content of Fe and Cr is higher,the passive film is thicker,and the surface is relatively stable.