Research On Effect Of Nitrogen On Corrosion Behavior Of 316L Stainless Steel In Simulated White Water Environment

Since the 1960s,sodium hydrosulfite(Na2S2O4)had been widely applyed as a main bleaching agent in the pulp and paper industry,with thiosulfate(S2O32-)presenting in the paper machine as the by-product of bleaching stage,then the metallic components suffered sever pitting corrosion.Intensive researches show that the accelerated corrosion is attributed to the synergistic effect of Cl-and S2O32-.The existing methods to corrosion inhibitation includes increasing the content of corrosion resistance alloys(Mo)and adding corrosion inhibiter,which are both inadequate in some aspects.Therefore,it is significantly important to find a material that can defense the corrosion in paper pulp environment containing Cl-and S2O32-.In the present research,the effect of N alloying on the corrosion resistance of 316L austenitic stainless steel in the modified pulp and paper environment with Cl-and S2O32-was studied.The deoxidation rates of Mg treatment and rare earth treatment were calculated through thermodynamic method.The precipitation law of second phases was studied by Thermo-Calc software.The variations of corrosion resistance and passive film with and without N alloying were evaluated by electrochemical tests.The corrosion products were predicted by FactSage software.The morphologies and corrosion products after palarization tests were investigate by optical microscope(OM),field-emission electron microscope(SEM),energy dispersive spectrometer(EDS)and X-ray photoelectron spectroscopy(XPS).The main conclusions are shown as follow:The results of Thermo-Calc calculation show that with the increase of N content,the precipitation temperature increased,and maximum precipitation weight percent decreased.Additionally,the results of solution treatment probe show that the grain size increased with the increase of solution treatment temperature and duration.The test steels all entered the γ phase region at the solution treatment temperatures,indicating the total redissolution of precipitates.The results of electrochemical polarization tests show that the addition of S2O32deteriorated the corrosion resistance of 0N steel in NaCl solution.However,the breakdown potential(Eb)and protection potential(Eprot)increased with the increase of N content,which indicated the optimization of pitting corrosion resistance by N alloying.Compared with 0N steel,Eb of 0.20N steel was improved significantly,while Eprot barely changed.For 0.3 1N and 0.51N steels,further enhanced Eb and Eprot were obtained.The results of critical pitting temperature(CPT)measurement show that CPT continuously increased with N content.The corrosion morphology investigation after cyclic polarization tests show that there were massive large-size pits distributed intensively on 0N steel,with a large amount of corrosion product accumulating around the pits,while no evident pits were observed on 0.3 1N steel.Additionally,the corrosion product analysis shows that the corrosion product formed on 0N steel consisted with S,NiS2 and FeS2.The results of electrochemical analysis on passive fim show that the addition of S2O32-degraded the passive film of 0N steel in NaCl solution.The film resistance(Rf)and metal/electrolyte interface resistance(Rct)decreased,and donor density(ND)increased.Nevertheless,with the increasing N content,Rf and Ret increased,and ND decreased,indicating the improved corrosion resistance and compactness of passive film.The XPS analysis of passive film shows that the addition of S2O32-decreased the content of corrosion resistant species(Cr2O3 and Fe2O3)in the film,while N alloying increased their content.Furthermore,NH3 and NH4+(products of N buffering reaction),were detected in N-bearing steels,and CrN(film self-healing specie)were detected in the passive film of high N content steel.