Investigation On Corrosion Behavior Of Medium-Entropy Alloy CrCoNiN

High-entropy alloys have broad application prospects because of the excellent performance.CrCoNi medium-entropy alloy(MEA)exihibits better strength and toughness than most high-entropy alloys and multi-phase alloys,which can be used as the foundation for future development of promising engineering alloys.However,CrCoNi MEA still cannot meet the requirements of high yield strength in engineering application.Therefore,N is considered to be introduced into the CrCoNi MEA to improve its mechanical properties and corrosion resistance.The study in respect of corrosion resistance in simulated seawater environment and the behavior of microbiologically influenced corrosion(MIC)of CrCoNiN MEA is of great significance for its application in marine and other special engineering fields.CrCoNiN MEA was studied in this paper,and the CrCoNi MEA and 316L austenitic stainless steel were used as reference steels.A variety of electrochemical experiments(DP,CP,EIS,OCP,LPR and EFM)were conducted to evaluate the corrosion resistance of CrCoNi and CrCoNiN MEA in simulated seawater environment and the microbiologically influenced corrosion behavior of CrCoNi,CrCoNiN MEA and 316 SS caused by Pseudomonas aeruginosa.XPS was used to study the structure and composition of the passive film.A variety of surface analysis techniques(OM,SEM,TEM and CLSM)were applied to observe the microstructure and precipitates of CrCoNi and CrCoNiN MEA,together with the P.aeruginosa biofilm and pitting morphology on the coupon surface.The main conclusions are obtained as follows:The addition of nitrogen expanded the austenite phase region of CrCoNi MEA,and also significantly reduced the grain size of austentite.Nitrogen existed in the form of Cr2N precipitates and soluted N atoms in CrCoNiN MEA.Metastable pitting corrosion was easily initiated around Ce oxide inclusions and Cr2N precipitates.Nitrogen could inhibit the initiation of metastable pitting corrosion of CrCoNiN MEA,significantly reduce the corrosion current density,as well as increase the pitting and repassivation potential and critical pitting temperature.The passive films of CrCoNi and CrCoNiN MEA were p-type semiconductors.The addition of nitrogen reduced the acceptor density,increased the thickness of passive film,and promoted the enrichment of Cr and Cr2O3 in passive film.CrCoNiN MEA enriched ammonia(NH3 and NH4+)on the outermost surface of passive film and CrN at the metal/passive film interface,which improved the stability and repair ability of passive film.Therefore the corrosion resistance of CrCoNiN MEA in high temperature chloride solution was significantly improved.P.aeruginosa accelerated the corrosion rate of CrCoNi and CrCoNiN MEA,which was demonstrated by a negative shift of EOCP,a decrease of polarization resistance and an increase of corrosion current density in P.aeruginosa medium.P.aeruginosa could also form an uneven biofilm on the surface of MEA coupons,which destroyed passive film on the surface of coupons and accelerated pitting corrosion.Compared with 316L SS,the passive films of CrCoNi and CrCoNiN MEA have excellent repairing ability.The microbial corrosion resistance of the two kinds of MEA in the environment containing P.aeruginosa was similar,and they were both superior to 316L stainless steel.