Study On Corrosion Behavior Of S32750 Super Duplex Stainless Steel

Duplex stainless steel is a kind of stainless steel consisting of ferrite and austenite.The contents of ferrite and austenite in duplex stainless steel are about 50%respectively,and the content of less one phase is not less than 30%.Therefore,duplex stainless steels combine the advantages of ferrite steel and austenitic steel,which means duplex steels have both good mechanical properties and corrosion resistance,and are widely used in marine oil industry aand coal-chemical industry.With the further expansion of the demand of energy,the environment that duplex stainless steels are used in is getting worse and worse.How to keep good corrosion resistance in various harsh environments has become an important direction of the development of duplex stainless steels.The effects of annealing temperature and holding time on the microstructure,phase composition and corrosion resistance of super duplex stainless steel have been studied.The results show that the ratio of ferrite in duplex stainless steel increases while the ratio of austenite decreases with the increase of annealing temperature in the range of 950?-1200?.When annealing temperature comes to 1050 C and the holding time is 30 min,the ferrite ratio is about53%,and the austenite mostly distributes in the ferrite matrix in strip shape.At this time,S32750 super duplex stainless steel has the best corrosion resistance in 1mol/L NaCl solution.In this paper,the stress corrosion process of S32750 super duplex stainless steel in 1mol/L NaCl solution was studied by electrochemical noise analysis.The results show that constant stress hinders the formation of passive film,inhibits the repair of passive film and promotes pitting corrosion.When the constant stress exceeds the yield strength,the S32750 super duplex stainless steel yields with fresh metal exposed continuously which promotes the occurrence of stress corrosion.Higher temperature of NaCl solution aggravates the effect of Cl~- on passive film,which makes stress corrosion more easily to happen at the pitting location on passive film,for the stress concentration occurs more frequently at the location and results in microcracks.