Effect Of Microstructure On Corrosion Resistance Of Duplex Stainless Steel

Due to consisting of both ferrite and austenite phase,duplex stainless steels(DSS)possess good mechanical properties and corrosion resistance,so they have been widely using in harsh fluid environments.For example,because of higher resistance to chloride ion corrosion,they have been often used as the substitutes for austenitic stainless steels(ASS)in the chloride ion environments in petrochemical and marine systems;Because of higher strength,they have been also the preferred materials for the fabrications of flow parts.However,in recent years,corrosion failures of DSS in chlorine-containing environments and cavitation failures in high-concentration acidic chlorine-containing solutions have been reported frequently.It has been shown that the corrosion resistance of the DSS is often deteriorated by the microstructural changes caused by cold and hot processing,welding and high temperature exposure.In view of the fact that the microstructural morphology can affect the corrosion resistance of DSS,this thesis took the 2205 DSS as the research subject,adopted the combination of heavy cold rolling and annealing treatment to initiatively modify the microstructural morphology of steel,and then investigated the influence of different microstructural morphologies on the corrosion resistance and cavitation corrosion resistance of the steel in chlorinecontaining solution,in order to provide a reference for improving the corrosion resistance and cavitation corrosion resistance of DSS.The main research contents and conclusions of this thesis are as follows:(1)The influence of annealing time(30 s and 1800 s)and annealing temperature(950 ℃,1000 ℃,1050 ℃,1100 ℃)on the microstructure of 2205 steel was studied after severe cold rolling deformation.The results show that cold-rolling deformation caused the two phases to be striped and continuous along the cold-rolling direction,the phase thickness reduced.and the microstructure to be refined.Low-temperature and short-time annealing made no obvious change in the morphology,however with the increase of annealing temperature or annealing time,the austenite strips started to break into discontinuous elongated islands.For high-temperature and long-time annealing,the austenite phase islands distributed more dispersedly and uniformly,impairing the feature of banded microstructure.Harmful phase precipitated in the steel at 950 ℃ and annealing time of 1800 s,but no precipitate appeared in the other time-temperature combined annealing treatments.Annealing temperature and time affected the phase content,austenite grain size and hardness of the material.With the increase of annealing temperature and time,the austenite phase reduced,the austenite grain size increased and the hardness decreased.(2)Electrochemical corrosion test and immersion corrosion test were used to study the influence of different micromorphologies produced by heavy cold rolling and annealing treatment on the corrosion resistance of 2205 DSS.The electrochemical corrosion test was conducted in 3.5% Na Cl solution.The results showed that the corrosion resistance of AT1100-1800 specimen was the best,and the corrosion resistance of AT950-1800 specimen was the worst.The corrosion resistance of the asreceived material was average.After immersion in 5% HCl solution,moderate corrosion occurred in the as-received specimen,90% cold-rolled specimen,AT1100-30 and AT1000-1800 specimen,showing good corrosion resistance.AT1050-1800 and AT1100-1800 specimen showed moderate corrosion and slightly poor corrosion resistance.AT950-30,AT1000-30,AT1050-30,AT950-1800 specimen suffered from very serious corrosion,their corrosion resistance is the worst.The SEM morphology of all the specimens showed that selective corrosion occurred.(3)The effect of different micromorphologies produced by heavy cold rolling and annealing on the cavitation corrosion resistance of 2205 DSS in 5% HCl solution was studied.The results show that the as-received material has the best cavitation corrosion resistance,the 90% cold-rolled specimen is slightly worse,and the specimen annealed at 950 ℃ for 1800 s showed the worst cavitation corrosion resistance.At the same annealing temperature,the cavitation corrosion resistance of the specimens annealed with short time(30 s)is better than that of the specimens annealed with long time(1800s).The SEM morphology results after cavitation show that all the surface materials of the specimens had been exfoliated and the damage was serious.It is the interaction between cavitation and corrosion that promotes the dissolution and exfoliation of the materials.