The Relationship Between Microstructure Evolution And Electrochemical Corrosion Behavior Of Multiphase Alloy

Stainless steels are iron-based alloys that contain a minimum of approximately 12 wt.% Cr and austenite stainless steels with higher content of Ni constitute the largest stainless steel family. With the requirements of resource saving, high quality and high corrosion resistance, duplex stainless steels (DSSs) have been developed in the past decades. Considering that the failure of stainless steels are mainly induced by pitting and intergranular corrosion, it is very important to evaluate the pitting and intergranular corrosion resistance of DSS during its manufacture process.Because there are two phases with different crystal structure in DSSs, the distributation behavior of alloy elements in ferrite, austenite and interface are quite different. In addition, DSSs are prone to form some unwanted secondary phases (carbides, nitrides, s,χand R) during exposure to elevated temperatures between 300 and 1300℃and their corrosion resistance are determined by the weakest phase. Thus, it is extremely important to clarify the underling relationship between alloy element distribution, secondary phase precipitation and the associated corrosion resistance. Based on the complexity in DSSs, the relative research work have been done from three viewpoints:Firtsly, the advanced techniques for corrosion evaluation should be established for detecting the role of secondary precipitates in multiphase alloys; Secondly, the relationship between new develpoed alloys and environment condition should be expored; At last, the rule for the chemical composition design and microstruture control should be put forward.Through understanding mechanism of pitting and intergranular corrosion of austenite stainless steel, we put forward new methods for evaluating the localized corrosion resistance of DSSs. And then we check the effect of chloride concentration on corrosion resistance of DSSs, investigate the effect of the chemical composition and heat treatment on microstructure evolution and the localized corrosion resistance and finally obtain the rule for the chemical composition design and microstruture control in typical DSSs, which is very useful for the development of DSSs. The detailed research contents and highlight of innovations are as follows:(1) Because there are seldom research focused on the relationship between pitting morphology, growth and stability, this work is to investigate the potentiostatic metastable and stable pitting behaviors of AISI 304L stainless steel. The results demontrate that the current-time follows the prevailing relationship I=at2 during initial growth and then their relationship changes as I=at0.5 during stable growth. The transition from metastable pitting to stable pitting is determined by the pitting stability products, e.g., ia*=(DΔC*)/(2π/3nF). At last, a model is put forward for understanding the whole pitting process.(2) The effect of chemical composition and heat treatment on microstructure evolution and pitting corrosion resistance of DSSs has been investigated by the integrated technique of critical pitting temperature, repassivation temperature and microscopic pitting control. The results show that the chemical composition can greatly affect the pitting corrosion resistance and polarization behavior of DSSs.In addition, different heat treatment will result in the alloy elements redistribution and secondary precipitates, which will change the value of critical pitting and repassivation temperature, the position of pitting initiation and propagation and polarization behavior of the alloy.(3) Intergranular corrosion (IGC) behavior of AISI 304 austenite stainless steel has been evaluated in relation to the influence of aging temperature and time. For this evaluation, double loop electrochemical potentiokinetic reactivation (DL-EPR) is performed to produce time-temperature-sensitization (TTS) diagram for AISI 304 stainless steel. The relationship between carbide morphology, precipitation position and the associated IGC has been determined by microstructure analysis. At last, we put forward a model for IGC evoluation of AISI 304 stainless steel based on the change of Cr concentration at the depleted zone.(4) The optimal test conditions using DL-EPR method for evaluating IGC susceptibility of UNS S32101 and UNS S31803 DSSs have been firstly defined on the basis of test response reproducibility, etching selectivity, high sensitivity to detect the dechromized zone and aggrement with IGC microstructure criteria. The results demonsrate that the optimal test condition of UNS S32101 is quite different from that of UNS S31803 due to the difference of chemical composition and microstructure. In addition, the IGC susceptibility of UNS S32101 DSS increases with increasing the aging time. However, the IGC susceptibility of UNS S31803 DSS approaches the highest value after aging up to 24 h and then decreases with aging time.