Investigation On The Mechanism Of Localized Electrochemical Corrosion Behavior Of Duplex Stainless Steel

Stainless steels are iron-based alloysthat contain a minimum of approximately12wt.%Cr and austenite stainless steels with higher contentof 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. Because the phase fraction and the alloying redistribution are strongly influenced by themal treatment, the thermal treatment can show great effect on corrosion resistance to DSS coupons. Secondly, the overall corrosion resistance of DSS is affected by local heterogeneities, such as the inclusion, precipitate, defect and dislocation, among which the inclusion can usually act as preferential site for pitting initiation and therefore determine the the final quality of DSS production. Additionally, in comparison with the single phased austenite stainless steel, the multiple structure in DSS increases its complexity of localize corrosion mechanism. Thus, it is extremely important to clarify the underling relationship between alloy element distribution, heterogeneity and the associated localized corrosion resistance of DSS. The relative work can also promote the further understanding on the corrosion mechanism of multiphase alloy.Based on the above viewpoints, various localized analysis methods and electrochemical techniques were used in this work to investigate the influence of typical oxide inclusions in DSS on the pitting initiation and propagation, the relationship between the structural evolution affected by annealling heat treat and the localized corrosion property of DSS and the effect of environmental parameters on the crevice corrosion resistance of DSS. Furthermore, a new electrochemical method to evaluate the resistance to crevice corrosion of DSS was put forward and the localized corrosion mechanism was explained by the interaction of diffusion and IR drop control process. At last, the rule for the chemical composition design and microstruture control was given out. The detailed research contents and highlight of innovations are as follows: (1). Because there are few research focused on the underline relationship between the typical non-metallic oxide inclusions and the pitting initiation, this work was to investigate the type, morphology, distribution of typical non-metallic oxide inclusions in DSS as well as their effect in triggering pitting initiation by the combination of SEM morphology analysis, AFM topology anlaysis and localized electrochemical study. The Result showed significance in understanding the initial stage of pitting for DSS.(2). The effect of chemical composition and heat treatment on microstructure evolution and pitting corrosion resistance of DSS2304has been investigated by the integrated technique of critical pitting temperature and microscopic pitting control. The result gives out important guaidence to solution annealing treatment of DSS production.(3). The influence of environmental parameters on the crevice corrosion resistence of DSS was determined by the use of critical crevice temperature and potentiodynamic polarization technique. According to the result, the electrochemical potentiostatic critical crevice temperature method was established to evaluate the crevice corrosion property of DSSs. Additionally, the relationship between the alloying redistribution and the unique polarization behavior of dual phase structure was investigated by the combination of in-situ technique, potentiodynamic method, localized electrochemical analysis and microscopic morphology analysis. The result contributes to the further understanding in crevice corrosion of DSS.