| Due to increased cost of raw materials, there has been an incentive to develop stianless steels with lower contents of nickel over the past 10 years. This type of so called "low-nickel stainless steel" shows explosive growth-rate. Up to now, its growth-rate per year has reached 20%. In order to substitute extensive applied austenitic stainless steel, which contents at least 8 percent of nickel, the research on low-nickel stainless steel mainly focus on two aspects:one is low-nickel austenite/ferrite duplex stainless steel (e. g. LDX2101 and 2304), the other is 200 series Mn-N substituted austenitic stainless steels. Due to localized corrosion is main failure form for stainless steel, the related research has been one of the most important field in corrosion of metal.The fact that the heterogeneousness of alloy elements distribution between two phases in duplex stainless steel (DSS) makes their corrosion resistance quite different. Additionally, at the heat treatment temperature from 300℃to 1300℃, quantitive second precipitates appear, together with originally existed inclusions and different single phases, the microstructure of DSS is highly complex. Up to now, there are two problems which should be solved during the research of localized corrosion of DSS: One is the fundation of new techniques on the evalutiaon of localized corrosion resistance for the complex microstructural systems. The key point is to distinguish the role of each phase (αphase,γphase, s phase,χphase, second austenite, M23C6 and Cr2N, et al.) played during localized corrosion. The other is to clear the effect of microstructural transformation of DSS on the related rules and theories of localized corrosion. The significance of the two respects research lie in its support to optimizaition of alloy design, microstructural control and proccessing confirmation (heat treatment and welding).Against that background, in this paper, pitting corrosion and intergrannular corrosion of DSS, e.g. LDX2101 are studied and localized corrosion resistance of severial 200 series stainless steel are comparatively and systematically investigated. After three years hard work, the main innovation of this paper are:Two techniques: DL-EPR and EIS are founded to evalute selective corrosion resistance of LDX 2101 DSS; Potentistaitic pulse technique are susseccfully applied to investigate regional pitting corrosion resistance of dual microstructure; Basis on this, combined with long working distance microscopy, the rule of pitting initiation and development are clearly elucidated.Research with respect to the susceptibility to selective corrosion of LDX2101, a kind of new-type economical DSS, has hardly been attempted. This paper focus on the effects of thermal aging treatments at 700℃for various aging times up to 100 h on the selective corrosion resistance of LDX2101 by two non-destructive electrochemical measurements:double-loop electrochemical potentiokinetic reactivation (DL-EPR) and electrochemical impedance spectroscopy (EIS). SEM in conjunction with EDS and XRD were used to examine the evolution of microstructure. The results showed that the two applied electrochemical measurements agreed very well. Both methods were able to reveal the relationship between microstructure and selective corrosion resistance, which was related to the formation of chromium-and molybdenum-depleted zones around the precipitates, especially the s phase. Nevertheless, more information could be obtained using EIS methods, including the interfacial charge transfer reaction and the corrosion product adsorption process. The results suggest that the susceptibility of the aged alloy to selective corrosion is presumably codetermined by the formation of chromium-and molybdenum-depleted areas, as well as by the replenishment of them, in these areas from the bulk during aging.For the first time, optimized potentiostatic pulse technique (PPT) was established and applied to study the rule of initiation of pits of solution annealed and aged DSS in 1.0mol/L NaCl solution. The results showed that the size of pits can be well-controlled by adjusting the condition of potential pulse. Combined with SEM/EDS system observation, it was found that for solution annealed specimens, sulphide and oxide mixed inclusions were the preferential pit initiation sites, while for aged specimens, majority pits occurred at inclusions in area impoverished in Cr/Mo, which is vicinal to s phase. The phenomena that growing pits is retarded by precipitition rich in Cr/Mo, i.e. s phase, was testified and explained.The electrochemical corrosion activity of the economic lean duplex stainless steel LDX2101 with three different microstructures was investigated in chloride solutions by long working distance microscopy (LWDM) in combination with PPT. In addition, the site and chemical composition of surface of material were also ex-situ determined by integrated SEM/EDS system. The results demonstrated that:(1) During aging process of LDX2101 at 700℃, the following reaction (a→Cr2N+γ2) firstly occurs, and then the reaction a→s+γ2 occurs. (2) The initiation site of electrochemical active point has been in-situ determined by LFLM in combination with potentiostatic pulse technique. (3) The sites of electrochemical activity points have been changed from inclusions (MnS/Al2O3) to the chromium depletion region, namely secondary austenite phase (γ2).It is still a problem to product low-nickel austenite stainless steel with reasonable cost performance and meets the demands on corrosion resistance and processability in our country. In this paper, many researches have been carried out to provide reference to develop the product at the early stage for Bao Steel. Main localized corrosion behaviors of one Mn-N substituted austenite stainless steel of Bao Steel were evaluated comparably with several commercially available products and 304 SS produced by Bao Steel. The effects of alloy elements on localized corrosion resistance were described and some advices on the product formulation were given.
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