| Duplex stainless steels (DSS), characterized by dual-phase ferrite-austenite microstructure, have important prospects for structural materials because of their attractive combination of high mechanical strength and good corrosion resistance in various aggressive environments. Economical duplex stainless steel is one of the trends of the development of DSS, containing nitrogen, low nickel. Generally speaking, DSS have excellent corrosion resistance, but under certain conditions it will happen to localized corrosion, including pitting, intergranular corrosion, stress corrosion cracking and so on. Compared to single-phase stainless steel, the localized corrosion of DSS is more complicated, mainly in the complex organization with a multiphase (ferrite phase, austenite phases, Cr2N,σ,χ,γ2).With the applications of the economical duplex stainless steel in a wide range, the investigation of corrosion resistance can not be ignored. Thus, a systematic study of the localized corrosion (pitting, intergranular corrosion) rules and mechanisms of 2101 duplex stainless steels were carried out in this paper. The heat treatment (high temperature solution and the temperature sensitivity of the area) and alloy composition on the microstructure evolution and localized corrosion resistance of DSS 2101 were illustrateed. The corrosion mechanisms of DSS 2101 in several media were given. The pitting model of duplex stainless steel was proposed. Based on the study of corrosion laws and mechanisms, the principles and methods of the alloy design and microstructure controlling were put forward, providing the experimental and theoretical basis of corrosion aspects for the new dual-phase steel design and development, process optimization, product performance. The main contents included the following aspects:①With the potentiodynamic polarization curve, potentiostatic critical pitting temperature (CPT), electrochemical Potentiodynamic Reactivation (EPR) and chemical immersion methods, the effect of phase ratios, precipitates and alloying elements on the corrosion behavior of DSS 2101 was investigated, respectively. The results showed that annealed in the temperature range of 1000～1200℃for 30 min, the ratios of two-phase of DSS 2101 changed notablely. Meantime, the lower values of pitting potential (Epit) and CPT were obtained after annealing at the higher temperatures. Pitting was initiated preferentially in the ferrite/austenite boundaries or inside the ferrite domains, indicating ferrite phase had inferior pitting corrosion resistance than austenite phase in as-received steel. This result can be explained through the PRE values of two phases together with the effect of supersaturation of nitrogen in the ferrite. Considering various factors, it should select 1050℃as the best solution treatment temperature for DSS 2101. After Aging in the temperature range of 300～900℃for 10min, DSS 2101 aged at 700℃had the most amount of secondary phase precipitates, the smallest CPT value, the largest ratios of reactivation current density and activation of the ratio of current density (Ra) value. Thus, the noise temperature of precipitation for DSS 2101 was 700℃. The analysis of X-ray diffraction analysis(XRD), Transmission Electron Microscope(TEM), coupled with the calculated phase diagram by Thermo-Calc software confirmed that the main precipitate of DSS 2101 aged at 700℃was Cr2N. The precipitation proceeded asα→Cr2N+γ2.Aging time before 30min, Epit and CPT decreased rapidly; aging time exceeding to 120min, Epit and CPT changed little. The pitting corrosion mechanism of aged DSS 2101 was theγ2 phase selective dissolution around Cr2N. Increasing the aging time increased the sensitivity degree of intergranular corrosion of DSS 2101. The results of CPT and chemical immersion found that Cr, Mo and Ni elements improved the corrosion resistance of DSS 2101, while the effect of N on the corrosion resistance was not obvious. The reason was that the corrosion resistance of duplex stainless steel was not determined by the total content of alloying elements in this steel, but by the distribution of alloying elements in the two phases.②The Electrochemical noise technique was used to examine the corrosion types of DSS 2101 in 0.1% NaOH,5% H2SO4, and 6% FeCl3 solutions and the piting corrosion behavior in lmol/L NaCl solution. It was found that the corrosion process of the steel in three solutions followed by passive state, uniform corrosion of the ferrite phase and pitting corrosion. The noise spectrum analysis can effectively predict the trend of corrosion of DSS 2101. The PSD slope of potential noises in three solutions were -0.63,-1.4,-2.3, indicating the different types of corrosion of DSS 2101 in these solutions. The pitting index (PI) did not provide meaningful information for DSS 2101 immersed in 0.1%NaOH,5%H2SO4 solutions. The corrosion rates in three media were reflected by the noise resistance (Rn), presenting good correlation with the results from linear polarization and EIS. Compared to the power spectral density (PSD) through FFT analysis, the energy distribution plot (EDP) from wavelet analysis can reflect better the metastable/stable pitting behavior of DSS 2101 under the condition with the potential of 250mV(SCE) and temperatures of 15℃,20℃,25℃,28℃and 35℃respectively. Under the different temperature, a relatively high energy fraction Ejd corresponded to different wavelet coefficient time-scale.③Potentiostatic CPT method was used to obtain the critical pitting temperature of DSS at different temperature change rates. The results demonstrated that CPT values increased as the temperature change rate (u) increased. A linear relationship between the critical pitting temperature (CPT) and the square root of temperature change rate (υ1/2) was found. Furthermore, deriving from the point defect model (PDM), this relationship received theoretical support at low temperature change rates, indicating that the occurrence of pitting corrosion for two kinds of duplex stainless steel followed the mechanism of passive film breakdown supported by the PDM.
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