| The second phase precipitation process of 316 LN stainless steel which aged at 650 ℃, 750 ℃ and 800 ℃ is studied in terms of Nb segregation on the high side.By means of Metalloscope, scanning electron microscope(SEM) and transmission electron microscope(TEM), numbers of observations on the process of second phase precititation process of 316 LN stainless steel aged at different temperature are carried out.he phase species of the stainless steel are measured by X-ray diffraction using electrolytic extraction method. The phases transition process of 316 LN stainless steel aged at the three specified temperatures is analysed; the stability and structures in the circustance of Cl- for 316 LN passive film are studied by applying DEIS, TAFEL and Mott-Schottky tests; After precipitating different phase, the intergranular corrosion sensitivity of 316 LN stainless steel is estimated by dint of EPR, chemical immersion and SEMThe results show that: When the 316 LN stainless steel with high Nb ages at 650 ℃, 750 ℃ and 800 ℃, the second phase is mainly precipitated along the grain boundaries, and with the extension of aging time, the amount of precipitation increase.750 ℃ is the most sensitive precipitation temperature at which the amount of precipitation is the most in the whole aging stage and incubation time(1%) is the shortest before precipitation. It gives priority to precipitate nitrides, of which are mostly Z-CrNbN phase with a small amount of ε-Cr2 N phase, along with χ-Cr12Fe36Mo10 and η-Fe2 Mo phases under the three selected aging temperatures. When aging at 750 ℃, χ phase and ηphase obtain the shortest incubation time, which are 25 h and 100 h respectively. While, the second constituents σ-FeCrMo and Cr23C6 have longer incubation time and less precipitation.. The σ phase is still unfound when aging 1000 h at 800 ℃, which indicates that the σ phase is in need of the longest incubation stage among the phases related above. In addition, Cr23C6 is dissolved out fastest at 650 ℃, however, it demands 1000 h before precipitated at 750 ℃ as well as 800 ℃. Of nitrides, CrNbN phase is preferred to precipitate with high Nb of which the nuclear proliferation activation energy is decreasing sharply as a result of the incubation period reduction. The precipitated CrNbN phase induces the Cr and N dilution in the substrate and results in shortening the time for incubation and accelerating precipitation of χ phase and Laves phase. Besides, it leads to the Mo-poor region generated in the substrate, meanwhile, it promotes precipitating of the σ phase as well as Cr23C6 in the stainless steel.The solid solution state of 316 LN stainless steel maintains the stability of the passive film in a wider range of anodic potential and simultaneously has the highest breakdown potential. However, the breakdown potential of the stainless steel with nitrides precipitation phase is relatively lower. As is indicated in the resistors of passivation film by means of electrochemical impedance spectroscopy measurement, the solid solution of passivation film keeps a higher resistance value and shows the best stability. Whereas to the stainless steel exist nitrides precipitation phase, the stability of its passivation film decreases. The data given by the Mott-Schottky tests demonstrate that 316 LN passivation film in the solid solution state obtains a lower donor density, which is 4.277×1023/cm3, with a lower concentration of defects inside and the best stability. After aging at 750 ℃ for 5 h and 10 h, the existence of nitrides(CrNbN) results in a rise of the defects concentration in the passivation film, coming along with the protection capability degradation.The degree of sensitization of 316 LN stainless steel after aging at selective temperature is evaluated by Electrochemical Potentiokinetic Reactivation(EPR), which illustrates that: With the increase of aging time, the amount of precipitation increases and the initial activation electric current of stainless steel increase in turn. Aging for a shorter time(5 h), 316 LN stainless steel exist CrNbN phase and the obvious reactivation peak can not be observed, the steel displays lower intergranular corrosion susceptibility. Aging at 750 ℃, the species of precipitated phases in stainless steel reflect the most sensitivity and complexity, of which the sensitization degree is the highest within the same aging time. Especially during a long period of aging, it reveals a more serious grain boundary sensitization and severe corrosion occurs within the grain boundary.One can see from the chemical immersion tests that: With an increasing number of precipitated phases in the stainless steel, the corrosion rate is rising. The minority CrNbN phase obtained by a short aging time can significantly reduce the intergranular corrosion resistant ability of the stainless steel. It’s pointed out that EPR is merely applied to the measurements of a sever degree of the sensitization in 316 LN stainless steel. |
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