Investigation On The Microstructure Evolution And Corrosionbehavior Of Duplex Stainless Steel

Duplex stainless steel(DSS) combining many of the beneficial properties of ferritic and austenitic steels has been widely used in petroleum, chemical industry, marine field and so on due to its excellent mechanical properties and corrosion resistance. The dual phase scale of marix(?+?) and the precipitation of harmful phase have significant effect on the performance of the duplex stainless steel in the range of 300?~1000?.In this study, four types of typical duplex stainless steels- 00Cr21Ni2Mn5 N, 00Cr22Ni5Mo3 N, 00Cr25Ni7Mo3 and 00Cr27Ni7Mo5N- were chosen. The microstructure, pitting corrosion resistance and surface passivated membrane characteristics of four duplex stainless steels after solution treatment were investigated by employing Thermo-Calc software, metallurgical microscopy, scanning electron microscopy(SEM), transmission electron microscopy(TEM), electrochemical process, auger electron spectroscopy(AES), Dynamic Electrochemical Impedance Spectrum(DEIS) and X-ray photo electron spectroscopy(XPS).The results show that:(1) After complete solid solution treatment, ?/? ratio is close to 1:1 and there is no harmful phase, especially ? phase in four stainless steels, which means the steels can obtain good pitting corrosion resistance.(2) Different secondary phases could be formed under different aging conditions. Precipitated phases in 00Cr21Ni2Mn5 N were mainly for Cr2 N and ? phase, while in 00Cr22Ni5Mo3 N, 00Cr25Ni7Mo3 N and 00Cr27Ni7Mo5 N ? phase was predominant phase accompanied with precipitation of ? and Laves phases.(3) In the aged duplex stainless steels, the relationship between the amount of harmful phases and temperature conformed to the inverted ?V? curve. With the increase of alloy elements content, the precipitation amount and solution temperature of harmful phases including ? phase raise.(4) The relationship between pitting potential of aging duplex stainless steel with temperature show a ?V? shaped curve and pitting potential decreases with the increase of harmful phase precipitation. In addition, the higher alloy element content in duplex stainless steels is, the more the ? phase content is, which results in a significant reduction in the pitting corrosion resistance.(5) The passive film formed on the four duplex stainless steels contained layered Cr and Fe oxide as well as hydroxide, and displayed a bipolar n-p semi-conductive characteristic. 00Cr21Ni2Mn5 N ? 00Cr22Ni5Mo3 N and 00Cr25Ni7Mo3 N had a double-layer structure film in which the outer was very little trivalent iron oxide and the inner was chromium oxide(Cr2O3 and Cr O2) with a little content of divalent iron oxide. By contrast, the composition of film on 00Cr27Ni7Mo5 N was given priority to M-OH compounds(the outer ?-Fe OOH, and the inner Cr(OH)3 and Cr OOH). When sputtering to the matrix, Cr, Fe, Ni, Mo and other elements was detected existing as metallic state. With the increase of alloy elements amount in duplex stainless steel, more excellent passivation capability of surface passivation membrane and stronger resistance to pitting corrosion can be obtained.