| The corrosion problem of metal materials has always been the great challenges of world economic development and human society progress. Duplex stainless steel(DSS) containing two-phase ferrite-austenite microstructures plays important roles in various fields of producing and living due to its combination of the properties that are characteristic of both ferrite and austenite stainless steels and its merits on corrosion resistance. 2205 DSS is most widely used because of its excellent corrosion performance in combination with good mechanical property. The excellent corrosion resistance property of DSS mainly depends on the microstructure of ferrite and austenite phase in solution-treated condition and the approximately equal volume fraction of the two constituent phases, while harmful precipitates can reduce the corrosion resistance property of DSS. Due to the higher content of alloy elements, 2205 DSS may precipitate a variety of secondary phases such as ? phase and ? phase when exposed to temperatures between approximately 300~1000°C, which will influence the corrosion resistance of DSS significantly, so the precipitates should be avoided in DSS intentionally. However, during materials fabrication, process and application in practice, some treatments such as welding and other thermal treatments could cause the DSS exposed in the range of precipitates sensitive temperature, resulting in secondary phases precipitating in different degrees, so it's necessary to investigate the influence of different contents and kinds of precipitates on materials corrosion resistance property. Besides that, it is known that cold rolling is a technique applied industrially to increase the strength of materials, while cold rolling produces important changes on microstructure and secondary phases precipitation behavior in 2205 DSS, which will also influence the corrosion resistance property of DSS consequently, so it is essential to research the correlation between changes on microstructure and corrosion resistance of coldrolled DSS as well.Based on previous work by other researchers, this paper aimed at the scientific problems remained to be solved, such as the influence of ? phase after short-term aging on pitting initiation position and overall corrosion property in 2205 DSS, the correlation between ? phase after long-term aging and selective corrosion property as well as corrosion morphology in 2205 DSS, the influence of cold rolling treatment on changes in precipitates volume fraction, corrosion resistance and corrosion morphology. The microstructure and chemical composition of 2205 DSS was investigated by optical microscope and scanning electron microscope, meanwhile potentiodynamic polarization as well as electrochemical impedance spectroscopy was employed to detect the corrosion resistance of 2205 DSS for forecasting its corrosion resistance properties. Besides that, the same microstructure before and after corrosion tests were contrasted to distinguish the mechanism of the selective corrosion of constituent phases, and relevant tests for detecting corrosion resistance property of cold-rolled 2205 DSS were also carried out, respectively. The final purpose of the paper was to provide scientific basis for welding temperature control, hot working process optimization, microstructure constituent prediction and corrosion resistance prediction.Results showed that only ferrite phase and austenite phase were detected in 2205 DSS after solution treatment in 1050°C, and the ratio of two phases was approximately equal. ? phase would preferentially precipitate at ferrite-ferrite boundaries after aging for 5 min at 850°C. With prolonging aging time, ? phase would precipitate at both ferrite boundaries and ferrite-austenite boundaries. As ? phase was meta-stable phase, it would dissolve and transform into ? phase along with prolonging aging time. ? phase could be transformed by eutectoid reaction from ferrite and the dissolution of ? phase as well. Both ? phase and ? phase were Cr-riched and Mo-riched phases, so precipitates would break the balance of original phases and lead to heterogeneity of alloy elements distribution.? phase after short-term aging could reduce the corrosion resistance of 2205 DSS slightly and also destroy the stability of potentiodynamic polarization curve. Due to the fact that the distribution of alloy elements was nonuniform between ? phase and matrix around and that ? phase acted as anode with respect to other phases, pitting corrosion would firstly initiate at ? phase boundaries and ? phase would be corroded preferentially. The grain size of ? phase influenced the position of pitting initiation, that the grain boundaries of ? phase with small grain size and large density of distribution would be sensitive for pitting initiation.2205 DSS with ? phase would show selective corrosion property, namely ? phase was corroded firstly, then the Cr-depleted zones around the ? phase, followed by ferrite phase, and primary austenite phase and secondary austenite phase were the last to be corroded during electrochemical corrosion. ? phase could reduce the whole corrosion resistance and the stability of passive film of 2205 DSS. Besides that, ? phase could also change the electrochemical corrosion morphology characteristics, that was, with the content of ? phase increasing gradually, the corrosion morphology changed from hemispherical shape to seriously destroyed inner morphology with small surface diameter and large cavity, then transformed into morphology with wide and shallow shape.The grains of 2205 DSS were lengthened and broken after cold rolling treatment. The precipitation content of ? phase and ? phase was increased, and the precipitation speed of precipitates was also accelerated after cold rolling treatment. Some part of austenite phase could transform into strain-induced martensite during cold rolling treatment. The corrosion resistance of cold-rolled 2205 DSS after aging treatment decreased significantly compared to that of hot-rolled specimens with the same aging condition, and the stability of passive film of coldrolled specimens was also weakened. The order of corrosion took place in constituent phases differed from that in hot-rolled specimens, that positions such as phase boundaries, defect sites and strain-induced martensite would be corroded preferentially. Addition to that, the corrosion morphology of cold-rolled specimens showed obvious differences compared to that of hot-rolled ones with the same condition. |
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