| Stainless steels refer to the iron-based alloys which have chromium content no less than12wt.%, and exhibit excellent oxidation resistance in air. Stainless steels could be classified to5series. Among these series, the austenitic stainless steels are the most widely used steels and the duplex stainless steels are irreplaceable in some area, so there is no doubt that the researches to these two series of steels are very hot. During the service process of stainless steels, various kinds of localized corrosions which induced by pitting and intergranular corrosion could cause huge damages. Some technologies have been introduced to the produce and service processes to enhance the resistance to localized corrosions, such as addition of alloy elements or choose appropriate heat-treatment conditions. Based on the said background, the resistance to localized corrosion of several typical stainless steels which had experienced such technologies was evaluated by electrochemistry methods and the microstructures were characterized by microscopy techniques in these paper, the detailed research contents are described as the following aspects:(1)With the testing of pitting potential (Eb) using potentiodynamic polarization curve and the ratio of reactivation(Ra) using double loop electrochemical potentiodynamic reactivation(DL-EPR), the effect of the addition of Nitrogen(N) and Niobium (Nb) on the pitting corrosion and intergranular corrosion behavior of304austenitic stainless steels was investigated. The results showed that, the specimens with N and Nb addition exhibited better resistance to localized corrosion than those without elements addition. Further researches revealed that, the addition of Nb to304austenitic stainless steels could enhance their resistance to intergranular corrosion, but will cause a deterioration of the resistance to pitting corrosion; the addition of N will enhance the resistance to pitting corrosion of the material, the resistance to intergranular corrosion will deteriorate while the content of N was below0.2%, but will meliorate while it was above0.2%.(2)The testing of critical pitting temperature(CPT) was applied to characterize the pitting behivior changes of duplex stainless steel(DSS)2205which were suffered solution treatment in the temperature range of1050℃to1200℃for24h. The microstructural evolution and pit morphologies of the specimens, especially the site pitting nucleated preferentially, were studied through optical microscopy and scanning electron microscopy (SEM). The potentiostatic CPT measurements show that the CPT was elevated as annealing temperature was increased from1050℃to1150℃and decreased as the temperature was continuously increased to1200℃. The specimens annealed at1150℃exhibited the highest CPT. The results were explained by the variation of pitting resistance equivalent number (PREN) of ferrite and austenite phase as the annealing temperature was varied, it would show the best resistance to pitting corrosion when the PRENs were the same in the two phases, and1150℃was the best solution treatment temperature for DSS2205.(3)With the CPT tests and Ra tests, the changes of pitting corrosion and intergranular corrosion of DSS2304specimens which were suffered2hours heat treatment in the temperature range from600℃to950℃were evaluated. The results showed that the resistance to localized corrosion will deteriorate observably while the heat treatment temperature was between650℃to800℃. The characterization to the microstructure evolution of the specimens by SEM revealed that the Chromium-depleted cauesd by the precipitation of Cr2N was the reason for the deterioration. Because2hours represented the typical time for the mechanical hot-work process, it was necessary to avoid the temperature range of650℃to800℃when mechanical hot-work process was applied on DSS2304. |
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