| 304 and 304L stainless steel has excellent corrosion resistance in the nitric acid medium, therefore, is widely used in nitric acid production plants and pipelines. But domestic nitric acid production plants have been troubled by the problems of stainless steel pipe weld corrosion failure and local erosion corrosion failure. In this thesis, 304 stainless steel's corrosion mechanism in nitric acid under different conditions was investigated for two problems of intergranular corrosion and flow corrosion. In the first place,The reasons of the corrosion failure in the stainless steel pipe welds at site were analyzed, the influences of temperature and time on the corrosion resistance of stainless steel in nitric acid were studied through simulating the heating process in the process of stainless steel welding by heat treatment method, and sensitizing behavior in stainless steel weld heat affected zone was explained. The change law of corrosion properties of 304 stainless steel in nitric acid along with the nitric acid concentration, temperature and soaking time was studied by polarization curve, Mott-Shottky analysis, EIS and soak weightlessness experiment. Besides, corrosion electrochemical behavior of stainless steel under the condition of flow was studied through self-developed tube flow corrosion experiment apparatus, through the above research and experiment, the main conclusions are as follows:(1) Relative Uniform intergranular corrosion occurred on the surface of and base metal of three-way tube and straight tube. Base metal of three-way tube and straight tube had the same content of Cr and Ni, and there were slight differences in corrosion current density and impedance values between these two. The corrosion resistance of two kinds of parent metal was good,But the carbon content of tee was 1.8 times of that of straight pipe. In the welding heat affected zone, there are many differences between two kinds of material corrosion resistance, the corrosion rate of the material in heat affected zone is significantly greater than that of the parent metal, the corrosion rate of tee in heat affected zone is obvious greater than that in other areas. Carbon content is the key factor which has an influence on the corrosion resistance of 304 stainless steel welding joints in nitric acid. The higher the carbon content, the easier the sensitization of stainless steel in the heat affected zone will happen during welding, which lead to the dramatical decline of corrosion resistance.(2) When the heating temperature was between 500? and 900?, even short heating time could also lead to the significant increase of sensitization degree of intergranular corrosion of 304 stainless steel. When the heating temperature was between 650 ? and 750?, the sensitization degree of intergranular corrosion of 304 stainless steel reached the peak value. When the heating temperature was blow 500 ? or above 900 ?, the sensitization degree of intergranular corrosion of 304 stainless steel changed slightly, the corrosion resistance would not reduce. Through the change of the electrochemical impedance spectroscopy stainless steel sample could be divided by different degree of sensitization, the reaction resistance Rct of samples whose sensitization degree of intergranular corrosion were relatively high decreased, integrity and protective of stainless steel passive film declined. The influence laws about how the corrosion resistance was influenced by heat treatment gained from electrochemical impedance test and DL-EPR test respectively were coincident basically, after heat treating at 650? , the value of interface capacitance and electric double layer capacitance on the surface of passive film reached the peak value, the protection ability of passive film in nitric acid medium became poor, the corrosion resistance decreased obviously. And along with the increase of holding time, the solution resistance in the corrosion crack and the charge transfer resistance on the corrosion interface decreased gradually.(3) The corrosion mechanism of 304 stainless steel at different temperature in different concentration of nitric acid is basically the same. As the increase of temperature and concentration of nitric acid, the corrosion potential of 304 stainless steel in nitric acid showed a trend of increase gradually, the range of passivation zone decreased, maintaining passivity current density increased gradually, corrosion rate increased gradually, the corrosion of stainless steel in nitric acid became activation corrosion, the transpassivation would be more likely to happen. As the increase of temperature, the formation of the 304 stainless steel passivation layer would be more and more difficult, it was more and more difficult to maintain the stability of the passive film state. As the increase of the concentration of nitric acid, reaction resistance decreased, the dissolution rate of passive film increased, protection of the passive film on stainless steel substrate weakened gradually. Mott-Schottky analysis of passive film formed on stainless steel showed p-type semiconducting properties. As the increase of the concentration of nitric acid, the linear slope in line region increased gradually, the concentration of the carrier increased first followed by an decrease. As the increase of temperature, the linear slope in line region decreased gradually, the concentration of the carrier increased gradually, the flat band potential has a negative shift. 304 stainless steel's capacitive reactance arc radius decreased as the increase of immersion time, the protection of passive film on the metal substrate gradually reduced. The dissolution rate of passive film on the surface of the stainless steel in nitric acid was smaller at the beginning of the soaking. The dissolution rate of passive film increased at a rapid speed as the increase of immersion time,ending at a stable value. The components of passive film on the surface of 304 stainless steel in nitric acid were mainly Fe-Cr-Ni metal compounds,what was also called ? phase.(4) The passivation behavior of stainless steel in the flow of nitric acid,at first, the passive film on the surface of the stainless steel was continuously expanding after immersed in nitric acid, passive film gradually stabilized after 4 hours. With the increase of temperature, velocity and impact angle,the corrosion rate of stainless steel in the flow of nitric acid increased gradually. With the increase of temperature, The thickness of the passive film decreased gradually, the density and integrity became lower. With the temperature of nitric acid increasing, the range of passivation narrowed down, the passivation potential decreased, the passivation behavior of stainless steel in nitric acid flow was instability, transpassivation would be easier to happen which would lead the occurrence of intergranular corrosion.With the increase of flow velocity, the thickness of the passive film decreased. The protective effect of the passive film on stainless steel weakened gradually. Speed has small influence on the corrosion potential,passivation range and passivation behavior of stainless steel, velocity of the flow and passivation current density increased at the same time, which illustrated that the dissolution rate of passive film will increase with the velocity of the flow. The corrosion resistance of stainless steel in nitric acid flow became smaller with the increase of the impact angle. The change of impact angle influented the initiating passive potential of stainless steel in nitric acid. The increase of initiating passive potential and passivation current density would lead the weaken of self-repairing capability of the passive film when the destruction and indication occurred.The flow corrosion features of stainless steel in nitric acid are mainly affected by the surface static pressure distribution, the erosion-corrosion is more serious in the region where the static pressure is larger. |
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