Study On The Corrosion Resistance And Electrochemical Impedance Spectroscopy Of Multicomponent Thermochemical Treatment With Doping Alloy Elements Layer

Low temperature gas multi-component thermochemical treatment technology has been applied in anti-corrosive field for its excellent corrosion resistance, but the anti-corrosive mechanism remains unclear yet. In this thesis, Q235steel and35CrMo steel were treated with droping alloy element composite processing and multi-component thermochemical treatment. Make use of different kinds of methods to evaluate the corrosion resistance and then discussed the corrosion mechanism.Specific content included observing metallographic organization of samples after the different technological processes; assaying the structure and composition of the modified layer with XRD and SEM; the corrosion resistance performance was evaluated by means of neutral salt spray corrosion test, electrochemical polarization curve and impedance spectroscopy testing technique; and by using EIS technical tested the impedance of the sample for immersion, to track the failure process of the modified layer. At last, mechanical propercties was characterized through micro-hardness and impact test.The result indicated that the modified layer formed on Q235steel is most of Fe3N and Fe4Nincluding some oxide, such as Fe3O4、Fe2O3. There are some more Al2O3and SiO2in the layer, which was formed on Q235steel after droping alloy element composite processing.35CrMo steel has the same surface structure as the Q235steel, but there are some more alloy nitrides. Therefore the thickness of the modified layer is thinner due to the alloy nitrides. The surface hardness of treated samples is much higher because of the solid-solution strengthening mechanism of N, C atoms. Conversely, the anti-impact propercties of the former system reduce substantially, while the35CrMo hardly changed.In the sense of anti-corrosion propercty, the corrosion resistance of samples was notably enhanced by combined treatment. And samples of treated showed good performance against the neutral salt spray corrosion since the formation of Fe3N and FesO4which own stable chemical propercty and compact molecular structure in the compound layer separated the surface of steels and ambient erosion substances. On the other hand, nitriding layer exhibited higher corrosion potential, lower corrosion current density and larger charge transfer resistance, which indicated that nitriding layer significantly modified the corrosion resistance of substrate.EIS test result showed that the impedance spectrums of samples which have been surface treated were different from the matrix, and the impedance is also much higher than matrix. Furcthermore, the impedance spectrums of the sample for different immersion periods changed step by step, which suggested that the performance and structure of the nitriding layer changed with the development of corrosion. At the early stage of immersion, the impedance spectrum was made of two capacitive loops for the film on the surface. As time prolonging, more corrosion occurred and the EIS remained two loops. However diffusion impedance appeared at late stage of immersion. Meanwhile, it can be seen from the fitted data that the value of CPEc increases while Rct reduced as the immersion time prolonging, which is indicative of the coating capacitance and electron transfer across the interface respectively.The structure of the modified layer and salt spray corrosion tracking experiment suggested that corrosion first occured at the loose area of infiltration level surface, and then developed to the internal direction of the modified layer gradually, eventually leading to modified layer failure.