| Metal is the most important structural material at present,while it's corrosion bring enormous losses to national economic.Metal corrosion caused not only the huge economic losses,but also significant pollution,casualty and a waste of resources.Thus,preventing metal corrosion and improving metal's anti-corrosion became one of the most arresting troubles right now.Gas oxynitrocarburizing treatment technology which made by Southwest Jiao Tong University has fine mechanical properties.Besides,it also has strengths such as low cost,easy to operate,corrosion resistance,and has wide application in future.Nevertheless,from the micro level(electronic level),there are still lack of adequate understanding of the technique's anti-corrosion mechanism.The electroless plating nickel on gas oxynitrocarburizing sample was carried out in order to further improve material's high corrosion resistance.This showed that corrosion resistance of material which by using combined treatment improved substantially compared to the previous one.Whereas,this phenomenon cannot be understood deeply.So,the surface of Q235 was treated by gas oxynitrocarburizing treatment and combined treatment.The corrosion behavior of samples was measured by potentiodynamic polarization test and electrochemical impedance spectroscopy(EIS)in the 3%wt NaCl solution.The anti-corrosion mechanism was discussed by applying Empirical Electron Theory(EET)and "the double layer"model.The result reveals that the corrosion resistance of Q235 carbon steels was considerably improved by gas oxynitrocarburzing and the pitting potential and corrosion potential of the oxynitrocarburized sample are +835 mV and-310 mV,respectively.According to the analysis of EIS and SEM,the main corrosion mechanism is pit formation.EDS results show that chloride concentration in the 3%wt NaCl solution,column crystal structure in compound layer,and micro cavities have a great influence on the pitting corrosion.The corrosion potential of samples treated by combined treatment are up to 1.364V.The anti-corrosion layer of sample treated by combined treatment in the salt spray test can be achieved a long-term effect protection about 300h,which the anti-corrosion is far greater than single gas oxynitrocarburzing treatment and electroless plating nicker treatment.The empirical electron theory(EET)was applied to evaluate the structure of valence electrons and the energy of covalent bonds.The calculated results reveals that the largest bond strength nA and the bond energy EA in the oxynitrocarburized layer are markedly higher than that in a-Fe after gas oxynitrocarburizing(nA is 0.9552?1.83?2.4239?0.5251?0.3835 for Fe4N?Fe3N?Fe2N?Ni??-Fe;EA is 156kJ/mol?361.4kJ/mol?437.8kJ/mol?201kJ/mol?107.4kJ/mol for Fe4N?Fe3N?Fe3N?Ni??-Fe),which indicates the close relationship between corrosion corrosion and the structure of valence electrons.The Fermi energy of Fe-N compound layer and the Ni layer was calculated,the results show that Fermi energy of Fe-N compound layer higher than that of the Ni layer.According to the definition given by the theory of electrochemical corrosion,we can understand that,essencially,the corrosion of metal is the process in which the electrons transfer from one atom to another.And the valance electrons are the basis of interatomic connections.In this case,the materials corrosion resistance can be represented by the bond strength of valance electrons between the atoms.That is,thermodynamically speaking,the greater n? and E? of materials means the better corrosion resistance.What's more,the Fermi energy of the Fe-N compound layer is much higher than that of the Ni layer.Electron of Fe-N compound layer transferred to the Ni layer and caused Fe-N compound layer with positive excess charges and negative excess charges on the Ni layer,thus forming "the double layer".It was "the double layer" that made samples after combined treatment improve significantly it's anti-corrosion. |
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