Preparation Of Rare Earth - Molybdate Synergistic Inhibition System And Its Related Mechanism

As the main engineering materials, the steel materials are wildly used in various fields. The steel materials have to contact with all kinds of corrosive medium during the using process. The corrosion and failure of steel materials is easy to occur due to its characteristic composition, resulting in huge economic losses, serious accidents and huge influence of the development of national economy and the safety of human beings. The research on the corrosion and protection of steel materials is a long-term, urgent and important job. As an economic and effective corrosion inhibition method, the corrosion inhibitor has been wildly used in industrial practice. At present, along with the progress of the society and the enhancement of human beings’consciousness of environmental protection, the research and development of green corrosion inhibitors which are environmentally friendly and harmless to the human body has become a major direction in the field of corrosion inhibitor research. Because of their characteristics of low toxicity, pollution-free, the molybdate and rare earth salts inhibitors become the focuses of this research field. At present, the related research of molybdate inhibitor has made some progresses, but its corrosion mechanism has not been fully figured out. As the leader of green corrosion inhibitor, the rare earth salts inhibitors got great importance to research scholars in this filed. However, the studies and applies of rare earth inhibitors are mostly confined to the aluminum alloy and magnesium alloy, etc., the corrosive medium is given priority to with neutral medium, and the research of steel’s corrosion in acid medium is less. Due to the difference of electronic structure, part of the rare earth elements have the characteristic of base conversion process, and which can enhance the corrosion inhibition effect of rare earth corrosion inhibitor in some certain circumstances. There is no report about the reasons of enhancing the inhibition efficiency of rare earth elements with different valances. The microscopic mechanism for the rare earth elements with different valances to enhance inhibition efficiency and increase effect of compound is not clear, which to a certain extent, restricts the indepth study of rare earth corrosion inhibitor and its widely using in industry. This thesis will focus on the inhibition effect of rare earth element with different valances on steel materials in acid solution to do some researches and studies. To prepare the rare earth-molybdate compound inhibitor with good corrosion inhibition effect for steel materials in acid solution on the basis of the synergistic mechanism with molybdate.This research uses cerium nitrate as the rare earth inhibitor with different valances to distribute with sodium molybdate which is one of the environmental friendly corrosion inhibitors for preparation of composite inhibitors and then studies the inhibition performance for steel materials in three kinds of acid solution(hydrochloric, phosphoric and sulfuric acid). Different kinds of oxidants are used to complete part of the valance transformation in order to improve the inhibition efficiency of composite inhibitors, the third kind of compound is used to prepare ternary composite inhibitor to further improve the corrosion inhibitive effect of Mo-Ce composite inhibitor. In the process of research, weight-loss method is used to study the inhibition efficiency of inhibitor in different systems, macro morphology observation and electrochemical tests are combined to evaluate and analyze the inhibition effect of composite inhibitors. Electrochemical workstation is used to test the electrochemical characteristics (polarization curves and impedance sprctroscopies) of X80pipesteel specimens in different corrosion system to analyze the influence on the electrochemical reaction of specimens’ corrosion dissolution and the electrochemical mechanism of different inhibitors. X-ray photoelectron spectrometer is used to test the composition and chemical valance of rare earth elements in the protective film on the surface of X80pipeline steel samples in order to discuss the possibility and conditions of the implementation of valance transformation. X-ray spectrometer is used to make analysis for the micro area of protective film on the surface of X80pipeline steel samples for the qualitative and quantitative analysis for the elements contained in the protective film. The results showed that:the valance transformation of rare earth element can be realized by persulfate in hydrochloric acid solution so as to improve the inhibition efficiency of the composite inhibitor; the valance transformation of rare earth element can’t be realized in phosphoric acid and sulfuric acid solution; the inhibition efficiency of Mo-Ce composite inhibitor can be improved by further synergy with small organic molecules (vanillin or alizarin red) in phosphoric acid solution; however the Mo-Ce composite inhibitor can’t be used as the corrosion inhibitor for X80pipeline steel in sulfuric acid solution.