| In order to extend the service life of the steel material, the excellent corrosion resistance apart from outstanding strength-toughness matching is required of the low carbon-structural steel (carbon steel) used in atmospheric environment. However, the usage of weathering steel is restricted by the increasing cost of alloying elements. Therefore, the research on economical weathering steel has gradually become a new focus of advanced steel. In general, most of the research was carried out by adding the low-cost alloying elements. It is conformed to the conditions of our country to develop high performance RE-bearing low carbon steel with better weatherability, which also possesses important significance in saving resource and for sustainable development of steel material.The early literatures showed that RE/S was an important parameters which could characterize the function of RE in steel, besides total RE contant. And the RE/S value could influence directly the existing form of RE and improvement effect on mechanical properties of RE-bearing steel. However, research reports on the relationships between the RE/S value and anti-atmospheric corrosion properties of carbon steel were few. Meanwhile, with the development of the metallurgical technology, the purity of the steel has been raised. The existing form and corrosion resistance mechanism of RE in the new carbon steel will certainly be corresponding changed. Therefore, it is practical significance to carry out the further study of the effect of the RE/S value and RE in different forms on carbon steel and the underlying mechanisms. In addition, since the influence of RE on the protection of rust layer of the carbon steel is not yet well understood, it is beneficial to engaged in systematically study in the contribution of RE to the rust layer of the carbon steel.The influence of RE on the industry and marine environmental corrosion resistance of low carbon-structural steel was studied by means of cycle immersion corrosion tests and atmospheric exposure tests. Meanwhile, considering the characteristics of the corrosive medium in different atmospheric environment, the rules and mechanisms of the different RE phase in improving atmospheric corrosion resistance of the steel substrate and the rust layer had been investigated by physical chemistry phase analysis, investigation of inclusions, in-situ observation of micro-area pH value as well as electrochemical methods.Based on the results of the SEM observation and the thermodynamic calculation, we found that RE could exist in low carbon steel in different forms, mainly including sulfides, oxides, oxysulfide, RE-Fe intermetallic and solid soluble RE. The properties of RE-inclusion and the RE alloying quantity were remarkably related to the RE/S value apart from the total RE. As far as experimental steel(RE-0.030wt%, O～0.0030wt%) was concerned, the amount of the inclusion was elevated and the shape of the inclusion was completely deteriorated, when the RE/S value was controlled in the range of 2.35 to 4.62. And the RE alloying quantity would account for about 50% of the total RE, while the RE/S value was higher than 8.57. Besides, the interval between pearlite slice was decreased with the increase of RE/S value. Moreover, the pearlite slice tended to degenerate in the steel with high RE/S value.As shown by the cycle immersion corrosion test and atmospheric exposure test, the addition of RE to low carbon-structural steel could effectively improve the industrial and marine atmospheric corrosion resistance. In the simulated atmospheric environment, the corrosion resistant effect of experimental steel was gradually improved with the increasing of the RE/S value.The in-situ observation result of micro-area pH value showed that a serious localized corrosion caused by the inclusion in the carbon steel could decrease the pH value of the thin liquid film of the carbon steel significantly. By contrast, RE sulfides and RE-Fe intermetallic as well as solid soluble RE were helpful to the inhibition of the acidification of the thin liquid film on RE-bearing steel. According to the corrosion kinetics, the corrosion current of the substrate was decreased with the increasing of the pH value of the thin liquid film of the steel. The inhibition of the acidizing caused by RE sulfides, RE-Fe intermetallic and solid soluble RE were not only able to decrease the anodic dissolution rate of RE-bearing steel in the industrial atmospheric environment, but also able to promote the formation ofα-FeOOH which was a steady protective corrosion product and make the rust layer more compact.Polarization curves of the bare steel were investigated in 0.2% NaCl solution. Experimental results revealed that both self-corrosion potential and corrosion current of experimental steels decreased with the increasing of the RE/S value, showing the developing trends of the characteristics of the cathodic polarization. Moreover, an obvious passivation phenomenon was found in the anode region of the polarization curves of high RE/S steel. As shown by the microscopic observation of pitting initiation, the initial pitting corrosion behaviors with various types of inclusion were different in chloride solution. The pitting morphologies of the experimental steel would be changed by adding RE and increasingthe RE/S value. Compared with the carbon steel, the corrosion influence scope of inclusion in RE-bearing steel had a much wider range. The dissociated Ce3+ form inclusion could be used as cathodal depositing inhibitor in NaCl solution with neutral pH value, and thus leaded to a much stronger corrosion resistance of the RE-bearing steel to Cl-Furthermore, based on the pitting micro-morphologies and electrochemical characteristics of experimental steel, the marine atmospheric pitting initiation models of four types inclusions were also established, respectively.The result of the industrial atmospheric exposure test showed that the rust phase of the experimental steels were quite similar, which were composed ofα-FeOOH,γ-FeOOH,δ-FeOOH, Fe3O4 and amorphous rust. Binding weakly with the steel matrix, the rust layer of the carbon steel was loose and porous, which contained a large percentage of active components. Whereas, RE could promote the formation of the steady protective corrosion products, decrease the ratio of conductive phase Fe3O4, which could inhibit the corrosion effectively. On the other hand, the result of the marine atmospheric exposure test showed that RE distributed unevenly in the rust layer and could be combined with Cl- and SO42-. Thus, in the presence of RE, the electromigration of corrosive anion in the corrosion product would be prevented effectively and the anion-selective tendency of rust layer was weakened. Meanwhile, RE had the capability to refine the rust particles and raise the ratio of amorphous in the rust.The measurement results of electrochemical studies showed that RE could inhibit the electrochemical reaction of corrosion of the carbon steel exposed in Jiangjin area. Based on the thermodynamics analysis, the introduction of RE in the carbon steel could cause the free corrosion potential shift positively, and reduce the ionization tendency of the steel matrix. On the other hand, kinetics analysis indicated that RE in the steel could reduce corrosion current, and make the rust layer tend to be passivated with the extending of the exposure time. The equivalent circuit of the electrochemical reaction was obtained by analyzing EIS of the experimental steel. The parameters of Rr and Rct could characterize the anti-corrosion of the rust layer. After 2 years exposure test, both Rr and Rct of RE-bearing steel were much bigger than those of carbon steel. The test indicated that the rust layer on the surface of RE-bearing steel had strong ability to lower the corrosion rate. |
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