| 其他摘要 | The excellent anti-corrosion property of weathering steel comes from an adherent and protective rust layer formed on its surface after long-term atmospheric exposure. The weathering steel is widely used in rail, truck, container, automobile, steel structure and towers for electric power, etc. Traditional weathering steels are alloyed with some expensive alloying elements, such as Ni and Ti, which increases the cost of weathering steels and hence limits their application. So relevant research concerning cost effective weathering steel has been soaring in recent years. Up to now, previous researches on weathering steel mainly are focused on the distribution of single alloying element, the effect of atmospheric conditions on the corrosion behavior and possible anti-corrosion mechanism of single alloying element. There are few investigations of the interaction of two or more alloying elements in weathering steel. Therefore, it is of great importance to investigate the interaction of cheap alloying elements and develop new cost effective weathering steel.
Synergistic effect of Mn and Cu alloying elements on the atmospheric corrosion resistance presented in this study is the first report in weathering steel field. The relationship among the contents of Mn and Cu, atmospheric environments and the synergistic effect of Mn-Cu-alloying were investigated systematically by wet/dry cyclic accelerated corrosion test. In order to elucidate the anti-corrosion mechanism of Mn-Cu-alloying and provide some useful information for the development of new cost effective weathering steel, the corrosion performance of rusted specimens, rust composition, the structure and property of rust layer were analyzed by some material analytical techniques, such as electrochemical measurement, SEM, XRD, EDX, XPS, and so on.
Based on the synergistic anti-corrosion effect of Mn and Cu, a kind of Mn-Cu-alloying cost effective weathering steel was developed and produced by two steel companies successfully.
Mn-Cu-alloying improves the corrosion resistance of steels both in industrial and oceanic atmospheric environments synergistically. Cu-alloying can improve the corrosion resistance of steels to certain content, while Mn-alloying reduces the corrosion resistance of steels. At the initial stage of corrosion in a sodium chloride solution, Mn-alloying accelerates the dissolution of the anode, while both Cu-alloying and Mn-Cu-alloying retard the anode dissolution and improve the rest potential of steel. After rust layers formed on steels, Mn-Cu-alloying synergistically suppresses the anode dissolution and accelerates the cathode reduction. The analyses show that Mn-Cu-alloying retards the crystallization of rust, suppresses the growth of rust phases, and accelerates the formation of an inner rust layer enriching inverse spinel oxides like Fe3O4, which improves the protection and the adhesion of the rust layer on Mn-Cu-alloying steel. Under simulated industrial atmosphere environment, all steels are corroded uniformly. And the effect of Mn-/Cu-alloying on the corrosion performance of steels is similar to that in simulated oceanic atmospheric condition.
No obvious Cu segregation is observed in all Cu-bearing steels when Cu content is between 0.1~0.5%. The microstructures of all Cu-bearing steels are composed by ferrite and pearlite. The concentration of Cu in pearlite is higher than that in ferrite and that at the grain boundaries. With increasing Cu, Mn and Si contents in steels, the volume fraction of pearlite increases. The addition of Cu also modifies the distribution and the shape of MnS inclusions. Based on the cost effective and practical principles, the optimal annealing temperature between 1100~1200 C and the optimal Cu content between 0.2~0.4% are determined.
In practical contrast corrosion tests, rolled Mn-Cu-alloying steel also shows excellent corrosion resistance. Its anti-corrosion property is slightly better than those of traditional 09CuPTiRE and of SPA-H weathering steels. It is also found that the corrosion resistance of rolled Mn-Cu-alloying steel slightly increases with deceasing grain size of steels. The results of XPS indicate that Mn exists as MnFe2O4 or CuMn2O4 in rust for rolled Mn-Cu-alloying steel. The mechanical properties of rolled Mn-Cu-alloying steel and of its weld joints are slightly better than that of 16Mn steel.
The anti-corrosion mechanism of single alloying element and the interaction in Al-Si-alloying and Cu-Ni-alloying were also investigated. Under Cl--rich wet/dry cyclic environment, Al-Si-alloying facilitates the formation of a dense oxide film near the substrate/rust interface, which enriches fine spinel oxide (FeAl2O4), Si or Si-rich compound; this kind of film has highly protective property. Al-Si-alloying can not enhance the density and stability of rust layer remarkably. In a NaHSO3 solution (pH~4), the recycle of acid accelerates the corrosion of steels at the initial stage. After rust layers formed on the steels, the leak of rust destabilizes the rust layer due to the dissolution of Al. Al-alloying or Al-Si-alloying is hence not suitable for improving the anti-industrial atmospheric corrosion of steels.
Ni-alloying and Cu-Ni-alloying can improve the corrosion resistance of steels both in salty atmosphere and industrial environment containing SO2. The atmospheric corrosion resistance increases with increasing Ni and/or Cu contents in steels. For the steel containing Cu, the synergistic anti-corrosion of Cu-Ni-alloying is excellent, persistent and stable if the content of Ni is above 4%. Ni-alloying mainly suppresses the anode dissolution, but it hardly affects on the cathode reduction. Cu-Ni-alloying not only suppresses the anode dissolution and the cathode reduction, but also remarkably improves the rest potential of steels. Furthermore, Cu-Ni-alloying refines the rust, retards the crystallization of rust, and leads the formation of fine inverse spinel oxide (Ni2FeO4) and the transformation of γ-FeOOH under salt-rich environment, which improves the ion-selectivity of the rust layer and other properties. The rust layer on 0.3%Cu-4%Ni-alloying steel is cation selectivity. In industrial atmospheric environment, Cu-Ni-alloying also affects the rust composition and refines its structure, leading to an enhanced corrosion resistance. |
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