| 其他摘要 | During the periods of the eighth and ninth five-year plan, the “reverse phenomenon” of corrosion rate of low alloy steel was observed in Wanning district with high relative humidity, high temperature and high chloride ion containing; that is the corrosion rate firstly augments and then decreases, subsequently presents accretion again. Although the causes leading to the “reverse phenomenon” were explained in terms of environment such as chloride ion, meteorologic parameter and corrosion products, it was not still convinced. In addition, weathering steel with perfect ability of anti-corrosion exposed at a certain exposure station in Wanning district suffered serious corrosion yet and the weatherability was undermined. However, the results obtained during the periods of the eighth and ninth five-year plan did not offer convincible explanation. In allusion to issue mentioned above, a new research project has been implemented with aims to investigating atmospheric corrosion kinetics of low alloy steel, to discussing the acceleration mechanism of chloride ion and to opening out the cause to “reverse phenomenon”, as well as to interpreting the reason of abnormal behavior about the weatherability of weathering steel. The whole period of exposure time is four years staring May 2005 to May 2009, with low alloy steel (Q235 and CortenA) as tested materials. The two exposure stations locate in Wanning station (tropical marine environment) and Sheyang station (industrial environment).
The atmospheric corrosion of two steels in different exposure station was investigated. The results show that the mechanism of atmospheric corrosion of two steels in Wanning district represents transition, which the more close to the sea line, the turning point moves onwards. The corrosion rate of low alloy steel presents maximum, which the closer to the sea line, the value of maximum in corrosion rate presents higher. However, the mechanism of atmospheric corrosion of two steels in Shenyang district represents no transition, where the corrosion rate reaches vertex and then gradually decreases.
As primary corrosive particle, chloride ion remarkably influenced the corrosion behavior of low alloy steel; however, its effect to the corrosion process was still confused according to the results obtained during the periods of the eighth and ninth five-year plan. Therefore, we attempted to analyze its acceleration mechanism to the corrosion behavior of low alloy steel by using surface analytical techniques and electrochemical measurement. The acceleration mechanism was presented that in high chloride ion deposition, it was instrumental in the formation of -FeOOH, which accelerated the corrosion process; while as to low chloride ion deposition, its effect was mainly conducive to the transformation of -FeOOH to -FeOOH.
During the proceeding of analyzing the acceleration mechanism of chloride ion, the -FeOOH phase among corrosion products was dominant factor to accelerate the corrosion process. Therefore, we especially focused on the evolvement of -FeOOH among the corrosion products, aiming to figure out the essence of reverse phenomenon. Among all the iron oxide, -FeOOH exerted significant influence. The presence of a monolayer of the rust as well as -FeOOH accelerated the corrosion process during the initial exposure stage. -FeOOH formed in the inner layer was gradually consumed and transformed to γ-Fe2O3 in the wet-dry cycle, which was beneficial to protect the substrate and reduced the corrosion rate.
As to the phenomenon that the weatherability of weathering steel (CortenA) exposed in Wanning station was sharply deteriorated, relative theory research was implemented in terms of exposure environment and alloy elements. The results show that weathering steel suffers serious corrosion in spite of the enrichment of Cr in the rust layer. The amount of chloride ion deposition and Cr jointly determines the weatherability of weathering steel in marine site. Under low chloride deposition, the enrichment of Cr plays an important role on improving weatherability; conversely chloride ion is primary factor to whittle weatherability in high chloride deposition.
In the present work, taking “reverse phenomenon” as research background, the research project was carried out in allusion to issues not to resolve and new phenomenon during the periods of the eighth and ninth five-year plan. The existence of “reverse phenomenon” was actually approved. The rust layer in-situ formed on the low alloy steel was studied and the acceleration mechanism of chloride ion was presented, making us to profoundly cognize its effect on the corrosion behavior. The evolvement of -FeOOH among the corrosion products was investigated for the first time and the reason to “reverse phenomenon” was clarified. The synergistic effect of exposure environment and alloy elements on weatherability was also discussed and the weatherability was explained in theory, offering the theoretical and practical foundation for designing commercial and applied weathering steel. |
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