Corrosion behavior of weathering steel in tropical atmosphere of Vietnam
Abstract
Weathering steel (WS) of Corten B type was exposed for three years at three test sites including urban and marine sites. After one year-exposure, WS showed the protective ability better than carbon steel (CS) did. The corrosion loss of WS fitted well power law with high correlations. The characterization of rust layer was studied by using X-ray diffraction, optical microscope and SEM-EDX. Goethite (α-FeOOH), akaganeite (β-FeOOH), lepidocrocite (γ-FeOOH), maghemite and magnetite were founded in corrosion product formed on WS for 3, 6, and 12 month-exposure; The structure of rust layer of WS composed of two layers – the inner layer dark-grayish, dense and good adherent to steel substrate; and the outer layer porous and reddish; The morphologies of rust surface showed the sandy and flowery structure typical for lepidocrocite and cotton balls structure typical for goethite; Cr and Cu distributed in the inner layer of rust – at distance of 40-60µm from steel substrate. The resistance of rust layers was evaluated by using electrochemical impedance spectroscopy measurement. This proved the predominance of WS over CS in corrosion resistance. The influence of environmental factors on protectiveness of WS was also discussed in this paper
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Introduction
Weathering steels (WS), also known as low-alloy steels, are mild steels containing a small amount of Cr, Cu, Ni and P (less than 3-4%) [1], they have been used very popular in the world from the viewpoint to reduce maintenance cost of steel structure. The enhanced corrosion resistance of WS is due to the formation of protective rust layer (patina), which is dense and good adherent to steel substrate, and therefore they impede the corrosion process. The formation of such protective rust layer has been shown to depend upon the existence of certain alloying elements in the steel and on exposure conditions [2, 3].
The environmental conditions which promote the formation of protective rust layers are existence of wet-dry cycling, absence of very long time of wetness, absence of marine component in atmosphere. It is now accepted that wet/dry cycling is necessary to form a dense and adherent rust layer, with rainwater washing the steel surface well, accumulated moisture draining easily, and fast drying action (low time of wetness) [2]. Surfaces protected from sun and rain (sheltered) tend to form the loose and poorly compact rust, while surfaces freely exposed to the sun and rain produce more compact and protective rust layers. The presence of atmospheric SO2 within certain limits favors the development of protective patina, while the chlorides suspended in marine atmosphere impair its protective properties [3,4]. For instance, in Japan, the use of unpainted conventional WS is not recommended in marine atmosphere with chloride deposition rate higher than 6mg/m2.day [5].