Low-alloy weathering steels resistant to SOx pollution show poor performance in environments where airborne salinity is high. Recent researches on rust and weathering steels, therefore, have been focused on the rusting mechanism in the presence of chloride and development of new steels and coatings for the use in marine/coastal environments. In the paper discussion and literature review are given for the following subjects: 1) tolerable limit of airborne salinity level for the use of uncoated weathering steels in infrastructure, 2) related case study of flake rusting on bridges in coastal area, 3) thermochemical aspect of rusting in chloride environments, 4) crystal structure of iron rust in high salinity, 4) electrochemical reaction of rust and its catalytic properties in chloride media, 5) protective nature of rust of weathering steels, and 6) quality evaluation (or stabilization) of rust on weathering steels.
Although the word ‘antei-sabi (stable rust)’ is often used in the Japanese weathering steel market, its clear explanation has not been available. Thus based upon accumulated survey data regarding rust conditions of weathering steels, engineering definition of ‘stable rust’ is proposed. In order to realize a so-called minimum maintenance bridge concept advocated by Ministry of Construction, practicality in terms of weathering steel application is assessed. Since long term prediction of corrosion loss for the steel, as well as application of adequate maintenance procedures can be established, drastic reduction in life cycle cost for bridges is considered to be plausible. Several issues to be overcome in order to realize super durable bridges by use of weathering steel are pointed out focusing on its atmospheric corrosion kinetics.
The protective ability of rust layers formed on weathering steels exposed in an industrial region for various periods has been studied by the potential measurement, and the rust composition and structure were also studied. Potential of weathering steel in 0.1mol/L Na2SO4 solution tends to become noble with exposure time. Potential of weathering steels with rust layers, which were exposed for a long time and had superior atmospheric corrosion resistance, became over -0.3V (vs. SCE). The rust layer possesses the double layer structure composed of inner dark layer of α-FeOOH and outer bright layer under reflected polarized light, which is observed typically in the protective rust layer. On the other hand, potential of mild steel exposed for a long time is less noble at about -0.45V (vs. SCE). These results mean that potential reflects the difference of characteristic of rust layer. Potential became noble with increasing amount of α-FeOOH in rust, which suggests that potential depends on the composition of the rust layer. Furthermore, potential correlates with the ratio of rust composition α/γ*: α-FeOOH/(γ-FeOOH+β-FeOOH+Fe3O4), and potential of -0.3V (vs. SCE) corresponds to α/γ*=2. Therefore potential as well as the ratio α/γ* is able to use for the evaluation of the protective ability of the rust layers formed on weathering steels.
Corrosion products of metals comprising of oxides protect metals from, corrosion, and it is thought that less soluble and more compact oxide films have higher protection ability. This investigation developed a model of the kinetics of oxide dissolution by assuming that the transfer of the lattice metal and oxide ions to solution couples to keep the electric neutrality. The model reproduced the time changes in the concentrations of metals dissolved from CuO and magnetite in chelating agent solutions and the peaking dissolution rate at a pH.
For a better understanding of the solid-state properties of the Cr-substituted goethite α-(Fe1-x, Crx)OOH as the final protective rust layer on weathering steel bearing chromium, Raman and FT-IR spectroscopies and magnetic measurement have been carried out for the synthetic Cr-substituted goethite rusts containing various amount of Cr. It is found from the Raman spectroscopy that addition of Cr(III) ions to Fe(II) ion alkaline aqueous solution delays the precipitation of Cr-substituted goethite in the air oxidation process. Despite the increase of Cr content, the behavior of FT-IR spectra suggests that the Cr-substituted goethite keeps a goethite (α-FeOOH) type oxyhydroxide crystal structure. Ferromagneticity appears over about 2 mass% Cr content though the Cr-substituted goethites without and with lower Cr content less than 2mass% possess the antiferromagneticity. This occurrence of magnetization proves that Cr atoms in the lattice of Cr-substituted goethite are substituted at the sites of Fe atoms. The particular content of about 2mass% Cr in the antiferro/ferromagnetic change corresponds to that producing finer particles of Cr-substituted goethite by the action of Cr(III) ions.
Alloy designing methods, such as Ca or enhanced Ni addition together with elimination of Cr, for anti air-born salinity weathering steel were established, utilizing various findings in terms of colloidal function of rusts. Addition of Ca to steel is a measure aiming at increasing pH at corroding interfaces, resulting in stabilization of iron oxy-hydroxides, alteration of ion exchanging properties of the colloidal corrosion products, and maintaining passive state of locally bare steel surfaces at defects of rust layers. Enhanced nickel prescription decreases active dissolution rates of the steel, to reduce acidification of the corrosion interface caused by hydrolysis of metallic ions. So formed rust may contain Ni(II) ions to stabilize Fe(II, III) oxides, which may be beneficial to alter its ion-exchanging properties to even facilitate alkalization at steel/rust interfaces. Elimination of chromium is essential to improve corrosion resistance of low alloy steels applied to the coastal atmospheres because of its acidifying properties. Practicality of the established concept was confirmed through 9 year exposure tests of so-prescribed steels to the severe coastal environment, resulting in the first commercialization of anti air-born salinity weathering steel. This paper mainly describes authors' seed investigating activities carried out from 1984 to 1989 together with the recently obtained penetration curves.
The degradation of protective rust layer on weathering steels was investigated by electrochemical method in chloride solutions. The samples used were weathering steels which had been exposed in rural atmosphere for about 35 years. Electrode potential changes were measured in some chloride solutions for the samples with protective rust layer. The rest potential shifted from original 200 to 300mV vs. SSE to less noble and fairly constant values after 48 to 72 hours immersion. The constant potential gave less noble values with increasing chloride ion concentration. From the results of corrosion monitoring and visual observation, it is concluded that the decrease in rest potential is attributed the increased number of active sites for anodic dissolution. The alloying elements such as Cr, P and Cu are concentrated in rust layer at corresponding portions where active dissolution occurred on metal/rust interface. It is not observed that chloride permeation under high chloride ion concentration environments is inhibited by the presence of these elements.
The iron rust phase has been analyzed by using the in-situ XRD and the alternating current (AC) impedance methods after wet/dry corrosion test using NaCl-MgCl2 complex which is main composition of airborne saline particles. The corrosion mass gain of the carbon steel depends on the concentration of Cl ion from environment in the test chamber, and it does not depend on the type of cations (NaCl or MgCl2). As the concentration of Cl ion increases, the content of β-FeOOH increases in iron rust phases. The transition of β-FeOOH from the green rust I (G.R.I) was observed directly by the in-situ XRD. The amount of G.R.I depends on the concentration of Cl ion, and β-FeOOH is transformed from G.R.I automatically in drying process, where there is no dependence of type of cations on the iron rust phase whichever cation Cl bonds with. It was found by the AC impedance that the resistance of the rust (Rrust) increases with the number of cycles in the corrosion test, and that the structural factor of the rust becomes predominant in Rrust. With the increase of the amount of rust, the resistance corresponding to corrosion rate (Rt) decreases, which is related to the reduction of β-FeOOH in rust phase.