The rust layer formed on the 27 years old weathering steel bridge exposed in coastal industrial zone was characterized. The rust layer indicated protective characteristics in terms of the color of rust, the corrosion loss of the steel and the anodic polarization curve. The rust layer consisted of the stacking layers which were dark and bright in the polarized light. The dark rust layer covered the steel surface continuously, while the bright one partly covered the dark rust layer. The outer bright rust layer was in rich of the constituents of γ-FeOOH and β-FeOOH according to the Raman spectroscopy. The inner dark rust layer consisted of α-FeOOH and amorphous substance. The α-FeOOH spread in the wide range of the inner dark rust layer. The elements of Cr, Ni and Cu were observed in the inner rust layer. Copper concentrated both at the crack of the inner rust layer and in pits of the steel. Sulfur concentrated simultaneously with Cu.
The influence of the chloride deposition on the protectiveness of rust formed on weathering steel in long term exposure was examined. Specimens of weathering steel containing Cu and P were exposed to a rural environment for 41 years. After the exposure, the corrosion behavior of the weathering steel with protective rust in the laboratory corrosion tests simulating coastal environments was observed. In the test, spots of orange rust appeared on the surface of the specimens, which increased in number and size with the increase of the deposited NaCl. Amount of dissolved ferrous ions also was enhanced when the deposited NaCl was over critical amount. Through EPMA inspection of the cross sections of the rust, it was confident that Cl ions penetrated into the rust to reach the rust/steel interface, which suggested that the dissolution of the iron occur at the same interface. From the result of X-ray diffraction analyses and polarized light microscopic observations, change in the structure of the rust was hardly seen. The protectiveness of the rust was found to have a certain limit regarding chloride deposition, although its basic structure was apparently maintained.
Iron oxyhydroxides were prepared by oxidation and hydrolysis of Fe2+ containing solutions. The types of product were different for different solution pH : α- and γ-FeOOH at pH=6.0-7.0 ; γ-FeOOH at pH=7.0-8.0 ; γ- and δ'-FeOOH at pH=8.0-9.2 ; and δ'-FeOOH at pH=9.2-9.5. Preparation of iron oxyhydroxides from solutions with Cu2+, Ni2+ and PO43- were also carried out at pH=7.5. It was found that the ion addition increased the specific surface area and decreased the crystallite size of γ-FeOOH. Adding Cu2+ formed α- and γ-FeOOH while γ-Fe2O3 was formed by Ni2+ and PO43- addition.
The corrosion product formed on weathering steel exposed in a rural environment in the United States for 16 years has been investigated using Mössbauer spectroscopy, X-ray diffraction and Raman spectrometry. Mössbauer spectroscopy was used to measure the fraction of each oxide in the rust layer and micro-Raman spectrometry was used to locate and map the oxides to 2 microns spatial resolution. The protective inner-layer closest to the steel substrate consisted of nano-sized goethite ranging in size from 5-30 nm. The outer-layer close to the rust layer surface, consisted of lepidocrocite and goethite with the former oxide being most abundant. Comparison of the goethite in the rust layer was made with synthetic chromium substituted goethite with nearly identical microstructural characteristics being recorded. It can be said that most of the so-called X-ray amorphous substance, generally considered as mixture of nano-phase oxides of crystal size less than about 15 nm and actual amorphous phase with very short-range ordered atomic arrangement, possesses goethite structure with particle size less than 15 nm. Considering this nano-phase goethite, new quantitative determination method of total goethite in rust layer is proposed. It is concluded that chromium-substitution in the goethite is important for formation of a nano-phase oxide layer which may help protect the weathering steel from further corrosion.
Non-painted bridge were weathering steel was built and exposure test panels were installed on it on the highway of the Tohoku mountainous area. Twelve years having passed since then, investigations were made on conditions of rust layers formed on those surfaces. It was found that rust on the bridge was deep reddish brown in general and protective rust seemed to have been formed steadily but ion properties resistance value was so low as 0.34 kΩ on average. On the other hand, in the investigations of rust layers of exposure test pieces, it was recognized there were almost uniform non-polarizing layers containing condensed Cr and Cu. But many cracks were found on those rust layers and condensed Cl existed along them. It is supposed this may be the cause which made the ion properties resistance value low. This Cl was supposedly produced from antifreeze agent spread on the road in winter time, and its mist may have accumulated and penetrated into the rust layers. In the future, it must be studied what effect this Cl will give to formation protective rust.
We have tried to show detailed description of the long-term stabilizing process of rust layer on weathering steel reported previously by considering Cr-substituted ultra-fine goethite. As a result, detailed long-term rust-phase change on weathering steel is newly proposed, that is the change from initial poorly protective rust to the final protective Cr-substituted fine goethite via Cr-substituted ultra-fine goethite.
The working group in JSCE : “Detection of the localized corrosions by the electrochemical noise” carried out a cooperative measurement by the group members, to investigate the potential noises on the condition under which the cracks occurred in the corroding crevice. A crevice corrosion specimen with two type-304 stainless steel slips that were spot-welded in each other was used for the specimen. 3.5 mass% NaCl solution at 80°C was employed under air exposed condition for the test solution. Potential noises were measured with an electrometer and a reference electrode every 0.5 s in typically. From the measurement results, it was found a potential noise of a rapid shift to the noble side followed by slow recovery occurred in the condition under which the corrosion cracks were formed inside the crevice. This fluctuation pattern-calls RR-type-is a symmetrical to the pattern of the potential noise by localized corrosions that were formed on the free surface. The RR-type noise was not observed in the condition under which only the crevice corrosion occurred without the cracking. To investigate the generation mechanism of the RR-type noise, the potential noise of the specimen was measured in scratching the specimen surface. And also, the short circuit current noise between the specimen and the counter electrode of type-304 stainless steel was measured in the same condition. Consequently, it was estimated the origin of the RR-type noise was hydrogen evolution reaction at the bare surface after the film breakdown by a crack formation, because cathodic local current, not anodic local current was required to make the potential shift noble side transitionally. The potential of inside a corroding crevice is a less noble level, and the pH in there is a lower value, so that hydrogen evolution reaction is easy to occur inside the crevice.