The largest third-generation synchrotron radiation beams show advantages of ultra-bright, highly directional, high energy and variable energy when compared with a conventional X-ray equipment. Recently, the application of synchrotron radiation analysis at SPring-8 has been expanded for the research on corrosion phenomenon, such as, structure analysis of the passive film in stainless steel, the rust layer generated on the weathering steel, etc. This report will introduce the SR-X ray analysis methods suitable for corrosion research and applications by XAFS, XRD, GIXS, HX-PES and others.
Recent works on formation processes of iron corrosion products are reviewed. Although green rust (GR) is sometimes observed as intermediate corrosion products of iron based alloys, it is easily transformed to ferric oxyhydroxides such as goethite and lepidocrocite through the aerial oxidation. This is because GR contains ferrous ions which are oxidized to ferric ions in aqueous solution. In particular, the characteristic features of the structure and transformation of GR are overviewed in this paper.
A local structural analysis of heavy rust layers with large swelling and laminated layers formed on weathering steel bridges using synchrotron radiation X-ray diffraction (SR-XRD) in SPring-8 have been performed. The main constituent in average composition of the whole layer was spinel-type iron oxide [mainly Magnetite (Fe3O4)] and the mass ratio was 30-40 mass%. In contrast the mass ratio of spinel in its local parts, i.e. outer layer, inter-layer and inner layer position was not higher in common but the mass ratio of β-FeOOH was higher. Therefore it indicates that these heavy rust layers have been composed of many layers of spinel poor, rich and poor - cell (SPRaP-cell). Thus SR-XRD is useful for the analysis of the constituent distribution in the rust layer.
Structure of the native oxide film and the thin corrosion product films, which formed in the early stage on hot-dip Zn-0.2%Al, Zn-5%Al and Zn-55%Al coatings in humid atmosphere under the presence of NaCl particles, were investigated by means of photoemission spectroscopy (PES) using synchrotron radiation (150 eV) and Al-Kα radiation (1487 eV). In 150 eV, the detection depth of Al2p, Zn3d, Cl3p and O2p electron is below approximately 1 nm, and is a third or less of that in 1487 eV. As a result, the dominant surface components of the native oxide films were aluminum oxide independent of Al content in Zn coatings. It was found that the dominant component in 1nm depth of thin corrosion product films on Zn-Al coatings was different from those in 4 nm. Thin corrosion product films were composed mainly of zinc oxide, zinc chloride, aluminum oxide, zinc chloride, and the ratio of those components was changed by the Al content in Zn coatings and the depth from surface.
Diagnosis of water quality for polarity reversal in galvanized steel pipes were carried out by using Mahalanobis Distance (MD). Cooling and heating water quality data (109 data of no corrosion trouble and 20 data of corrosion trouble) and coolant water quality data (173 data of no corrosion trouble and 12 data of corrosion trouble) were used for the diagnosis of water quality. In the diagnosis of cooling and heating water quality, the misjudgment was 1 among 20 troubled data, thus this diagnosis was high accuracy. In diagnosis of coolant water, the misjudgments were 3 among 12 troubled data. Since many kinds of dosing are implemented in coolant water, dosing influenced the water quality remarkably and the accuracy of diagnosis of water quality using MD decreased.
Silver, a typical electronic material, corrodes in sulfur gas released from rubber. To evaluate the corrosion damage, we established a method for estimating concentration of sulfur gas released from rubber. Firstly, we showed that the silver corrosion behavior in sulfur gas released from rubber is similar to that in sulfur gas released from sulfur flower using a boxy-vessel test. Moreover, the corrosion product on silver was Ag2S only, and the thickness of the corrosion product increased linearly with exposure time. We also found that the silver corrosion rate in the boxy-vessel test depends on both the corrosion reaction and the diffusion of sulfur gas. The former mainly determines the silver corrosion rate. The corrosion rate was therefore calculated by corrosion analysis considering the sulfur gas diffusion and corrosion reaction. Under the condition that the corrosion analysis assumed an inverse problem, the concentration of sulfur gas released from rubber was estimated from the corrosion rate measured by a pre-experiment. By using the corrosion analysis under the boundary condition of concentration of sulfur gas released from rubber, we can estimate the silver corrosion rate in the space of electronic equipment where silver coexists with rubber. We can therefore verify effective countermeasures against corrosion and ensure the corrosion reliability of electronic equipment.