Analysis of iron rusts by X-ray diffraction and X-ray absorption fine structure measurements

Abstract

Recently, much attention has been devoted to steels with better atmospheric corrosion resistance especially in chloride environments of the various industries. For example, new weathering steels have been researched and developed, according to the increase of requirements of reduced initial construction and maintenance costs for steel bridges [1,2]. So far, uncoated conventional weathering steels for bridges have not been adopted in the chloride environments such as marine and coastal areas. This is because the steels encountered the formation of flaky rust with poor adherence, resulting in deterioration of corrosion resistance to the same level as that of mild steel. The amounts of de-icing salts have also been much increased due to the establishment of domestic laws banning tires with spikes being used on expressways since 1991. Thus, there have been great demand for new weathering steels that are resistant to chloride environments. It is thought that the improved resistance to atmospheric corrosion of steel could be due to the formation a protective rust layer preventing the intrusion of chloride species. β-FeOOH, one of the polymorphs of ferric oxy-hydroxides, is known as a component of corrosion products of steel in the presence of Cl- ions [3]. The authors have recently found that corrosion resistance in chloride environments was well correlated to the formation of β-FeOOH rust and increased with the decrease of the fraction [4-6]. Shiotani et al. [7] and Yamashita et al. [8] have also confirmed good correlation between fraction of β-FeOOH rust and corrosion thickness loss of weathering steels involving on-site exposure tests at 41 bridges for periods of 17-18 years by Public Works Research Institute, Kozai Club, and Japan Association of Steel Bridge Construction [2]. The authors have found that Ti was effective as an alloying element to improve corrosion resistance of steel in chloride environments by decreasing the formation of β-FeOOH [4-6]. From the above, numerous studies on atmospheric corrosion of weathering steels have been carried out from the veiwpoint of composition, structure or various properties of the iron rusts, by using X-ray diffraction (XRD), electron diffraction (ED), transmission electron microscopy (TEM), infra-red spectroscopy (IR), laser Raman spectroscopy (LRS), Mössbauer spectroscopy, X-ray absorption fine structure (XAFS), electrochemical impedance spectroscopy (EIS), and so on. On the other hand, the authors have also been conducting fundamental experiments involving artificial rust synthesis to elucidate the influence and role of various alloying elements on the formation of β-FeOOH rust [9]. As well as β-FeOOH, α-FeOOH, γ-FeOOH, Fe3O4 and poorly crystallized iron oxide (XRD amorphous rust) are also known as a major component of corrosion products of steel formed by atmospheric corrosion [1]. So far, Cr, Cu, and Ni have been widely used as alloying elements to improve resistance to atmospheric corrosion of steels [1]. Atmospheric corrosion resistance of steel is thought to be improved through the control of not only β-FeOOH but also α-FeOOH, γ-FeOOH, Fe 3O4 and poorly crystallized iron oxide (XRD amorphous rust) by the addition of alloying elements such as Ti, Cr, Cu, and Ni. This chapter reviews characterization techniques of iron rusts by X-ray diffraction (XRD) such as quantitative measurement of rust composition and estimation of rust crystallite size. Analysis of iron rusts by X-ray diffraction (XRD) and X-ray adsorption fine structure (XAFS) using ultra-bright synchrotron radiation (SR) are also explained. In addition, this chapter also introduces the study of artificial rust synthesis to elucidate improvement mechanism of resistance to atmospheric corrosion of weathering steel.