(Invited) Structural Analysis of Rust Layer Formed on Carbon Steel Exposed to Atmospheric Corrosion Environments Using Synchrotron Radiation X-Rays
Authors: Yamashita Masato , Fujimoto Shinji 
Published: December 2020

Carbon steel has been a beneficial material to humankind, but unfortunately is easily oxidized back to iron ore if the steel touches with an atmospheric environment. This phenomenon is well-known as atmospheric corrosion which proceeds under an electrolyte film containing oxygen and water covering the steel surface. Through atmospheric corrosion, surface of a carbon steel becomes covered with rust layer which largely affects the corrosion process of the steel. In this study, structure of rust layer formed on carbon steel exposed to atmospheric environment is explored using synchrotron radiation X-rays.

The X-ray diffraction (XRD) and X-ray adsorption fine structure (XAFS) analyses for the rust layer containing Cr revealed that Cr is incorporated in the corrosion product with goethite (alpha-FeOOH) structure and resultantly the particle size of which becomes less than ten nanometer. This ultrafine Cr-goethite 1) works as a strong protective barrier for the corrosives in the atmosphere. XAFS spectra at Cr K-edge explain the probable site of Cr3+ in goethite and this Cr3+ gives cation selectivity to goethite in the following way. It was shown by the extended X-ray absorption fine structure analysis that the incorporated Cr3+ is not located at a definite distance from Fe3+ and is coordinated with O2-; therefore Cr3+ forms CrOx
3-2x complex ion having negative charge. Namely, CrOx
3-2x complex ion is adsorbed to the goethite crystal surface and its negative charge gives cation selectivity which reduces penetration of corrosive anion such as Cl- through the rust layer.

In-situ XRD observation of the formation process of rust layer directly indicates the Fe2+ - Fe3+ change during the corrosion processes of a carbon steel. An addition of metallic salt to the electrolyte film covering the steel surface changes the rust formation process. For instance, Cl- promotes the formation of akaganeite (beta-FeOOH) and some other metallic salts help goethite formation 2).

The addition of nickel sulfate to the surface of carbon steel in the chloride and sulfuric acid mist environment promoted the formation of goethite and akaganeite 3). It was assumed that this akaganeite was not conventional β-FeOOH with a tetragonal crystal structure, but monoclinic akaganeite containing Ni2+. This monoclinic akaganeite might reduce the corrosion rate of the carbon steel since the monoclinic akaganeite incorporates corrosive Cl- in the crystal structure of the akaganeite.

Based on the arguments mentioned above, it is said that the rust structure is greatly affected by the coexisting ions on the steel surface and resultantly controls corrosion process of a carbon steel.

References

1) M. Yamashita, H. Konishi, J. Mizuki and H. Uchida, Materials Transactions, 45, No.6, 1920-1924(2004).

2) M. Yamashita, H. Konishi, T. Kozakura, J. Mizuki and H. Uchida, Materials Transactions, 46, No.5, 1004-1009 (2005).

3) S. Hayashida, M. Takahashi, H. Deguchi, H. Tsuchiya, K. Hanaki, M. Yamashita and S. Fujimoto, Zairyo-to-Kankyo, 69, No.5, 148-153(2020).