Synchrotron Radiation Study on Structure of Atmospheric Corrosion Products Formed on Steel Surfaces

A rust layer formed on a low-alloy steel surface is generally considered to be responsible for protecting the steel against corrosives in the atmosphere. Therefore, the rust layer plays an important role in corrosion control. Certain alloying elements promote the formation of a so-called “protective rust layer” on the surface of the steel. For example, it is well known that weathering steel, which contains small amounts of anticorrosive alloying elements such as Cr, P, Cu and Ni, possesses a high corrosion resistance of approximately twice that of carbon steel. Consequently, weathering steel is recognized to be a structural material that does not require painting. We have only a nominal understanding of the mechanism of the protective ability of the rust layer. The properties of a material can generally be explained on the basis of its atomic structure. Therefore, it is meaningful to study the structural properties of the protective rust layer of weathering steel. It is particularly necessary to analyze the local structural and chemical properties of anticorrosive alloying elements and corrosive ions in the rust layer. In addition, it is also desirable to observe the corrosion process directly. Information obtained in this manner can be the guiding principle of the research and development of high-performance weathering steel. As is well known, X-rays and gamma rays are easily available for analyses of the chemical state and structure of a material on a nanometer scale. Recently synchrotron radiation has been particularly useful for this purpose in a wide variety of scientific fields. Synchrotron radiation is electromagnetic radiation emitted from a fast electron moving at approximately the velocity of light following a curved trajectory under a magnetic field in an accelerator. It has a number of unique properties as follows: