The behavior of phosphorous during passivation of weathering steel by protective patina formation

Abstract

Although it is well known that the protective patina on weathering steel normally has the two layer structure which consists of an outer crystalline FeOOH layer and an inner amorphous iron oxy-hydroxide layer, its formation mechanism has not been clarified well. Therefore, the analyses of this type of rust formed by 19 year exposure were carried out using Computer Aided X-ray Micro Analysis (CMA) and Laser Raman Spectroscopy. From these analyses, the existence of a thin intermediate layer composed of phosphatic rust was found between the outer and the inner layers showing that phosphorus in steel is converted to phosphatic compound during the protective rust formation. In-situ observation of colloidal rust formation by Video Enhanced Microscope told us that the phosphatic rust layer was formed at the initial stage of rust formation. It can be considered that the amorphous inner layer gradually grows up inside the initial phosphatic layer, whereas the crystalline FeOOH layer is formed outside.