PROTECTIVE RUST LAYER FORMED ON WEATHERING STEEL BY ATMOSPHERIC CORROSION FOR A QUARTER OF A CENTURY

The rust layers formed on weathering and mild steels by atmospheric corrosion in an industrial region for a quarter of a century have been characterized by means of observation of the reflection behaviour of polarized light, scanning electron microscopy, infrared transmission, X-ray diffraction and Raman spectroscopy, and electron probe microanalysis. It was found that the protective and stable corrosion product which covered the surface of weathering steel mainly consisted of alpha-FeOOH (ferric oxyhydroxide). The protective rust layer contained a considerable amount of Cr, and was composed of densely packed rust particles in contrast with the rust layer formed on the mild steel. The fact that the protective rust layer was mainly composed of alpha-FeOOH can reasonably be interpreted in terms of the transformation process of a corrosion product formed by the atmospheric corrosion of steels.