Role of Nanophase Oxides in Short-Term Atmospheric Corrosion of Structural Steels

Abstract

Systematic studies on the development of nanophase iron oxides in the corrosion products of carbon and weathering steel were performed to understand the role of nanophase oxides in short-term atmospheric corrosion. Similarities and/or differences between short-term and long-term atmospheric corrosion were established by studying carbon steel and weathering steel coupons exposed in mild marine environments for short-term and comparing it with previously established long-term data. Influence of substitutional elements, in particular chromium, in forming nanophase goethite was investigated. Crystallographic, magnetic and morphological properties of nanophase chromium substituted goethite have been characterized in order to understand the protective nature of chromium-substituted goethite in a naturally weathered steel surface. Spectroscopic investigation of the corrosion products of both carbon and weathering steel indicated that lepidocrocite and goethite were the predominant oxides to form following short-term exposures. The corrosion coatings were well layered for exposure times as early as 2 months. The layering was very similar to that observed on steel coupons exposed for more than 8 years. The outer layer was composed of lepidocrocite and occasionally goethite. The inner layer was mainly composed of nanophase goethite. The relative fraction of nanophase goethite was significantly higher in weathering steel compared to carbon steel at the end of six months of exposure. The data analysis also revealed that during the first two months of exposure weathering steel corrodes faster than carbon steel. However carbon steel corrodes more rapidly after 6 months of exposure. At the end of one year, the corrosion rate of carbon steel is higher than weathering steel. It is proposed that during the couple of months, nucleation of oxides is the dominant process in both carbon and weathering steel. At the end of six months, a considerable amount of nanophase goethite formed on carbon steel continues to grow into bigger crystals. On the other hand, in weathering steel the crystal growth of a significant fraction of initially formed nanophase goethite is inhibited. It is proposed that substitutional elements like chromium inhibit the crystal growth in weathering steel. The crystallographic and spectroscopic data for showed that with increasing chromium concentration, the crystallite size of synthetic goethite measured from X-ray diffraction and particle length of goethite measured from Mossbauer spectroscopy and Transmission Electron Microscopy, became smaller