Microstructural Analysis of Rust Layers on Weathering Steels Exposed at Rural and Coastal Areas

Abstract

Weathering steel has been widely used for the bridge materials because of its superior atmospheric corrosion resistance. It has been, however, pointed out that the amounts of corrosion are greater used in coastal area than rural area. In order to clarify the differences of corrosion resistance between rural and coastal area, rust layers were analyzed in details by combining with FIB (Focused Ion Beam) and electron microscope. Analytical results indicate as follows:
Inner layer of the rust exposed in rural area consists of multiple layers and thickness of each layer is less than 2μm. These layers have different composition and degree of enrichment in alloying elements, especially chromium. Though these layers consist of mainly FeOOH fine particles, crystalline FeOOH layer exists between fine particle layers. Rust layer exposed in coastal area did not show such multiple-layer structure. The layer consists of crystalline β-FeOOH and FeOOH fine particles. The crystalline β-FeOOH is not a single crystal but is an aggregation of ordered fine rust particles. A size of the fine particles of rust exposed in coastal area is not much different from that exposed in rural area. Major differences existed are structure and packing state of fine rust particles.