Composition and protective ability of rust layer formed on weathering steel exposed to various environments

The compositional change of rust (corrosion products) layer formed on weathering steel exposed to atmosphere with different amount of air-borne sea salt particles in Japan have been investigated by the X-ray diffraction method. The mass ratio (alpha/gamma) of crystalline alpha-FeOOH to gamma-FeOOH, in the rust layer formed on the weathering steel exposed in an industrial environment, increases with an increase in exposure duration. The alpha/gamma is closely related to the corrosion rate in environments when the amount of air-borne salt is less than 0.2mg NaCl/dm(2)/day (2.31 x 10(-7) gNaCl/m(2)/s). However this is not the case in seaside environments with a higher amount of air-borne salts. The mass ratio (alpha/gamma*) of crystalline alpha-FeOOH to the total mass of gamma-FeOOH, beta-FeOOH and Fe(3)O(4), in the rust layer formed on the weathering steel is related to the corrosion rate even in seaside environments certainly more than 0.2 mg/dm(2)/day (2.31 x 10(-7) g/m(2)/s) of air-borne salt particles. When the alpha/gamma* is more than 1, a higher corrosion rate more than 0.01 mm/year (3.17 x 10(-13) m/s) is not observed. The alpha/gamma* is a protective ability index of rust formed on weathering steel. (c) 2005 Elsevier Ltd. All rights reserved.