Compositional gradient and ion selectivity of Cr-substituted fine goethite as the final protective rust layer on weathering steel

Abstract

The compositional gradient and ion selectivity in the final protective rust layer of the Cr-substituted goethite, α-(Fe1-x, Crx) OOH, on a weathering steel have been discussed by examining the spontaneously and artificially formed Cr-substituted goethites. It was found that the Cr content in the spontaneously formed final protective rust layer increases gradually toward the [rust/steel] interface. This increase in the Cr content decreases the crystal size of the goethite and leads to a denser Cr-substituted goethite rust layer which provides higher protectiveness against atmospheric corrosives. It also became evident that the goethite with Cr content higher than approximately 3 mass% possesses cation selective ability. Thus, it is supposed that the spontaneously formed Cr-substituted goethite final protective rust layer has the ability to suppress corrosive anion penetration, at least near the [rust/steel] interface where the Cr content is estimated to be more than approximately 5 mass%. The structure of the [rust/steel] interface was also examined by transmission electron microscopy and the presence of a thin interfacial layer was pointed out. A detailed structure of the final protective rust layer is newly proposed with considering the compositional gradient and the structure of [rust/steel] interface.