Influence of Cu and Ni on the morphology and composition of the rust layer of steels exposed to industrial environment

Four samples of steels with alloying elements were exposed to an industrial environment during 1,955 days, aiming to elucidate the effect of the alloying elements Cu and Ni on the resistance of weathering steels to corrosion processes. The samples were characterized with optical microscopy, scanning electron microscopy (SEM), powder X-ray diffraction (XRD), saturation magnetization measurements and with energy dispersive (EDS), infrared, Mössbauer and Raman spectroscopies. All the steels originated orange and dark corrosion layers; their thicknesses were determined from the SEM images. EDS data of such rust layers showed that the alloying element content decreases from the steel core towards the outer part of the rust layer. Moreover, in the dark rust layer some light-gray regions were identified in the W and Cu-alloy steel, where relatively higher Cr and Cu contents were found. XRD patterns, infrared, Raman and Mössbauer spectra (298, 110 and 4 K) indicated that the corrosion products are qualitatively the same, containing lepidocrocite (γFeOOH; hereinafter, it may be referred to as simply L), goethite (αFeOOH; G), feroxyhite (δ′FeOOH; F), hematite (αFe₂O₃; H) and magnetite (Fe₃O₄; M) in all samples; this composition does not depend upon the steel type, but their relative concentrations is related to the alloying element. Mössbauer data reveal the presence of (super)paramagnetic iron oxides in the corrosion products. Saturation magnetization measurements suggest that feroxyhite may be an occurring ferrimagnetic phase in the rust layer.