SEM/Micro-Raman Characterization of the Morphologies of Marine Atmospheric Corrosion Products Formed on Mild Steel

The exposure of carbon steel to marine atmospheres gives rise to the formation of various corrosion products (lepidocrocite, goethite, magnetite, akaganeite, etc.) that present a wide variety of morphologies depending on the specific conditions of their formation. The literature on this topic is scarce and lacks a rigorous analytical characterization of the different morphologies seen in microscopic observation of rust surfaces under a scanning electron microscope. The research presented in this paper uses XRD and the powerful integrated SEM/Micro-Raman technique to characterize a series of morphologies commonly found in the rust that forms when mild steel is exposed to marine atmospheres. Two atmospheres with highly different chloride deposition rates (atmospheric salinity) have been considered. In the lower salinity atmosphere an adherent rust layer was formed on the steel, the outer surface of which layer exhibited a variety of SEM morphologies corresponding to lepidocrocite (mainly) and goethite. In the higher salinity atmosphere the thick rust layer formed on the steel was easily exfoliated (detached) and the morphologies on the inner surface observed using SEM were very different, corresponding mainly to akaganeite and magnetite.