Characterization of corrosion products formed on mild steel in deoxygenated water by Raman spectroscopy and energy dispersive X-ray spectrometry

The corrosion of steel parts of heating systems is nowadays not commonly observed failure because of the lack of oxygen in circulated media. In rare cases corrosion rapidly initiates and propagates due to various, mostly unexplained mechanisms. A voluminous corrosion product that was found in the vicinity of a perforation to a pipe wall was investigated in order to understand the reasons for the observed rapid corrosion processes. Raman spectroscopy and energy dispersive X-ray microanalysis were used to examine this corrosion product across the whole cross-section of the pipe. It was observed that the inner part of the corrosion product at the contact with the pipe's inner surface mostly consisted of magnetite. The outer part of the corrosion product, which was in contact with the water in the pipe, was mostly goethite. These findings confirmed the hypothesis that the magnetite sediments on the surface of the new pipe caused oxygen concentration cell formation, which triggered corrosion dissolution of the pipe wall and led to a rapid perforation.