Morphology and phase composition of corrosion products formed at the zinc-iron interface of a galvanized steel

The corrosion products formed on the inner wall of pipes made of galvanized low carbon steel, exposed for similar to 2 years to water flowing in a large household heating system, were analysed using X-ray diffraction, Mossbauer and Raman spectroscopic techniques, as well as metallographic techniques. Products grew in the form of large-sized tubercles that gradually developed causing base metal losses up to perforation of the steel pipe. Considerable differences in the phase composition were found between the products formed in contact with the steel and those constituting the outer part of tubercles. The former were mainly made of FeCO3 (siderite), with small amounts of Zn-5(CO3)(2)(OH)(6) (hy&Ozincite), ZnCO3 (smithsonite), (Fe,Zn)CO3 mixed carbonate and CaCO3 (calcite), the latter mainly by Fe(III) oxyhydroxide goethite. Both parts of the tubercles also contained small amounts of other ferric oxyhydroxides, gamma-FeOOH (lepidocrocite) and beta-FeOOH (akaganeite), and very small amounts of hematite. The procedures used proved effective for an adequate identification of both the iron-containing and iron-free compounds in the corrosion products as well as for suggesting a corrosion mechanism. (c) 2008 Elsevier Ltd. All rights reserved.