An X‐ray examination of internal corrosion products from power station boilers

Abstract

AbstractThe composition and structure of corrosion products from Power Station boilers have been investigated, mainly by X‐ray diffraction methods, but employing also petrographic and polarising metallurgical microscopes and optical and X‐ray spectrographic techniques. Particular emphasis has been given to the investigation of pit formation in high‐pressure boiler tubes.The results from the experimental boiler indicate that magnetite would be the main coating formed on a clean steam generating tube, and that the haematite found on the power station tubes derives from manufacturing heat treatment.The number of specimens showing pitting corrosion was very limited, but a number of very thin films (∼ 1μ thick) were found in the hard black material (mainly magnetite) filling the pit. Layers of copper and copper‐iron sulphides were identified as well as sodium sulphate and a small pocket of cuprite (Cu2O) crystals. Delafossite (CuFeO2) was also present in small amounts, but copper metal— very prevalent in mud drum deposits—was not detected.There were no striking differences between the scale and deposits on uncorroded parts of tubes and those in or near pits. The variation in the composition and structures of the scale is considerable in both cases.The most frequently occurring phases present in tube deposits, other than magnetite and haematite, were copper, cuprite, hydroxyapatite, delafossite and zinc‐iron spinel. The experimental boiler has shown that apatite may be formed when the calcium ion concentration is very low.