Formation of mixed cation spinels on corroding carbon steel surfaces

Carbon steel surfaces removed from the boiler inlet and outlet of the coolant system of the nuclear power generating station at Douglas Point, Ontario, have been examined by x-ray diffractometry, scanning electron microscopy, and energy dispersive x-ray spectrometry. In addition to the expected composition product, magnetite, two other phases have been detected: (1) individual particles up to 100μm in diameter on the outermost surface, rich in aluminum and silicon, and having the mica type structure, and (2) a second phase uniformly distributed over the surface at the corrosion film-water interface. The latter phase, present in greater quantity at the boiler inlet, has the same structure (spinel) as does magnetite, and contains in decreasing order of magnitude, Fe, Ni, Al, Si, Mn, and Cr. The composition and structure of the corrosion products are discussed in terms of the composition of the materials in the heat transport system, and current knowledge of corrosion mechanisms and corrosion product stabilities in high temperature aqueous systems