Growth of Magnetite on Steel in High Temperature Aqueous Environments

Abstract

Previous studies on the oxidation of steel in high temperature aqueous environments have shown that the morphology and kinetics of formation of the corrosion product are dependent to a large extent on the environment. In alkaline solutions, oxidation in the presence of an external cathode resulted in the formation of a duplex layered oxide, whereas in the absence of an external cathode only a single layered oxide was formed. Furthermore, oxidation in ferrous chloride environments produced very much faster rates of oxidation. This work was undertaken to provide more information on the principles involved in the production of various oxide forms. The oxidation of mild steel in deionized water, 3.5MNaOH and ferrous chloride solutions at 316oC have been investigated using principally electron microscopy and Fe55 and Fe59 radio active tracers. These new data are used to discuss differences in oxide morphology. A mechanism for the oxidation of steel in the absence of an external cathode has been proposed, which involves a two stage process of pore blocking followed by solid state diffusion. Regarding the oxidation of steel in the presence of an external cathode, the kinetics and mode of outer layer formation have been determined; these results are consistent with oxidation controlled by solution transport of iron. The oxidation of iron in ferrous chloride solutions is discussed, emphasis being placed on the effect of nickel in the early stages.