In previous work on the oxidation of mild steel to magnetite in high‐temperature alkaline solutions different workers have obtained markedly differing types of oxide under apparently similar conditions. The types of oxide obtained, the conditions under which they were grown, and possible reasons for the difference in oxide morphology are discussed. Experiments are described which support the idea that the type of magnetite growth obtained upon an area of mild steel depends upon the presence or otherwise of a suitable cathodic surface in contact with it. For example, the oxidation of a mild steel specimen in a new pressure vessel will take place in a different manner to that of a specimen in a vessel with a pre‐formed protective magnetite film coating its sides, since the magnetite film in the latter case probably acts as a cathode for the oxidation of the specimen. The properties of the types of growth, and in particular their healing properties, are discussed with particular reference to the practical problem of pitting growth in steam generator tubes.

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