The Fast Linear Growth of Magnetite on Mild Steel in High-Temperature Aqueous Conditions

Abstract

Unstressed mild steel placed in unstirred aqueous solutions of ferrous ion at 300–400° oxidises at a rapid linear rate with formation of a firm scale of magnetite. Weight-change data show that this non-protective oxidation is active in certain chloride and bromide solutions, particularly if small amounts of metallic nickel or cobalt have deposited on the steel or its incipient magnetite coating. Whereas, with protective growth of magnetite on mild steel in liquid alkaline solutions at high temperature, only about half of the oxide adheres to the metal, all of the non-protective magnetite produced under the influence of chloride and nickel adheres to the metal. This finding, together with topographical evidence that the bulk of the non-protective magnetite forms at the metal/oxide interface, leads to the conclusion that prohibitively high stresses develop in this magnetite as it grows, causing the continual development of porosity in the oxide. This cohesive failure is sometimes inadequate to relieve all growth stresses and is then accompanied (especially at 300°) by adhesive loss leading to lamination of the magnetite layer. Beside accounting for the linear oxidation rate, the porosity of the magnetite is believed to place the oxidation under control of the cathodic reaction occurring at the environment side of the magnetite layer. The efficiency of nickel and cobalt as cathodes at the magnetite surface is suggested as an explanation of the accelerating effect of these metals on the oxidation reaction. At 400° the oxidation often takes place with decarburisation and embrittlement of the underlying steel by the hydrogen released during the action. This effect is discussed in detail and the conditions for its occurrence are defined in terms of the competition for the available hydrogen between the embrittlement reaction and various processes for the dispersal of the hydrogen.