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

Abstract

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 reli...