Formation and Transformation of Magnetite (Fe3O4) on Steel Surfaces Under Continuous and Cyclic Water Fog Testing

Formation and transformation of magnetite on two selected low-alloy structural steels were studied using cyclic and continuous water fog tests. It is shown that continuous wetting of steel surfaces results in the formation of magnetite as the main constituent of rust formed. However, in wet/dry fog cycles with drying periods of more than 2 min in every 30-min cycle, a defective spinel phase similar to γ-Fe2O3 forms. Continuous wetting and fast rusting appear to be the main criteria for the stability of magnetite. Both the magnetite and the defective spinel phase γ-Fe2O3 transform to γ-FeOOH first and to α-FeOOH ultimately under repeated drying conditions, mostly controlled by electrochemical oxidation process. Magnetite formed on steel surfaces in crevices in the open atmosphere is held stable by the lack of electrochemical oxidation conditions or activation energy for spontaneous oxidation in air.