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 st ructural steels were studied using cyclic and continuous water fog tes ts. It is shown that continuous wetting of steel surfaces results in t he formation of magnetite as the main constituent of rust formed. Howe ver, in wet/dry fog cycles with drying periods of more than 2 min in e very 30-min cycle, a defective spinel phase similar to gamma-Fe2O3 for ms. Continuous wetting and fast rusting appear to be the main criteria for the stability of magnetite. Both the magnetite and the defective spinel phase gamma-Fe2O3 transform to gamma-FeOOH first and to alpha-F eOOH 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 elec trochemical oxidation conditions or activation energy for spontaneous oxidation in air.