The mechanism of formation of green rusts, Fe3O4, α-FeOOH, γ-FeOOH, δ-FeOOH and amorphous ferric oxyhydroxide, which are important atmospheric corrosion products of iron and steels, in aqueous solution at room temperature, has been investigated. The formation processes on which end products are determined are strongly affected by the oxidation rate, pH and the structure and composition of initial and intermediate species of iron. The schematic diagram of formation processes of intermediates, oxide and oxyhydroxides of iron has been presented, in which both dissolved and solid species of iron are included. The rusting process of mild and low alloy steels during atmospheric exposure test can be interpreted by the mechanism of formation of γ-FeOOH, amorphous ferric oxyhydroxide and α-FeOOH on the basis of presented diagram. A large amount of amorphous matter found in rust was identified by infrared and far infrared spectra as amorphous ferric oxyhydroxide, FeOx(OH)3-2x. The amorphous ferric oxyhydroxide rust formed on low alloy steel was characterized by considerable amount of bound water. It was concluded that the amorphous ferric oxyhydroxide obtained by reprecipitation after dissolution of γ-FeOOH is able to cover uniformly on low alloy steel, because Cu and P contained in low alloy steel contribute to uniform dissolution of steel and facilitate uniform formation of γ-FeOOH on steel surface. The amorphous ferric oxyhydroxide containing bound water forms a dense, crack-free rust layer, which acts as protective barrier against atmospheric corrosion. The wet-dry cycle and the atmosphere containing SO2 favor the formation of stable protective rust layer on low alloy steel due to the acceleration of dissolution of iron and γ-FeOOH and precipitation of amorphous ferric oxyhydroxide in rusting process.
