Mechanism of formation of magnetite from ferrous hydroxide in aqueous corrosion processes

The stoichiometric conditions for the formation of ferrous hydroxide Fe(OH)₂, by mixing Fe²⁺ ions with caustic soda NaOH, leads by oxidation to magnetite, irrelevant of the foreign anions, e.g. Cl^? or SO₄ ^2?, as demonstrated from Mössbauer spectroscopy. The electrochemical potential E_h and pH value of the initial conditions correspond to the drastic change from basic to acidic medium, observed when varying the initial Fe²⁺/OH^? ratio. Mössbauer analysis of the end products of oxidation at various temperatures shows that magnetite is only obtained at stoichiometry at very low temperature, but extends off stoichiometry at higher temperatures. The mechanism of formation of magnetite through an intermediate compound is discussed.