Environmental factors affecting the corrosion behaviour of reinforcing steel: I. The early stage of passive film formation in Ca(OH)2 solutions

Oxide film thickening on reinforcement steel at early stage of formation is followed in naturally aerated Ca(OH)2 solutions, recalling the natural behaviour in concrete, by measuring the open-circuit potential, E, with time up to 4 h. The final potentials, Efin, are reached from negative values indicating oxide film growth. E varies with the Ca(OH)2 concentration according to a straight line relationship. Oxide film thickening, at early stage of immersion, follows a direct logarithmic growth law as evident from the linear relationship between E and log t. The rate of oxide film thickening deceases by increasing the concentration and pH of the solution and by raising the temperature. The free activation energy of oxide film thickening is determined and found to be 29.28 kJ/mole, indicating that the process of oxide film growth is under diffusion control.

Research highlights
► Oxide film thickening on reinforcement steel at early stage of formation in naturally aerated Ca(OH)2 solutions can be measured.
► The final measured potentials are reached from negative values.
► The oxide film thickening at early stage of immersion follows a direct logarithmic growth law.
► The rate of oxide film thickening deceases by increasing the concentration and pH of the solution and by raising the temperature.