Influence of Fluoride Concentration and pH Value on the Corrosion Behaviour of Iron

The growth kinetics and electrochemical properties of the oxide film formed on iron were studied in 0.01 and 0.1 mol L-1 NaF solutions with different pH values (4.5, 5.5 and 6.5) by cyclic voltammetry and electrochemical impedance spectroscopy. The growth of the oxide film on iron in potentiodynamic conditions, characterized by the occurrence of "current plateau" recorded on the cyclic voltammograms, showed that the growth of the oxide film takes place by low-field migration mechanism. The ionic conductivity of the oxide during its growth (similar to 10(-12) S cm(-1)) and the electric field strength through the oxide (similar to 10(6) V cm(-1)) were calculated. Thickness of oxide film is dependent upon the solution pH and fluoride concentration; thinner and more compact film forms in solution with lower fluoride concentrations and higher pH values. Impedance measurements were carried out on the potentiostatically formed oxide films after electrode stabilization for 30 minutes at the open circuit potential (E-OCP). The charge transfer resistance and the electrode capacitance were determined. The resistance value, as well as E-OCP value, decreases with increasing concentration of F- and lowering the pH. Corrosion rate values were computed from charge transfer resistance and it was found that corrosion rate increased with increased F- concentration and decreased pH, being highest in 0.1 mol L-1 NaF, pH=4.5 (60.69 mu g cm(-2) h(-1)) and lowest in 0.01 mol L-1 pH=6.5 (0.25 mu g cm(-2) h(-1)).