Effect of molybdate ions as corrosion inhibitors of iron in neutral aqueous solutions

The effect of molybdate ions on the corrosion of Fe in neutral solutions was investigated by electrochemical measurements (dc polarisation and impedance spectroscopy) together with gravimetric determinations. Studies were conducted in solutions containing sodium hydrogen/sodium sulphate salts with molybdate concentrations ranging between 10(-4) and 10(-2) M at pH 8 and 9. Mass loss measurements indicated that about 10(-3) M of molybdate was necessary in order to inhibit completely the corrosion of Fe at room temperature. The potentiodynamic polarisation and electrochemical impedance studies gave indications about the mechanism of action of the MoO42- ion . They also showed that the inhibiting effect of the oxyanion is increased in the presence of dissolved oxygen. Spontaneous passivation of the corroding Fe electrode could happen only in the presence of dissolved oxygen at concentrations greater than 10(-4) M. However, passivity was also obtained under potentiodynamic polarisation conditions in deaerated solutions. The effect of the oxygen was attributed to displacement of the corrosion potential into the region of self passivation of the steel. Finally, the results indicated that the corrosion inhibition of Fe in neutral solutions by molybdate ions was largely insensitive to pH over the range from 8 to 9.