The potentiodynamic behaviour of iron in alkaline solutions under carefully controlled perturbation conditions reveals that the overall electrochemical process is more involved than was thought earlier. The electrochemical characteristics of the systems are explained through a series of successive conjugated redox coupks principally involving Fe(&), Fe(OH), gnd FeOOH as limiting stoichiometric species. The yield of soluble species such as either FeO:-or HFeO; increases with the pH. Ageing effects of reactants and products are also distinguished through the potentiodyoamic E/I records.

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The electrochemical and ellipwxnetric responses of iron electrodes in 0.04 M NaOH and in saturated Ca(OH), are investigated at 25"C, in the potential range where the passivating layer in the absence of oxygen under potential controlled and non-equilibrium conditions is formed. From the correlation of results a composite structure of the passivating layer is envisaged involving an inner layer which is difficult to electroreduce, probably related to FesO,, and an outer gelatinous iron hydroxide layer where a relatively fast, Fe(II)/Fe(III) redox process can be voltammetrically followed. The inhibitive properties of saturated Ca(OH), as compared to 0.04 M NaOH are explained through the absorption of Car' ions at the outer plane of the inner part of the passivating film and the corresponding transport of water into the gelatinous part of the passivating film.

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