A comparative electrochemical and ellipsometric study of the iron electrodes in different alkaline electrolytes

Abstract

The electrochemical and ellipsometric responses of iron electrodes in 0.04 M NaOH and in saturated Ca(OH)₂ 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 Fe₃O₄, 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)₂ as compared to 0.04 M NaOH are explained through the absorption of Ca²⁺ 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.