The mechanism of the passivation of iron in neutral solutions: An ellipsometric and coulometric investigation

Ellipsometric and electrochemical transient techniques have been used to investigate the pre- and post-passive region on iron in neutral solutions. In particular, a new ellipsometric transient technique allows the examination of the dissolution and film formation on iron, even though this is undergoing roughening by etching. Stationary states of the metal surface were measured by classical ellipsometry. Measurement of transient states depend on the variation of light intensity, when the initial offset angle of analyser and polarizer are respectively at an angle of 25, 0 and 90°. Potentiostatic and galvanostatic coulometry and capacitance measurements were applied. Oxide-free surfaces with visible unidirectional scratches gave different values of Δ and ψ if the position of the sample was changed with respect to the oncoming light; apparent Δ and ψ were dependent also on the degree of polishing. If the metal was held at potentials during which it dissolved in the active region for more than ca 7 min and i > 10 μA/cm2, Δ and ψ changed with time and the original values were not re-attained on reduction; the double-layer capacitance was increased by 2–3 times by such treatment. A sufficient degree of polishing gave Δ and ψ for oxide-free iron in solution, which showed refractive index and adsorption coefficient the same as those determined by others in vacuum. The corresponding metal was taken as an oxide-free optical reference state. Coulometry showed 1–2 monolayers of a ferrous oxide at the peak of the current/potential curve. Far from this peak on the positive side, the galvanostatic transients during reduction are in two sections which correspond to a charge of 1 : 2 in the charge reduced. At less positive post-peak regions coulometry allowed evaluation of the ratio Fe2+ : Fe3+ in the film. Δ/potential and ψ/potential relations showed three regions; pre-peak, peak to 0 mV(she) and 0–900 mV(she). Δ/potential and ψ/potential relations measured at short (1 s) and long (300 s) times are consistent. Δ/time measurements down to 10−2 show changes of Δ from the beginning of anodic polarization. Ellipsometric measurements are affected by roughness except at an asymptotic degree of polishing. Metal dissolution may also effect roughening to an extent which completes ellipsometrically with effects due to oxide growth. A transformation of a ferrous to a ferric oxide begins at the i/V peak and is complete at + 500 mV to the peak. Such transformations are controlled by potential, and not by field. The film before the peak is a ferrous oxide phase, probably Fe(OH)2. Its growth kinetics indicate it is three dimensional, and formed by nucleation of adsorbed OH−. The ferric film is γ-Fe2O3. Film thickness l, refractive index n, and absorption coefficient, k, can be indicated within certain limits as functions of potential from ellipsometry. Application of coulometric thickness measurements to allow solution of the ellipsometric equations shows a growth of ferrous oxide to the peak, a region during which the oxide is transformed to a ferric oxide and then linear growth with potential to 40 A. In the pre-peak region, iron dissolves from oxide-free sites to solution. The current peak corresponds, essentially, to full coverage of the surface with Fe(OH)2. The fall of current after the peak could arise from the complete coverage. It is accompanied by the beginning of conversion to Fe2O3. In the passive region Fe2O3 growth continues.