The transpassive behaviour of the anodic film on Fe-Cr alloys.
Tonkinson, Charles Henry Llewelyn.
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This work was undertaken to investigate the transpassive behaviour of the anodic film on two Fe-Cr alloys, namely Fe18Cr and Fe18Cr2Mo in acidic aqueous media in the pH range 0.5 to 3.8. Two electrochemical techniques were used, namely cyclic voltammetry and chronoamperometry. The two primary experimental variables in the cyclic voltammetric experiments were pH and sweep rate (2 - 800 mV/s). The main variables in the chronoamperometric experiments were the size of the potential step, the number of potential steps and the starting and ending potentials. Secondary experimental variables were temperature (25, 90°C), rotation rate (0, 150 rad/s), and the artificial addition of cations (Fe2+, Fe3+ and Cr3+) to some of the solutions. A voltammetric anodic peak, referred to as peak A, occurs in the transpassive region of the above Fe-Cr alloys, followed by a region of secondary passivity and then oxygen evolution. It was this peak that was investigated by cyclic voltammetric methods. The peak A current response was independent of rotation rate at pH 3.8 but was dependent on rotation rate at pH 0.5 with solutions of intermediate pH showing a gradual change in rotation rate dependence. This indicated a predominantly solid state process in less acidic solutions (pH 2.4 & 3.8) whereas in strongly acidic solutions (pH 0.5) the action of ions in solution must contribute significantly towards peak A processes. A method was developed to correct the peak A current response for the current due to oxygen evolution. The results of this method indirectly confirmed the hypothesis that more than one oxidation process contributes to the peak A current response. A diagnostic plot for diffusion control was applied to the peak height of peak A. The diagnostic involves plotting the peak height over the square root of the sweep rate versus the square root of the sweep rate. A process under diffusion control would give a horizontal line for this diagnostic plot. At pH 0.5 and at slow sweep rates (less than or equal to 60 mV/s) the diagnostic plot gave a positive deviation from the horizontal and this deviation was enhanced as the temperature was increased. As the pH was increased (towards pH 3.8), the deviation from the horizontal at slow sweep rates gradually became negative and this deviation was again enhanced when the temperature was increased. This phenomenon is explained in terms of the role of the hydronium ion. From the addition of Fe2+, Fe3+, and Cr3+ to pH 0.5 and pH 3.8 solutions it was noted that ferrous ions increased the peak A current response more than chromic ions of the same concentration. Ferric ions slightly decreased the peak A current response. Based on these results, reports in the literature, and the apparent role of the hydronium ion, a partial scheme was proposed in order to explain the role of Fe and Cr, from the alloy substrate, in the anodic film in the transpassive region. In chronoamperometric experiments, stepping to the transpassive region confirmed the phenomenon of the rising transient. A quantitative nucleation model - which was based on previous models from the literature - was generated. The model was successfully fitted to two rising transients, one from the pH 3.8, and the other from the pH 0.5 solution. The model also allows for the presence of a pre-existent laver at the starting potential of a chronoamperometric experiment after the electrochemical cleaning procedure. The model incorporates both diffusion controlled and charge transfer controlled steps. A key concept in the model is that of nucleation and "slow death" of corrosion pits growing into the electrode. "Death" of a pit occurs when it is covered by a nucleating and or growing passivating film. The rising transients were only obtained on Fe-Cr alloys (with one exception) when stepping to the transpassive region and also only in solutions where peak A was obtained in a cyclic voltammetric experiment. The exception to this was that in the pH 0.5 solution and at 90°C, rising transients were obtained when stepping to the passive region. This did not occur at 25°C. Rising transients were also obtained on pure iron when stepping to the passive region. In addition to the rising transient, a reverse rising transient was discovered. This reverse rising transient (which generated a cathodic current) was obtained when stepping the potential cathodically from the transpassive region. It was shown that the occurrence of the reverse rising transient was dependent on the presence of a stable, transpassive anodic film before the potential step. One indirect result from the discovery of the reverse rising transient was that it indicates that secondary passivity exists at least 200 mV into the oxygen evolution region.