The dissolution and passivation of Fe and FeCr alloys in acidified sulphate medium: Influences of pH and Cr content

The steady state polarization curves of ferritic FeCr alloys, containing 7–12% Cr and immersed in 1M H₂SO₄, exhibit two current maxima, i.e. two passivating processes take place at the electrode interface within two different voltage ranges. The origin of these two passivation phenomena was investigated by plotting the steady state polarization curves and also by measuring the electrode impedance along with different polarization points. These results are then compared to those obtained with Fe electrode immersed in sulphate solutions of various pH value (0–5) since the steady state polarization curves of this electrode exhibit also two current maxima in a weakly acid medium, i.e. at solution pH greater than 4. However, even in a strongly acid solution in which only one current maximum is observed for the Fe electrode, the electrode impedance showed two passivation processes are occurring at the electrode interface. Therefore, the Fe dissolution involves at least two dissolution paths and the current at which one dissolution path overtakes the other is dependent upon the solution pH. The impedance diagrams of FeCr alloy electrode are similar to those of Fe electrode especially when the comparison is made with the Fe electrode immersed in a less acid solution, i.e. the addition of Cr in Fe enhances the appearance of two passivation phenomena with increase of solution pH.