The influence of copper on corrosion and passivation of iron-chromium alloys. I. Electrochemical behaviour in sulphuric acid solutions

The kinetics of dissolution and passivation of alloys of the type FeCrX-CuY (X=5, 7, 9, 11%; Y=0, 0.5, 1%) was investigated in 1N H sub 2 SO sub 4 (pH 0.3) and 0.5M Na sub 2 SO sub 4 (pH 3), using rotating disc electrodes (RDE) and hydrodynamically modulated rotating ring disc electrodes (HMRRDE). Nonstationary current density-potential curves (10 mV/s) were measured. For both types of electrodes the transients of current density after pulse polarisation into the passive range were investigated. The self-activation of 1 h prepassivated electrodes (open circuit breakdown) was investigated by potential scans. The dissolution of Cu bearing samples led to the formation of a visible film of Cu lowering the current densities in the active and prepassive range as well as the critical current and charge densities of passivation. Copper accelerates the decline of current transients after pulse passivation (similar to chromium), whereas the partial current for film formation was increased as deduced from studies with the HMRRDE, leading to the conclusion of a larger film thickness. However, according to the accelerated open circuit breakdown of Cu bearing samples, films on these alloys are less stable than expected, which is related to a faster removal of the oxide film. The results demonstrate an ambivalent effect of Cu in the passive state of these alloys.