ON THE ATMOSPHERIC CORROSION OF METALS WHICH ARE COVERED WITH THIN ELECTROLYTE LAYERS .2. EXPERIMENTAL RESULTS

In this study, the corrosion properties of iron and the alloys Fe-0.5Cu and Fe-3.4Cu are investigated during several wet/dry transitions. The corrosion potential is measured with a Kelvin probe without touching the surface under investigation, the corrosion rate is followed by the decay of the partial-pressure of oxygen in a closed volume. It could be shown that the corrosion rate of pure iron during drying is determined by three competing reactions: the acceleration of the diffusion limited oxygen reduction in the beginning, the passivation of the surface as drying continues and the decrease of the rate of the oxygen reduction during the last stage of drying. The latter process is coupled to the re-oxidation of the previously reduced Fe3+ and therefore with Fe2+ enriched oxyhydroxides, as oxygen is reduced on these Fe2+ doped semiconducting iron oxyhydroxides. In opposite to pure iron the two FeCu alloys show low corrosion rates during drying, but the reason of this is quite different for both alloys. The alloy Fe-0.5Cu shows rather negative corrosion potentials during drying, which is interpreted by a slow kinetics of the oxygen reduction on the oxide scale formed on this alloy. In contrast to this, the corrosion potentials of the alloy Fe-3.4Cu are shifted anodically after several wet/dry cycles, the surface being passivated. No activation is observed, even if the surface is covered by thick electrolyte layers. Activation occurs only, if again SO2 is adsorbed onto the metal surface. © 1990.