THE PASSIVATION OF IRON AND IRON CHROMIUM-ALLOYS IN ACID AND NEUTRAL SOLUTION

The pH-dependence of the passivation of Fe, Fe10Cr and Fe20Cr was investigated in borate buffer solution (pH 8.4) and H2SO4 (pH 0.3) under potentiostatic conditions. The influence of the pH is due to the pH-dependence of the potential of the oxide electrode and the formation of CrO4(2-) which results in additional surface charge. The potential drop within the Helmholtz layer will be enlarged and a higher corrosion rate takes place. In the passive range 0.2 V less-than-or-equal-to U less-than-or-equal-to 1.2 V the layer properties are independent of pH. The rise of the potential drop at the phase boundary yields an increase of the passive corrosion current density. In the range of transpassive dissolution of Cr (1.2 V < U less-than-or-equal-to 1.7 V) the layer dissolution increases with decreasing pH. The start of the oxide growth occurs earlier due to a thinning of the initial layer in acid media. The transpassive Cr-dissolution out of the stationary oxide film depends upon the transport through the Fe-oxide matrix. It increases with decreasing pH due to the higher corrosion rate of the Fe-oxide and the resulting greater amount of Cr-ions at the surface.