IMPEDANCE AND TRANSIENT STUDY OF IRON AND IRON PHOSPHORUS ALLOY IN CARBONATE BICARBONATE SOLUTION WITH PHOSPHATE ANIONS

Anodic oxidation of iron and Fe-1.82% P was studied at 75-degrees-C in 0.5 M Na2CO3 + 1.0 M NaHCO3 with the addition of 0.5 M and 2.0 M Na2HPO4. Polarization curves and potentiostatic transients showed that anodic currents for the Fe-P alloy were higher than for iron. The charge of the active dissolution preceding the onset of passivation was, for the Fe-P alloy, about four times bigger than for iron, whereas current decays following the start of the passivation were slower. Addition of 2.0 M Na2HPO4 resulted in an anodic activation of iron similar to that caused by phosphorus, demonstrated by higher current of polarization curves, higher initial currents at potentials of the passive region, slower current decays and higher interfacial capacitance C(if) in the active-passive transition region. High C(if), reaching 60 F m-2 at the potential of -0.65 V (sce), can be ascribed to redox reactions involving ferrous phosphate and Fe(III) species. It is suggested that the effect of phosphorus on the anodic behavior of iron is associated mainly with the formation of phosphates as a result of oxidation of phosphorus on the metal surface.