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 Na2H PO4. Polarization curves and potentiostatic transients showed that ano dic 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 o f 2.0 M Na2HPO4 resulted in an anodic activation of iron similar to th at caused by phosphorus, demonstrated by higher current of polarizatio n 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 th e potential of -0.65 V (sce), can be ascribed to redox reactions invol ving ferrous phosphate and Fe(III) species. It is suggested that the e ffect of phosphorus on the anodic behavior of iron is associated mainl y with the formation of phosphates as a result of oxidation of phospho rus on the metal surface.