IMPEDANCE STUDY OF EFFECT OF PHOSPHORUS ON ANODIC BEHAVIOR OF IRON IN CARBONATE BICARBONATE SOLUTIONS

Anodic behavior of iron and Fe-1.82%P was studied in 0.5 M Na2CO3 + 1.0 M NaHCO3 at 75-degrees-C with the use of ac impedance and dc methods. In the active and passive regions, anodic currents and interfacial capacitance (C(if)) of the Fe-P alloy were higher than those for iron, and the charge transfer resistance (R(ct)) was lower. Rotation of the disc electrodes resulted in an increase of anodic currents and C(if), and in a decrease of R(ct). C(if) passed through a maximum as the potential increased, reaching values up to about 100 F m-2 in the active-passive transition region. Lower values of C(if) were roughly proportional to R(ct)-1 whereas higher values of C(if) (above about 10 F m-2) occurred at -0.70 or -0.65 V (sce) when R(ct)-1 was decreasing. It is suggested that the high values of C(if) are associated mainly with reverse electrochemical reactions, involving Fe(II) and Fe(III) species in corrosion products. The effect of phosphorus on anodic oxidation of iron can be explained by a decreased protectiveness of surface layers.