IMPEDANCE STUDY OF EFFECT OF PHOSPHORUS ON ANODIC BEHAVIOR OF IRON INCARBONATE 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 c apacitance (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 pote ntial increased, reaching values up to about 100 F m-2 in the active-p assive transition region. Lower values of C(if) were roughly proportio nal to R(ct)-1 whereas higher values of C(if) (above about 10 F m-2) o ccurred at -0.70 or -0.65 V (sce) when R(ct)-1 was decreasing. It is s uggested that the high values of C(if) are associated mainly with reve rse electrochemical reactions, involving Fe(II) and Fe(III) species in corrosion products. The effect of phosphorus on anodic oxidation of i ron can be explained by a decreased protectiveness of surface layers.