EFFECT OF OXYANIONS OF PHOSPHORUS ON THE ANODIC BEHAVIOR OF IRON AND IRON PLUS PHOSPHORUS ALLOY IN BORATE SOLUTION

Electrochemical and ellipsometric investigations of anodic oxidation a nd film growth on Fe and Fe + 1.1 wt.%P have been performed at 25-degr ees-C in a deaerated borate solution (pH 8.5) with additions of sodium hypophosphite (NaH2PO2), potassium phosphite (K2HPO3) and sodium phos phate (Na2HPO4) at concentrations between 0.01 M and 0.5 M. In the act ive-passive transition region anodic oxidation of Fe and the Fe + P al loy was retarded by small additions of Na2HPO4 (up to 0.05 M) but was accelerated by larger additions and also by the hypophosphite and phos phite salts at all concentrations. Addition of NaH2-PO2 caused pitting of the Fe + P alloy. In the active-passive region two-layer films gre w on the metal surface; the optical constants of inner layers were low er in the presence of P oxyanions, indicating a lower film density. In the presence of the oxyanions the films were usually thicker. The inh ibiting effect of phosphate anions at low concentrations may result fr om pore blocking in the film, whereas the activating effect of all the oxyanions may be due to the formation of soluble complexes, incorpora tion of P-containing salts and/or an increased water content in the pa ssivating oxide film.