AN ELLIPSOMETRIC STUDY OF THE EFFECT OF PHOSPHATE ANIONS IN BORATE SOLUTION ON ANODIC FILMS GROWN ON IRON

Corrosion and film growth on Fe were examined at 25-degrees-C in a de-aerated borate solution of pH 8.5 with an addition of 0.01 M and 0.1 M Na2HPO4. In the active-passive transition region corrosion and film growth were retarded by the small addition of Na2HPO4, but accelerated by the higher addition. In the passive region corrosion and film growth increased with the increasing Na2HPO4 concentration. Optical constants of anodic films in the passive region were lower in the presence of Na2HPO4, suggesting incorporation of iron phosphates and an increased content of hydroxides or oxyhydroxides. The inhibiting effect of Na2HPO4 may result from better compactness of hydroxide and oxyhydroxide films owning to codeposition of iron phosphates, whereas its deteriorating effect can be due to defects introduced by the phosphates into the passive oxide film.