CHEMICAL-COMPOSITION, CHEMICAL-STATES, AND RESISTANCE TO LOCALIZED CORROSION OF PASSIVE FILMS ON AN FE-17-PERCENT-CR ALLOY

The resistance to pitting of an Fe-17%Cr alloy in neutral chloride sol ution was studied, as a function of chloride concentration and of pola rization time (aging), using combined electrochemical measurements and XPS analyses. Potentiodynamic experiments (at 1 V/h) show that pittin g occurs at chloride concentrations above 5 X 10(3)M. Potentiostatic p olarizations of the alloy in the passive region were performed in a 2 X 10(-2)M chloride solution (pH 6), at +390 mV/SHE (in the passive reg ion), for different times (5 min, 1 h, and 27 h). Prolonged polarizati on induces better protective properties of this film (lower residual c urrent) and better resistance to pitting (higher pitting potential). X PS analyses of the passive films show that these properties are relate d to transformations of the passive layer with time. The increased pit ting resistance observed is attributed to chromium enrichment in the p assive layer and a thickening of the inner oxide layer of the passive film. Chloride ions are not incorporated in the oxide layer, but are p resent in the outer layer of hydroxide.