AES ANALYSIS OF PITS AND PASSIVE FILMS FORMED ON FE-CR FE-MO AND FE-CR-MO ALLOYS

A single pit was produced on binary iron-chromium (Fe17Cr), iron-molybdenum (Fe1Mo, Fe3Mo, and Fe5Mo) and ternary iron-chromium-molybdenum alloys (Fe17CrxMo, x = 1, 3, 7 w%) in 1 N H2SO4 with Cl- additions (0.02 to 0.08 molar) at room temperature. Compositional changes at the bottom of the pit and in the surrounding passive film were determined using high resolution Auger-Electron Spectroscopy. The chromium and molybdenum content of the passive layers are not changed in the presence of Cl- and there is no incorporation of chlorine in the passive film as long as the films were formed in a chlorine free electrolyte and Cl- was added later. At the bottom of the pits precipitated salt films could be determined after removing the electrolyte with filter paper. These salt films are strongly enriched in chromium, molybdenum and chlorine. The salt film was not detectable for FeCr after rinsing the samples with destilled water, whereas in the case of molybdenum additions the salt film could not be washed away with water. At the salt free bottom of pits in Fe17Cr we found an enrichment of chromium which is less than that in the passive film but similar to that determined for active corrosion. © 1990.