ON THE MECHANISM FOR IMPROVED PASSIVATION BY ADDITION OF MOLYBDENUM TO AUSTENITIC STAINLESS-STEELS IN O-PHOSPHORIC ACID

In order to clarify the mechanism for increased corrosion resistance in O-phosphoric acid solutions by addition of molybdenum to 17Cr-13Ni stainless steel, anodic curves polarization and electron spectroscopy for chemical analysis (ESCA) have been performed on the Mo-containing steels passivated in 40 wt% H//3PO//4 with chlorine and fluoride ions as impurities. The addition of Mo to austenitic SS affects the nature and composition of the passive film. The studies by ESCA have shown that the passive film is formed in an overlap of two layers. The external layer contains phosphates and is rich in MO(VI) and Si(IV). The improved resistance of SS alloyed with Mo is caused by that layer probably composed of a silico-molybdic-hydrate complex which is particularly stable in acidic pH. The similarity of results obtained between Mo-alloyed steels and steels without Mo but passivated in the MoO//4**(**2** minus **) solutions suggest that the protective layer is formed by soluble Mo compounds.