XPS CHARACTERIZATION OF THE CORROSION FILMS FORMED ON NANOCRYSTALLINENI-P ALLOYS IN SULFURIC-ACID

X-ray photoelectron spectroscopy (XPS) was used to examine the corrosi on films formed on nanocrystalline (nc) and amorphous Ni-P alloys in 0 .1 M H2SO4. Neither the nc nor the amorphous alloys were found to exhi bit passivity. An enrichment of elemental P compared to Ni was observe d at the surface of both materials, suggesting that Ni is preferential ly dissolved during anodic polarization. The high volume fraction of g rain boundaries and triple junctions on the nc specimens resulted in e nhanced dissociative adsorption of oxygen and hydroxyl species from so lution, The films formed were, however, non-protective because the def ective nc surfaces also facilitated atom dissolution and oxidation of surface P atoms from hypophosphite to soluble phosphate anions. At hig her applied potentials, the thick, porous films formed on the nc speci mens provided a small kinetic barrier to further dissolution, resultin g in slightly lowered anodic current densities compared with amorphous Ni-P.