The nature and composition of the passive film formed on Ni-P-Cr metallic glasses

The presence of both Cr and P in transistion-metal metalloid glasses has been observed to promote the development of very stable, protective passive films. In order to investigate the interaction of Cr and P in the formation of passive films four alloy systems where chosen for study; Ni-20P, Ni-20P-3Cr, Ni-20P-6Cr and Ni-20P-10Cr. All the alloys were prepared by melt spinning and the glassy structure was confirmed by X-ray and electron diffraction, and by transmission electron microscopy. The authors have examined the anodic behavior and the composition of the passive film formed on these alloys in 0.2N HCl (pH=0.7) and 0.2N H/sub 2/SO/sub 4/ (pH=1.2) using electrochemical techniques and surface analysis (XPS and AES). A very interesting property of the glassy alloys containing P or P and Cr together is their insensitivity to Cl ion attack. The authors have speculated that this may be due to a cation selective effect which is induced by phosphate anions in the outermost portion of the passive film. Increasing Cr additions enhances this effect. The alloys exhibit wider passive potential regions and lower passive current densities as the Cr content is increased. A second interesting property of the Ni-P-Cr alloys is the composition and chemical make-up of the passive film. XPS results obtained from the surfaces of the polish-formed films and passive films on the glassy alloys show the polish-formed films to contain Cr/sub 2/O/sub 3/, which is considered by many to be the main passivating species of stainless steels