Polarization Behavior of Pd₇₈Si₁₆Cu_6-xCr_x Amorphous Alloys in an Artificial Body Fluid

Ribbons of amorphous alloys, Pd₇₈Si₁₆Cu_{6 x}Cr_x (x = 0, 2, 4 and 6 mol%), were polarized in an artificial body fluid, deaerated PBS ( - ), as the electrolyte to estimate the performance of the alloys as biomaterials with the intention of developing new high corrosion resistant amorphous alloys. Three kinds of surface conditions of amorphous alloys were prepared: as-received (by single roller melt spinning method), polished and just-polished (within 300 s after polishing). Polished crystalline alloys were also polarized as controls. I_corr (corrosion current density) and I_crit (critical passive current density) of the as-received amorphous alloys and the polished crystalline alloys decreased with increasing chromium content. Corrosion resistance of these alloys was improved by the addition of chromium. I_corr, I_crit and I_ps (passivation current density) of the polished amorphous alloys were lower than those of the polished crystalline alloys and these values of the former alloys were independent of chromium content. This indicates that corrosion resistance of the polished amorphous alloys is higher in the artificial body fluid than that of the polished crystalline alloys. To determine the cathodic reactant on the as-received amorphous alloys in deaerated PBS ( - ), one of the alloys was polarized in deaerated 2.5 kmol%H₂SO₄. The cathodic Tafel slope in PBS( - ) was the same as that in H₂SO₄, indicating that the cathodic reactant in PBS ( - ) was identified as hydrogen ion. In addition, polarization curves of pure palladium, pure copper and pure chromium were examined to determine the anodic reaction. E_corr (corrosion potential) of the pure palladium was similar to those of the alloys, suggesting that palladium ion is preferentially released from the alloys under the immersion in deaerated PBS ( - ). I_corr, and I_crit of the polished amorphous alloys were lower than those of the as-received amorphous alloys. To discuss the improvement in corrosion resistance by polishing, just-polished alloys were also polarized. The curves of the as-received and the just-polished alloys contained negative loop except the polished alloys. I_corr and I_crit of the just-polished alloys were slightly larger than those of the polished alloys. These results indicate that the surface morphology of the amorphous alloys changed during the exposure to air, causing the increase in corrosion resistance.