Electrodeposited nickel-molybdenum, nickel-tungsten, cobalt-molybdenum, and cobalt-tungsten were characterized for the hydrogen evolution reaction (HER) in the electrolysis of 30 w/o KOH alkaline water at 25~ The rate-determining step (rds) of the HER was suggested based on the Tafel slope of polarization and the capacitance of electrode-solution interface determined by ac impedance measurement. The HER on the nickel-and cobalt-based codeposits was enhanced significantly compared with that on the electrolytic nickel and cobalt with comparable deposit loadings. The decrease in the HER overpotential was more pronounced on the molybdenum-containing codeposits, particularly on cobalt-molybdenum which also showed a high stability. The enhancement of the HER was attributed to both the synergetic composition and the increased active surface of the codeposits. The real electrocatalytic activity of the electrodes and the effect of their surface increase were distinguished quantitatively. The linear relations between HER overpotential and surface roughness factor of the electrodes on a Y-log(X) plot were obtained experimentally and interpreted based on the Tafel law.

Because orits good radiopacity, superelasticity, and shape memory properties, nickel-titanium (NiTi) is II potential material for fabrication of stents because these properties Clln facilitate their implantation and precise positioning. However, ill vitro studies of NiTi alloys report the dependence of alloy biocompatibility and corrosiun behavior on surface conditions. Surface oxidation seems to be very promising for improving the corrosion resistancc and biocompatibility of NiTi. In this work, we studied the effect on corrosion resistancc and surface chanlcteristics of electropolishing, heat treatment, and nitric acid passivation of NiTi stents. Characteri zation techniques such as potentiodyna mic polarization tests, scanning electron microscopy, Auger electron spectroscopy, and X-ray photoelectron spectroscopy were used to relute corrosion behavior to surface ch.mlcteristics and surface treatments. Results show that all of these surface treatments improve the corrosion resistance of the alloy. This imprOl'ement is attributed to the plastically deformed nath'e oxide layer removul und replacement by a newly grown, more uniform one. The uniformity of the oxide layer, rather than its thickness and composition, seems to he the predominant factor to explain the corrosion resistance improvement. 0 t998 John Wiley & Son~. Inc.

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