Effect of Tantalum on Corrosion Resistance of Ni–Nb(–Ta)–Ti–Zr Glassy Alloys at High Temperature

Ni60Nb20� xTaxTi15Zr5 (x ¼ 0, 5, 15 and 20 at%) glassy alloys with high thermal stability and high corrosion resistance have been synthesized by the melt spinning technique. The substitution of tantalum for a portion of niobium in the Ni60Nb20Ti15Zr5 glassy alloy causes a significant increase in the supercooled liquid region (� Tx ¼ TxTg) from 52 K at 0 at% Ta to 67 K at 20 at% Ta accompanied by an increase in Tg and Tx. The glassy alloys are spontaneously passivated in a wide passive region and at low passive current densities of the order of 10 � 2 Am � 2 in pH 2 H2SO4 and pH 2 H2SO4 + 500 ppm NaCl solutions at a high temperature of 353 K. On the other hand, in pH 2 H2SO4 + 500 ppm NaF solution at 353 K, the alloys without Ta and with 5 at% Ta suffer general corrosion, while the passive current densities decrease by about two orders of magnitude with the addition of 15-20 at% Ta to the Ni-Nb-Ti-Zr glassy alloy. It indicates that the effect of the addition of tantalum is more pronounced for improving the corrosion resistance in fluoride containing solutions than the other two types of solutions.