Ni-NiMo<SUB>2</SUB>B<SUB>2</SUB> 2相合金のHNO<SUB>3</SUB>水溶液中での耐食性に及ぼすCr添加の効果

Abstract

The effect of addition of Cr on the corrosion resistance of a Ni-NiMo2B2 two-phase alloy in 6%HNO3 aqueous solution was examined by measuring the mass loss, the corrosion potential and the anode polarization curve, and also by metallography. When the content of Cr in the alloy was 15 mass%, Cr was distributed evenly to the both phases of the Ni matrix and boride NiMo2B2, and consequently the chemical composition of Ni matrix became Ni-15 mass%Mo-15 mass%Cr while the boride phase changed from the orthorhombic boride NiMo2B2 to the tetragonal boride (Ni, Mo, Cr)3B2. The corrosion resistance of the Cr-containing Ni-(Ni, Mo, Cr)3B2 two-phase alloy in the 6%HNO3 aquoeus solution was about 10 times superior to the Ni-(Ni, Mo, Cr)3B2 two-phase alloy which does not contain chromium. The corrosion of the Cr-free Ni-NiMo2B2 two-phase alloy proceeded by a mechanism in which the Ni matrix worked as the anode while the boride NiMo2B2 phase worked as the cathode. On the other hand, in the Cr-containing Ni-(Ni, Mo, Cr)3B2 two-phase alloy, the Ni matrix became passivated, and as a result, its corrosion potential became more noble than that of the boride (Ni, Mo, Cr)3B2. Consequently, the corrosion of the Ni-(Ni, Mo, Cr)3B2 two-phase alloy proceeded through the preferential corrosion of the boride (Ni, Mo, Cr)3B2. Furthermore, the corrosion current density of the boride (Ni, Mo, Cr)3B2 was found to be as small as about 0.2 A· m−2 from the polarization curve. Also, the difference in the corrosion potential between the Cr-containing Ni matrix and (Ni, Mo, Cr)3B2 was 60 mV smaller than that between the Ni matrix and NiMo2B2 in the Ni-NiMo2B2 two-phase alloy. Therefore, the driving force for the local cell corrosion of the Cr-containing alloy was reduced by the addition of Cr. That is to say, the lower corrosion current density of the boride (Ni, Mo, Cr)3B2 and the smaller corrosion potential difference improved remarkably the corrosion resistance of the Ni-(Ni, Mo, Cr)3B2 two-phase alloy.