Corrosion Behavior of Amorphous Nickel-Valve Metal Alloys in Boiling Concentrated Nitric and Hydrochloric Acids

The corrosion behavior of amorphous nickel-base alloys containing titanium, zirconium, niobium, tantalum and/or phosphorus in boiling 9 N HNO_3 solutions with and without Cr^ ion and in a boiling 6 N HCl solution was investigated. In boiling 9 N HNO_3 solutions alloys containing 20 at% or more tantalum were immune to corrosion, maintaining the metallic luster, and Ni-40～60Nb alloys showed low corrosion rates of the order of μm/year. In the boiling 6 N HCl solution only tantalum-containing alloys were immune to corrosion although a higher tantalum content was required in the 6 N HCl solution in comparison with 9 N HNO_3 solutions. Protective surface films on the Ni-Nb and Ni-Ta-(P) alloys were composed exclusively of NbO_2(OH) and TaO_2(OH). Consequently, the corrosion resistance is not provided unless alloys contain elements, the passive film of which is highly stable in these aggressive boiling acids, such as tantalum