The Crevice Protection Potential for 90-10 Copper Nickel

The potential and pH variations for a simulated "crevice” on 90-10 Cu-Ni in 0.1M NaCl at pH 10.3 were determined for a range of external potentials, and related to the experimental potential-pH diagram for the alloy in 0.1 M NaCl. The range of values of steady state potential and pH for "crevice” samples undergoes a discontinuity at an external potential of 0.140 ±0.010 volts (SHE). The electrode potential at which this discontinuity occurs has been called the "crevice protection potential”. At potentials more noble than the crevice protection potential, the metal in the crevice corrodes actively, and the occluded cell electrolyte becomes more acid. At more active potentials, the metal in the crevice passivates, and the occluded cell electrolyte becomes more alkaline. This crevice protection potential corresponds closely to the potential of the intersection of the "corrosion region” on the experimental potential-pH diagram and the locus of the primary passivation potential as a function of pH for the alloy. Cautiously, this concept might be applied to all copper base alloys exhibiting "copper type” potential-pH diagrams.