Corrosion of Cu-Ni Alloys in Sulfide-Polluted Seawater

A study of the corrosion of copper-nickel (Cu-Ni) alloys showed hydrodynamic conditions increased the corrosion rate (I_corr) significantly by reducing the sulfide concentration polarization. Pre-exposure to oxidation products of dissolved sulfide and elemental sulfur (S) increased I_corr of the alloys in seawater significantly. Pre-exposure to nitrite and chromate decreased I_corr by almost 2 orders of magnitude for alloys of 90% Cu-10% Ni and 70% Cu-30% Ni (UNS C70600 and UNS C71500, respectively) However, cyclic polarization curves for both alloys pre-exposed to chromate and nitrite showed the passivators caused severe localized corrosion for the 70% Cu-30% Ni alloy only by transition from the active to passive state. The corrosion mechanism of the 90% Cu-10% Ni alloy differed from that of the 70% Cu-30% Ni alloy in quiescent sulfide-polluted seawater. A four-part corrosion mechanism was proposed. The rate of sulfide ion diffusion to the alloy-solution interface always was the rate determining step for corrosion of the alloy of higher Ni content, whereas different kinetics of the reaction determined I_corr of the second alloy.