Effect of Flow on Corrosion of Copper-Nickel Alloys in Aerated Sea Water and in Sulfide-Polluted Sea Water

The corrosion of 90:10 Cu:Ni and 70:30 Cu:Ni alloys in aerated sea water and deaerated sea water containing ~0.2 g/m³ sulfide has been studied using a recirculating flow loop. Tubular specimens of each alloy were tested under well characterized hydrodynamic and mass transfer conditions. At sea water velocities up to 5 m/s (Reynolds numbers up to 74,000), the corrosion rates in the sulfide polluted sea water were lower than in the aerated sea water. Thus, even under comparatively adverse flow conditions, the mere presence of sulfide does not cause accelerated attack of copper-nickel alloys. These results can be compared with those of other workers who found that when both sulfide and oxygen were present in flowing sea water, the rates of corrosion of copper-nickel alloys were higher than those obtained in unpolluted aerated sea water. Under most of the experimental conditions examined, corrosion occurred fairly uniformly across the exposed surfaces of the specimens. However, in aerated sea water, the high nickel alloy suffered localized corrosion at 3 m/s and higher velocities. The rate of localized corrosion was high and became more intense as the velocity was increased from 3 to 5 m/s. This breakaway phenomenon was associated with a breakaway potential (E_B) which is defined as the potential at which a sudden increase in anodic current occurs when the potential is swept in the active to noble direction. When the corrosion potential (E_corr) was more active than E_B, corrosion was uniform and corrosion rates were low; but when E_corr>E_B, localized corrosion of the 70:30 Cu:Ni occurred. A working model is advanced for this breakaway corrosion phenomenon in which the results are interpreted in terms of the electrochemical (not mechanical) stability of the surface film.