Mechanism of Sulfide-Accelerated Corrosion of Copper-Nickel (90-10) Alloy in Seawater

摘要

The mechanism of sulfide-induced accelerated corrosion of 90-10 copper-nickel (iron) alloy is investigated. Samples of the alloy are exposed to flowing (2.4 meters per second) seawater, with and without 0.01 milligrams per liter sulfide, for various periods of time. The resulting surfaces are examined by means of Auger electron spectroscopy coupled with inert-ion-bombardment. A detailed depth profile is thereby obtained of concentrations in the surface region of a total of nine elements. The results are consistent with the hypothesis that iron hydroxide segregates at the surface to form a protective gelatinous layer against the normal chloride-induced corrosion process. Trace sulfide interferes with formation of a good protective layer and leaves the iron hydroxide vulnerable to ultimate partial or complete debonding. When the alloy is first exposed to pure seawater for a prolonged period of time, however, subsequent exposure to sulfide is no longer deleterious. This is apparently due to a layer of copper-nickel salt that slowly forms over the iron hydroxide.