Deposition Corrosion of Galvanized Steel in the Presence of Copper

Deposition corrosion, via formation of microgalvanic cells from copper ions (e.g., Cu[I], Cu[II]) on iron or galvanized (zinc-coated) steel pipe, has been linked to disastrous field corrosion failures. Key factors expected to control deposition corrosion, including soluble copper concentration, copper ion speciation, and flow pattern (stagnant versus recirculating), were examined. The mass of copper plating was directly proportional to the soluble copper concentration in solution. The presence of flow, which allowed greater mass transport of reactants to the pipe surface, proved to be crucial to replicating deposition corrosion in the laboratory: tests with flow demonstrated up to 7× more zinc release and 55× more iron release when copper was present than when it was absent, compared to increases of only ≈2× in the same water under stagnant conditions. Scale dissolution, x-ray fluorescence, and scanning electron microscopy were used to characterize copper-rich deposits on the surface of both field and laboratory samples that were consistent with metallic copper, supporting a deposition corrosion mechanism.