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ABSTRACT
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 [asymptotically =]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.
KEY WORDS: aqueous environments, deposition corrosion, dissolved-copper effect, drinking water, galvanized steel, localized corrosion, pitting, premise plumbing, scanning electron microscopy, water mains
INTRODUCTION
Corrosion of drinking water distribution systems can cost water utilities and homeowners tens of billions of dollars each year in infrastructure damage, adversely impact public health, and cause water loss through leaks.1 The corrosion of iron is particularly costly because the majority of water mains currently in service are composed of iron and its alloys.2-3 Despite early reports that the presence of dissolved copper could dramatically increase the rate of failure for iron and galvanized steel, this "dissolved copper effect," or deposition corrosion, has received little recent attention.4-7 Mechanistically, plating of copper ions (e.g., Cu[I], Cu[II]) onto an iron pipe surface forms microgalvanic cells, dramatically accelerating corrosive attack and causing rapid pipe failures.4-5 If enough copper "islands" are present, an overall acceleration of the corrosion rate may be observed electrochemically or by depletion of chlorine/oxygen in water resulting from cathodic reactions. Although deposition corrosion has been acknowledged as "disastrous" in the field,4 laboratory work has not always been successful in reproducing the phenomenon, limiting the development of remedial measures.
Sources of Upstream Copper
In spite of predicted damage to iron infrastructure from dissolved copper in water, a...