Rapid Pitting of Copper Tube from Deposits of Cathodic Manganese in Hot Water Systems: Mechanism and Effects of Water Chemistry

Abstract

A large building with water frequently above 65°C experienced at least 300 leaks in a hot water recirculating line. The leaks were unusual, as they occurred along the very top of horizontal pipes that were heavily coated with iron and manganese sediment on the bottom half of the pipe. This etiology of the leaks is inconsistent with under-deposit corrosion but consistent with a previously hypothesized mechanism of chemistry-related hot water pitting. We verify this mechanism for the first time by: (1) developing a macrocell setup to test the proposed anodic and cathodic reaction and (2) determining how water chemistry can promote or inhibit this form of nonuniform corrosion. The macrocell demonstrated that copper or stainless steel coated with manganese oxides were highly cathodic relative to an anodic exposed copper surface. Pitting current densities as high as 200 µA/cm2 were found in the worst-case conditions at pH 8.2 in waters with a high ratio of sulfate to bicarbonate. High levels of silica inhibited pitting corrosion currents, but orthophosphate inhibitors did not. No iron oxide coatings tested in this work had cathodic tendencies.