Scale formation on polypropylene and copper tubes in mildly supersaturated tap water

This paper presents an experimental study of scaling of polypropylene and copper tubes exposed to flowing tap water over an eight week period. The study was motivated by the recent interest in switching from copper to polymeric materials in heat exchangers and absorbers used in solar water heaters. The chemistry of the tap water is controlled to provide mildly supersaturated conditions at pH equal to 8 and 40 °C. Supersaturation is 5.0 with respect to calcite and 7.9 with respect to hydroxyapatite. These experimental conditions are a departure from the common practice of accelerated testing with distilled water highly saturated with respect to calcium carbonate and thus better represent conditions expected in solar systems. The microstructure and composition of the scale as it grows are examined with scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and ion chromatography. The results show scaling in tap water is slower than scaling in distilled or deionized water with a single supersaturated component. In the case of copper, prolonged exposure to tap water oxidizes the surface of the tube and subsequently affects scaling. The oxidized copper surface is more susceptible to calcium phosphate scaling and this tendency results in the higher scaling rate for copper than for polypropylene in water containing phosphate. The scaling rates for polypropylene and copper tubes are 0.8 × 10^-10 and 1.7 × 10^-10 (mol Ca)/(m²s), respectively.