Impedance Analysis on Initiation and Growth of Ant-Nest Corrosion of Copper

Abstract

The branched pitting corrosion often occurs on copper tubes used in heat-exchanger units in air-conditioners and leads to leakage of heat-exchanger fluid for several months. Figure 1 shows a photograph of this unique localized corrosion formed by galvanostatic polarization in the laboratory. The morphology is characterized by the fractal structure in which pits of several ten micro-m diameter are formed at copper surface and smaller pores are developed on the pit walls randomly. This localized corrosion has been reported with some names: ant-nest corrosion, formicary corrosion, unusual corrosion, branched pits and pinhole corrosion [1]. It’s well known that the occurrence of this localized corrosion is related to trace phosphorus in copper and presence of low molecular weight carboxylic acid such as formic acid. However, the origin to form branched structure has not been clarified. We reported an impedance method to analyze the branched (fractal) pore electrode with transmission line model (TLM) [2, 3]. Impedance spectrum of simple cylinder pore electrode on Nyquist plane normally describes straight-line of 45 degree slope in the high frequency range. In the case of electrode with fractal pores, the slope of straight-line decreases depending on the fractal dimension [3]. In this study, we employed this impedance analysis to investigate the initiation and growth of branched pores in ant-nest corrosion of copper.