Abstract
The tendency of pitting corrosion on copper tubes of heat exchangers was investigated by analyzing the quality of cooling water sampled from air-conditioning units at different cities in Japan. A two-dimensional diagram (2DD) was constructed with two parameters chosen from water quality conditions, i.e., total hardness, acid consumption, pH, concentrations of SO42― and Cl―. The results of 2DD analyses demonstrated that the acid consumption was strongly related to the susceptibility of copper pitting corrosion. In order to make a diagnosis of copper pitting corrosion, we proposed a three-dimensional diagram (3DD). The axes of 3DD were the acid consumption, pH and ratio of amount of Cl― + SO42― ions to total anions (HCO3―, Cl― and SO42―). This diagram indicated that the cooling water sampled from air-conditioning units could be divided into high and low pitting corrosion risk categories precisely. In addition, the simulated cooling water was prepared in the laboratory to assist the effectiveness of the proposed 3DD analysis. The anodic polarization curves of copper electrode were measured in the simulated cooling water as the accelerated test for pitting corrosion susceptibility. The 3DD analysis was applied to the discrimination of corrosive properties of simulated cooling water after accelerated test. The surface morphology of copper electrode was analyzed by the optical microscope. The results of 3DD analyses were in good agreement with those of surface morphologies of copper electrode after anodic polarization curve measurement. These results implied that 3DD allowed the screening of cooling water related to the risk of pitting corrosion on copper tubes of heat exchangers.
