Copper corrosion in mildly alkaline water with the disinfectant monochloramine

Abstract

The corrosion of pure copper for a period of 2-30 days was investigated in simulated tap water containing monochloramine (normally at 4.0 mg/1) in the pH range of 7.6-8.4, using electrochemical impedance spectroscopy, potentiodynamic scan, X-ray photoelectron spectroscopy (XPS), inductively coupled plasma and scanning electronic microscopy (SEM). The XPS spectra and SEM indicated that after 8 days of immersion, the copper surface was covered by both cupric and cuprous oxides. Diffusion in the oxide film was found having a significant effect on the corrosion process. The results showed that at pH 8.0 the polarization resistance (Rp) increased with time during the first 6 days and reached relative stability after 8 days. An equivalent circuit model was proposed to fit the corrosion process, and the elements extracted from the model predicted a corrosion rate in the order of 0.1 μm/yr at pH of 8.0. The values of Rp increased with higher pH for all 2, 4, 6, and 8-day immersions, but decreased at a higher monochloramine concentration of 20 mg/l. In the control experiment, Rp had the largest value after a 30-day immersion time. Dissolved inorganic carbon (DIC) enhancess the dissolution of copper by a factor of 2, when the concentration of DIC increased from 0.59 to 4.0 mM. Increasing ionic strength from 0.005 to 0.02 M reduced Rp, but the increase of Rp with immersion time was faster at higher ionic strengths. © 2002 Elsevier Science Ltd. All rights reserved.