The Effect of Free Carbon Dioxide on Copper-corrosion Behavior

Abstract

Copper tube is generally used in water supplying systems, however the copper tube rarely suffers from Type I pitting-corrosion. In this study, the effect of free carbon dioxide (CO₂ (aq.)) on the copper-corrosion behavior was examined.
Copper specimens were immersed in simulated tap-waters in atmospheres of 79%N₂-21%O₂, 78.8%N₂-21%O₂-0.2%CO₂, 77%N₂-21%O₂-2%CO₂ and 59%N₂-39%O₂-2%CO₂ (artificial airs). The concentrations of CO₂ (aq.) and dissolved oxygen (DO) in the waters were changed by these methods. After the immersion tests, electrochemical impedance spectroscopy (EIS) measurements were carried out. The electrochemical parameters were determined by curve fitting after EIS.
The pHs of waters were constant in 78.8%N₂-21%O₂-0.2%CO₂, 77%N₂-21%O₂-2%CO₂ and 59%N₂-39%O₂-2%CO₂ throughout the immersion tests. Generally, H⁺ is consumed for reduction of DO with oxidation of copper. On the other hand, H⁺ is supplied by CO₂ (aq.) and H₂O as follows.
CO₂ (aq.)+H₂O↔H₂CO₃
H₂CO₃↔HCO₃^-+H⁺
It is considered that the concentration of CO₂ (aq.) and pH are held constant when the partial pressure of CO₂ gas is constant. The values of charge-transfer resistance and Warburg impedance decreased with the increasing partial pressure of CO₂ gas and/or O₂ gas. Consequently, the copper-corrosion rate increased with the concentrations of CO₂ (aq.) and/or DO in the water. H⁺ was supplied by CO₂ (aq.) and H₂O. H⁺ might assist the reduction of DO to promote the oxidation of copper as follows.
O₂+4H⁺+4e^-→2H₂O