Issue 26, 2023

The influence of calcium on copper corrosion and its by-product release in drinking water

Abstract

Copper is a high-quality material commonly used in drinking water supply pipes. Calcium is a prevalent cation found in drinking water. However, the effects of calcium on copper corrosion and its by-product release remain unclear. This study discusses the influences of Ca2+ on copper corrosion and its by-product release in drinking water under different conditions of Cl, SO42−, and Cl/SO42−, using electrochemical and scanning electron microscopy techniques. The results indicate that Ca2+ slows down the corrosion reaction of copper to some extent in comparison with Cl, and the Ecorr shifts positively by 0.022 V, while Icorr decreases by 0.235 μA cm−2. However, the by-product release rate increases by 0.5 μg cm−2. The addition of Ca2+ causes the anodic process to become the controlling factor for corrosion, with an increase in resistance observed in both the inner and outer layers of the corrosion product film through SEM analysis. The corrosion product film becomes denser due to the reaction between Ca2+ and Cl, forming a product that inhibits the entry of Cl into the passive film on the copper surface. Adding Ca2+ promotes copper corrosion with the help of SO42− and the release of corrosion by-products. The anodic reaction resistance decreases while the cathodic reaction resistance increases, resulting in a small potential difference of only 10 mV between the anode and cathode. The resistance of the inner layer film decreases, while that of the outer layer film increases. SEM analysis shows that the surface becomes rougher with the addition of Ca2+, and 1–4 mm granular corrosion products are formed. This is due to the fact that Cu4(OH)6SO4 has low solubility and forms a relatively dense passive film that inhibits the corrosion reaction. The added Ca2+ also reacts with SO42− to form CaSO4, which reduces the amount of Cu4(OH)6SO4 generated at the interface, thus damaging the integrity of the passive film. Adding Ca2+ promotes the corrosion of copper by Cl and SO42− and enhances the release of corrosion by-products, with the highest corrosion rate observed under the Cl/SO42−/Ca2+ conditions. The resistance of the inner layer membrane decreases, while the mass transfer resistance of the outer layer membrane increases. Under the Cl/SO42− conditions, the SEM surface of the Cu2O particles is uniform in size, arranged in an orderly and compact manner. After adding Ca2+, the size of the particles becomes uneven, and the surface becomes rough and uneven. This is because Ca2+ firstly combines with SO42−, thus promoting corrosion. And then the remaining Ca2+ combines with Cl, which inhibits corrosion. Despite the amount of remaining Ca2+ being small, it still promotes corrosion. The amount of released corrosion by-products is mainly controlled by the redeposition reaction that occurs in the outer layer membrane, determining the amount of Cu2O to which the copper ions are converted. The increase in resistance of the outer layer membrane means that the charge transfer resistance of the redeposition reaction increases, and the reaction rate slows down. Consequently, the amount of Cu(II) converted to Cu2O decreases, leading to an increase in Cu(II) in the solution. Therefore, adding Ca2+ in all three conditions results in an increase in the release of corrosion by-products.

Graphical abstract: The influence of calcium on copper corrosion and its by-product release in drinking water

Article information

Article type
Paper
Submitted
15 Mar 2023
Accepted
31 May 2023
First published
13 Jun 2023
This article is Open Access
Creative Commons BY-NC license

RSC Adv., 2023,13, 17842-17855

The influence of calcium on copper corrosion and its by-product release in drinking water

P. Xu, Q. Fu and M. Zhao, RSC Adv., 2023, 13, 17842 DOI: 10.1039/D3RA01696J

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