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

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 Ca(2+) on copper corrosion and its by-product release in drinking water under different conditions of Cl(−), SO(4)(2−), and Cl(−)/SO(4)(2−), using electrochemical and scanning electron microscopy techniques. The results indicate that Ca(2+) slows down the corrosion reaction of copper to some extent in comparison with Cl(−), and the E(corr) shifts positively by 0.022 V, while I(corr) decreases by 0.235 μA cm(−2). However, the by-product release rate increases by 0.5 μg cm(−2). The addition of Ca(2+) 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 Ca(2+) and Cl(−), forming a product that inhibits the entry of Cl(−) into the passive film on the copper surface. Adding Ca(2+) promotes copper corrosion with the help of SO(4)(2−) 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 Ca(2+), and 1–4 mm granular corrosion products are formed. This is due to the fact that Cu(4)(OH)(6)SO(4) has low solubility and forms a relatively dense passive film that inhibits the corrosion reaction. The added Ca(2+) also reacts with SO(4)(2−) to form CaSO(4), which reduces the amount of Cu(4)(OH)(6)SO(4) generated at the interface, thus damaging the integrity of the passive film. Adding Ca(2+) promotes the corrosion of copper by Cl(−) and SO(4)(2−) and enhances the release of corrosion by-products, with the highest corrosion rate observed under the Cl(−)/SO(4)(2−)/Ca(2+) conditions. The resistance of the inner layer membrane decreases, while the mass transfer resistance of the outer layer membrane increases. Under the Cl(−)/SO(4)(2−) conditions, the SEM surface of the Cu(2)O particles is uniform in size, arranged in an orderly and compact manner. After adding Ca(2+), the size of the particles becomes uneven, and the surface becomes rough and uneven. This is because Ca(2+) firstly combines with SO(4)(2−), thus promoting corrosion. And then the remaining Ca(2+) combines with Cl(−), which inhibits corrosion. Despite the amount of remaining Ca(2+) 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 Cu(2)O 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 Cu(2)O decreases, leading to an increase in Cu(ii) in the solution. Therefore, adding Ca(2+) in all three conditions results in an increase in the release of corrosion by-products.