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 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 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.