Impact of elevated Ca²⁺/Mg²⁺ concentrations of reverse osmosis membrane desalinated seawater on the stability of water pipe materials

Hardness and alkalinity are known factors influencing the chemical stability of desalinated water. This study was carried out to investigate the effect of Ca²⁺ and Mg²⁺ on corrosion and/or scale formation on the surface of different water distribution pipe materials under tropical conditions. The corrosion rates of ductile iron, cast iron and cement-lined ductile iron coupons were examined in reverse osmosis (RO) membrane desalinated seawater which was remineralised using different concentrations of Ca²⁺ and Mg²⁺. The changes in water characteristics and the coupon corrosion rates were studied before and after the post-treatment. The corrosion mechanisms and corrosion products were examined using scanning electron microscope and X-ray diffraction, respectively. We found that the combination of Ca²⁺ and Mg²⁺ (60/40 mg/L as CaCO₃) resulted in lower corrosion rates than all other treatments for the three types of pipe materials, suggesting that Ca²⁺/Mg²⁺ combination improves the chemical stability of desalinated seawater rather than Ca²⁺ only.