Impact of flow rate on corrosion of cast iron and quality of re-mineralized seawater reverse osmosis (SWRO) membrane product water

Abstract

Remineralization of desalinated water as part of post-treatment has attracted great attention lately because of the need for high quality water and for the protection of the network pipelines. Laboratory experiments of remineralization using Ca2+ and Mg2+ containing substances were carried out to investigate the effect of flow rates on corrosion behaviour of cast iron coupons and water quality of re-mineralized SWRO product under tropical conditions. Experiments demonstrated the clear effect of increased corrosion rate at higher flow rate, i.e., the flow rate of 1, 10 to 20L/d increased the corrosion rate from 4.72, 6.74 to 9.17mpy, respectively. The turbidity and apparent colour of flow rate at 1L/d were much higher than those of 10 and 20L/d because of iron rusting materials formation and accumulation at the low flow rate. Cast iron corrosion and various flow rates did not significantly influence water quality parameters associated with SWRO product remineralization, such as total alkalinity and hardness. Microstructure observation showed that the main corrosion product was small crystalline globules of lepidocrocite, which formed a thick orange layer on coupon surface. Crystal fiber of white calcium composite was also found on some areas of the coupon surface which functioned as a protective layer.