Corrosion control and inhibition mechanism of sodium polyphosphate on highly eroded cement coating pipe surfaces

摘要

Cement coating has been widely used in water distribution systems to protect metal pipes. However, little attention has been paid to the eroded cement coating surface and the iron release control. A complicated biochemical reaction exists between the bulking pipe water and the cement coating surface. While the treated water may meet the standards for drinking water quality, water chemical stability cannot always be guaranteed, resulting in the occurrence of discoloration and sand particles in tap water and complaints from customers about water quality. In this study, the effects of adding sodium hexametaphosphate on pipe water quality and corrosion inhibition from cement coating surfaces were investigated. It was shown that adding sodium hexametaphosphate not only provides an immediate remedy for the highly eroded cement coating surface and inhibits iron releasing, but it also improves water chemical stability and water quality by reducing hardness and total iron content in the bulking pipe water. A short-time inhibition treatment cannot provide residual protection for the eroded cement coating surface. Adding sodium hexametaphosphate does not promote bacteria regrowth in water distribution systems. Surface analysis using SEM, EDS and XRD showed that a protective layer developed on the cement coating surface and was composed of CaFe2P2, Ca2Fe(PO4)(2)(H2O)(4), Ca(Fe-6(OH)(6)(H2O)(2)(PO4)(4))(2) and Fe2P2O7 center dot 2H(2)O by removing Ca, P, and Fe from the bulk pipe water.