Research On Influence Of Phosphates On Corrosion Of Carbon Steel In Reclaimed Water

Reclaimed water has become one of the key measures to solve the water crisis in the city, but the pipeline corrosion problems caused by the process of recycling have caused widespread concern. Reclamied water contained more organic matter and nutrients, and its effect on the metal surface dirt and corrosion mechanism are unclear. Reclaimed water quality have a greater impact on the corrosion rate of carbon steel, corrosion product structure and components, and corrosion of carbon steel can also cause deterioration of water quality regeneration. In this study, the impacts of phosphate and its mechanism on corrosion of carbon steel were studied comprehensively, from the corrosion rate, corrosion product structure and composition, iron release, microbes, particularly under the circumastance of Larson Ratio and Hardness. This work provided a scientific basis for the control of corrosion problems and improves the quality of the reclaimed water.The results showed that: phosphates in the reclaimed water inhibited the corrosion of carbon steel and the inhibition efficiency was related to the concentration of phosphates. When concentration of PO43- was 0.8mg/L, the inhibition of corrosion of carbon steel attained the best and the corrosion inhibition efficiency was 15.66%. With the concentration of phosphates rising, the corrosion layer on the surface of carbon steel became thicker, the area of coverage was also larger. “Bathing spherical” structure was found in the corrosion products which had more phosphorus. At the same time, phosphate influenced the iron release and different concentrations of phosphate brought different inhibitory efficiency. When concentration of PO43- was 0.8mg/L, the rate of iron release attained the best. Phosphates accelerated iron release in the initial 4 hours. The phenomen of "initial acceleration" iron release appeared in short time, especially in the initial 4 hours.Larson Ratio directly affected the inhibition efficiency of phosphates. Higher value of Larson Ratio made the inhibition efficiency of phosphates worse. Under the circmastance of phosphates without proteins, higher value of Larson Ratio made the proportion of iron oxide higher and proportion of calcium oxide lower. Under the circmastance of phosphates with proteins, higher value of Larson Ratio also made proportion of calcium oxide lower.Hardness directly affected the inhibition efficiency of phosphates.When the hardness was 50 degrees(concentration of calcium ion was 20 mg/L), the inhibition efficiency of phosphates attained the best. Under the circmastance of phosphates without proteins, higher hardness made the proportion of iron oxide lower and proportion of calcium oxide higher. Under the circmastance of phosphates with proteins, higher hardness also made proportion of calcium oxide higher. In the water with low hardness, inhibition efficiency of phosphates was worse, which may because phosphates affected the crystallization of calcium; proteins may compete with the phosphate on the adsorption sites on the solid surface so that affected the absorption of phosphate, thereby affecting inhibition effect of phosphate. However, due to the adsorption of proteins and microorganisms, proteins still inhibited corrosion well.Influence on the corrosion of proteins was divided into two parts. On the one hand, proteins were adsorbed on the surface of corrosion products and formed into proteins film which worked; on the other hand, microbial action which formed by proteins affected the corrosion. Proteins inhibited the formation of calcium carbonate, but the effect was only effective in the early period. When concentration of calcium ion was too low, the calcium carbonate crystals precipitation and growth process, phosphate molecules as impurities, on the one hand directly combined with calcium ions formed into metastable calcium phosphate which reduced the activity of corrosion layer and inhibited the formation of calcium carbonate, on the other hand may impeded the growth and expansion of calcium carbonate by adsorbed on the active growing sites of calcium carbonate.