The principle oxidants in the drinking system are dissolved oxygen and chlorine, and the corrosion of water pipe is going alone because of oxidants reaction. Since 1930, pH control chemicals such as NaOH, Ca(OH)₂, NaHCO₃, and phosphate corrosion inhibitors have been used to prevent the corrosion of water pipes. However, to directly add Ca(OH)₂at inlet of water well in production process of drinking water may cause a problem to make clouding phenomenon in tap water. The objective of this study removes such problems as the turbidity, strengthen more safety of water.

Experimental

Materials – The iron(carbon steel) was in the form of sheet 0.04 cm in thick and the following analysis: Fe 99.62%, Mn 0.26%, Cu 0.08%, Ni 0.05%, Mo 0.01%. specimens cut from this sheet measured 15cm×1.2cm. The chemicals used as corrosion inhibitors were NaOH, Ca(OH)₂, and H₃PO₄ and tap water used as blank. NaOCl was used to disinfect the drinking water made by the demo plant.  In order to examine the effect of disinfectant residual concentration according to adding lime water and corrosion inhibitor such as ortho phosphate(H₃PO₄), the residual concentration of disinfectant was measured in carbon steel water pipe 50 m in length at the corrosion pilot plant and the temperature of solution was in room temperature between 5°C in the winter to 35°C in the summer. Chlorine residual concentration in the water pipes 50 m in length was measured for 5, 10, 20, 40, 60 min, respectively. The electrochemical tests was measured using Solartron Potentiostat 1480 and the test was performed  in a typical three electrode cell which consist of carbon steel as working electrode, saturated calomel electrode as reference electrode, platinum counter electrode.

Results

HOCl and OCl^- in drinking water which chlorine was added as disinfectant reach in equilibrium state at nearly pH 7.5. As pH increases, more of OCl^- exists than the strong disinfectant, HOCl^-, Increasing HOCl below pH 7.5 is more benefit in terms disinfectant efficiency. Figure 2 above shows the concentration of free residual chlorine with time. Running drinking water in the pipe, adding PO₄ decreased concentration of chlorine from 0.48 to 0.28 ppm for 40 min, NaOH decreased concentration of chlorine from 0.45 to 0.10ppm, Ca(OH)₂ decreased concentration of chlorine from 0.24 to 0.10 ppm, but tap water as blank degreased concentration of chlorine from 0.40 to 0.07 ppm for only 10 minutes. Of three corrosion inhibitors, PO₄ showed the highest reduced efficiency for the concentration of residual chlorine. To inhibit the reaction of chlorine with metal water pipe is similar to inhibit the corrosion reaction.

Reference

US EPA, Disinfection Profiling and Benchmarking, EPA Guidance Manual, August 1999.

American Society for Testing and Materials, G 4-95(1996).