| Summary: | 碩士 === 國立交通大學 === 環境工程系所 === 107 === Brass dezincification corrosion of water meter is a significant problem in potable water systems due to corrosion product (scalants) build-up and pipe blockage, and pitting failure. In addition to cause internal blockage of the water meter, the scaling may also reduce the cross-sectional area of the water inlet. This might cause the increase of water flow of the water meter, even affecting the accuracy of the record of water consumption. It was pointed out that brass alloys are prone to corrosion in certain water quality conditions and to form scalants.
The formation of scaling is deeply affected by water quality. This study was thus to investigate the effect of the water quality on the scaling formation in the water meter in Taiwan. In addition to the Ruifang area, this study also selected the Banqiao area as the control group, which was found comprising insignificant scaling inside water meter. This study can be divided into three parts. The first part was to analyze various water quality in Ruifang area and Banqiao area, e.g., pH, turbidity, alkalinity, temperature. Then the second part was the analysis of organic matter and inorganic compositions of the scalants. The third part was to identify the correlation between water quality and scaling by statistical analysis.
It was found that different areas have different conditions of scalant due to the differences in the water quality. The results showed that the scalants mainly contained zinc carbonate, zinc hydroxide, zine oxide, a small amount of aluminum oxide and bismuth oxide and fatty acids. It is inferred to be the product of dezincification corrosion, as known as, meringue. The results also showed that the meringue dezincification was favored by higher pH (between 8.0 and 9.0), alkalinity and conductivity. Conversely, increased free chlorine and chloramine may inhibit meringue dezincification because of the high oxidation-reduction potential. the water meter in Ruifang area is in the corrosive pH range, and the residual chlorine content is low, which induced the possibility of microbiologically influenced corrosion(MIC). The high metal content of water increased the conductivity then rose the corrosion current of the surface of water meter, which aggravated the corrosion condition. However, there was negligible phenomenon of scaling formation inside the water meter in Banqiao area because the pH of water in this area was in neutral. Furthermore, the residual chlorine was higher (above 0.3 mg/L), which caused the oxidation-reduction potential to rise, resulting in a passivation reaction on the surface of the water meters. Therefore, the high residual chlorine and neutral pH keep the water meter away from MIC.
From the results of this study, the pH, alkalinity, and conductivity have positive correlation with scaling, and the inhibitors are residual chlorine, oxidation-reduction potential, and dissolved oxygen. Therefore, if the pH of the water is in neutral, and the chlorination amount is increased to increase the oxidation-reduction potential, the water meter might generate the passive layer, at the same time, avoid MIC. In this way, the water meter can be prevented from meringue.
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