AWWA ACE58167

This work demonstrates that aluminum residuals in drinking water canincrease susceptibility to copper pitting corrosion under certain conditions of pH andchlorine residual. In the first set of experiments examining the role of pH, free chlorine wasincreased incrementally every two days from zero to 4.8 mg/L as Cl2 using bleach (6%NaOCl). The target level of chlorine was increased as follows: 0, 0.5, 0.8, 1.2, 1.5, 1.8,2.1, 2.4, 3.6, and 4.8 mg/L as Cl2. Targeted pH values of 6,7,8, and 9 were tested induplicate (8 cells total), and the pH was adjusted and Cl2 residuals were replenished twicedaily. pH values were continuously held to within 0.3 units of the target value.In all experiments, free chlorine was measured using the DPD colorimetric testper standard method 4500-Cl G. The water was completely changed on day 12 andtests were ended on day 20 of the experiment. Cumulative copper release to the waterwas measured at these times using the bicinchoninate method. Corrosion potential (Ecorr)and polarization resistance (RP) measurements were collected hourly using a multiplexerconnected to a personal computer, using a polarization scan rate of 0.5 mV/sec from -5 to+5 mV of the open circuit potential. Platinum counter and Ag-AgCl reference electrodeswere checked and maintained regularly to insure accuracy. Instantaneous corrosion rateswere estimated using potentiodynamic scans and Tafel analyses.The combined impacts of Al and Cl2 were examined by repeating the abovetests, but with addition of 2 mg/L as Al of Al(OH)3 solids. The Al(OH)3 stock solutionwas created by raising the pH of 3000 mg/L as Al-AlCl3 solution to 8.0 using NaOH,allowing the solids to settle, decanting the water, re-suspending the solids in freshwater,until the conductivity of the suspension was equal to that of the experimental water. Thesame chlorine dosing schedule was used, except an additional level of 6.0 mg/L Cl2 wasadded on day 20 for all pH conditions. Copper release was measured and water waschanged on day 22, and chlorine was again dosed to 6.0 mg/L. The pH for all conditionswas then raised to 9.0, and Ecorr and Rp were measured for another 2 days.To account for possible deposition of aluminum solids within the experimentalapparatus, aluminum solids were re-dosed once approximately 24 hours after each waterchange. Thus, the actual level of aluminum in the water could have been as much as 50%higher than the target levels, and lower than the target levels dependent on losses to theapparatus surface or pipe samples. At the end of the experiments, samples were cut in halfand photographs were taken of the corroded copper surfaces using a digital camera. Includes 7 references, figures.

Product Details

Edition:

Vol. - No.

Published:

06/15/2003

Number of Pages:

13

File Size:

1 file , 650 KB