GVWD corrosion control initiative, phase II: inhibitor chemical testing at Seymour Dam
Creators: name: MacQuarrie, Doug M.
Publisher: name: University of British Columbia
Year: 2008

description: Greater Vancouver Regional District (GVRD) waters have some of the highest corrosion potentials in North America. A previous pilot study determined that copper corrosion could be reduced by 60 to 80 percent and lead corrosion could be reduced by10 to 60 percent by disinfecting with chloramine instead of chlorine and by adjusting pH and alkalinity to 8-8.5 and 20 mg/L as calcium carbonate (CaCO3) respectively. The study also recommended that further corrosion control pilot testing be carried out with chemical inhibitors as an adjunct to pH and alkalinity adjustments. The purposes of the inhibitor testing would be to determine their effectiveness in further reducing lead levels at the tap, reducing iron pipe deterioration in some of the older municipal systems, and to determine the impact of inhibitors on re-growth potential. A literature search, and extensive discussion with chemical suppliers determined that zinc orthophosphate and Type N sodium silicate offered the best potential as corrosion inhibitors with GVWD type waters. Thus it was decided to evaluate (within the limitations that the seven loop pilot plant would allow) zinc orthophosphate, type N sodium silicate and a commercial blend of the two. pH and alkalinity were adjusted by the addition of lime [Ca(OH)₂] and sodium bicarbonate (NaHCO₃) respectively, and the water was disinfected with 2.5 mg/L of monochloramine (NH₂Cl). The copper and cast iron corrosion rates were measured over the course of 12months on pipe inserts removed at 3 month intervals. The removed inserts were measured for weight loss, pitting corrosion, and interior biofilm was monitored. In addition, corrosion rates were monitored weekly using an electrical resistance measuring device. Standing water samples taken regularly from lead/tin solder jointed soft copper plumbing coils, submerged free standing coils of 50/50 lead/tin solder, and from faucets on each loop were measured for lead, zinc and copper leaching. The results of the weight loss determinations from the pipe inserts and the weekly resistance measurements indicate that all of the inhibitors, particularly the zinc orthophosphate, work very favorably with copper, but they offer negligible additional benefit over that obtained from the pH and alkalinity adjustments alone in the case of the cast iron coupons. Some very high metal levels were measured in some of the leaching samples. It is postulated that these high levels were due to the redissolution and sloughing (during the 24-hour standing period) of some of the protective scale that formed during periods when the water was flowing. Previous studies have shown that inhibitors such as phosphates and silicates work best in a constant flowing situation. A further suggestion of this sloughing was demonstrated by the appearance of sediment in the samples even though the water was clear prior to isolation of the standing samples. The protective scale which is formed by inhibitors is generally a metal and silicate combination or a metal, phosphate and zinc combination. Thus if the scale sloughs off, more metal will be in the sample when it is digested. Another problem which can occur with zinc orthophosphate at pH above about7.5 is that the zinc orthophosphate can precipitate out before forming a protective scale (EEC 1990). Sometimes, zinc levels were found to be a great deal higher than can be attributed to the feed itself (0.37 mg/L). It seems likely that these high zinc levels were due to a combination of precipitation and sloughing. The occurrence of sediment further corroborates this hypothesis. Generally, in the leaching samples from the solder coils, the lowest lead levels occurred in the raw water. It could be that the lead reacted adversely to all of the treatments tried. Peak metals levels occurred in several loops at the same time. The reason for these coincidental peaks is not clear. There is no obvious pattern. It may be that the degree of scale dissolution and sloughing is dependent on pH and/or alkalinity levels and/or fluctuations, or it may be due to some other factors or combinations of factors. During the standing period the scale appears to weaken but it may not come off until several weeks later. There did not appear to be a correlation between metal levels and total chlorine levels. More work is needed in this area to try and ascertain what causes the extremely high metal levels at certain times and not others. The scale formed seems to be very much a dynamic and ever evolving component in the corrosion situation. The biofilm examinations showed no significant differences between loops with the copper coupons, but higher growth levels were found with the cast iron coupons in the loops with the zinc orthophosphate feed.