The water chemistry of Thunder Bay treated water poses unique problems for additional treatments and especially corrosion control.
This report describes the effects of three fluoridating agents on the chemistry of Thunder Bay drinking water and in particular their effect on lead-pipe corrosion. Static corrosion tests were performed with hydrofluorosilicic acid (H2SiF6), sodium silicofluoride (Na2SiF6), and sodium fluoride (NaF), each at concentration levels of 0.5, 0.7, 1.5 parts per million as fluoride ion. One additional experiment was conducted using a pH ( pH=8.5) adjusted solution of hydrofluorosilicic acid at each of the three levels. The static corrosion tests used lead coupons that were fabricated from common lead water supply pipe. The experiments measured the amount of lead leached into solution from the coupons immersed in the solution of the fluoridating agent. Fixed samplings at 6, 24, 96, 168, and 360 hours were analysed.
The fluoridating agent that caused the greatest amount of lead leaching was the solution of un-buffered hydrofluorosilicic at a concentration of 0.7 ppm. The rate was about 2.6 times the control. When the same hydrofluorosilicic acid is used in a pH adjusted solution (pH=8.5) the relative corrosion rate was about 8 times lower, than the un-buffered acid.
Sodium silicofluoride was found to have a relative corrosion rate of about 1.4 times lower then free hydrofluorosilicic acid.
The solution of sodium fluoride at 0.7 ppm as fluoride had the lowest corrosion rate.
The findings show that all the fluoridating agents chosen increase the corrosion of lead pipe to some extent in Thunder Bay tap water. The pH adjusted hydrofluorosilicic acid appears to be the best compromise since it had the next lowest corrosion rate. This choice would require an additional neutralizing agent, sodium hydroxide for pH adjustment.