The effect of pH adjustment on the internal corrosion rate of residential cast-iron and copper water distribution pipes

The Greater Vancouver Regional District distributes drinking water that displays several attributes of an aggressive water: low pH, low alkalinity, and high dissolved oxygen. A study, consisting of two experimental sessions, was conducted at the University of British Columbia to examine the effects of pH adjustment on internal corrosion of residential cast-iron and copper water distribution pipes.Because of its aggressive nature, this water accelerates the corrosion of water pipes, which not only increases maintenance costs, but also encourages high levels of metal in the water. This last finding was confirmed by a preliminary survey wherein, after one month's sampling of six dwellings, it was found that the recommended maximum level of 1.0 mg/L was exceeded in 67% of the morning cold water first-flush samples.Adjustment of pH with hydrated lime, Ca(OH)2, was used for corrosion control. Cast-iron and copper samples were exposed to pH-adjusted water for varying lengths of time, in two flow-through experimental systems (gravity-fed and system-pressure-fed).Although the corrosion rates were different for the two experimental sessions, analysis of the pH-related corrosion rates variation led to the same two findings. The corrosion rates of cast iron were 10 times those of copper; the increased pH enhanced these cast-iron corrosion rates by approximately 15%. However, a pH increase reduced copper corrosion by as much as 68%. The effects of increased pressure on corrosion were different for both metals; a higher pressure greatly enhanced cast-iron corrosion but had little effect on copper corrosion. Key words: aggressive water, cast iron, corrosion, copper, lime, pH adjustment, water pipes.