Amrou Atassi, Marc Edwards, Jeffrey Parks, and Anusha Kashyap

OBJECTIVES:

The Water Research Foundation funded this study to systematically investigate the interactions between water chemistry, temperature, cavitation, and phosphate corrosion inhibitors with cement-based pipes and linings. Specifically, the project was intended to accomplish the following objectives:

- Determine the impact of phosphate chemicals, water chemistry, temperature, and gaseous cavitation on the formation of protective scales, lime leaching, and water quality for cement-based pipes and linings

- Evaluate implications for Lead and Copper Rule (LCR) optimization and corrosion control

- Assess the current practices of select utilities using phosphate or silicate corrosion inhibitors and provide information for utilities to make informed decisions regarding the use, effectiveness, and application of phosphate corrosion inhibitors on cement-based pipes and linings

BACKGROUND:

Concrete corrosion has serious societal and economic impacts and is an important concern in a utility's overall corrosion control strategy. Concrete or cement-lined pipes make up over 50 percent of the drinking water transmission and distribution system infrastructure (USEPA, 2007), and therefore it is extremely important to understand and evaluate ways to increase its life while minimizing any unintended consequences in the distribution system.

One important consideration is the effect of cement-based pipe corrosion on a utility's ability to comply with the LCR. Many utilities have controlled lead and copper corrosion by adjustment of pH and/or alkalinity, but others have addressed lead and copper corrosion by dosing orthophosphate or polyphosphate blends at the treatment plant. Degradation of concrete infrastructure can affect lead and copper corrosion control programs by release of lime into the bulk water. This lime dissolution can cause a substantial increase in pH causing the phosphate corrosion inhibitors to become ineffective. Therefore, an improved understanding of the corrosion mechanisms involved and steps that can be taken to mitigate concrete corrosion at different water quality conditions are very important.

APPROACH:

Work was performed in three distinct tasks. In Task 1, water quality and field data were gathered from 19 water utilities to examine current practices regarding phosphate and silicate corrosion inhibitors. As part of that evaluation, water quality data, information concerning concrete or cement-lined pipe failures, and compliance with the LCR was collected. The results obtained during this portion of the study were synthesized to identify the key elements that were studied as part of Task 2.

Task 2 used bench-scale testing to study the interactions of phosphate, zinc, magnesium, and silicate with cement. Further, the effects of gaseous cavitation, temperature, pH, and alkalinity were evaluated. The relative significance of zinc in concrete and cement-lined pipe was also assessed. Bench-scale test rigs were designed to simulate essential features of water hydraulics through a typical distribution system main at small scale, while achieving good corrosion signal-to-noise ratio, representative concrete surface area-to-water volume ratio and water contact times, reasonably aggressive hydraulic conditions, and a range of passive and aggressive water chemistries.

Task 3 evaluated laboratory findings in relation to real system samples and failures for select utilities identified in Task 1. This involved evaluating harvested pipe sections or coupon specimens from various water distribution systems. Scale from these samples was analyzed by scanning electron microscopy with energy dispersive spectrometry (SEM-EDS) for chemical composition.

RESULTS/CONCLUSIONS:

The bench scale testing results are summarized in Table 5.1 in the report, which outlines all of the water quality conditions that were tested and the impacts on the concrete/cement-lined pipe and the bulk water quality. The conclusions from this work also include the following:

- The case studies from the 19 participating utilities represent the diversity and complexity of corrosion control issues. The findings of the bench scale tests generally agree with the utilities' experiences and demonstrate the effectiveness of corrosion inhibitors in the protection of concrete and cement-based pipes. It is also important to note that it is unlikely for a utility to have one type and material of pipe in their distribution system, as can be seen from the case studies, which further complicates the utility's corrosion control program.

- Low alkalinity (approx. 20 mg/L as CaCO₃) and low pH (approx. 7.0) water conditions can be extremely aggressive to concrete and cause rapid degradation of concrete by lime leaching into the bulk water.

- High alkalinity (approx. 200 mg/L as CaCO₃) and high pH (approx. 8.3) water conditions are non-aggressive to concrete but scaling of pipes by calcite precipitation can be a major concern to utilities due to potential impacts on the hydraulic capacity of the distribution system.

- Higher concentrations of magnesium and silicon can be effective at preventing corrosion of concrete, but only at higher bulk water pH of around 9.5.

- The kinetics of concrete corrosion degradation reactions increase at higher temperatures, but this effect is countered by reduced calcite solubility in waters with higher pH/higher alkalinity (with no inhibitor added).

- Gaseous cavitation did not increase concrete corrosion, but vaporous cavitation can be extremely detrimental to concrete.

- Both non-zinc-based phosphate inhibitors (orthophosphate and polyphosphate) were effective at reducing concrete corrosion at near neutral pH. At a pH of 8.3 neither orthophosphate nor polyphosphate were effective at reducing corrosion.

- Zinc is an active compound in reducing corrosion of concrete, but it behaves as a better inhibitor when used in conjunction with phosphate from the perspective of corrosion control and scaling. The addition of 0.25 mg/L Zn produced a 33 percent reduction (in comparison to control) in calcium leaching relative to use of orthophosphate alone.

- Increased dosing of zinc (0.5-1 mg/L Zn) provided increased protection to concrete for corrosive water conditions.

APPLICATIONS/RECOMMENDATIONS:

Regardless of the water condition, utilities should consider the following tasks prior to making any changes to their corrosion control programs:

- Conduct bench scale or pipe loop testing for baseline (or control) and different corrosion inhibitor and pH/alkalinity adjustment conditions to confirm the results and develop a corrosion control strategy that meets the utility's goals

- Continuously monitor water quality changes in the distribution system (for example, if pH and/or alkalinity increase in the distribution system, it is possibly due to corrosion of concrete or cement-lined pipe)

- Continuously monitor hydraulic changes in the distribution system to assess decrease in hydraulic capacity caused by lower than expected C-values (for example, if the C-value is lower than expected for the distribution system, it is possibly due to deposition or a scaling forming on the distribution system piping walls)

- Continuously monitor and assess the impact of changes on LCR compliance from distribution system sampling

- Evaluate surge conditions in the distribution system to avoid vaporous cavitation conditions that could be detrimental to concrete or cement-lined pipes

Based upon the monitoring data, a utility can assess its current corrosion control practices and whether it is effective for protection of distribution system infrastructure and LCR compliance.

RESEARCH PARTNER:

USEPA

PARTICIPANTS:

Eighteen utilities from throughout the United States participated in this project.