Comparison of Zinc Versus Non-Zinc Corrosion Control for Lead and
Copper [Project #4103]
ORDER NUMBER: 4103
DATE AVAILABLE: April 2011
PRINCIPAL INVESTIGATORS:
Orren D. Schneider, Jeffrey Parks, Marc Edwards, Amrou Atassi, and Anusha Kashyap
OBJECTIVES:
The objective of this project was to determine if there are corrosion control advantages
between zinc orthophosphate (ZOP) and non-zinc orthophosphate corrosion inhibiting
compounds (CICs) under realistic distribution system and domestic plumbing conditions.
This project focused on a mix of metals typically used in water systems that can impact
compliance with the Lead and Copper Rule (i.e., leaded brass, lead-tin solder, copper,
and lead). The project also addressed possible secondary benefits of ZOP in reduced
cement corrosion.
BACKGROUND:
Corrosion of lead and copper in drinking water distribution system is a known public
health concern. One option for controlling pipe corrosion is the use of ZOP. ZOP is
thought to work by depositing a layer of material that prevents the water from coming
into contact with the pipe surface. While some zinc may become incorporated into these
protective films, most of it, however, passes through the distribution system and
ultimately ends up in wastewater. While many communities nationwide use ZOP, it is
expensive and the zinc becomes concentrated in wastewater sludge, which is an
environmental concern. An alternative to ZOP is orthophosphoric acid (non-zinc
orthophosphate), which has been successfully used for corrosion control at a number of
utilities nationwide.
However, there is a lack of scientifically valid data comparing the performance of ZOP
and non-zinc CICs for controlling the rate of corrosion and levels of metal release from
iron, lead, brass, and copper piping and plumbing devices. Studies have not established if
CIC performance (metal release, scale stability) varies between fresh and scale-covered
surfaces. When trying to select and evaluate CICs based on analogous system
performance, water utilities are disadvantaged by lack of adequate information on
adverse effects on water discharges. Clarifying the role of zinc in CIC performance for
lead, copper, and brass will help utilities and state regulatory agencies more effectively
balance the potential risk of lead and copper release with that of zinc discharge to the
aquatic environment.
The testing that was conducted was meant to highlight any differences in performance
among CICs with different levels of zinc rather than to determine if any particular CIC
could be used to achieve regulatory goals. Thus, for the pilot work, the test loops
contained enough pipe material (2 feet each of lead pipe, lead-tin solder inside copper
©2011 Water Research Foundation. ALL RIGHTS RESERVED.
tubing, and leaded brass) to measure any released metal, but was not representative of
typical domestic plumbing systems that could contain more than 20 feet of lead service
line and over 100 feet of copper tubing. Hence, when presenting results, no comparisons
will be made to the Lead and Copper Rule (LCR) action levels of 15 µg/L for lead and
1.3 mg/L for copper.
APPROACH:
The project team utilized a multiphase approach to address these issues. The first phase
was a series of bench-scale experiments performed to examine the impact of corrosion
variables including pH, orthophosphate dose, and chloride to sulfate mass ratio (CSMR)
on lead and copper release. Experiments were also performed to examine the impact of
corrosion variables on cement. The second phase of the research consisted of pilot testing
at five systems that currently use orthophosphate. These pilot tests focused on examining
differences in the performance of CICs (under a constant orthophosphate dose and
varying zinc concentrations) using side-by-side testing in pipe loops containing segments
of lead, copper tubes containing lead-tin solder, and leaded brass. Biweekly
measurements were made to ensure that the pH and orthophosphate dose was equivalent
in all of the pipe loops at each location. Performance differences among the CICs were
explored using an electrochemical method (linear polarization resistance – LPR
electrodes), water quality analyses for dissolved and particulate metals, and qualitative
comparisons using metal coupons analyzed by x-ray fluorescence (XRF) and scanning
electron microscopy with electron dispersive spectroscopy (SEM-EDS). The third phase
of the research involved the collection of historical and operational data from utilities that
use ZOP along with analysis of samples from distribution systems to examine the fate of
zinc and orthophosphate in the distribution system.
RESULTS/CONCLUSIONS:
Results
Bench-scale experiments showed that ZOP may have some advantages over non-zinc
orthophosphate in stopping galvanic attack and reducing lead leaching in waters prone to
galvanic corrosion. The experiments conducted in this study used water from a Tennessee
utility experiencing problems complying with the LCR. This water had very low
alkalinity (8 mg/L as CaCO3) and the pH was 7.3. Zinc did not appear to affect lead
leaching when chloride was low (3 mg/L); however, when 15 mg/L chloride was present
(simulating a brine leak at the utility) lead leaching was reduced by 90% when 0.1 mg/L
zinc was dosed in conjunction with 1 mg/L phosphate (as P). However, ZOP did not
always have benefits relative to non-zinc orthophosphate, even in waters where lead
corrosion was primarily driven by galvanic currents.
ZOP addition provided better corrosion protection to cement than zinc alone or non-zinc
orthophosphate for water with low alkalinity (20 mg/L) and low hardness (5 mg/L as Ca)
at pH 7. During bench-scale testing less calcium and aluminum leached from cement
coupons and less internal carbonation was observed when 0.25 mg/L zinc and 2 mg/L
orthophosphate (as PO4) was added to the water. Higher doses of zinc (0.5 and 1.0 mg/L)
©2011 Water Research Foundation. ALL RIGHTS RESERVED.
provided even more protection. ZOP addition to non-aggressive water (i.e., 200 mg/L
alkalinity and 120 mg/L calcium hardness) had little effect on cement corrosion.
For the pilot testing, measurements were collected on a bi-weekly basis for corrosivity (as
measured by LPR electrodes), dissolved metal release, and particulate metal release.
Statistical analyses were then performed for each of the individual sites as well as for the
pooled data. The results of these analyses indicated that there is no statistically significant
difference in performance among high-zinc orthophosphate (1:1 or 1:3
zinc:orthophosphate mass ratio), low-zinc orthophosphate (1:10 zinc:orthophosphate
mass ratio), and non-zinc orthophosphate for corrosion control. Where site specific
differences were noted, there were no systematic differences.
The results of the utility case studies indicate that release of zinc in wastewater residuals
and/or receiving streams can be a concern for some utilities. The rising cost of ZOP (in
relation to non-zinc orthophosphate) also has a number of utilities wondering if zinc can
be eliminated without degrading corrosion control performance. Three participating
utilities eliminated or reduced their zinc levels for various reasons with no impact on lead
and copper levels. A number of the participating utilities have corrosive water quality and
use the zinc for protection against corrosion of asbestos cement pipe and cement-lined
pipe, and not necessarily for prevention of lead and copper corrosion.
The results of the field testing for three participating utilities that have been feeding ZOP
for past 15+ years indicate that there is some zinc uptake in the distribution system, but
that there is still residual zinc that ends up in homes and eventually the wastewater
treatment plants’ receiving streams.
Conclusions
The results from the statistical approach suggest that, for general corrosion of lead and
copper in most locations, there does not appear to be a significant difference in
performance between ZOP and non-zinc orthophosphate. This conclusion is based on
analyses of electrochemical measurements, dissolved metal release, and particulate metal
release.
Bench studies have shown that zinc may be beneficial for preventing some types of
copper pitting corrosion. Furthermore, results of this study suggest that addition of a zinccontaining CIC is beneficial in reducing cement degradation and aluminum release to
water when treated water is low in calcium and alkalinity. There appears to be little
advantage in adding zinc to treated water high in calcium and alkalinity as non-zinc
orthophosphate alone can inhibit calcium carbonate scaling of cement. If zinc dosing is
discontinued, calcium leaching from cement can return to levels that correspond to those
present when no zinc protective scale is present in as little as five weeks.
Based upon calcium and aluminum leaching results, it appears that a 0.1 mg/L zinc dose
is sufficient to provide continued corrosion protection once a protective zinc-containing
scale layer has been formed.
©2011 Water Research Foundation. ALL RIGHTS RESERVED.
The results of these studies should be interpreted with caution however, and bench-scale
and/or pilot studies should be conducted to determine if zinc addition is beneficial for a
specific water quality condition. Additionally, a cost/benefit analysis should be conducted
with regards to the benefit of adding zinc (to increase the life of concrete infrastructure)
versus the cost of zinc treatment and disposal. Non-cost factors, such as the
environmental impact of zinc, should also be included in the evaluation.
APPLICATIONS/RECOMMENDATIONS:
For utilities that wish to reduce costs and/or environmental impacts of their corrosion
control programs, switching from ZOP to non-zinc orthophosphate may be desirable.
However, before doing so, these utilities should conduct a survey of their distribution
system to see how much cement is present, either as transmission piping (as cement or
asbestos cement pipe) or linings of metal pipes. Additionally, because of the possible
public health impact, utilities should conduct side-by-side pilot tests to determine if there
is a difference in CIC performance. Before any testing is conducted, the utility should
consult with their primacy agency to determine what type/length of testing would be
adequate for the primacy agency to allow a change in the corrosion control program.
These pilot tests could consist of coupon testing, electrochemical testing (using linear
polarization resistance probes), water quality testing (using several lengths of lead and
lead-soldered copper pipes), or any combination thereof. The tests should be run for at
least a three-month period (to allow for a steady-state to be established), include replicate
measurements, and be conducted during the warmest water periods.
If a utility does choose to change their CIC, corrosivity measurements at a number of
locations in the distribution system should be made using pipe coupons or linear
polarization resistance electrodes to establish a baseline level. Thus, when a change in
CIC is made, the utility will be able to relate changes in corrosivity due to this change in
chemicals. Prior to the change in CIC, the utility should plan and execute a thorough
flush of their distribution system to remove loose scales and sediments. In addition,
utilities may consider a gradual decrease in zinc levels rather than a sudden stop in zinc
feed. This gradual change in zinc concentration may reduce perturbations of chemical
equilibria at the pipe walls and may mitigate release of established scales.
After the change in CIC, utilities should continue to monitor the distribution system in
terms of lead and copper, as well as pipe failures due to internal corrosion, on a more
frequent basis following the change in orthophosphate feed.
RESEARCH PARTNER:
U.S. Environmental Protection Agency
©2011 Water Research Foundation. ALL RIGHTS RESERVED.