United States
Environmental Protection
Agency
Municipal Environmental
Research Laboratory
Cincinnati, OH 45268
Research and Development
EPA-600/S2-84-130 Sept. 1984
&ER& Project Summary
Seattle Distribution System
Corrosion Control Study:
Volume VI. Use of a Rotating
Disc Electrode to Assess
Copper Corrosion
Ronald D. Hilburn
The uniform corrosion of copper
tubing used for transport of Tolt River
water is characterized in this study as a
heterogeneous rate process composed
of metal oxidation and oxide film
growth, interfacial chemical reactions,
and mass transport in the liquid phase.
Quantitative rate expressions were
developed to characterize each of these
rate processes. Experiments designed
to measure the temperature and pH
dependence of corrosion under rate
control by each process were conducted
using steady-state electrochemical
techniques. The persistent and unex-
pected influence of solution transport
of a reaction product, presumed to be
OH~, complicated characterization and
identification of underlying rate pro-
cess. Surface pH could be characterized
empirically as a function of solution
temperature, pH, and diffusion layer
thickness.
This empirical correlation for surface
pH along with solution mass transport
models developed for turbulent and
laminar pipe flow were combined to
form a steady-state pipe flow model for
uniform copper corrosion. Predictions
made using the model under stagnant
and low flow rate conditions show a sta-
ble and low corrosion rate of 0.2 mils
per year (MPY) in water of pH > 6.0. At
lower pH, predicted rates are substan-
tially increased as the pH is reduced and
temperature is increased. At high flow
rates, tremendous acceleration of cor-
rosion rate occurs, which again increas-
es with increasing temperature and de-
creasing pH. Only at pH > 8.0 are the
dramatic pH and temperature effects
dissipated so that the rate is stabilized
at a minimum value of approximately
0.2 MPY.
Steady-state electrochemical tech-
niques gave rapid, reliable, and repro-
ducible corrosion rate measurements
and provided the versatility necessary
to characterize quantitatively a hetero-
geneous rate process like aqueous cop-
per corrosion.
This Project Summary was developed
by EPA's Municipal Environmental
Research Laboratory. Cincinnati. OH.
to announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
Background
Corrosion of materials used to transport
drinking water is both a public health
concern and an enormous economic
problem. In 1975, the estimated national
cost for corrosion in industrial, commer-
cial, and public establishments was $82
billion. This figure included monies spent
for replacement of deteriorated parts and
equipment, maintenance and repair, and
direct expenditures for corrosion control.
Costs as high as $375 million per year
-------
have been estimated for replacement
parts and repair of drinking water
distribution systems transmitting corro-
sive water, with an additional cost of $27
million per year for water treatment to
control corrosion. The Seattle Water
Department has estimated repair costs to
consumer plumbing systems to be 10 to
20 times higher than costs associated
with the distribution system. Annual
expenditures of $500,000 for corrosion
control in Seattle are estimated to reduce
consumer costs by $2 million.
Corrosion occurs in the distribution or
plumbing system — the part of the drink-
ing water cycle closest to the point of
customer use. Water quality degradation
and contamination occur because of the
release of metal cations and other
corrosion products into the water. Public
health concerns involve ingestion and
bodily accumulation of metal cations
from drinking water. Dissolved lead and
cadmium are two contaminants likely to
be present above permitted levels as a
result of aqueous corrosion in plumbing
systems using galvanized steel or copper
tubing with lead/tin solder.
Adverse aesthetic effects often result
from the leaching of copper, iron, zinc,
and manganese from corroding pipelines.
For example, drinking water may be
undesirable because it has an unpleasant
taste or color, or because it promotes
staining of porcelain bathroom and kit-
chen fixtures. All of these effects arise
from a chemical reaction between a
structural material and a chemical com-
ponent of the transported natural water
As the reaction proceeds, the metal is
thinned and/or pitted, thus shortening its
useful lifetime according to the rate of the
overall reaction. Reaction products may
also promote precipitation of solids,
which accumulate on the metal surface
and in some cases reduce water pressure
and pipeline carrying capacity. Corrosion
control efforts are then aimed at slowing
down the reaction to an acceptable rate or
stopping it completely without creating
further ecological or water quality prob-
lems.
Purpose and Scope of Work
This study characterizes aqueous
copper corrosion as a heterogeneous rate
process by combining principles from
various disciplines to develop quantitative
rate expressions for each component rate
process involved. Principles taken from
solid-state, electrochemical, and corro-
sion sciences are combined with those of
water chemistry and environmental
engineering to describe and explain the
overall corrosion process as a composite
result of fundamental rate processes.
Although the component rate processes
are coupled, each is influenced by a
distinct set of environmental variables
that affects the rate at which it proceeds.
Thus this work presents an overview of
copper corrosion in drinking water as
a composite result of several fundamental
rate processes, and it determines through
laboratory experiments which rate pro-
cesses exert the greatest rate-controlling
influence on the overall process. Environ-
mental variables most important in influ-
encing the rate-controlling processes are
also evaluated.
Corrosion rate was measured with
standard steady-state electrochemical
techniques augumented with special
instrumentation necessary for measure-
ment in natural waters of low conductiv-
ity. Measurements were made under
varying conditions of rate control to
evaluate the dependence of component
processes on temperature, fluid motion,
and chemical composition of the system.
Results
General Observations
The likelihood of rate control by each of
the component rate processes is shown
in a comparison of the magnitude of the
three types of data as a function of pH
(Figures 1 and 2) at 25°C. The following
general observations are made:
a) Corrosion rates measured at pH >
7.0 on electrodes without oxide films
were much greater than those mea-
sured on oxide-covered electrodes.
b) Corrosion rates measured in stag-
nant solution were considerably
lower than those made at 3OOO RPM.
These observations demonstrate both the
influence of oxide film growth and solu-
tion mass transport on the overall rate
and, thus, suggest that a model of the
overall process include the coupled
effects of these two rate processes.
Kinetic data may be important, but only at
sites where the oxide film has been
damaged or removed. The fact that solu-
tion mass transport control data were of
smaller magnitude than the other two
sets of data at corresponding pH and
temperature values indicates that some
species exist in solution whose diffusion
affects the rate of the overall corrosion
process
The rate of the process thus depends
not only on the chemical reactions
involved and the growth of oxide film, but
also on the hydrodynamic flow regime of
the corroding system since it inevitably
influences the rate of solution mass
transport. A definite solution mass
transport influence exists, and the A
species exerting that influence may very "
well be OH" diffusing away from the sur-
face into bulk solution. In situations
involving a reaction product that must
diffuse away from the reacton site, a per-
sistent influence is exerted on the overall
process rate (even at very high Reynolds
Numbers) to the point of masking the
effects of the underlying process. A slow
diffusion of OH" away from the surface
increases the pH of the solution adjacent
to the oxide surface and reduces the oxide
film growth rate. As the diffusion rate is
increased (at higher Reynolds Numbers),
the surface pH can drop, allowing a faster
rate of oxide film growth and greater
corrosion rate. The high pH values that
arise at the oxide surface may promote
precipitation of such solids as Cu(OH)2,
which have low solubilities. If such is the
case, actual rate control of the corrosion
process may occur in the solution
adjacent to the oxide surface, not within
or on the oxide itself. In addition, a variety
of Cu(OH)n"n complexes may form in the
solution adjacent to the oxide surface,
reducing the "free OH"" concentration and
lowering the surface pH.
Pipe Flow Model
The principal value of corrosion research A
performed in the laboratory lies in its \
application to real-life corroding systems,
such as copper tubing used for cold water
plumbing. A simulation model is presented
for predicting the rate at which copper
tubing will corrode under a given set of
environmental conditions. The model is
based on (1) the presumption that mass
transport of OH" is the rate-controlling
process and (2) the combined results of
laboratory studies with quantitative
models for mass transport in laminar and
turbulent pipe flow. The result is an ability
to predict the rate of uniform copper
corrosion in cold water plumbing systems
under varied conditions of flow, tempera-
ture, and pH of the water.
Results of the calculation for varying
Reynolds Numbers at 25°C appear in
Figure 3. The rates increase with flow
rates at Reynolds Numbers greater than
2000. Effects of pH < 6 are apparent at all
flow rates. Rates become very high at low
pH and high flow rate.
The influence of temperature appears
in Figure 4, which shows corrosion rates
versus pH at a high Reynolds Number
(50,000). The corrosion rate is much
more pH dependent at 15°C and 25°C
than at 5°C.
Model predictions provide valuable
insight into identification of principal A
variables affecting the overall process, ™
-------
1.5
T-25°C
* *
6.0
6.5
8.5
7.0 7.5
pH
Figure 1. Comparison of corrosion rates measured with and without Oxide film at 25°C.
the extent to which each variable exerts
an influence, and the range of magnitude
in which the maximum effect is mani-
fested. Possible strategies for corrosion
control (or at least corrosion rate reduc-
tion) are also suggested.
Conclusions
Over the pH range of 6.0 to 8.5, the
rate of copper corrosion is reduced as pH
is increased. Over the temperature range
of 5°C to 25°C, the rate of copper
corrosion is reduced as temperature is
reduced. The presence of a Cu20 film on
the copper surface reduces the corrosion
rate at all pH and temperature values
studied.
The transport of a reaction product
(presumed to be OH~) away from the
oxide/solution interface is the principal
process controlling the overall corrosion
rate. At low pH (6.0), rate control by mass
transport is nearly complete. At higher pH
values (>8.0), the influence of the under-
lying rate process exerts a greater
influence on the overall rate.
In pipe flow under stagnant or low rate
(laminar flow) conditions, the corrosion
rate of copper is stabilized for pH > 6.0 at
a value of 0.2 MPY. Only when the pH is
reduced below 6.0 do the accelerating
effects of low pH and high temperature
manifest themselves. At high (turbulent)
flow rates in pipe flow, corrosion rates are
accelerated dramatically with reduced pH
and increased temperature. Under these
conditions, only at pH >8.0 is the
corrosion rate stabilized to an acceptably
low value of 0.2 MPY.
The use of steady-state electrochemical
techniques gave reliable and reproduci-
ble corrosion rate measurements, even in
water of low conductivity. In addition,
they provide the versatility needed for
experimental design of adequate sophis-
tication to provide data that can be used
in mechanism determination and model
development. These qualities greatly
enhance the research capabilities of the
investigator.
Recommendations
The ability to characterize complex
metal-oxide-solution systems as hetero-
geneous rate processes is an important
step in determining the dependence of
the corrosion rate on system variables.
Crucial to the development of aqueous
corrosion science is the gathering of
appropriate multidisciplinary information
to develop quantitative rate expressions
for processes involved in the general
corrosion of metals other than copper
used in the transport of drinking water.
The model presented for aqueous
copper corrosion should be extended and
refined. Extension to a broader range of
aqueous species and concentrations to
include species involved in water treat-
ment processes such as chlorination
seems advisable. Also in order are (1)
more sophisticated mathematical model-
ing of the oxide/solution interf acial inter-
actions controlling the solution pH just
next to the interface, and (2) precipitation
studies involving Cu2+ and OH~ species.
An evaluation could then be made of the
effect of precipitated hydroxide and
carbonate solids in altering the corrosion
rate by depositing on the metal surface.
The steady-state methods used here
should be further applied to the study of
other metal-oxide-solution systems
along with the testing and development
of transient electrochemical techniques
such as the A.C. impedance method. The
latter, because of its possibility for instan-
taneous measurement, can provide in-
valuable insight into the coupled rate pro-
cesses involved in aqueous corrosion.
The full report was submitted in fulfill-
ment of Contract No. R806686-010 by
the University of Washington, under the
sponsorship of the U.S. Environmental
Protection Agency.
-------
0.5
0.4
I
„• 0.3
I
.1
8-
o
o.i
o t
6.0
T-2S°C
Oxide film growth
Stagnant diffusion
6.5
7.0
7.5
8.0
8.5
Figure 2. Comparison of corrosion rates measured with and without solution mass transport
effects at 25°C.
I.OO-i
2.500
3.000
4.000
4.500
3.500
Log RE
Figure 3. Corrosion rate estimates for 1/2-in. copper tubing at varied flowrates and 25°C.
-------
1.00
O.80
ti 0.60
i
fc 0.40
o
0.20
RE-50000
5.O 6.O 7.0 8.0
Figure 4. The pH dependence of corrosion rates estimated for turbulent flow conditions.
*USGPOs 1984-759-102-10697
-------
Ronald D. Hilburn is with University of Washington. Seattle. Washington 98195.
Marvin C. Gar dais is the EPA Project Officer (see below).
The complete report, entitled "Seattle Distribution System Corrosion Control
Study: Volume VI. Use of a Rotating disc Electrode to assess Copper Corrosion,"
(Order No. PB 84-229 707; Cost: $19.00, subject to change) will be available
only from:
National Technical Information Service
5285 Port Royal Road
Springfield. VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Municipal Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
Official Business
Penalty for Private Use S300
ACCT - C06324C
LCD W TiLLrY
REGION V EPA
LIBRARIAN
LAB - M€RL
ST
60604
------- |