United States
Environmental Protection
Agency
Municipal Environmental
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-84-133 Sept. 1984
SER& Project Summary
In Situ Analysis of Corrosive and
Passive Surfaces by Laser-
Excited Raman Spectroscopy
Chris W. Brown
From both laboratory and field invest-
igations it has been found that
protective films can be formed or
deposited on lead and asbestos-cement
pipes. The purpose of the present
project was to determine the chemical
and/or crystalographic composition of
surface films on these pipes. Both
laboratory and field samples of lead
were investigated by Raman and
infrared spectroscopy. A number of
laboratory samples were investigated in
situ using laser-excited Raman
spectroscopy. A limited number of field
samples of asbestos-cement pipes were
studied using infrared spectroscopy. In
addition, surface films from field sam-
ples of galvanized iron pipes were
measured.
The results on lead pipes indicate that
the surface films are primarily
composed of lead monoxide, PbO. This
compound exists in two crystalograph-
ic forms, orthorhombic and tetragonal.
The latter should be the stable form
under ambient conditions; however,
orthorhombic is "stabilized" by some
types of impurities and is the dominant
form present at pH values below 7. Due
to its unstable nature, orthorhombic
PbO has a strong tendency to enter the
water column. The stable tetragonal
form of PbO was observed at pH values
greater than 7.5. This form appears to
be much more protective of the surface,
i.e., it acts as a protective film.
Variations in the chemical composi-
tion of asbestos-cement pipes make it
impossible to identify the composition
of surface films using spectroscopic
techniques; however, surface films on
field samples of galvanized iron were
identified as zinc carbonates using
jnfrared spectroscopy.
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 document-
ed in a separate report of the same title
(see Project Report ordering informa-
tion at back).
Introduction
Drinking water is often transferred to
homes and businesses through pipes
made of lead or asbestos-cement. These
materials are durable and are ideal for
fabricating pipes; however, both lead and
asbestos present known health hazards.
Under certain conditions, lead pipes can
corrode and lead enters the water.
Moreover, in asbestos-cement pipes, the
cement can corrode allowing the
asbestos to enter the water.
The purpose of the present project was
to identify the composition of films formed
on lead and asbestos-cement pipes that
were subjected to a number of different
aqueous treatments. Visible films either
form or they are deposited on these pipes.
These films can be part of the corrosion
process or they can be protective, i. e.,
inhibit further corrosion. Our goal was to
correlate the composition of these films
and the rate of corrosion with the
experimental conditions.
Procedure
Eleven laboratory experiments were
performed on lead systems for a duration
of 1 to 47 days; experimental conditions
for the experiments are summarized in
Table 1. Seven of these experiments were
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Table 1. Summary of Flow-Thru Cell Experiments
Experiment Type of
No. Cell
Duration
Days
Conditions
1 Small 1 Distilled water
2 Small 8 Tap water, pH = 5, hardness = 12 mg/L
3 Small 28 3/1 Distilled/Tap, alkalinity = 4 mg/L andpH = 9.5 (with
NaOH)
4 Small 10 3/1 Distilled/Tap, alkalinity = 4 mg/L and pH = 10.5
5 Large 34 3/1 Distilled/ Tap, alkalinity = 4 mg/L and pH = 9 3
6 Small 29 3/1 Distilled/Tap, alkalinity = 4 mg/L andpH = 9.0 with
100 ppm phosphate added as buffer.
7 Large 34 3/1 Distilled/Tap, alkalinity = 4 mg/L and pH = 8.0 with
phosphate buffer
8 Small 24 3/1 Distilled/Tap, alkalinity = 4 mg/L, 1000 ppm
phosphate, pH = 70.4 (with NaOH)
9 Small 14 .3/1 Distilled/Tap, alkalinity = 4 mg/L, 50 ppm phosphate,
pH = 8.6
70 Large 47 3/1 Distilled/Tap, buffered with phosphate to pH = 7 8
11 Large 40 3/1 Distilled/ Tap, alkalinity = 4 mg/L, pH = 8.4 with NaOH
performed with a small flow-thru cell
using 1 -inch-square lead coupons as the
test samples and a 2-liter water reservoir.
Four experiments were performed with
1 -inch OD lead pipes as the test sa/nples.
In these experiments, water was pumped
through the pipes from a 100-gal
reservoir.
Raman spectra of test samples in both
the flow-thru eel Is were measured in situ,
i.e., the cell was inserted into the Raman
spectrometer and spectra of the surfaces
were measured without removing the
samples from the cell. Infrared spectra of
surface films were measured by scraping
off some of the surface film and forming a
KBr pressed pellet.
Raman spectra were measured using a
Coherent Radiation Laboratory* Argon-
ion laser emitting at 488 nm or 514.5 nm,
a Spex Industries Model 1401 double
monochromator, and photon counting
detection Infrared spectra were meas-
ured on a Beckman Model 4260 Infrared
Spectrometer. Corrosion rates were
determined in mils per year using the
PAIR technique on a Petrolite Corporation
Model M-3010 Corrosion meter.
A number of field samples of lead,
asbestos-cement, and galvanized pipes
•Mention of commercial products or trade names
does not constitute endorsement or recommenda-
tion for use
were also analyzed. Where possible
Raman spectra were measured;
however, the scrapings from a number of
samples were analyzed by infrared
spectroscopy
Results and Discussion
Lead Pipes
One of the goals of this project was to
show that similar results could be
obtained using a miniature flow-thru cell
with small lead coupons or a large flow-
thru cell with 1-inch OD lead pipes.
Before initiating the project, we knew
that it would be much easier to measure
Raman spectra of samples in the small
cell but we were not sure the results
would be the same as those obtained in
the large system. By the end of the project,
we were convinced that identical results
would be obtained with both systems.
This was confirmed by the similarities in
the corrosion rates and Raman spectra of
the surface species.
The analytical results on lead pipes
indicate that corrosion is minimized
when tetragonal PbO is formed on the
surface. Tetragonal PbO is the
thermodynamically favored form at
temperatures below 488°C; however, the
other crystallographic form, orthorhom-
bic PbO, is genera.lly observed. The orth-
orhombic form may appear for a number
of reasons: it is apparently stabilized by
impurities, which will be present in every
"real" system; it also appears to be
produced in the decomposition of other
oxides, e.g., Pb304.
Orthorhombic PbO is less protective of
the surface and apparently corrodes
away, i.e., gets into the water system.
Some of our earlier results indicated that
orthorhombic PbO may be formed in
solution and redeposited on the surface.
Under conditions favoring the formation
of orthorhombic PbO, the oxide was
found on the surface of a Pt reference
electrode in the same container. Under
conditions favoring tetragonal PbO, this
oxide was observed only on the surface of
the working lead electrode. In the present
program, our goal was to determine those
chemical conditions that produced
tetragonal PbO by using Raman spectra
to monitor the surfaces. In addition, we
used Raman to identify any other oxides
or compounds present on a surface.
Orthorhombic PbO was produced in
experiments 1 and 2 when the pH was
< 7. In both experiments, a precipitate
was formed, although this was more
pronounced at the lower pH 1. In
experiment 2, the pH of the aqueous
medium was increased to 7.5 after the
fifth day, and the Raman spectrum of the
surface was that of tetragonal PbO.
The other nine experiments were
performed over the pH range of 8 to 10.5
In every case, tetragonal PbO was
identified as a component of the surface
film. In addition, the spectrum of lead
carbonate was observed in experiment 5
and there was an indication of a small
amount of orthorhombic PbO in experi-
ment 6 at day 12, which seemed to in-
crease by day 34. The corrosion rates for
this experiment ranged between 1 and 2
mils per year, which was significantly
higher than other experiments. It is
difficult to explain the results in experi-
ment 6 since similar conditions produced
tetragonal PbO in other experiments.
In experiments 6 through 10, phosphate
was added as a buffer. With the exception
of experiment 6, adding phosphate
lowered the rate of corrosion at lower pH
values. For example, the lowest overall
rates were obtained in experiment 7 with
pH = 8 and phosphate buffer. In this case,
the rate was > 0.5 mpy only on one day.
These results are very optimistic.
The general tendency of the corrosion
rates was to start high, decrease for
several days, increase rapidly, and start to
decrease again. In each experiment,
there was a considerable amount of oscil-
lation, i.e., increases and decreases in the
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corrosion rate. We suspect that initially a
film freely forms on the bare metal. As the
film becomes thicker, the rate decreases.
At some point, the film becomes too thick
and some of it flakes off, causing an
abrupt increase in the rate. As more film
forms on the surface, the rate decreases
again.
Specific correlations between the
corrosion rate and Raman spectra could
not be found. For example, it was not
possible to correlate the oscillations in
the corrosion rate with Raman spectral
intensities. Typically, the Raman intensi-
ties increased with the time of the
experiment, although there were some
significant changes in intensities. Gener-
al correlations between corrosion and
Raman spectra could be seen For
example, the corrosion rates were higher
than expected in experiment 6, and
orthorhombic PbO was observed.
Although conditions do not favor either of
these, they correlate well together.
In summary, it appears that the
tetragonal form of PbO is the more stable
and protective form of the lead oxides.
The rate of corrosion is decreased
considerably when tetragonal PbO is
present. The only apparent difficulty with
the formation of tetragonal PbO is that
the film may become too thick and flake
off. The evidence of this flaking is not
conclusive; however, it does seem
reasonable in light of the fluctuations in
the rate of corrosion. Tetragonal PbO was
formed and the fluctuations were
minimized with a phosphate buffer and
pH of 8.0.
In addition to measuring in situ Raman
spectra of laboratory experiments on lead,
Raman spectra of seven field samples of
lead pipes supplied by EPA were
measured. Some samples contained
tetragonal PbO, others contained
orthorhombic PbO From the information
supplied, it appeared that the tetragonal
form was produced at higher pH values,
or with phosphate buffer, or with both.
Asbestos-Cement Pipes
Fifteen field samples of asbestos-
cement pipes were also supplied by EPA.
We were unable to obtain adequate
Raman spectra of surface films.
Moreover, we could not obtain Raman
spectra of scrapings from the surface. We
were able to measure infrared spectra of
the scrapings pressed into KBr pellets.
Our intentions were to determine the
differences between spectra of "coated"
surfaces and those of clean surfaces.
However, differences between spectra of
clean surfaces were as great as the differ-
ences between clean and "coated"
surfaces. Thus, reasonable comparisons
could not be made.
Galvanized Iron Pipes
Five galvanized pipe samples were also
supplied. Again, we had difficulty in
obtaining Raman spectra. Good infrared
spectra were, however, obtained. Since
we knew that carbonate was present in
some of these samples, we measured the
spectrum of zinc carbonate as a standard.
Comparison of this spectrum with those
of the field samples indicated that zinc
carbonate was the major component in
two samples and was present to a lesser
extent in the other three samples.
Conclusions
The results on both laboratory and field
samples of lead suggest that corrosion is
reduced at high pH values, or with the
addition of phosphate buffer, or with
both. The reduction in the corrosion rate
correlates with the formation of the more
stable tetragonal form of PbO. At lower
pH values, the presence of carbonates
seems to influence the formation of the
less stable orthorhombic crystalline form
of PbO. The only major concern about
the formation of tetragonal PbO is that
under certain conditions the films can
become rather thick and flake off.
However, there is no direct evidence that
this happens.
Raman spectroscopy proved to be an
ideal analytical method for monitoring
the composition of filmson lead, although
it was not effective on asbestos-cement
or galvanized iron pipes. We circumvent-
ed this problem by measuring infrared
spectra of surface films on these
materials after scraping film from the
surface. Infra red spectra were very useful
in identifying the composition of films on
the galvanized pipes as zinc carbonate.
The general conclusions from this
project suggest that lead corrosion can be
at least reduced with higher pH values or
with the addition of phosphate buffers
Moreover, the spectroscopic results
support the use of these molecular
spectroscopic techniques for identifying
the compositions of surface films
The full report was submitted in
fulfillment of Contract No. R806686 010
by the Seattle Water Department under
the sponsorship of the U.S. Environment-
al Protection Agency.
Chris W. Brown is with Department of Chemistry, University of Rhode Island,
Kingston, Rl 02881
Marvin C. Gardels is the EPA Project Officer (see below).
The complete report, entitled "In Situ A nalysis of Corrosive and Passive Surfaces
by Laser-Excited Raman Spectrometry," fOrder No. PB 84-229 756; Cost:
$14.50, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield. VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Municipal Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
~tf U S GOVERNMENT PRINTING OFFICE, 1984 — 759-015/7805
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Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
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