September/October 2004
Preventing Corrosion in Steel Bridges
by Shuang-Ling Chong
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(Above) A painter applies a protective coating to a bridge to protect it from corrosion. |
FHWA researchers evaluate the accuracy and reliability of three chloride test kits to determine their performance and accuracy.
Each year the Federal Government
and State departments
of transportation (DOTs)
spend billions of dollars on bridge
rehabilitation and maintenance due
to corrosion. On bridges, corrosion
is most often caused when steel is
exposed to atmospheric conditions,
such as salt, moisture, and oxygen.
To prevent corrosion on bridges,
transportation agencies apply a protective
coating to the steel.
But according to Dr. Bernard
Appleman, a consultant at KTA-Tator,
Inc. and former executive director of
the Society for Protective Coatings,
if the steel has a corrosive agent on
it before painting, the protective
coating may fail prematurely. "Soluble
salts, especially chloride salts that are
not removed before painting, are a
major source of early and often catastrophic
paint failure," says
Appleman. If the paint fails prematurely,
the resultant corrosion will
eventually compromise the structural
integrity of the metal. "Ultimately,
this paint failure can require extensive
bridge maintenance, which is
not only costly but also an inconvenience
to the driving public," he
adds. Therefore, before the bridge
painter applies the protective coating
to either new steel or a rehabilitated
bridge, the surface needs to be
evaluated for cleanliness.
Presently, painting specifications
almost all rely on visual (or qualitative)
measurements to determine
readiness for applying protective coatings.
However, researchers at the Federal
Highway Administration (FHWA)
are looking for a more accurate, quantitative
measurement that can be used
again and again to determine if corrosive
elements are on steel prior to
applying a coating. One such method
may be to test for chloride.
To help bridge coating inspectors
better assess the condition of steel
prior to painting, FHWA recently
evaluated three commercially available
chloride test kits that are used
to determine the cleanliness of steel
surfaces. The objectives were to
assess the accuracy and precision of
the tests and to identify the factors
that influence their performances.
From Visual to Quantitative Assessments
Because contaminants can affect the
performance of bridge coatings,
inspectors need accurate techniques
to assess the cleanliness of the steel
surfaces prior to painting. Most
methods used today, however, are
qualitative or semiqualitative at best.
"All of the cleaning standards
today are visual," says Bob Kogler,
team leader for bridge design and
construction research at FHWA. According
to Kogler, assessing steel
cleanliness using visual standards
can lead to disputes. "An inspector
may look at the steel and see indications
that it is not clean enough,
while the contractor may argue that
it is clean enough," he says. "To some
degree, even though we have standards,
it is almost a matter of opinion
because the standards themselves
are qualitative."
In 2001, the FHWA Nondestructive
Evaluation Validation Center
completed a study that evaluated the
accuracy of the visual inspection
method for determining the condition
of bridges. The study showed
that inspectors vary considerably in
how they complete routine inspections.
In particular, they vary in how
they assign condition ratings.
"Eventually we need to make our
evaluation of steel surface cleanliness
a quantitative measure, because
it would clear up a big area of disputes
on bridge painting jobs,"
Kogler says. "The measurements
[derived from the testing kits] will
tell us how chemically clean [the
surface] is, not just how clean it
looks. And that will give us a much
better measure of the potential performance
of the paint."
Some applications, such as those
in the marine industry, already are
moving toward quantitative methods
to assess chloride concentrations on
steel surfaces.
The Problem with Chloride
Because chloride is the primary surface contaminant and is usually the most corrosive agent to steel, inspectors may be able to test for it before painting steel surfaces. High concentrations of chloride can cause early coating failures, such as rust and
delamination, a process in which the coating begins to separate from the steel. Ultimately, the rust and coating delamination can destroy the structural integrity of the metal. Chloride is of particular concern for structures that are salted during deicing operations or are located in a marine environment, where the concentration of chloride salts can be high in seawater and spray.
After the steel surface is blasted clean with abrasives or cleaned with high-pressure water, and before a coating is applied, the inspector should assess or test the steel surface for chloride. If the visual inspection or testing indicates high chloride concentrations, the metal must be cleaned again and retested.
Three Chloride Test Kits Evaluated
Currently where specified, coating inspectors use one of three commercial test kits to evaluate chloride levels quantitatively. Generically, the kits are the swab test, the patch test, and the sleeve test.
All three tests use a liquid, either acidic fluid or de-ionized water, to dissolve or extract chlorides on the surface of the steel into a solution. The inspector then tests the solution for chloride concentrations. The swab test relies on wet cotton balls to extract the chloride from the surface of the steel. The patch test uses a syringe containing extraction fluid to
draw chloride from the patch test area. And the sleeve test extracts the chloride in a fluid-containing sleeve that is attached to the steel.
According to State DOTs and bridge inspectors, all three tests have shown inconsistent and highly variable results. These inconsistencies may be due to different extraction efficiencies and detection sensitivities in the tests, as well as
operator variability.
Therefore, FHWA researchers investigated the variability and limitations of the test methods to establish techniques that may be used to obtain reliable and accurate chloride concentration test results.
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If the steel surface of a bridge is not cleaned adequately before painting, the protective coating can fail prematurely. The steel then will develop corrosion, such as the rust shown on the underside of this bridge. If the
bridge is not rehabilitated, the corrosion eventually will compromise its structural integrity. |
Experimental Procedures
The researchers analyzed steel panels in a vertical position. Four different levels of chloride concentration, ranging from 3 to 30 micrograms per centimeter squared (mg/cm2), were applied to the panels to determine if the chloride concentration affected the validity of the results. An industry rule of thumb is that after blasting, a bridge should be painted within 4 hours. Therefore,
the researchers performed tests under three conditions that fell within this timeframe: within 1 minute after panels were doped
(that is, artificially contaminated with chloride), after aging doped panels at high heat and moderate humidity for 4 hours, and after aging
doped panels at high heat and high
humidity for 4 hours.
The detectors for the swab, patch,
and sleeve tests are an ion detection
strip, four bottles of titration liquids,
and an ion detection tube, respectively.
Because the patch test can use
two different fluids, acidic fluid or
de-ionized water, the researchers
conducted additional tests to determine
which fluid recovered the most
chloride. Since the researchers found
that acidic fluid extracted more chloride
than de-ionized water, acidic
fluid was used in the patch test.
In all, the researchers evaluated
each kit under 12 different conditions
(4 chloride concentrations and
3 aging conditions) to determine
how chloride concentrations and
aging affect the accuracy of the test.
Each test was performed three times
by three different operators at the
Paint and Corrosion Laboratory at
FHWA's Turner-Fairbank Highway
Research Center (TFHRC) in
McLean, VA.
Three Chloride Test Methods
Following are detailed descriptions of how the researchers conducted tests using the three kits.
Swab Test
The researcher extracted chloride from a surface area of 150 cm2
(23.25 in2)-a 15-cm-long by 10-cm-wide steel panel-using 15
milliliters, ml (0.51 fluid ounce, fl oz) of de-ionized water for cotton
swabbing. To reduce dripping, only one-third of each of the four
cotton balls used for swabbing was soaked with de-ionized water.
The researcher then absorbed the remaining liquid with an additional
cotton ball and used an ion detection strip to measure the
concentration of chloride in the extracted solution.
Patch Test
The researcher glued a patch securely onto the steel surface, covering
an area of 12.25 cm2 (1.89 in2). The researcher then injected 1.5
ml (0.05 fl oz) of extraction fluid into the patch, then extracted two-thirds
of the fluid from the patch and reinjected it to mix the fluid
more thoroughly. The researcher then rubbed the patch with a finger
for 1 minute to promote chloride solubility. Next the patch was
rinsed with an additional 1.5 ml (0.05 fl oz) of extraction fluid.
Finally, the researcher combined the two extractions and titrated the
resulting 3 ml (0.1 fl oz) of extract with reagents included in the kit.
Sleeve Test
The researcher poured 10 ml (0.34 fl oz) of extraction fluid into a
sleeve and then attached the sleeve firmly to the steel panel. The
researcher then lifted the free end of the test sleeve and held it
upright with one hand to allow the extraction fluid to make contact
with the test surface. With the other hand, the researcher massaged
the solution through the test sleeve against the steel surface for 2
minutes. The researcher then removed the test sleeve and used an
ion detection tube to test the solution for chloride concentration. |
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A technician uses high-pressure water to clean a section of steel. |
Comparing the Results
The researchers identified strengths
and shortcomings for each test. The
research team acknowledges that
since these tests were conducted in
a laboratory, where procedures were
carefully controlled, the experimental
results may be better than would
be expected in the field.
Of the three tests, the swab test
recovered the highest amount of
chloride and also offered the most
reproducible data. For freshly doped
steel (that is, a specific amount of
chloride applied to the steel for
testing), the swab test recovered
approximately 70 to 100 percent of
the chloride. This test also had the
least variability.
The swab test uses an ion detection
strip, which can detect chloride
concentration only above 30 parts
per million (ppm). The large extraction
area, however, compensates for
the detection limit. The researchers
found that the lowest level of chloride
detection possible under the
laboratory conditions employed
was 3 mg/cm2.
One shortcoming of the swab test is that it must be conducted very carefully, which may be challenging in the field where a test operator or inspector may be high on a ladder testing steel overhead. Because it is conducted in an open environment, water
can drip and evaporate easily, which will result in reduced chloride recovery and therefore imprecise results.
Unlike the swab test, the patch test is a closed extraction system, which prevents fluid evaporation and loss. However, the operator may still lose fluid if either the patch is not adhered to the steel surface firmly, or if the syringe, which is used to extract fluid, is improperly inserted into the patch. In either case, the loss of even a small amount of extraction fluid will
result in inaccurate chloride measurements.
The patch test, with titration liquids used as a detector, also provides high chloride recovery. But the results were found to be the most unreliable of the three tests, as indicated by a higher margin of error. One potential cause for the error is the variability in drops needed to reach the titration end point (that is, color change). If the color change falls between two drops, some operators will use an extra drop while others will use one drop less. The number of drops used may vary by operator, or the same operator may use a different number of drops for each test conducted. This variation in the number of drops will affect
chloride concentrations.
An additional shortcoming of the patch test is that it only indicates minimum and maximum values rather than actual values. However, a coating inspector could be conservative and use the maximum value to determine whether to proceed with a painting job.
A final shortcoming of the patch test is that the acidic fluid requires mercury nitrate as one of the titrants.
Because mercury is a hazardous
waste, the operators or inspectors
must follow strict guidelines
when disposing of the fluid.
The sleeve test, like the patch
test, is a closed system with little
risk of fluid loss, but extraction fluid
can be lost if the sleeve is not adhered
to the steel surface firmly. If
fluid is lost, the test will generate
unreliable chloride measurements.
The sleeve test was more effective
at recovering chloride at higher
concentrations than at lower concentrations.
The low rate of chloride
recovery at lower concentrations
may be due to the low sensitivity
and unclear color separation at the
low reading end of the ion detection
tube. The sensitivity can be increased
if the extraction volume is
reduced or extracted area is increased.
The sleeve test had a margin
of error that fell between that of
the swab and patch tests.
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For the patch test, a researcher injects chloride extraction fluid into a patch on a vertical
steel plate. |
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For the swab test, a researcher uses a cotton ball and deionized water to detect and extract chloride from the steel plate. |
Parameters of the Three Chloride Extraction Test Methods
| Swab Test |
Patch Test |
Sleeve Test |
System type |
Open |
Closed |
Closed |
Detection method |
Ion detection strip |
Titration |
Ion detection tube |
Area, cm2 |
150 |
12.25 |
10 |
Extraction fluid, ml |
15 |
3 |
10 |
Area (cm2)/volume (ml) |
10 |
4.1 |
1 |
pH value of extraction fluid |
De-ionized water, pH = 6.7 |
Acidic, pH = 3.9 |
Acidic, pH = 4.2 |
The table shows the system type, detection method, area of extraction, extraction fluid
volume, ratio of area to fluid volume, and pH value of the extraction fluid for each of the three test kits. Source: FHWA. |
Humidity Affects Test Results
An important finding of the research
is that heat and humidity will affect
test results. When the doped panels
were aged at a high temperature-
37 degrees Celsius (98.6 degrees
Fahrenheit)-for 4 hours under two
different humidity conditions, the
chloride recovery was less than that
of freshly doped panels. However,
the researchers noted a considerable
difference between moderate and
high humidity. At 57 percent relative
humidity, the chloride recovery was
reduced only slightly. But for all
three tests, chloride recovery decreased
considerably at 78 percent
relative humidity.
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The bar graph compares the chloride recovery by the swab, patch, and sleeve
tests at four different chloride levels: 3, 5, 9 and 30 micrograms per centimeter
squared (mg/cm2). For all tests, the chloride recovery decreased with decreasing
chloride concentrations; it decreased from 100 to 70 percent, 120 to 85 percent,
and 100 to 25 percent for the swab test, patch test, and sleeve test, respectively.
The graph also shows the error bar for each of the tests, indicating how
reproducible the results are. A low standard error indicates high reproducible
results, while a high standard error indicates low reproducible results. All three
tests showed some standard error in the tests. The standard error was lowest
for the swab test and highest for the patch test. Source: FHWA. |
The researchers speculate that
low levels of rust formed after the
steel was exposed to high levels of
heat and humidity for 4 hours. The
invisible rust entrapped the chloride,
making it more difficult to extract.
These results suggest that inspectors
should determine the surface
chloride concentration as soon as
possible after blasting. Any delay,
especially in hot and humid environments,
may result in erroneously low
chloride values. If these low chloride
values fall within the acceptable
limits for the protective coating, an
inspector may decide to apply the
coating to a steel surface that in
reality has unacceptable levels of
chloride trapped under invisible rust.
Using the Research Findings
Although additional research is warranted,
the findings from the FHWA
study may provide valuable information
that coating inspectors can
apply in the field today.
Summary of Test Results
| Swab Test |
Patch Test |
Sleeve Test |
Chloride concentration |
High (10)a |
Medium (4.1) |
Low (1) |
Loss of extraction fluid |
Yes |
No, if patch adhered
to steel firmly |
No, if patch adhered to steel firmly |
Minimum threshold |
3µg/cm2 |
~ 1 µg/cm2 |
~ 5 µg/cm2 |
Reproducibility of results |
High (for tests above 30 ppm) |
Low (only minimum and maximum values given) |
High (for tests above 9 ppm) |
Detection sensitivity |
> 30 ppm |
> 1 µg/cm2 (4 ppm) |
> 5 ppm |
Detection range |
30 - 600 ppm |
1 - 50 µg/cm2 |
1 - 50 ppm (= 1 -50 µg/cm2 |
a: Ratio of extracted area to extraction fluid volume. |
"A coating inspector could utilize the results of [the] research by selecting a technique based on the levels of chloride expected and the environmental conditions encountered," Appleman says. "For example, if the surface was exposed to the sun on a hot day, the coating inspector might choose to use a method other than swabbing, since that method would result in inaccurate chloride recovery due to water evaporation during extraction in open air. As another example, if the acceptance
criterion was low-such as 3 micrograms per square centimeter-a coating inspector would avoid using a method with low sensitivity."
Tips to Improve the Accuracy Of Chloride Tests
- Because all chloride detection tests are highly sensitive to the testing procedure, the research team developed the following suggestions to help improve the accuracy of the tests.
- Conduct extractions as soon as possible after the steel is blasted because exposure of clean steel to high heat and humidity can reduce the amount of chloride extracted.
- Obtain complete training in the extraction method being used.
- Verify the accuracy for each batch of detector using known chloride standards before performing field tests. Furthermore, extract steel panels that are freshly doped with a solution of known chloride concentration to test the operator's technique.
- Use extreme caution to avoid losing fluid during the extraction.
- Extract large areas using smaller amounts of extraction fluid to increase the sensitivity of the test.
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From the Lab to the Field
The next step would be to evaluate the performance of the chloride recovery test kits in the field to determine whether quantitative tests should be incorporated into specifications for bridge painting. "This research [was conducted] in a laboratory under controlled conditions, which is appropriate for basic research," Kogler says. "[But] we need to evaluate these methods under field conditions, in cooperation with State bridge owners, to see what kind of improvements we need to make to these tests or procedures before we put [them] in our specifications."
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The bar graph shows the effect of aging on the chloride recovery for the swab, patch, and sleeve tests. For the test panel that aged at 37 degrees Celsius (98.6 degrees Fahrenheit) and 57 percent relative humidity (RH) for 4 hours, the chloride recovery is almost 100 percent, similar to that from the freshly doped panels. However, after the test panels were aged at 37 degrees Celsius and 78 percent relative humidity for 4 hours, the chloride recovery decreased significantly for all the tests. The recovery decreased from 100 to 40 percent, 100 to 80 percent, and 100 to 60 percent for the swab, patch, and sleeve tests, respectively. The graph also shows the error bar for each of the tests. All three tests showed a similar, moderate standard error for the aged tests. Source: FHWA. |
Future research will not only evaluate the accuracy and reliability of the tests but also the usability. "Imagine climbing up on a ladder to a bridge abutment, with traffic overhead on the bridge," Kogler says. "You have a syringe of water, and
you need to glue this sticky thing on the bridge, and use a dropper. It takes a while to run the test. Even though [the tests] were designed to be used in the field, we need to find out how usable they really are."
The big question is whether bridge inspectors should be required to test for chlorides for all bridge painting jobs right now, and whether the tests are accurate and user-friendly enough. In laboratory tests, the researchers found that the testing must be conducted very carefully to ensure consistency of results. "This research provides some of the first real unbiased data on
these test kits," Kogler says. "The analysis gives us a snapshot of where the technology is now and where we need to go to improve it."
Shuang-Ling Chong, Ph.D., has been a senior research chemist at FHWA since 1989. Chong's responsibilities
have included managing the Paint and Corrosion Laboratory, studying accelerated testing of bridge coatings, and developing
methods for characterizing coating materials and failures. Chong earned her Ph.D. in physical chemistry in 1969 from Rutgers, The State University of New Jersey.
Additional details regarding the experimental methods used in the study are available in Dr. Chong's article, "Intra-Laboratory Assessment of Commercial Test Kits for Quantifying Chloride on Steel Surfaces," published in the Journal of Protective Coatings and Linings, p. 42, August 2003. For more information, contact Shuang-Ling Chong at 202-493-3081.
The author would like to thank Yuan Yao and Muriel Rozario of Soil and Land Use Technology (SaLUT), Inc. for their input in preparing this article. The author also would like to acknowledge the vendors of the three test kits evaluated, who were very cooperative in this study.
Other Articles in this issue:
Taking the High Road
The Space Between
Designing Tomorrow's Pavements
Learning from the 2003 Blackout
Rustic Pavements
I-95 Shutdown—Coordinating Transportation and Emergency Response
Traffic Safety Information Systems
Preventing Corrosion in Steel Bridges
The Uncertainty of Forecasts
Testing Truncated Domes