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|Federal Highway Administration > Publications > Public Roads > Vol. 68 · No. 2 > Preventing Corrosion in Steel Bridges|
Preventing Corrosion in Steel Bridges
by Shuang-Ling Chong
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.
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.
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.
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.
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.
"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."
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."
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.
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