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.

FHWA researchers evaluate the accuracy and reliability of three chloride test kits to determine their performance and accuracy.

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. 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. 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 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.

Left picture- A painter applies a protective coating to a bridge to protect it from corrosion.
Right picture- 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.