Researchers at FHWA set out to find bridge coatings that will last 100 years. Here’s what they found.

Corrosion is the bane of many a bridge designer’s existence. When steel bridge elements are inadequately protected from the natural environment, they face the risk of corrosion due to intrusion of moisture and salt. This corrosion can impair the long-term function and integrity of the structure.

To ward off corrosion, engineers have developed a variety of coatings that can be applied to steel surfaces to protect them from the elements. These coatings are critical to ensuring the long-term durability and integrity of bridge structures. Yet their application and maintenance pose a number of challenges. The current state of practice involves multilayer coatings typically consisting of a zinc-rich primer over an abrasive blast-cleaned surface and one or two additional coating layers on top of the primer. A typical three-coat application for rehabilitating an existing steel bridge can account for as much as 20 percent of the cost of fabricating a new steel bridge. Repainting a bridge in the field is highly labor intensive and expensive, due to the need for scaffolding and containment measures, where the paint blasted from the surface needs to be collected and disposed of.

Current three-coat systems offer a service life up to 30 years before a major touchup is required. Researchers at the Federal Highway Administration (FHWA) want to more than triple that. In August 2009, staff in FHWA’s Coatings and Corrosion Laboratory at the Turner-Fairbank Highway Research Center (TFHRC) in McLean, VA, initiated a study to identify a coating system that could provide 100 years of virtually maintenance-free service life at a cost comparable to existing coating systems. The goal was to identify coating systems that could dramatically extend the service life of the original shop-applied coating and thereby reduce the frequency of pricey repeat applications down the road.

Choosing the Coatings

The mission of the Coatings and Corrosion Laboratory is to use proven methods--and develop innovative new methods--for evaluating the durability of new coating systems, especially environmentally compliant materials, for their ability to protect steel bridges from corrosion over the long term. Researchers typically evaluate the coatings using both accelerated laboratory tests and natural outdoor exposure.

For this study, the researchers selected eight coating systems--three three-coat systems consisting of organic, inorganic, and moisture-cured zinc-based primers; four two-coat systems with various combinations of zinc-based primers and organic top coats; and a single-coat system of calcium sulfonate alkyd.

The researchers chose these coating systems based on previous FHWA studies and past experience at the laboratory. In particular, earlier studies indicated that the two-coat systems might compare favorably with the high-performing legacy zinc-rich three-coat systems. Another FHWA study revealed that single-coat systems could meet requirements similar to the three-coat systems for specialty applications such as overcoating and maintenance purposes.

Of the eight coating systems chosen for the study, two of the three-coat systems were used as controls, and the remaining six coating systems were selected for testing. The FHWA researchers devised names for the coating systems based on their chemical compositions, such as inorganic zinc primer/epoxy/aliphatic polyurethane, and referred to the coatings by their acronyms, in this case, IOZ/E/PU.

Preparing the Test Panels

The researchers used two types of test panels for the study. The first, described as type I, was a conventional 4- by 6-inch (10- by 15-centimeter) steel plate, rectangular in shape. Research staff also developed a new type of test panel, dubbed type II, to more closely simulate the detailing of steel bridge members. Each type II panel was an 18- by 18-inch (46- by 46-centimeter) square and had V-shaped and inverted T-shaped welded attachments, an overlap joint, an angle attachment, and five bolt-nut assemblies affixed to the panel.

The researchers prepped the steel substrates using white metal blast cleaning, according to a surface preparation standard (SSPC-SP5) prescribed by the Society for Protective Coatings. Next, they coated all of the type I test panels according to the manufacturers’ recommendations for dry film thickness.

For the larger type II panels, the researchers identified three areas on each panel that would be coated with varying dry film thicknesses. The majority of each type II panel was coated according to the dry film thickness recommended by the manufacturer. This area was termed nominal. One 7- by 8-inch (18- by 20-centimeter) square area within each panel received a lower amount of coating, a dry film thickness 20 percent less than target dry film thickness. Another 7- by 8-inch (18- by 20-centimeter) square area received a higher percentage of coating, a dry film thickness 20 percent greater than the target.

A Variety of Test Conditions

The researchers tested the panels under three types of conditions: accelerated laboratory testing, outdoor exposure testing at TFHRC, and outdoor exposure testing at the Golden Gate Bridge in San Francisco, CA.

Accelerated laboratory testing. The researchers performed accelerated laboratory tests consisting of 360-hour-long cycles that included three types of exposure: freezing, ultraviolet/condensation, and prohesion (related to protection and adhesion of protective coatings under certain corrosion test methodologies). The tests were carried out for 300 days, consisting of 20 cycles. The researchers examined the panels upon completion of every cycle and also at the conclusion of the accelerated laboratory testing.

Outdoor exposure testing at TFHRC. For the outdoor testing in Virginia, the research team installed the type I and II panels on two wooden racks that were inclined at 30 degrees and faced toward the south. To simulate natural weathering, all panels were left to weather outdoors. The researchers also designed an automatic system with a timer to dispense salt spray for a short period every 24 hours, which allowed for simulating natural weathering with salt spray.

Outdoor exposure testing at the Golden Gate Bridge. San Francisco’s severe fog conditions and airborne chlorides create a harsh climate for steel structures, making the Golden Gate Bridge an ideal location for this study.

“This bridge is exposed to a number of microclimates because temperatures, wind speeds, and fog conditions can change within a fraction of a mile,” says Dennis Dellarocca, paint superintendent for the Golden Gate Bridge.

As at TFHRC, researchers placed the type I and II panels on wooden racks inclined at 30 degrees. The racks were deployed on top of the south anchorage house near the south abutment of the Golden Gate Bridge. The research team evaluated the test panels every 6 months to assess coating performance.

Characterization Tests and Evaluation Techniques

A series of tests helped characterize the performance of the coatings on the panels before, during, and after the accelerated laboratory testing and outdoor exposure tests. In particular, the researchers looked at characteristics including color, gloss, dry film thickness, and adhesion strength. They also employed a technique known as Fourier transform infrared spectroscopy, which helps identify the chemical composition of paint coatings and metal surfaces. Further, they reviewed the samples to detect any discontinuities, such as pinholes, voids, cracks, thin spots, or contaminants in the coating film.

Specifically, the researchers evaluated the performance of the eight coating systems under the four test conditions--accelerated laboratory testing, natural weathering with and without salt spray at TFHRC, and natural weathering at the Golden Gate Bridge--according to several parameters. Those include gloss reduction, change of color, change of adhesion strength, development of surface defects, and growth of rust creepage at the scribe. (A scribe is a line scratch made with a cutting tool through the overcoat surface down to the steel substrate to simulate defects in the paint coating.)

How the Coatings Stood Up

In tests of this type, reductions in color and gloss are typically regarded as changes in the physical properties of a coating system. Changes in adhesion strength, the development of coating defects, and creep from the scribe are generally seen as indicators of the performance of a coating system in an exposure condition.

Results in gloss reduction/color change. Two two-coat systems (thermally sprayed zinc primer/linear epoxy and experimental zinc primer/linear epoxy) and the one-coat system (high-ratio one-coat calcium sulfonate alkyd) demonstrated the greatest gloss reduction. Both of the two-coat systems failed prematurely due to significant loss in color and gloss, and both had the same linear epoxy topcoat.

The two coating systems that exhibited the fewest gloss changes after tests were moisture cured urethane zinc primer/epoxy/fluorourethane (a three-coat system) and inorganic zinc primer/polysiloxane (another two-coat system). The three-coat control systems (inorganic zinc primer/epoxy/aliphatic polyurethane and zinc-rich epoxy primer/epoxy/aliphatic polyurethane) exhibited less than 30 percent gloss reduction after tests. Type II panels in natural weathering tended to show the greatest gloss reduction among all the coating systems.

The one-coat system (high-ratio one-coat calcium sulfonate alkyd) exhibited the most dramatic change in color, ranging from a 2.7 percent change observed in the type I panels under accelerated laboratory testing to nearly 12 percent in the type II panels under natural weathering with salt spray. Two of the two-coat systems (thermally sprayed zinc primer/linear epoxy and experimental zinc primer/linear epoxy) also showed noticeable changes in color, ranging from about 3 percent to 4.5 percent across all samples and testing sites. These three coating systems also demonstrated the greatest gloss reductions.

The others exhibited less than 2 percent color changes after the tests. Because of scattered data among the different test conditions, the effects of salt spray and the type of test panel on color change were inconclusive, except that most coating systems exhibited the least percentages of color change under accelerated laboratory testing.

Results in adhesion strength and surface defects. Except for the high-ratio one-coat calcium sulfonate alkyd and moisture-cured urethane zinc primer/epoxy/fluorourethane, all of the two-coat systems, including the controls, showed varying degrees of reduction in the adhesion strength at the end of testing in every test condition. The researchers did not observe a clear trend between reduction of adhesion strength and test conditions. Further, they were unable to obtain data on adhesion strength for the thermally sprayed zinc primer/linear epoxy coating at the end of the accelerated laboratory testing because the coating surface had become blistered.

The two-coat experimental zinc primer/linear epoxy panels showed the greatest reduction in adhesion strength. Both two-coat systems--thermally sprayed zinc primer/linear epoxy and experimental zinc primer/linear epoxy--which showed high variations in gloss and color, also suffered from the largest reductions in adhesion. The two control coatings (inorganic zinc primer/epoxy/aliphatic polyurethane and zinc-rich epoxy primer/epoxy/aliphatic polyurethane) and the zinc-rich epoxy primer/aliphatic polyurethane revealed the least amount of loss in adhesion strength. The other two-coat system, inorganic zinc primer/polysiloxane, showed a moderate loss (less than 40 percent) in adhesion strength.

Although the initial assessment of all coating systems showed no defects or evidence of invisible discontinuities on the surface, the accelerated laboratory testing resulted in severe surface deterioration as indicated by blistering, rusting, and cracking.

The two-coat thermally sprayed zinc primer/linear epoxy coating developed four defects after 1,080 hours (or 45 days) of accelerated laboratory testing, and the number of defects increased dramatically after 1,440 and 1,800 hours (60 and 75 days) of testing. Following the appearance of the defects, excessive blistering, which covered more than 50 percent of the surface area, and cracking of the surface developed. The zinc-rich epoxy/linear epoxy  coating showed no defects until 2,880 hours (120 days) of testing, followed by progressive changes leading to surface microcracks, which transformed into macrocracks. The three-coat moisture-cured urethane zinc primer/epoxy/fluorourethane coating did not develop any defects, while other coating systems developed only minimal coating defects (less than 1 percent of the surface area) upon completion of the accelerated laboratory testing at 3,600 hours (150 days).

Most of the type I panels did not develop any defects during outdoor exposure in natural weathering and natural weathering with salt spray. However, the thermally sprayed zinc primer/linear epoxy coating exhibited defects covering more than 80 percent of the surface area on most of the type I panels under natural weathering and natural weathering with salt spray.

The experimental zinc primer/linear epoxy coating developed a few defects (less than 1 percent of the surface area) in the type II panels during both natural weathering and natural weathering with salt spray after 10 months of outdoor testing.

Type II panels also showed coating defects in areas such as nuts, bolts, the underside of the T-attachment, and the wide-angle attachment. The crevices and cracks at these joints are difficult to paint compared to flat metal surfaces. As expected, these defects became rust spots.

This finding confirms that it is difficult to avoid initial coating defects from applications on bridge structures in the field because of the complex shapes of the structural elements. Thus, these imperfect sites are then prone to advanced coating failures and subsequent steel corrosion in service environments.

Results in rust creepage from the scribe during accelerated laboratory testing. The researchers measured creep in millimeters from the scribe and calculated it as an average value over the creep area. Based on the mean creepage values at the end of 3,600 hours (150 days) of accelerated laboratory testing, the researchers ranked the coating systems from highest to lowest in terms of rust creepage.

The high-ratio one-coat calcium sulfonate alkyd coating showed the lowest amount of creep, which means it was the best performing coating system for creepage in the lab, while the moisture-cured urethane zinc primer/epoxy/fluorourethane coating demons-trated the worst creepage.

Results in rust creepage from the scribe during natural weathering and natural weathering with salt spray. During the outdoor exposure testing, none of the type I panels developed rust creepage during natural weathering or natural weathering with salt spray, except for the experimental zinc primer/linear epoxy coating, which had a low rust creepage of 0.16 inch (0.4 centimeter) at the end of the salt spray exposure.

For the type II panels, after 10 months of exposure testing in Virginia and 6 months of exposure at the Golden Gate Bridge, only the experimental zinc primer/linear epoxy coating system exhibited recognizable rust creepage. The portions of the panels with the higher percentage of dry film thickness, in particular, showed extremely high creepage--5.5 inches (14 centimeters)--under exposure to salt spray. Similarly, the nominal dry film thickness areas of the same panels showed creepage of about 0.23 inch (0.6 centimeter). The rest of the type II coating systems did not show any rust creepage.

Concluding Thoughts

After 3,600 hours (150 days) of accelerated laboratory testing, 10 months of natural weathering with and without salt spray exposure, and 6 months of outdoor exposure testing at the Golden Gate Bridge, the researchers reached the following conclusions. First, the test results from this study indicate that none of the selected coating systems, including the two three-coat control coatings, will provide maintenance-free corrosion protection to steel bridge structures for 100 years.

Premature failures of two of the two-coat systems--thermally sprayed zinc primer/linear epoxy and experimental zinc primer/linear epoxy--occurred in accelerated laboratory testing as well as outdoor exposure testing. Three of the coating systems--moisture-cured urethane zinc primer/epoxy/fluorourethane, zinc-rich epoxy primer/epoxy/aliphatic polyurethane, and inorganic zinc primer/polysiloxane--performed satisfactorily in some categories but poorly in others, compared to the best performers.

Further, the researchers concluded that high-performance coating technology, regardless of cost, is not ready at this time to deliver superdurable coating systems that can last more than 100 years without significant interventions for maintenance.

Until future research and development efforts produce coating systems with extended service life, the main goal should be to use the proven legacy three-coat systems correctly by reducing human errors and improper applications and following applicable specifications and manufacturers’ literature.

Pradeep Kodumuri, Ph.D., is a senior research chemist at Henkel Corporation in Madison Heights, MI. He has experience with development, characterization, and evaluation of functional and protective coatings for many market segments. He has a Ph.D. in chemical engineering from Cleveland State University and an M.S. in chemical engineering from the University of Louisiana.

Seung-Kyoung Lee, Ph.D., is president of SK Lee & Associates in Fairfax, VA. Dr. Lee is the chair of the Transportation Research Board’s Committee on Corrosion and also chair of the Bridge Steel Coatings subcommittee. He has a Ph.D. and M.S. in ocean engineering from Florida Atlantic University and a B.S. in naval architecture from Inha University in Korea.

Y. Paul Virmani, Ph.D., is a program manager with FHWA’s Office of Infrastructure Research and Development. He has focused on reinforced and prestressed concrete and cable-stay bridge corrosion research for the last 30 years. Currently, he is responsible for research in the area of developing cost-effective corrosion protection systems for new construction and rehabilitation of existing salt-contaminated concrete bridges. Virmani has a Ph.D. from the University of Bombay (Mumbai), India, and an M.S. from the University of Rajasthan, India, both in physical chemistry.

For more information, contact Paul Virmani at 202–493–3052 or paul.virmani@dot.gov, Pradeep Kodumuri at 248–577–2056 or pradeep.kodumuri@henkel.com, or Seung-Kyoung Lee at 848–445–2977 or sk.lee2011@rutgers.edu. For the final report, visit www.fhwa.dot.gov/publications/research/infrastructure/structures/bridge/12044.