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|Federal Highway Administration > Publications > Public Roads > Vol. 70 · No. 2 > Are Two Coats As Effective As Three?|
Publication Number: FHWA-HRT-2006-006
Are Two Coats As Effective As Three?
by Shuang-Ling Chong and Yuan Yao
This research studies two-step painting systems as a possible alternative for protecting steel bridges and overpasses from corrosion.
Until the late 1970s, virtually all steel bridges in the United States were protected from corrosion by paint systems that consisted of three to five thin coats of alkyd paint containing toxic lead and chromate. Over the course of several days, bridge workers would apply the paint directly onto the mill scale (black corrosion analogous to rust) that adheres to the formed steel when it is heated. Subsequent painting for preventive maintenance and corrosion protection was rare and generally reserved for larger spans.
Because the majority of the steel bridges in the interstate system were built between 1950 and 1980, many have little protection from corrosion because their coating systems have outlasted their useful lives. Often, harsh environments and exposure to roadway deicing chemicals (salts) intensify the effects of the natural aging process. Further, the presence of potentially hazardous substances in the existing paint complicates maintenance processes and dramatically increases related costs.
Nearly 20 years ago, research led to the current standard, which is a three-coat system of zinc-rich primer/epoxy/polyurethane paint. Many States use the three-coat paint system as the preferred method of protection. In humid environments, for example, maintenance personnel use three coats of zinc-rich moisture-cured urethane (MCU)/MCU/polyurethane paint system.
A new class of coating systems consisting of a zinc-rich primer topcoated with fast-dry, high-build (thick film) polyaspartics, polyurethane, or polysiloxane promises anti-corrosive results that are comparable in some situations with the three-coat systems. These two-coat systems eliminate the intermediate epoxy layer, so painting a steel overpass can be completed overnight. When application specifications are followed, two-coat systems can reduce labor as well, increasing worker productivity and decreasing the overall cost of coating applications.
To assess the performance of these new two-coat systems, researchers at the Federal Highway Administration (FHWA) recently conducted a series of laboratory and outdoor tests that compared the performance of 11 two-coat rapid deployment systems with that of traditional three-coat systems.
Experimental Procedure at FHWA
The FHWA study used various combinations of primers and topcoats, applying the zinc-rich coating systems on clean steel panels according to the coating manufacturers' specifications. Each steel panel met the industry benchmark for near-white blast cleaning of painted or unpainted surfaces with abrasives, as set forth by the Steel Structures Painting Council, now the Society for Protective Coatings, under Surface Preparation standard 10 (SSPC-SP 10). The test panels measured 10.2 x 15.2 x 0.48 centimeters (4.0 x 6.0 x 0.19 inches). Prior to testing, the researchers scribed a 5-centimeter (2-inch) scratch diagonally on all of the coated panels to assess how each coating performed in terms of rust creepage—the growth of rust from a scribe through the coating.
The drying times of all the topcoats, including dry-to-touch times and dry-to-handle times, were tested using the American Society for Testing and Materials (ASTM) Method D 1640 (the industry standard for drying, curing, and film formation). The researchers measured the adhesion strengths by a pneumatic pulloff adhesion tester under ASTM Method D 4541 (again, the industry standard). They measured the gloss following the ASTM Method D 523. Gloss enhances appearance, and the reduction of gloss can signal chemical changes that may affect the paint's corrosion resistance.
The study involved both laboratory and outdoor tests to evaluate the coatings. The researchers prepared 8 replicate panels for each of the 11 coating systems, using 4 in the lab test and 4 outdoors. In the laboratory test, the panels were cycled through freeze, ultraviolet light (UV)/condensation, and salt-fog/dry-air conditions over the course of 500 hours. The researchers repeated this process 10 times, for a total test duration of 5,000 hours.
The researchers used a hot salt fog, generated with a 5-percent-by-weight solution of sodium chloride, and alternated that with ambient air at 1-hour intervals during the third phase of each cycle. Next, they examined the panels for surface failures such as blistering, rusting, or other imperfections. They also measured the panels for rust creepage at the scratched scribes after each test cycle (every 500 hours), using an FHWA-developed imaging technique designated as ASTM Method D 7087-05a.
The FHWA researchers evaluated eight two-coat, zinc-rich primer/topcoat systems, including four original manufacturer-recommended systems and four product-interchange systems. Three three-coat, zinc-rich systems served as controls. The volatile-organic-compound (VOC) content of all the coating materials was less than or equal to 340 grams per liter, g/L (2.8 pounds per gallon).
The researchers also exposed another set of coated panels for 2 years at a marine site in Sea Isle City, NJ. All the panels were placed at a 45-degree angle on wooden racks, facing directly south, and were sprayed every day with natural seawater to accelerate corrosion.
The Texas Department of Transportation's (TxDOT) experience with the two-coat painting system suggests such coastal testing is crucial and could have very influential results. "Two-coat painting does offer the advantage of saving time and money, but we haven't seen a system yet that we feel confident will give better or equal performance with the standard three-coat system for marine environments," explains Johnnie S. Miller, P.E., director of TxDOT's Construction Division, Materials and Pavements Section, Traffic Materials Branch. "Our use of the organic zinc/acrylic—vinyl or latex—topcoat system has worked out well for most of our State since Texas is predominantly a low-corrosive environment; however, we do not use this system along our coast."
And, like a few other States, "We are just now exploring more two-coat systems for marine environments as paint companies develop alternatives to the standard three-coat system," Miller says.
Clocking Drying Times
In the first part of the study, the FHWA researchers applied various primers and topcoats to the steel plates and then clocked the drying times. The dry-to-touch times for all the topcoats ranged from 0.5 to 1.6 hours. The dry-to-handle times ranged from 3 to 5 hours. The drying times proved similar for the topcoats in both the two- and three-coat systems. Especially for dry-to-handle times, however, the polyaspartics dried more quickly than conventional topcoats, given that they were applied as much thicker films than the topcoats in the three-coat systems. The drying times of the zinc-rich primers were similar, all drying in 2 hours.
a: Targeted dry-film thickness in the dry-time test
None of the coating systems showed surface failures except system 6, a combination of organic zinc-rich epoxy primer and polyaspartic topcoat, which developed extensive topcoat wrinkling. After the full, 5,000-hour laboratory test, the topcoat gloss diminished for the two-coat systems but not for the three-coat systems. System 6, with the polyaspartics topcoat, lost the highest amount of gloss (21 percent), suggesting that it might be affected by the surface wrinkling.
In general, the adhesion strength remained nearly constant. Among all the coating systems, those using inorganic zinc alkyl silicate primer (systems 5, 8, and 9) displayed the lowest adhesion strength (about 5.0 megapascals), which the researchers expected, given that inorganic zinc is known to have a low cohesive strength. The researchers found the other systems to be at least two or three times as strong in terms of adhesion.
All of the coating systems and the controls developed rust creepage at the scratched scribe after the 5,000-hour test, and the mean creepage distance grew linearly with test time. The researchers obtained the mean creepage by averaging the creepage of each set of four replicates and used a statistical linear regression analysis to obtain relatively high correlation factors, indicating a good linear fit.
Three-coat systems. The three-coat systems (systems 3, 4, and 5) developed scribe creepage in the amounts of 1.7, 1.4, and 2.8 millimeters (mm) (0.067, 0.055, and 0.110 inch), respectively, after the full lab test. The researchers consider these lengths quite small, indicating good overall coating performance on the SSPC-SP 10 steel surfaces. The inorganic zinc system (system 5) did not perform as well as in an earlier test, indicating that the inorganic system can be sensitive to application techniques and curing conditions. The creepage measured 1.6 mm (0.063 inch) after 3,000 hours instead of the zero obtained previously. How-ever, the larger scribe creepage found during this study actually included both rust and topcoat delamination (that is, a separation between the topcoat and primer where portions of the primer were not rusted).
Two-coat systems. The two-coat systems (systems 1, 2, 10, and 11) exhibited scribe creepage of 3.1, 3.3, 1.6, and 1.6 mm, (0.122, 0.130, 0.063, and 0.063 inch), respectively, after the 5,000-hour test. The first two systems, using polyaspartics as topcoats, performed similarly to or slightly worse than the three-coat systems with intermediate coats.
On the other hand, systems 10 and 11, using a different type of zinc-rich primer and topcoats with slightly longer drying times than polyaspartics (polyurethane and polysiloxane), performed as well as the three-coat systems (systems 3, 4, and 5) in terms of the small amount of scribe creepage—less than 2 mm (0.079 inch). This suggests that when using a two-coat system, the proper formulation of paint primer and topcoat can make a difference in the results.
Interchange of products from different manufacturers. Bridge owners have applied coating systems with organic zinc epoxy primer and inorganic zinc alkyl silicate primer on many steel bridges across the United States. Part of the mission of FHWA's test was to gauge the viability of using a polyaspartic topcoat in combination with such zinc-rich primers from various manufacturers. At the scribe, the organic zinc primers topcoated with polyaspartics (systems 6 and 7) performed better than the systems using zinc-rich MCU primers (systems 1 and 2). The creepage was small—0.8 and 1.6 mm (0.031 and 0.063 inch)—for the two systems, respectively. These creepage values are equal to or less than those developed by the three-coat systems.
However, because the organic zinc primer topcoated with polyaspartics (system 6) developed surface wrinkling, the researchers concluded that the combination does not make for an effective coating system. Likewise, the inorganic zinc primer appears to be sensitive to topcoat type; that is, it is not compatible with polyaspartics. Using polyaspartics with that primer (systems 8 and 9) reduced the coating performance at the scribe, where rust creepage increased to as much as 4.0 and 5.4 mm (0.157 and 0.213 inch), respectively.
System 9 panels developed delamination at the scribe in addition to the creepage, further suggesting low compatibility of the polyaspartic topcoat with the inorganic zinc primer. Among the four interchange systems, only system 7 performed well. There-fore, to ensure effective coating performance, both the organic and inorganic zinc primers should be used only with topcoats recommended by their manufacturers.
After 2 years of outdoor exposure at the Sea Isle site in New Jersey, systems 6 and 7 showed cracking over all of their coating surfaces. Cracking is a more severe failure mode than the wrinkling alone, observed in the laboratory test for system 6. The inhospitable environment and intense UV light at Sea Isle probably caused the failure because no such faults occurred in the lab test.
All the coating systems exhibited zero or some rust creepage at the scribe after the outdoor test, but these creepage amounts were smaller than those found in the laboratory because of the accelerated conditions of the laboratory test. The rust creepage also grew linearly with exposure time in both the laboratory and the field tests.
Exposure to corrosive elements reduced the gloss of all topcoats except polysiloxane (System 11) by 60 to 90 percent. The researchers attribute this large reduction to the high UV light intensity at the outdoor site. Among the six topcoats, therefore, the polysiloxane shows the greatest ability to retain gloss under intense UV conditions.
The adhesion strengths before and after the 2-year outdoor exposure followed a similar pattern to that of the laboratory tests. Essentially, these results demonstrate that all the coating systems retained their mechanical strength throughout the test period.
All the panels showed zero or some scribe creepage in both the laboratory and outdoor tests. After one- third of each test period had elapsed (5,000 hours for the laboratory test versus 2 years for the outdoor exposure), the indoor creepages measured much larger than the outdoor creepages, indicating that the laboratory conditions had greatly accelerated corrosion. Linear regression fitted to the lab and outdoor creepage results yield a correlation coefficient of 0.65, which the researchers consider to be fairly strong, especially because the outdoor environment is highly variable compared with the controlled conditions in the lab. Therefore, the researchers conclude that the accelerated laboratory test employed in this study appears to reliably predict the relative field performance of these coating systems.
Use as Recommended
The FHWA study showed that with regard to physical and chemical properties, all the topcoats in the two-coat systems dried quickly. The gloss of the two-coat systems diminished after the laboratory test but stayed the same for the three-coat systems. Conventional aliphatic polyurethane showed slightly better performance than the fast-dry polyaspartics, polyurethane, and polysiloxane after the lab test using a UVA lamp. However, only polysiloxane retained much of its gloss under the intense UV conditions at Sea Isle. And adhesion strength showed little variation in either the laboratory or outdoor tests. Even though the "aesthetic appeal" of the paint diminished as the gloss decreased, the integrity of the coating systems remained approximately the same; however, any decrease in gloss may be an indicator of material deterioration.
In terms of rust creepage, the study revealed that the currently available two-coat, zinc-rich primer/fast-dry topcoat systems (where both primer and topcoat are provided by the same manufacturer) all performed well, without any surface failures, but with zero or a small amount of rust creepage at the scribe after both the 5,000-hour accelerated laboratory test and the 2-year outdoor exposure in a salt-rich environment.
Ultimately, the FHWA researchers concluded that the two-coat systems performed comparably to the conventional three-coat, zinc-rich primer/epoxy/polyurethane systems. The results obtained in the FHWA study indicate that the new two-coat, zinc-rich coating systems can replace the three-coat systems to protect steel structures without sacrificing much corrosion resistance. At the same time, painting costs and traffic congestion will be reduced. Shop painting of new steel bridge structures using two-coat systems is recommended by FHWA to ensure good performance.
In addition, the researchers found that the organic zinc epoxy primer topcoated with two different polyaspartics performed as well at the scribe as those topcoated with the matched intermediate coat and topcoat designed by the same manufacturers. However, one of the two systems developed topcoat-wrinkling failures after the laboratory test, and both systems displayed cracking after the 2-year outdoor exposure. As a result, the researchers advise that the organic zinc epoxy primer as well as the inorganic zinc alkyl silicate primer should be used only with their own matched topcoats; otherwise, their performance may be reduced when they are topcoated with polyaspartics. Further, these results indicate that the new polyaspartic topcoats should be used with the MCU primers as specified by their manufacturers and not as topcoats for the organic zinc epoxy primer or inorganic zinc alkyl silicate primer.
These are the results to date. To collect additional data, FHWA will continue working with State DOTs to evaluate the field performance of the two-coat system on existing bridges.
Shuang-Ling Chong, Ph.D., has been a research chemist at FHWA since 1989. Chong's responsibilities have included managing the Paint and Corrosion Laboratory, studying accelerated testing of various bridge coatings, and developing methods for characterizing coating materials and failures. She earned her doctorate in physical chemistry in 1969 from Rutgers, The State University of New Jersey.
Yuan Yao is a chemist employed by Soil and Land Use Technology, Inc. She works onsite at FHWA's Turner-Fairbank Highway Research Center. Yao earned her M.S. degree in chemistry from the University of North Carolina at Charlotte in 1991.
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