Laboratory and test-site testing of moisture-cured urethanes on steel in salt-rich environment.

2000Final report; Oct. 1,1996-Sept. 30, 1998.PDFTech ReportFHWA-RD-00-156PavementsPavement performanceTestingSeal coatingSeal coating methodsProtective coatingsPaintPigmentsUrethaneMicaceous soilsIron oxidesZincZinc coatingsCorrosionPaintCoatingsTestingPerformanceBarrier pigmentsUrethanesMicaceous iron oxideOrganic zincSealerNew steelPower tool-cleaned steelCorrosionEnvironmental compatibilityCoating repairAccelerated testOutdoor exposureUnited StatesTurner-Fairbank Highway Research CenterChong, Shuang-LingYao, YuanUnited States. Federal Highway Administration. Office of Infrastructure Research and DevelopmentNTL-HIGHWAY/ROAD TRANSPORTATION-Pavement Management and PerformanceNTL-HIGHWAY/ROAD TRANSPORTATION-MaterialsNTL-HIGHWAY/ROAD TRANSPORTATION-Construction and MaintenanceFederal Highway AdministrationThree 3-coat moisture-cured (MC) urethane commercial products formulated for protecting new steel (SSPC-SP 10) and powertool-cleaned steel (SSPC-SP 3) surfaces against corrosion were evaluated; the total coating film thickness was about 75microns. Zinc-rich MC-urethane primers were used for SSPC-SP IO steel surfaces whereas the primers for SSPC-SP 3surfaces contained zero or a small amount of zinc: the same midcoats and topcoats were used for both steel surfaces. Sealerswith film thickness of 25 microns were also studied for any potential effect on coating performance of the coating systems forpower tool-cleaned surface. The volatile-organic-compound content of all the coating materials was below 340 g/L. TheFederal Highway Administration (FHWA)-developed cyclic testing method was conducted to compare the performance of thesecoating systems. The test included freeze, ultra-violet light/condensation, and salt-fog/dry-air cycles. An aggressive outdoormarine exposure at Sea Isle City, New Jersey, was also performed for all the coating systems for comparison. A.number ofphysical and chemical properties of the MC-urethanes were examined to study their effect on coating performance. All of thecoating systems maintained their total film thickness, topcoat hardness, and adhesion strength throughout the 4,000-h testperiod. The three commercial products were found to contain different chemical compositions and performed differently. All thecoating systems had an adhesion strength about 10.5 MPa. No surface failures were observed on any of the test panels afterthe 4,000-h laboratory test; however, all of them developed creepages at an intentional scribe (undercutting). The laboratory testresults in conjunction with the chemical analysis results suggested that pigment particle size distribution in the primers played amore important role than other factors in the formation of creepage at the scribe. A proper pigment particle gradation improvedthe coating performance. A chloride concentration of 20 micrograms per square of centimeter on the steel surfaces was foundto be significant to reduce the coating performance at the scribe of SSPC-SP 10 steel surfaces; however, the chloride effect wasfound to be minimal for SSPC-SP 3 steel surfaces. Interestingly, all the scribe creepages increased linearly with the laboratorytest time up to 4,000 h. Furthermore, the addition of a sealer to the primedmidcoafftopcoat systems unexpectedly impaired theperformance at the scribe on SSPC-SP 3 steel surfaces when compared with those systems without a sealer. On the otherhand, a coating system with sealer/primer/topcoat without a midcoat developed severe undetfilm corrosion probably due toinsufficient film thickness