Laboratory and Test-Site Testing of Moisture-Cured Urethanes on Steel in Salt-Rich Environment
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2000-12-01
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Abstract:Three 3-coat moisture-cured (MC) urethane commercial products formulated for protecting new steel (SSPC-SP 10) and power tool-cleaned steel (SSPC-SP 3) surfaces against corrosion were evaluated; the total coating film thickness was about 75 microns. Zinc-rich MC-urethane primers were used for SSPC-SP 10 steel surfaces whereas the primers for SSPC-SP 3 surfaces contained zero or a small amount of zinc; the same midcoats and topcoats were used for both steel surfaces. A chloride concentration of 20 micrograms per sq cm on the steel surfaces was found to be significant to reduce the coating performance at the scribe of SSPC-SP 10 steel surfaces; however, the chloride effect was found to be minimal for SSPC-SP 3 steel surfaces. Interestingly, all the scribe creepages increased linearly with the laboratory test time up to 4,000 h. Furthermore, the addition of a sealer to the primer/midcoat/topcoat systems unexpectedly impaired the performance at the scribe on SSPC-SP 3 steel surfaces when compared with those systems without a sealer. On the other hand, a coating system with sealer/primer/topcoat without a midcoat developed severe underfilm corrosion probably due to insufficient film thickness.
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