Evaluation of Galvanized and Galvalume/Paint Duplex Coating Systems for Steel Building Panels

摘要

Standing seam metal roofing systems are becoming increasingly popular because of lower life-cycle cost and esthetic appeal. The coating systems most commonly used in metal roofs and metal buildings are galvanized or Galvalumed steel, factory-coated with polyvinylidene fluoride (PVF) or silicone modified polyester (SMP). Specimens having these coating systems were subjected to ASTM B 117, conventional salt spray exposure testing and ASTM D 5894 Testing in the laboratory, which combines the environmental effects of salt fog and ultraviolet exposure, alternately, and the results were compared. The ASTM D 5894 testing was used to simulate the typical corrosive atmospheric conditions of outdoor exposure. Electro-chemical impedance spectroscopy (EIS) of specimens exposed to ASTM D 5894 accelerated laboratory weathering provided a means of modeling the equivalent circuit parameters, and detecting the onset of incipient corrosion (coating degradation). Results show that both of these systems (galvanized vs. Galvalume ) provide the same corrosion protection, and that EIS can be used to predict the long-term service life based on short-term field tests. Results indicated that ASTM D 5894 testing provides a better means of predicting real- world performance of a coating system than ASTM B 117.