Evaluation of coatings applied over corroded structural steel surfaces (FHWA-OK-91-05) 2722

Abstract

Results are presented for accelerated corrosion tests of five coating systems evaluated for use over corroded structural steel surfaces. This was the second series of such tests in a testing program initiated in 1988. The coatings were applied over both clean (non-corroded) steel panels and pre-corroded panels. The coated panels were then exposed for 1200 hours ( 50 days) in a salt fog chamber. Visual observations were made on a daily basis to obtain data on blister size, blister frequency, rust rating, and scribe rating. Using an overall performance index which is a sum of the four individual ratings, the best performing coating in this second series was the Carboline Carbozinc 11, Polyclad 936 system. The remaining four coatings were judged to have poorer overall performance, but they exhibited mixed results that made ranking more difficult. Comparison of results obtained in the two test series demonstrated that coating thickness was a significant test variable. Hence, changes in performance index with time were given more significance than the absolute values of the index. On this basis, DuPont 25P/Imron, from the first test series, and Carboline PC936/CZ11, from the second test series, were the best performing coatings evaluated. In reviewing the literature on maintenance painting strategies and cost considerations, it was concluded that surface-tolerant maintenance coatings could be of economic benefit even if their performance is inferior to that of coatings applied over clean grit-blasted surfaces.