Minimum Film Thickness for Protection of Hot-Rolled Steel: Results after 23 Years of Exposure at Kure Beach, North Carolina

Abstract

This study was initiated by Dr. Wouter Bosch, University of Missouri at Rolla, in 1957, more than 25 years ago, under the direction of the Corrosion Committee of the Federation of Societies for Paint Technology. It has been continued by the Steel Structures Painting Council (SSPC) from 1966 to the present, under the sponsorship of the Federation. Originally, the purpose of this study was to determine the optimum film thickness for a few representative coatings in typical atmospheres, with the hypothesis that there were likely to be some differences in film thickness requirements among coatings, surface preparation grades, and atmospheres of varying severity. Four progress reports have been published, the most recent in 1969 by the Steel Structures Painting Council. This 10-year SSPC report showed clearly (1) an increase of about 20 months of paint life for each additional mil of paint thickness, applicable to all oil-based and alkyd paints in three environments (industrial, marine, and rural); (2) a critical minimum thickness for each generic type of paint; and (3) the value of applying sufficiently heavy films at the beginning rather than undertaking more frequent maintenance painting. The results given in this work refer only to panel exposure tests in the marine environment of Kure Beach (800-ft lot), North Carolina, while previous studies also considered rural and industrial environments. The variables studied include type of paint (oil, alkyd, phenolic, vinyl, chlorinated rubber, and epoxy ester), surface condition of steel before and after surface preparation, and a wide range of film thicknesses (primer and topcoat). The main conclusions of the study are: • For surface preparation, the degree of cleanliness is much more important to the performance of chemical-resistant coatings than to the performance of conventional coatings. • Total dry film thickness (primer and topcoat) is the most important factor. Obviously, the minimum paint film thickness required for an effective protection is a function of the required protection duration. • Nine mils of oil paint film thickness (primer and topcoat) do not afford protection for more than 10 to 12 years (until Rust Grade 8, SSPC-Vis 2/ASTM D 610, Evaluating Degree of Rusting on Painted Steel Surfaces, is reached). • With other paint systems, film thicknesses of 10 to 12 mils, or even less, afford effective protection after more than 23 years.