Atmospheric Corrosion Resistance of Skyward- and Groundward-Exposed Surfaces of Zinc- and 55% Al-Zn Alloy-Coated Steel Sheet

Abstract

Zinc-coated (galvanized) and 55% Al-Zn alloy-coated steel sheet was tested for atmospheric corrosion resistance for 20 years in rural (Saylorsburg, Pennsylvania), industrial (Bethlehem, Pennsylvania), moderate marine (250-m [880-ft] lot, Kure Beach, North Carolina), and severe marine (25-m [80-ft] lot, Kure Beach, North Carolina) environments. Corrosion losses were determined separately for skyward-and groundward-exposed surfaces and compared to results for conventional two-sided coupons, which gave only an average of skyward and groundward surfaces. Results were fitted to exponential equations of the form C = AT B , where C is the corrosion loss, T is time, and A and B are constants. Coating lives were estimated based upon extrapolation of these equations to the time to reach a corrosion loss equal to the coating thickness. On average, the estimated life of a skywardexposed 55% Al-Zn alloy coating was 12 times that of a galvanized coating of equal thickness (25 µm).

Cut-Edge Corrosion Behavior of Prepainted 55% Al–Zn Steel with Chromate-free Primers in Various Atmospheric Environments

“…13) Townsend reported on the corrosion resistance of galvanized steel sheets and 55% Al-Zn steel with natural exposure tests in different environments. 14) Nomura et al reported on an example of the corrosion resistance of 55% Al-2% Mg-Zn alloy steel sheets compared with 55% Al-Zn steel. 15) However, there are only a few reports of systematic exposure tests of chromate-free prepainted steel sheets in different climatic environments.…”

The Electrochemical Polarization and Galvanic Couple Behavior of the Primary Phase of 55% Al-Zn Coating

“…55% Al-Zn metal coated steel is a widely used alternative to galvanized steel, offering improved atmospheric corrosion performance in some service environments (1). The coating consists of two phases (Figure 1) and it is often observed that surface corrosion of the coating initiates and proceeds in the zinc rich ß-phase (95 wt % Zn, 5 wt % Al) (2,3).…”