Thirty-year atmospheric corrosion performance of 55% aluminum-zinc alloy-coated sheet steel

In 1964, a series of aluminum-zinc (AI-Zn) alloy coatings mere applied to steel sheet on a laboratory continuous hot-dip coating pilot line. The coated sheets were exposed in outdoor corrosion tests in severe marine, moderate marine, rural, and industrial atmospheres. Following eight years of testing, the 55% Al-Zn composition was selected as the optimum composition because it combined excellent long-term durability with the ability to provide cut-edge protection to the steel substrate. Now, after 30 years of continued outdoor testing, the results show conclusively that the 55% AI-Zn alloy coating has better that twice the life of an ordinary zinc coating of equal thickness, and that it provides enduring cut-edge protection. Following identification of the optimum composition in 1972, steel sheet with the 55% AI-Zn alloy coating was produced commercially by Bethlehem Steel. Large quantities of this material have been put in service as unpainted roofing on metal buildings. Inspections of these buildings show that the corrosion performance is excellent for roofs that have been in service for up to 22 years in a variety of US environments. These results confirm the conclusions of the earlier outdoor tests.