Factors Affecting Durability of Ferrous Metals in the Atmosphere

The corrosivity of the atmosphere in which a light gage ferrous metal is to be used must be considered. The corrosivity of unalloyed steel can vary through wide limits with a range as great as 8,500 to 1. It is probably more useful to compare the slopes of weight loss versus time curves for different steels rather than the ratios of weight losses at a particular time because alloy steels use a finite amount of thickness to form a protective layer of corrosion products (rust). The formation of this protective layer is favored by bold rather than partially sheltered exposure. The advantages of using alloy steels increase with the increasing corrosivity of the atmosphere. Paint coatings on alloy steels are superior to those on unalloyed steels. The austenitic stainless steels do not suffer a significant loss in thickness or mechanical properties when exposed for long periods of time to very corrosive atmospheres. The corrosion resistance and the properties of stainless steels can be utilized to achieve lighter structures for use in the atmosphere or in fresh water.