Estimating the Life of Galvanized Coatings on Steel Sheet.

Flat, horizontal coupons of galvanized steel sheet exposed to outdoor atmospheres corrode faster on the top surfaces than on the bottom surfaces. The coating fails first on the top side, and the time to first rust is determined by the upper rate. Most corrosion rates reported in the literature are calculated from the total loss on both surfaces. The resulting corrosion rates are an average of top and bottom rates. Use of the average rate to calculate coating life leads to overestimates of 20 to 80%. Thus, estimates of galvanized coating life based on flat coupon corrosion rates should be reduced accordingly. [ABSTRACT FROM AUTHOR]