Using this Time to First Maintenance chart, developed using the Zinc Coating Life Predictor Model developed by Dr. Gregory Zhang of Teck Cominco, let’s look at the galvanized coating’s protective life within the five atmospheric environments defined by ASTM International. A galvanized coating’s protective life is determined primarily by the thickness of the coating and the severity of the exposure conditions. The thickness of a galvanized coating is expressed in “mils.” One ‘mil’ is equal to one thousandth of one inch.
So following the chart, let’s say you have a galvanized structure in an industrial environment. According to ASTM A123, a piece of steel 1/4 inch thick or greater must have a minimum of 3.9 mils of zinc. More often than not, you will get greater than the minimum requirement when hot-dip galvanizing, but for ease of using the chart, let’s say you picked up 4 mils on this piece. If you follow the 4.0 mils up to the industrial (gray) environment line and across, you’ll see you have approximately 72 years until first maintenance (or 5% rust of the substrate steel). In other words, 95% of your coating is still intact, so the structural integrity is not threatened. It is merely time to apply a corrosion-resistant coating on the structure to extend its life.