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CORROSION water, oxygen and atmospheric pollutants, as well as electrolytes in the environment. As long as the barrier is intact, the steel will be protected and corrosion will not occur. However, if the barrier is breached, corrosion will begin. The impervious nature of zinc makes it a very good barrier coating. Aged zinc corrodes approximately 1/10 to 1/40 the rate of steel depending on the environment, making the corrosion rate of a thin zinc coating equivalent to a much thicker steel piece. In addition to barrier protection, galvanizing also provides cathodic protection to steel, which means zinc will preferentially corrode to protect the underlying base steel. The zinc coating cathodically protects the steel from coating imperfections caused by accidental abrasion, cutting, drilling or bending. Zinc coatings "age" with time and exposure to the atmosphere. This aging results in a tightly bonded, thin layer of zinc carbonate on the surface that is often referred to as the "zinc patina." This patina is impervious, and passive, which slows the corrosion rate of the zinc. The zinc patina, which is a critical part of coating's longevity, requires natural wet and dry cycles to form. Accelerated tests (salt spray) do not 54 E&MJ; • FEBRUARY 2013 accurately predict the life of a galvanized coating because they are conducted under a constant salt fog, inhibiting the formation of the passive zinc patina. Using a Time to First Maintenance Chart (See Figure 1), developed using the Zinc Coating Life Predictor Model by Dr. Gregory Zhang of Teck Cominco, the galvanized coating's protective life is shown 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." A 'mil' is equal to 1/1,00th of an inch. According to ASTM A1234, 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, so for ease of using the chart, would equate to 4 mils. Backtracking the 4 mils up to the industrial (gray) environment line and across, the piece will have approximately 72 years until first maintenance (or 5% rust of the substrate steel). In other words, 95% of the 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. Cost Comparisons A common misconception is that galvanizing is cost prohibitive. When specifying steel corrosion protection systems, it is important to consider not only the initial cost, but also the life-cycle costs (including all future maintenance). When life cycle costs are considered, hot dip galvanizing is the most economical system for corrosion protection. Even if you think designing equipment members to fit into the galvanizing kettle may mean more erection time in the field and slightly more steel and connections, (by the way, 60-ft kettles are now common) those costs are generally small when compared to the savings indicated. When the hidden (indirect) costs (estimated to be five to 11 times the direct costs) associated with the maintenance of paint systems is added to the analysis, galvanizing is clearly the most economical choice. Joseph Langemeier (joelanemeaier@azz.com) is marketing manager for AZZ Galvanizing Services, headquartered in Ft. Worth, Texas, USA. www.e-mj.com