Steel is an abundant, efficient building material that provides specifiers design freedom. However, for projects exposed to the atmosphere and other harsh environments, it is critical to coat the steel for corrosion protection. Often large construction projects target a 50-100 year design life, highlighting the need for durable, long lasting corrosion protection. Hot-dip galvanizing (HDG) provides three levels of corrosion resistance to steel: barrier protection, cathodic protection, and the zinc patina.
Hot-dip galvanized coatings isolate the steel from the environment, similar to other coatings. As long as the barrier is intact, corrosion will not occur. Once the barrier is breached, the steel can be attacked, leading to rust. Two important properties to consider with barrior protection are adhesion to the base metal and abrasion resistance, as this will lead to a stronger barrier.
Hot-dip galvanizing's second level of corrosion resistance is cathodic protection. Zinc is anodic to steel, and therefore will preferentially corrode to protect the underlying metal. Zinc's sacrificial action will also offer protection where small areas of steel may be exposed due to cut edges, drill holes, scratches or suface abrasion. In fact, even if the HDG coating is damage to the point bare steel is exposed (up to about ¼ inch in the maximum diameter) no corrosion will begin unti all the surrounding zinc is consumed. For instances where a larger area were exposed, the zinc would keep the corrosion isolate to that area, rather than allow spreading across the part.
The final key to hot-dip galvanizing’s corrosion protection is the development of the zinc patina. Zinc, like all metals, corrodes when exposed to the atmosphere. During exposure to moisture and free flowing air (natural wet/dry cycles in the environment), a series of zinc corrosion byproducts will form on the coating surface. The formation of these byproducts (zinc oxide, zinc hydroxide, and zinc carbonate) develop into the zinc patina, which acts as an additional passive, non-soluble barrier for the hot-dip galvanized coating. Once developed, the zinc patina will slow the corrosion rate of the zinc to about 1/30th the rate of steel in the same environment. Thus, a relatively thin coating of zinc can last much longer in the environment to protect a much thicker piece of steel.