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.
The first line of corrosion defense is barrier protection. Like paints, the hot-dip galvanized coating provides protection by isolating the steel from the electrolytes in the environment. As long as the barrier is intact, the steel is protected and corrosion will not occur. However, if the barrier is breached, corrosion will begin.
Because a barrier must remain intact to provide corrosion resistance, two important properties of barrier protection are adhesion to the base metal and abrasion resistance. The tightly-bonded, impervious nature of zinc metal makes it a very good barrier coating. Coatings such as paint that have pin holes are susceptible to penetration by elements causing underfilm corrosion to spread rapidly.
Furthermore, 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, hot-dip galvanizing protects steel cathodically, which means zinc will preferentially corrode to protect the underlying base steel. The Galvanic Series of Metals is a list of metals arranged in order of electrochemical activity in seawater (the electrolyte). This arrangement of metals determines what metal will be the anode and cathode when the two are put in an electrolytic cell. Metals higher on the list are anodic to the metals below them meaning they provide cathodic or sacrificial protection when the two are connected. Therefore, zinc protects steel. In fact, this cathodic protection ensures even if the HDG coating is damaged to the point bare steel is exposed (up to ¼ inch in diameter), no corrosion will begin until all the surrounding zinc is consumed.
As previously explained, zinc is anodic to steel; therefore, the galvanized coating will provide cathodic protection to exposed steel. When zinc and steel are connected in the presence of the electrolyte, the zinc is slowly consumed while the steel is protected. The zincs sacrificial action also offers protection where small areas of steel may be exposed due to cut edges, drill holes, scratches, or as the result of severe surface abrasion during rough handling or job site erection. Cathodic protection of the steel from corrosion continues until all the zinc is consumed.
And the final factor in HDGs long-lasting corrosion protection is the development of the zinc patina. The zinc patina is the formation of zinc corrosion by-products on the surface of the steel. Zinc, like all metals, begins to corrode when exposed to the atmosphere. As galvanized coatings are exposed to both moisture and free flowing air, corrosion by-products will naturally form on the coating surface. The formation of these by-products (zinc oxide, zinc hydroxide, and zinc carbonate) occurs during natural wet and dry cycles in the environment. The zinc patina, once fully developed, slows the corrosion rate of zinc to about 1/30th the rate of steel in the same environment and acts as an additional passive, impervious barrier for the hot-dip galvanized coating.
Corrosion Protection Case Study
Suncor Energy Odyssey Project - Commerce City, CO, 2006
Suncor Energys Odyssey Project focused on upgrading the Commerce City, Colorado refinery to produce products that meet the newly regulated emission levels for low sulphur diesel fuel. The refinery provides a vital link between the companys large scale oil sands resource base and the growing energy market. The Commerce City plant is a major supplier of gasoline and diesel for the area and jet fuel for Denver International Airport.
Hot-dip galvanized steel has a proven track record in the Petro-Chemical industry and is the industrys coating of choice. Galvanized steel is the only coating that has been able to hold up to the corrosiveness and abrasiveness of refined oil sands. The turnaround time for this project was critical as it required the shutdown of two operations. Hot-dip galvanized steels naturally quick turnaround time meant the galvanized steel was there on time, every time. The hot-dip galvanized steel also ensures Suncors vital link in Commerce City will continue to function safely and without maintenance interruptions for generations.