Most power projects (generation, transmission, and distribution) are designed for a service life of 50 years or more. Furthermore, these structures are often exposed to some of the most corrosive and varied environments (i.e. above and below ground, blazing sun, wind, snow, etc.) which highlights the need for durable, long lasting corrosion protection. Hot-dip galvanizing (HDG) provides three levels of corrosion resistance for power structures: 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 barrier 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.
The impervious nature of zinc makes it a very good barrier coating. Chemicals and gases inside coal and natural gas generation facilities are extremely corrosive. Coatings such as paint that have pin holes in their barrier coating are susceptible to penetration by these elements and underfilm corrosion can spread rapidly. HDGs zinc coating is tightly bonded to the steel and provides impervious, uniform protection providing no weak spots for corrosive elements to attack.
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 (left) is 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.
Galvanizings cathodic protection provides a second level of corrosion resistance for the power market in harsh corrosive environments ensuring there will be no maintenance shutdowns. The sacrificial action of zinc means even if the coating is damaged by abrasive elements such as coal rock or debris, no corrosion will begin until all of the surrounding zinc is consumed. The natural cathodic protection of hot-dip galvanizing limits the need for maintenance of structures within and around the generation facility, which is important especially in the nuclear market where safety is critical.
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 byproducts 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 byproducts will naturally form on the coating surface. The formation of these byproducts (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.