Protecting Steel Against Corrosion
At US $2.2 trillion, the annual cost of corrosion worldwide is over 3% of the world’s GDP. This figure reflects only the direct cost of corrosion – essentially materials, equipment, and services involved with repair, maintenance, and replacement. It does not include the environmental damage, waste of resources, lost production, or personal injury caused by corrosion. With proper corrosion protection, much of this damage is avoidable.
The World Corrosion Organization estimates that a net of 20 to 25% of this annual cost (approx. $400 billion) can be saved by applying currently available corrosion control technologies. When left unprotected, steel will corrode in almost any environment. More harsh environments will hasten corrosion, ruining the appearance of steel while threatening its structural integrity. No method of corrosion protection is more effective than coating steel with zinc. Zinc coatings protect steel by providing a physical barrier as well as cathodic protection for the underlying steel. Zinc is so important in this role that the coating market represents nearly 60% of total zinc consumption each year.
How Zinc Protects Steel
Passively
Placing a barrier coating between steel and the corrosive environment.
Actively
Through electrochemical processes.
Barrier systems attempt to guard against rust by preventing moisture and oxygen from reaching the steel surface. However, paints are inherently permeable and when the coating is penetrated or mechanically damaged, corrosion occurs at the point of penetration and spreads. This leads to a layer of rust at the steel interface beneath the coating.
Sacrificial systems, such as zinc coatings, also provide barrier protection but protect in an entirely different way as well. Unlike a purely barrier system, zinc coatings protect galvanically, corroding in preference to steel. Damaged areas are contained, and there is no corrosion of adjacent exposed steel until all the surrounding sacrificial coating has been consumed. The presence of zinc is the key to cathodic protection.
Types of Zinc Coatings
A wide range of methods can be used to apply zinc coatings to steel including galvanizing, zinc thermal spray metalization, electroplating and zinc-rich paints, among others. These coatings can be used individually, or in combination with a paint topcoat for a synergistic effect.
Zinc thermal spraying often complements galvanizing. For example, it can be used to coat existing steel structures and to treat steel too large to be hot dipped. It can be used to restore corrosion protection to areas where galvanizing is removed during fabrication, such as along cuts or threaded ends; and to assure coverage of welds and rivets. It can also repair mechanical damage to galvanized surfaces. Similarly, the application of zinc-rich paints over galvanized or thermal sprayed coatings, known as a duplex system, can also extend barrier coating longevity.
This subject has been researched for many years and the literature is well supplied with reports on zinc’s performance in different climates, with different alloy additions to the coating and at different coating thicknesses. Each system comes with its own set of advantages so it is important that the correct zinc coating is specified to provide optimal performance under the exposure conditions to which the coating will be subjected.
