GALVANIZED STEEL SURFACESFor years, protecting steel from corrosion typically involved either the use of hot-dip galvanizing or some type of paint system. However, more and more specialists are utilizing a combination of both methods of protection in what is commonly referred to as a “duplex system.” A duplex system is simply painting or powder coating steel that has been hot-dip galvanized after fabrication.
When paint and galvanized steel are used together, the corrosion control provided is superior to either system used alone. The galvanized coating protects the base steel, supplying cathodic and barrier protection. Additionally, the galvanized coating prevents the underfilm development of iron-oxide, the outward pressure of which can cause paint to blister and peel. Paint, in turn, gives barrier protection to the galvanized coating. The paint slows down the rate at which the zinc is consumed, greatly extending the life of the galvanized steel. In return, once the paint has been weathered down or damaged, the zinc is available to provide cathodic and barrier protection so rust will not grow and paint will not peel. With this synergistic effect, it is typical for a duplex system to provide corrosion protection 1.5 to 2.5 times longer than the sum of the lifetimes of zinc and paint used individually. For example, if the galvanized coating is expected to last 40 years and the paint system is expected to last 10 years, the galvanizing and paint together should last 75 years without maintenance, or, conservatively, 1.5 times the sum of both systems.
Read more about the topic in the article PREPARING HOT-DIP GALVANIZED STEEL SURFACES FOR PAINTING OR POWDER COATING: A PRIMER
By Kimberlie J. Dunham, M.S.