Thomas J. Langill, Ph.D., Technical Director
American Galvanizers Association
12200 E. Iliff Ave. Suite 204
Aurora, CO 80014
Abstract: Duplex systems, also known as painting over hot dip galvanized steel, are extremely effective corrosion protection systems. However, careful surface preparation techniques need to be used to alleviate potential coatingfailures. Use the age and characteristics of the galvanized coating to determine what type of surface preparation is needed.
Hot-dip galvanized steel parts or assemblies are often required to be painted. The reason for painting can be to identify the particular structure, for architectural reasons, to provide a particular type of protection, or to extend the service life of anexisting structure. The combination of a paint system with a hot-dip galvanized coating is often referred to as a “duplex system” (1). When paint and galvanized steel are used together, the corrosion protection is superior to either protection system used alone (2).
The application of a paint system onto a hot dip galvanized surface requires careful surface preparation and a good understanding of bothcorrosion protection systems. The margin for error is very small when dealing with newly galvanized steel surface preparation. However, there have been many examples of paint adhesion problems on older or more moderately aged galvanized steel surfaces, and the most common cause is improper or incomplete surface cleaning and preparation (3). When the surface is cleaned and prepared correctly thecombined paint and galvanized steel corrosion protection system gives extremely long lifetimes (4). If the paint is properly maintained on the galvanized surface there is practically no limit on the life of the structure in terms of corrosion attack.
The adhesion of paint onto galvanized steel becomes a very small problem when the galvanized coating has weathered for at least a one-year period. Thezinc corrosion products form a very dense, insoluble protective layer that accepts a paint coat readily. A brand new galvanized coating also experiences few adhesion problems within the first 24 to 48 hours after coating.
Hot-dip galvanized coatings can be applied in two different ways. The parts can be fed into a liquid zinc bath in a continuous roller process where thecoating characteristics are highly dependent on the speed of the steel through the liquid zinc bath. The two most common steel products that are hot-dip galvanized using the continuous process are sheet and wire. The objective of the continuous process is to deposit a zinc coating that is smooth, thin and composed of nearly all zinc bath metal with very little zinc-iron intermetallic. The coating canbe alloyed to form a dull gray intermetallic coating which has a good surface profile and can be easily painted.
The second type of hot-dip galvanizing process is often called the “batch” process since individual steel pieces or assemblies are dipped in a molten zinc bath as individuals or as groups. The coating is formed by the interdiffusion of zinc and iron. A micrograph of a typical batchhot-dip galvanized coating is shown in Fig. 1. The coating forms four distinct layers or intermetallics. The first layer is called the gamma layer and has 75 % zinc and 25% iron. The next layer is called the delta layer and has 90% zinc and 10% iron. The third layer is called the zeta layer and has 94% zinc and 6% iron. The final layer is called the eta
layer and has 100% zinc and no iron.
Duringthe batch hot-dip galvanizing process, a number of process variables can affect the coating thickness. The primary determiner of coating thickness is the steel chemistry, with the most influential elements
Fig. 1 – Microstructure of Hot-Dip Galvanized Coating
being silicon and phosphorous. These two elements promote the interdiffusion of zinc and iron and cause the hot-dip galvanized...