Pipe drafting

PAINTING OVER HOT DIP GALVANIZED STEEL
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 coating failures. Use the age and characteristics of the galvanized coating to determine what type of surface preparation is needed.
INTRODUCTION
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 an existing 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 both corrosion 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 the combined 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. The