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Galvanized steel: New guide warns against poor choices of paint products

To prevent paint delamination and under-film corrosion from happening, the Galvanizers Association of Australia (GAA) has updated its product recommendation guide for paint systems used over galvanized steel in specific environments. The product guide is a companion reference to the earlier duplex coatings guide (it’s available from the technical publications page – free registration required), which maps specific products to the paint systems for the various corrosion categories from the Standard AS/NZS 2312 Part 2.

Aside from giving a different colour to galvanized structures, painting hot dip galvanized steel (duplex coating) can provide improvements in overall durability and performance. In highly corrosive environments, such as coastal or offshore locations, the additional years of service life added by the right paint system can be invaluable. Due to a synergistic effect, the service life of duplex coatings is greater than the sum of the paint coating plus the galvanized coating, and the right duplex coating can exceed 25 years in a C5 corrosivity environment.

However, choosing the right paint system is crucial when working with galvanized steel. Compared to uncoated steel, the protective zinc layer of galvanized steel is far more reactive. The wrong choice of paint system or incorrect surface preparation can strip away its protective properties rather than extending them.

“About once a month we come across issues in duplex coatings where either the surface preparation wasn’t correct, or the wrong paint system was used for that environment. It happens more often than you’d think,” says Ann Sheehan, Corrosion and Sustainability Officer at GAA.

It all begins with the correct surface preparation because the wrong technique or abrasive can blast off the galvanized coating and reduce the effectiveness of your protective layers. The GAA recommends only using non-metallic abrasives. If steel abrasives are used and painted over, the untreated metal can be embedded in the zinc and make a structure more susceptible to corrosion.

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