Paint delamination and under-film corrosion can strip years of service life from duplex coated structures. It can come down to a simple mistake – choosing the wrong 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.
“Engineers and specifiers often ask us for recommendations around which paint products to use for a particular system. We decided to save the industry some time by sourcing that information direct from paint suppliers and grouping them within service requirements and environments as laid out in AS/NZS 2312 Part 2,” says Sheehan.
“While more people might be aware of popular paint manufacturers, the guide also introduces a range of other manufacturers that specialise in industrial protective coatings. This helps our members to find the best fit for their next duplex coating project, no matter where it might be located.”
The product guide is available for download at the GAA’s technical publications page or through the design manual online. It recommends a range of paint systems for decorative or industrial finishes within each service requirement. It also includes the advised surface preparation for which details can be found in the full duplex coatings guide. For service requirements that need to meet specific industrial chemical or solvent exposure requirements, you should contact the paint manufacturers directly for further advice. Their contact details can be found at the back of the product guide.
Visit the Painting Downloads section of GAA’s Technical Publications page to access the Paint Product Guide.
