Painting metal substrates provides an attractive appearance and also imparts corrosion resistance. Painted metal articles, such as railings, mailboxes, athletic stadiums and window frames, last longer than unpainted metals when exposed to outdoor weather conditions, including rain, snow, sleet and hail. It is possible to achieve outstanding corrosion resistance for metal surfaces with waterborne paint resins.

Kathy Allen, technology manager; Margaret Kendi, associate research scientist; and Peter Schmitt, senior technology manager, coatings, adhesives, specialties, all of Bayer MaterialScience LLC, tested the corrosion resistance of metals coated with waterborne paint resins. Schmitt presented their findings, based on the paper, “Advances in Waterborne Resins for Metal Coatings with Improved Corrosion Resistance,” in New Orleans, as part of the Corrosion session during The Waterborne Symposium 2012.

What makes a successful coating for metal worth its mettle? It must adhere to the substrate, withstand weather and prevent corrosion. To ensure a successful coating system, several elements must be considered:

• The coating and surface need to be compatible;
• The substrate must be prepared properly; and
• The coating has to be applied at the proper thickness, in the correct manner and given the appropriate amount of cure time.

Paints based on one-component (1K) waterborne polyurethanes (PUDs) can protect metal surfaces and keep them looking good for an extended period of time. Paints based on 1K PUDs provide outstanding corrosion resistance along with excellent solvent and abrasion resistance.

During the presentation, Schmitt reviewed the chemistry of PUDs, demonstrated the flexibility of PUD technology and the effect of polymer makeup on corrosion resistance. He also discussed the testing done on 1K waterborne polyurethane and waterborne polyacrylate resins on metal after a variety of pre-treatments with and without a primer.

“Two-component waterborne polyurethane studies showed that good corrosion resistance can be achieved with the proper choice of co-reactants when these systems are applied directly on treated steel panels,” said Schmitt, who points out that the formulations with the highest crosslink density provide the best corrosion resistance.

Furthermore, testing showed waterborne polyurethane formulations without primer can provide a better combination of adhesion, humidity resistance and corrosion resistance than the other commercially available systems that were tested. The authors discovered that waterborne polyacrylates provide inferior corrosion-resistance properties when compared with polyurethane resins. However, their testing showed it is possible to protect metal surfaces using a blend of polyurethane and polyacrylate but at least 50 percent of the formulation must be polyurethane to achieve good corrosion-resistance properties.