Some manufacturers have shied away from using pre-painted metal parts when there is an exposed cut-edge in the design, for fear of corrosion. The National Coil Coating Association (NCCA) set out to put this misconception to the test. The NCCA, along with the help of PPG Industries, Inc., and the North American Zinc-Aluminum Coaters Association (NamZAC), worked together to compare the corrosion resistance of pre-painted metal louvers on air conditioning units to those coated with three types of post-paint.

Pre-painted louvers with exposed cut edges were tested in the field alongside three sets of metal louvers, one with a post-painted electrocoat, a second with a powder finish and a third with a spray finish. The parts in all instances were made of hot-dipped galvanized steel; louvers were exposed to the same environment over time in Daytona Beach, FL.

Comparisons made after 16 months, 39 months, 44 months and 68 months of exposure to the elements showed pre-painted parts were repeatedly and significantly better than the post-painted parts in terms of the corrosion resistance, uniformity of treatment and film thickness.

The reason that the pre-painted parts outperformed each of those with a post-painted application is due to the fact that pre-painted metal sheet is uniformly cleaned, pretreated and painted as a flat surface, so the edge-to-edge and side-to-side variability is virtually eliminated. In addition, the pre-painted part also has two coats of paint, tightly bonded to the metal as compared to a post-painted part, which generally has only one coat of paint. In most cases, the pre-painted part has paint (one or two coats) on the backside as compared to the post-painted part, which at best has a minimal amount of overspray. Uniform pretreatment and paint on the backside of a part increases the corrosion resistance of the part, enhancing long-term durability. This can be seen in Photos 1 and 2.

Cut or sheared edges expose the metal to the environment, thereby creating a reactive area at which corrosion can begin. The degree to which corrosion will occur depends on the type of metal, the thickness of the sheet, the pretreatment system and uniformity, the paint system and thickness, application method and the environment, as well as the angle of exposure (vertical or horizontal).

Areas where moisture is retained generally have the most severe corrosion. The moisture allows the ingress of contaminants (chemicals, salts, debris, etc.), which will attack the metal. The initial sign of corrosion is usually visible as microblistering on the surface of the painted panel/part. With time, this microblistering causes the paint to creep back from the cut edge. The pretreated metal is exposed as the paint creeps back. The pretreatment is the final defense for corrosion resistance.

The enhanced corrosion resistance observed when using pre-painted sheet as compared to other post-painted methods of application is so significant, that in many cases metallic coating weights may be reduced.

Once corrosion has been initiated, it usually continues at a steady rate, progressing through a metallic coating (if present) and into the metal (Photos 3 and 4).

The study findings conclude that cut edge protection of pre-painted parts has repeatedly proven to be significantly better than post-painted ones because of the uniformity of treatment and film thickness (Photos 5-8). With proper selection of paint and substrate, a pre-painted part should provide a superior, cost-effective product in corrosion resistance, appearance and durability as compared to parts painted using other paint application methods.

For more information about this study, or converting to pre-painted metal, visit www.coilcoatinginstitute.org or call the NCCA at 216/241.7333.

About Pre-Painted Metal

Pre-painted metal sheet has touched every aspect of daily life over the past 30 years. Its versatility allows it to be used almost everywhere - on building exteriors and interiors, appliances, HVAC, furniture, automotive, etc. It is functional and durable, and can be combined with high-performance paint systems with an unlimited spectrum of colors and an array of properties. Designs can be eye appealing - simple or complex - without affecting long-term durability, corrosion and mar resistance, or functionality.

These conversions to cost-effective, quality-enhanced pre-painted products occur almost daily from costly, labor-intensive and inefficient post-paint systems (electrocoat, spray and powder). Long-term durability is a key property of any painted metal product. In particular, protection of a cut or exposed edge is critical, no matter what type of paint application method.