It’s just another fact of life—sooner or later, all coil-coated metal is going to be fabricated (cut, punched, bent, stamped, etc.), and as a result there will be, in most cases, an exposed edge that one would think would be susceptible to corrosion. Given that long-term durability and corrosion resistance are key performance properties of any painted metal product, this is a big deal.
The type of metal and its coating type have a major impact on the prepainted metal’s ability to follow through on its promise of long-term durability. Decisions made at the beginning of the manufacturing process dictate whether the metal will last or start to corrode after just a few short years. NCCA has studied the long-lasting performance of cut edges on prepainted metal; more details on the study can be viewed in Tool Kit #5, Cut Edge Protection Using Prepainted Sheet.
Cut edges typically jump-start corrosion because a sheared edge exposes bare metal to attack by salt, moisture, and other materials. The first sign of corrosion is often red rust emanating from the face of the cut edge. If not counteracted (more on that later), this corrosion may cause the paint to creep back from the cut edge, exposing even more metal. Protecting an exposed edge is critical, and the prepaint process is built to better withstand this in-the-field assault. Due to the uniformity of cleaning of the metal, and pretreatment and the typical two-coat prepaint system, a prepainted finish provides superior corrosion resistance and protection.
Layers of Protection
Prepainted metal sheets actually have several layers of protection, working from the inside out to counteract corrosion. We start with a base metal that may be enhanced by a metallic coating layer; for instance, a hot-dipped galvanized metal. The initial cleaning and pretreating process uses a multistage, high-efficiency system that is specific to each substrate. Spa treatments, even for metals, aren’t one-size-fits-all. A high-temperature alkaline cleaner removes any mill oils, oxides, dirt, or other contaminants, increasing surface reactivity of the metal and thus allowing excellent bonding of the pretreatment. This tight adhesion is what pays dividends farther down the line, when the cut edge is subjected to the elements. The pretreatment also promotes adhesion between the primer and the metal coating, adding yet another barrier to corrosion.
A high-performance primer is applied to the pretreated surface uniformly across the strip. This usually contains corrosion inhibitors, which enhance protection from cuts, scratches, and bends while bonding tightly to the top coats.
Finally, we’re ready for the coating. Top coats are colorful, durable paints specifically selected for performance requirements of the material’s end use. Typical top coats include acrylics, polyesters, siliconized polyesters, and fluoropolymers for strength and durability.
The back side of the strip is painted with a primer and a backer coat to enhance performance and appearance by minimizing abrasion during shipping and handling.
Protection is built into each step in the process.
Prepainted metal sheet is uniformly cleaned, treated, and painted as a flat surface, so dirt is efficiently removed and imperfections are effectively eliminated. The result is a prepainted material that provides consistent edge-to-edge appearance and protection.
Post-Paint Doesn’t Cut It
“Post-painted” refers to a metal article that is coated after the metal is formed. It is generally a labor-intensive batch painting process, whereby several formed parts are hung on racks for processing. The problems begin almost immediately, as these parts can have forming lubricants, mill oils, dirt, and metal filings attached, which are not easily removed. These contaminants may become trapped in bends and crevices and may result in non-uniform pretreatment, creating weaknesses that corrosion can sneak through later.
Study Proves Prepaint Outperforms
Finally, remember that study mentioned back at the beginning? We took prepainted metal louvers with exposed cut edges and tested them in the field, head-to-head against post-painted metal louvers: one with an electrocoat, a second with a powder finish, and a third with a spray finish.
All test parts were made of hot-dipped galvanized steel, and the louvers were all exposed to the same environment in Daytona Beach, Fla. As you might imagine, it’s humid, rainy, and ripe with corrosive salt air.
The test samples were checked after 16, 44, and 68 months of exposure to the elements. Comparisons showed the prepainted parts were significantly more resistant to corrosion than the post-painted parts.
You can see the results for yourself below. In this case, seeing is believing.
So the answer to the question “Does prepainted cut edge metal stand up to corrosion?” is a resounding yes! To learn more about the coil-coating process and the benefits of prepainted metal, visit the NCCA website: www.coilcoating.org.
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David A. Cocuzzi
NCCA Technical Director