Pretreating for Outdoor Performance
Posted on Thursday, May 30, 2019
Since its introduction, powder coating continues to grow in industry acceptance as a reliable, affordable, and durable coating. As the quality, corrosion resistance, UV resistance, and surface finish improve, many manufacturers that produce products and equipment that spend their life cycle outdoors are converting to powder coating from liquid coating as their finish of choice.
As topcoat technology has evolved, so have pretreatment processes. Today there are more choices than ever that allow finishers to produce the best quality product that fits their systems and standards.
By nature, products designed for use outdoors need the most durable finish the industry can provide. Generally, these applications are expensive, large, and have long life spans. Protecting the appearance and integrity are primary goals. Traditionally, the industry has relied on zinc phosphate, chromates, and chrome phosphates to produce high corrosion resistance and durability. But due to environmental, safety, and economic concerns, the market is shifting to alternative technologies. There are now available choices that are free of heavy metals, require less energy, less water usage, and less maintenance than the old technology, and offer similar performance.
There are many considerations a manufacturer must consider when selecting a pretreatment for an outdoor product. High performance is certainly a prerequisite, but substrate type and application method (spray, immersion, or manual) must also be considered.
Let’s discuss by metal type.
Overall outdoor durability and corrosion go hand-in-hand. No other construction material is better for lightweight and corrosion resistance than aluminum. Hexavalent chrome has been the standard treatment for aluminum and continues to be used in many places. This would be true for both industrial and military applications. Patio furniture, outboard boat motors, automotive wheels, and building materials represent a variety of end uses for powder coated aluminum. These products have to take abuse and look good doing it.
Architectural and marine installations are the most demanding. Intense and constant UV exposure, humidity and corrosive, salty air dictate that finished aluminum for these applications be extremely robust. Because of aluminum’s use as door frames, window frames, fencing/railing, ladders, boat railing and trim, and decorative building components, it tends to be considered a permanent application, and therefore has the highest set of performance standards. To replace or refinish these components is difficult at best and very expensive; therefore, producing the highest quality finish from the start is critical. Chromate and chrome phosphate are still widely used, but zirconium (often referred to as nanotechologies, thin film, or transition metal treatments) is making serious inroads into this market and is becoming more accepted. The industry standards for architectural applications are AAMA (American Architectural Manufacturers Association) and Qualicoat. Zirconium pretreatments are proving they are just as effective when tested to these standards, and architects, who many times specify pretreatment, are beginning to allow for its use more widely.
For preparation of aluminum for these demanding outdoor applications with zirconium pretreatments, there are some general guidelines to follow.
Surface preparation is the first major key to robust performance. A clean, oil-free surface is necessary for good paint adhesion, as with any substrate. Removal of oil is always the first step, but oil is just one of the soils we encounter that we need to remove. Aluminum naturally oxidizes to protect itself, and that layer needs to be removed prior to treatment. This layer is not wettable to further treatment or topcoats, thus reducing pretreatment efficacy, and mechanical adhesion of topcoats. Also, in the outer layer of oxides, there may be trapped or embedded impurities that may act as corrosion sites where galvanic corrosion cells can begin. This oxide layer can be removed with a deoxidizer (either alkaline or acidic) or a controlled etch (either alkaline or acidic). Extruded and sheet alloys (like 5000, 6000, and 7000 series) generally respond very well to aggressive treatments in this stage of the process. Aluminum castings are generally much more sensitive to extreme alkalinity or acidity, and if not treated carefully, will produce a non-adherent smut on the surface that can cause paint adhesion issues.
Once the cleaning and deoxidizing is complete, surfaces should be rinsed thoroughly. If an alkaline cleaner/deoxidizer was used, it should be confirmed that the surface has been neutralized before further treatment, as alkalinity does not rinse nearly as effectively as acidic solutions.
Zirconium treatment is available in many forms these days. There are commercial products that are applied like a traditional conversion coating and then must be rinsed after treatment, but probably more common recently are the dry-in-place aluminum treatments. These products often contain zirconium, titanium, and/or a combination of other metal salts, as well as resins and silanes. These materials bond to the reforming oxide layer on the aluminum surface. Once they are dried in the dry-off oven, they form a hydrophobic and very corrosion-resistant layer on the surface that provides a very receptive layer for powder coat adhesion.
For the highest performance on aluminum, one of the simplest keys to the whole treatment program is high quality water. The use of deionized or reverse osmosis water for rinsing and pretreatment make-up is key. Hard water salts left on the surface are hydrophilic and will reduce the efficacy of any good treatment program. These foreign materials will also likely be the initiation sites of future corrosion.
Steel, particularly hot rolled, would be the most common material of construction for products made to live their life outdoors. Examples include tractors and farm implements, construction and mining equipment, playground and recreational fixtures, trailers, ATVs, and under-the-hood automotive components. For all these products, powder coated steel has to be durable for the long haul.
The standard treatment for steel in high performance situations has been zinc phosphate. It provides a heavy, insulating layer on the steel surface, a galvanic layer of protection, and an increased surface area offering a mechanical advantage for superior paint adhesion. With the increase in zirconium pretreatment performance, coupled with the environmental and process advantages, these newer coatings seem to be driving a shift in the market.
There are many specifications that are generally defined by each individual manufacturer. Overall, specifications include ASTM B-117 neutral salt spray, 500-2000 hours, and/or 20-60 cycles cyclic corrosion (SAE-J2334, GMW14872, etc). The industry tends to be shifting towards the cyclic tests, as they are truly more representative of the kind of corrosion seen in actual part aging/use.
For the preparation of steel for paint, as with any substrate preparation, cleanliness is key. But, when applying a zirconium pretreatment, cleanliness is even more crucial. With the iron and zinc phosphate process, they offer the benefit of relatively strong acid rinsing the surface during the application of the conversion coating. In this first part of the coating process, there is some pickling action which helps remove oxides, carbon, and other acid soluble soils. With zirconium coatings, we do not necessarily have this benefit. The focus of cleaning steel is primarily the removal of oils, greases, stamping fluids, coolants, metal fines, etc. Many outdoor products also contain a lot of welds, which require cleaning as well. Burnt carbonaceous oils, weld anti-spatter, and oxides formed around the weld in the heat-affected zone are some of the most tenacious soils that can be encountered. These soils, if not properly removed, will be the first areas of corrosion failure on the finished part because they will not receive uniform pretreatment, nor will paint adhesion be optimal.
Rinsing with fresh water after cleaning is important to completely remove loosened soils and the alkaline residue from the surface to allow for complete zirconium deposition.
Similar to aluminum, there are several options for zirconium pretreatment. A mid-process product that requires post rinsing is very popular, but dry-in-place options are also becoming more popular, particularly as zirconium products are being used in manual spray wand applications as well.
One of the biggest issues with zirconium treatments in steel coating lines is the accumulation of iron in the treatment stage. Zirconium treatment solutions are acidic; thus, when in contact with steel surfaces, they dissolve iron. This also happens in iron and zinc phosphate processes, but this dissolved iron is consumed and redeposited back on the part as part of the conversion coating. Acidic zirconium solutions can dissolve the iron, but since it is not used as part of the coating, it builds up in the bath. This iron that has been dissolved from the steel can plug nozzles and coat screens and the inside of plumbing and the washer housing. Most detrimentally, it gets deposited back onto the parts, and does not rinse off very well in subsequent stages. This loose iron particulate causes streaking, and in extreme cases, it can cause paint defects. Because the acidity of the zirconium solution is entirely independent of the coating deposition, there are zirconium products commercially available that will allow users to control iron content in the bath to less than 20 ppm in a working bath, thus preventing these issues.
The other very important benefit many zirconium and zirconium/silane products offer is the ability to pretreat over a weld. If the welds are properly cleaned, the zirconium pretreatment can wet these surfaces and bond to the oxide layer present, offering corrosion resistance and paint adhesion where traditional pretreatments cannot form a coating. Powder coatings continue to evolve with better finishes, more UV resistance, and more corrosion protection. Because of this, their use will increase for outdoor products. The pretreatments that these powder coatings rely on continue to evolve as well, giving manufacturers many options to refine their process and improve overall finished part quality.
Chris Berger is vice president/director of product development for Calvary Industries, Inc