Pretreating Aluminum for Powder Coating
Expert Rodger Talbert explains how to get better adhesion and corrosion performance from powder over aluminum.
Q: We are a job shop that powder coats both steel and aluminum. We have a five-stage pretreatment system that uses an alkaline cleaner, rinse, iron phosphate and rinse, and then finishes with a seal rinse. We want to be able to take on any job, but we have had some failures with coating aluminum parts that are used for outdoor products. How can we get better adhesion and corrosion performance of powder over aluminum?
A: This type of questions has been a common one for me and others who offer advice on powder coating. It is hard to give a simple answer on the best way to treat aluminum, but I will offer you good, better and best approaches.
Poor to fair method: blast only. Blasting the aluminum surface will remove the initial oxide layer and contamination and promote good initial adhesion. It also does enhance corrosion protection, but if the powder layer is too thin, moisture will break through and cause failure. Use of a powder primer layer under the topcoat can substantially prolong the adhesion and corrosion performance. Your method, chemical clean and iron phosphate, is not a very good method either. The addition of free fluoride in the bath will help to etch the surface, but it will not promote good long-term performance on outdoor products.
Good to very good method: chemical clean, de-oxidation and dry-in-place pretreatment (DIP). The de-oxidation step does a good job of removing the oxide layer, and the DIP adds a layer of corrosion resistance. This can be a solid way to treat the aluminum if the powder layer is complete and thick enough and performance expectations are not too extreme, such as in a coastline environment. Chemical clean, de-oxidation and zirconium pretreatment can also be effective in treating aluminum for outdoor use. Like the DIP, zirconium or other transitional metals are not as robust as some non-chrome and chrome options, but the performance can be good to very good if the powder layer is done right.
Excellent methods: chemical clean, de-oxidize, and chrome or non-chrome pretreatment. The addition of a good chrome or non-chrome pretreatment will prevent the formation of oxidation on the surface and provide superior adhesion properties over time. This is the best option for long-term performance, especially if the product will be used in a harsh environment.
Get familiar with the standards published by the Architectural Aluminum Manufacturers Association (AAMA). They have three levels of performance and testing methods to determine the goals associated with preparation and coating for outdoor markets.
How to Limit Orange Peel
Q: We have heard that orange peel is related to heavy film, and we could use some advice on how to better control smoothness.
A: Powder coatings may be a little less smooth than some liquid coatings due to the fact that the material is almost 100 percent solids. Without solvent, the coating has to depend on the melt and flow phase of the cure cycle to achieve a smooth finish. The ultimate smoothness of a powder film is related to a number of other factors as well:
The composition of the coating is an important factor. Sometimes the flow characteristics can be altered to provide smoother appearance but the performance needed from the powder may force the formulator to balance different ingredients to achieve the necessary properties, and that may mean more orange peel.
The particle size distribution may also be a factor. Each powder has its own target grind size, and there is a reason for going finer or coarser in many cases. A finer grind size may help make the finish smoother, but it also may make it harder to apply in Faraday areas (inside corners) and make it harder to get edge coverage. The formulator has to balance application characteristics with the need for a smoother film.
The coating thickness is also a factor. The optimum range of film thickness provides the most uniform mix of particles and the smoothest finish. If the film is too thin or too thick, the distribution of the particles may not be correct to produce the smoothest film. Also, as the film gets thicker, electrostatic behavior can impact the smoothness. The amount of free ions inside the powder layer increases and may begin to back-ionize. Back-ionization occurs when the electrostatic strength within the powder layer causes air molecules to split and the positive protons in the air molecule are attracted back to the gun electrode. These eruptions in the powder layer disturb the smoothness and can cause excess orange peel. In addition, larger particles have more cumulative charge. As the layer builds and insulates the surface, these more intensely charged larger particles build more readily than smaller particles with less cumulative charge, adding more texture to the powder surface.
Application is a key part of the smoothness of the powder layer. Hitting the target thickness and controlling the film to a uniform build will help you achieve more consistent results and limited orange peel. Micro-amps need to be limited to avoid back-ionization. The gun should not be held too closely to the part to help avoid excess texture. Training is needed to help operators better understand film-build control.
In a nutshell, the control of orange peel depends on the powder, how much is applied and how the spray gun is controlled.
You can purchase a set of panels from the Powder Coating Institute (PCI) that are numbered from 1 to 10 with gradient levels of smoothness to serve as a visual example of an acceptable standard. Meters also are available for measuring orange peel, but they are a lot more expensive.
Can a Powder Coated Part Be Recoated?
Q: Can powder coaters recoat an entire part to take care of an isolated defect?
A: Yes, it is a common practice to recoat a powder coated part, but there are a few steps you should take to ensure a good job. First, make sure you recoat the entire part to avoid inconsistent appearance and a rough finish. It will be more challenging to get coverage due to the insulating properties of the first layer, but you can get it if you follow a few simple steps: increase flow rate slightly, pull the gun back a little, and control micro-amps between 20 and 40.
Always test to make sure that the particular powder you are using will adhere when a second coat is applied. Most powders will have good inner-coat adhesion, but some may not, and you will want to confirm that. Polyesters tend to work better than epoxy for recoat adhesion. In some cases, you may need to do some light sanding to promote inner-coat adhesion.
Be aware that a second coating will likely produce some rejects from orange peel, light coat and other sources. Work had on first-pass yield so you do not have to invest the material and labor required to apply a second coat.
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