Don't Blame the Powder

I spend part of my life correcting other people’s mistakes. All the jumping to conclusions provides plenty o’ exercise for many a powder coating enthusiast. Here are some of the maladies attributed to our fair technology.

Corrosion Resistance

Countless coaters have beseeched me to help them remediate a catastrophe of coating failure apparently due to poor corrosion resistance. Their first inkling is that they must have used a substandard powder coating. Almost never is this the case. The most common cause of corrosion failure is due to poor metal preparation prior to the powder coating being applied to the part. Often a coater feels a solvent wipe will suffice for an article to be coated that will eventually be exposed to an extreme environment. It is rare for a powder to provide any modicum of corrosion protection with simple surface cleaning. Epoxy-based powder coatings are the only products capable of good corrosion resistance on clean bare metal. Virtually all other chemistries will fail.

Another scenario where a powder will fail in a corrosive environment is when the wrong metal pretreatment is used for a given substrate. Don’t be duped into believing a high-quality cleaning/pretreatment system will provide corrosion resistance if it is not specifically designed for your substrate. I have seen fabricators who have been told that a well-maintained iron phosphate pretreatment system will provide an excellent anchor for an aluminum alloy substrate. Don’t be fooled. Iron phosphate (and zinc phosphate for that matter) are designed for ferrous substrates. They will not work on aluminum, no matter how good its quality control.

Another recipe for corrosion resistance disaster is undercuring the powder coating. The metal can be clean and properly pretreated, however if the powder doesn’t see the right time at the correct temperature then all bets are off. I have tested exceptional powder coating formulas for corrosion resistance and found that a lack of complete cure will cause serious corrosion failures. A recent study by our lab demonstrated how an AAMA (American Architectural Manufacturers Association) 2605 grade powder prematurely failed salt fog testing (ASTM B-117) due to a less-than-complete cure cycle.

Powder chemistries most susceptible to lack-of-cure are polyurethanes and non- TGIC systems based on HAA (hydroxyl-alkyl amide). These products require full bake at the prescribed temperature (typically 375 to 400°F). TGIC polyesters, hybrids and epoxies have better low cure capability. however this doesn’t give you license to be cavalier in control of your part temperature.

Please keep in mind that heavy, dense parts require longer oven dwell times than light gauge metal parts. Ditto for higher loading of parts. And be careful when increasing the line speed of your continuous conveyor finishing system. Make sure your parts and therefore the coating get ample thermal energy to completely cure the coating. Temperature recorders, infrared pyrometers and simple solvent rub tests are instruments to ensure you’re getting the coating quality you desire.

The lesson here is: Do all things right if you want the ultimate performance from your coating system. And by “system” I mean everything from the type and condition of the metal to the cleaning/pretreatment and coating cure conditions—not just the powder coating.

Cratering

When a coater experiences film defects— specifically, craters—some jump to the conclusion that the powder coating must be the culprit. Very seldom is the powder the problem. Craters are circular interruptions in the coating that look like—you guessed it—lunar impressions. These defects are caused by a significant differential in the surface tension of the molten coating and a foreign material. The coating pulls away from the contaminant leaving a void that can persist all the way down to the substrate.

Craters almost always emanate from an environmental contaminant. The worst actors are lubricants. Silicones and penetrating oils (e.g., WD-40™) are on the top of the list. The intrepid maintenance guy who loves to spray everything in his sight to keep things lubricated is the first suspect to interrogate. Materials like these can persevere in the air for hours and sometimes days. And it doesn’t take a high concentration to spell disaster. Indeed, I have seen contaminants cause cratering at parts per million concentration levels.

Not only can environmental contaminants wreak cratering havoc, other sources can be the cause. Poorly filtered compressed air will guarantee the generation of craters in your powder finish. It is essential to install and maintain a high quality filter system on your compressed air supply. Integral to this system is a moisture separator installed just after your air compressor. Aftercoolers that refrigerate the air and thereby condense moisture work best. Very close to the point of your spray gun compressed air supply should be a filter series comprised of a particulate filter followed by a coalescing filter to trap any oil particles before they can contaminate your powder spray.

Just as important to installing a high quality compressed air filtering system, is maintaining it. Keep replacement filter elements on hand and establish a regular inspection routine to catch problems before they occur.

Another unsuspected source of cratering is contaminated blast media. Blast media can work wonders in preparing a metal surface for adhesion of a powder coating. It can erode the oxides that form on a metal surface and provide a textured surface for the powder to “bite” into to enhance adhesion. It also can become contaminated if the metal surface is oily or greasy. Contaminated media can transfer foreign materials such as oil and lubricants to subsequent metal surfaces. Compounding this problem is the potential of embedding contaminants into the surface. These are some of the worst contaminants to remove from a substrate.

Oils, lubes, silicone sealers and other powders (especially acrylics and silicone- based ones) can cause catastrophic fisheye cratering. Something as seemingly innocuous as WD-40™ can wreak havoc on a shop. Contaminated media in a blasting cabinet can also be a source of fisheyes. The solution for this lies in isolating the cause and eliminating it. The source can be the coating environment or the compressed air. Overloaded air filters and traps can harbor compressor lubricants that can be fatal to a powder coating operation.

In summary, when coating defects arise it is wise to conduct a thorough analysis of the root cause of the problem. Remember that the powder quality is but a minor factor in causing coating defects. More often the defect is attributed to an environmental or process shortcoming. Judicious investigation of your application and oven environment and review of your processes will usually uncover the culprit and mitigate further defect generation.

Kevin Biller is technical editor of Powder Coated Tough and the president of The Powder Coating Research Group. He can be reached at kevinbiller@yahoo.com.