Resolving Chemical Pretreatment Issues For Powder Coating & Industrial Painting
Introduction: A Fab Shop Needs Help
Recently we were contacted by a well-known fabrication shop that was having quality issues with their finishing work. They purchased a powder coating equipment package from a web outlet that sells booths and ovens for entry-level professional use. Unfortunately, they couldn’t get adequate support after the sale, so they reached out to Reliant for help.
Using their new powder coating equipment, the company coated several steel parts that were permanently installed outdoors in a hot, humid, sunny location. After installation, they discovered that some of the parts had blemishes in the coating when viewed in bright sunlight. Within a few weeks, the finish started flaking off and there was rust underneath.
Parts Inspection
We started by talking with them about the way they inspected their parts. We learned they did not have a dry film thickness (DFT) meter for checking coated parts. Also, they hadn’t been performing crosshatch adhesion tests or solvent rub tests. They would visually inspect finished parts for obvious cosmetic defects, but they weren’t doing anything else to confirm the thickness or integrity of the finish.
Quality Control Testing
We emphasized the need for QC which is outlined in our article about quality control testing:
https://reliantfinishingsystems.com/an-introduction-to-powder-coating-quality-control-testing/
Finish Defects
In addition to the coating peeling off in large flakes after the parts were installed, there was a rust problem. When they examined flaws in the coating, the metal underneath would sometimes be quite rusty and rust stains would show through at the edges of the defect. There could be two slightly different causes for this:
- The initial defect was caused by anything that led to the part’s bare steel surface being exposed: inadequate powder coverage, finish damage due to rough handling, the powder not being properly cured, etc. Once the original defect occurred and the integrity of the finish was compromised, water was getting through the defect to the part’s surface and creating rust. This could cause discoloration and contribute to the edges of the defect peeling back even more.
Because of the very large size of the flakes, the significant amount of rust, and the lack of obvious handling damage, we didn’t feel this was the cause.
2. It seemed likely the coating wasn’t adhering properly due to rust. There was either rust forming underneath the unbroken coating due to condensation, or they were possibly coating over existing flash rust. Defects formed as the cured powder “let go” from the metal substrate and eventually cracked and peeled off in large sections. Once the coating was broken, blemishes could grow due to water intrusion just as with #1.
This seemed to be the most likely answer based on the information they provided.
Equipment Shortcomings & Poor Pretreatment Practices
We spoke with the shop manager about their pretreatment practices. In rare cases, they had an off-site media blasting company prep badly rusted steel parts for chemical pretreatment. They sometimes used an older general-purpose pressure washer to remove dirt and grease if blasting wasn’t warranted. For large parts, they applied pretreatment chemistry with a pump-up sprayer and hosed it off. They weren’t precisely measuring chemistry titration or timing exposure once the chemical was applied.
For smaller parts, they used the inexpensive steam unit that was included with their powder coating equipment package. It was suitable for small parts and occasional use, but it wasn’t designed for big jobs or high-volume shops. It didn’t provide precise metering and didn’t have the ability to easily swap between the various chemicals the company would need to use if they wanted to consistently get professional quality results.
Our discussion revealed they weren’t taking time to document exactly how they were pretreating different batches of parts. Making the situation worse, the operator was purposefully preparing chemistry that was overly-concentrated. We explained that mixing pretreatment chemistry “hotter” (more concentrated) doesn’t mean it will automatically work better. You should follow the guidance provided by your chemistry supplier.
We also learned they weren’t attentive to dwell times, leaving chemicals on for up to a few hours before rinsing, and sometimes not rinsing them at all. Although there are no-rinse products on the market, these are very specific chemistries and not what was being used. According to the coater, almost every part had some level of residue remaining on the surface of the part at the time it was coated. They also sometimes coated over very mild flash rust, rather than remove it.
Once this was revealed, we were on the path to resolving the issue!
More Isn’t Always Better
The surfaces of the parts often had a powdery film on them after being chemically treated. This could seriously inhibit powder adhesion. A chalky residue appearing on a part after chemical pretreatment is usually caused by incorrect chemical concentration or pH levels (for example, the chemistry hasn’t been diluted enough), leading to too much metal attack and the formation of a powdery phosphate layer on the surface. This is sometimes referred to as “over-phosphating.” The acidic residue can also promote flash rust.
If you powder coat over metal that has a dried phosphate residue on the surface, the powder coating may not adhere properly, leading to potential peeling. The dried residue can act as a barrier between the metal surface and the powder coating, so a thorough rinse after the phosphate treatment is recommended to remove any residue and ensure optimal adhesion of the powder coating. Rinsing also helps prevent flash rust, which this company observed on some of their parts. If rinsing doesn’t remove the residue, further steps are needed to resolve the problem.
Pretreatment Basics
We talked with the company about how they should change their approach to preparing their parts for coating. First, we shared a brief article covering 3 guidelines for pretreatment:
Use Care With Iron Phosphate
Then we used this article to discuss iron phosphate pretreatment:
https://reliantfinishingsystems.com/iron-phospate-better-powder-coated-finish/
More Advanced Testing For Coated Parts
We shared an article about testing standards that may be required by more sophisticated customers. This helped them understand what’s common in the industry:
https://reliantfinishingsystems.com/cleaning-pretreatment-primer-part-two-meeting-standards/
Best Practices
We explained a typical approach might be to clean the part using a detergent, rinse it, use a phosphate to treat it, rinse it, and then apply a sealer or perform one or more secondary rinses. At the highest end, shops may use a pretreatment routine that has as many as 9 or more steps.
For manual pretreatment, we typically suggest purpose-built spray wand systems from PEM or steam units from Electro-Steam (Reliant is a national level distributor for both).
When parts are being transported by conveyor during the finishing process, a multi-stage pretreatment washer is typically the best choice if your acquisition budget allows. Reliant manufactures these and also partners with other companies that build them.
A Comprehensive Solution
Although they were having poor luck getting support from their equipment provider, we suggested they reach out to the chemical supplier they were working with to see if they would help get things back on track. Unfortunately, their chemistry was also being supplied by the online vendor who sold them their equipment. This long-distance “5-gallon bucket at a time” approach saved money, but it meant they weren’t interacting with a chemical company rep that could provide hands-on support at their facility or send parts back to a company laboratory for testing. We suggested they should interview at least two or three local/regional representatives from established companies like DuBois, PPG, Chemetall, Calvary, and Coral in order to start a productive relationship with a company that would be a good fit for their future growth.
We also suggested they consider a PEM spray wand system. Reliant partners with PEM to provide everything from spray wands to large multi-stage automated wash tunnels. PEM offers multiple products based on their P-300 system. This system has a base unit that includes an adjustable pump and an LP or natural gas fueled heater with temperature control. The base unit is connected to wands that let you switch between chemicals by simply swapping to a different spray wand. This eliminates the need for purging the system or swapping the pick-up between sources. Each wand has its own metering insert, so you can set each wand for the appropriate amount of chemistry or detergent dilution. This system also allows you to choose whether each stage is heated or not.
We recommended buying a DFT tester, performing crosshatch and solvent rub tests, and starting a documentation process where they log information about every coating job they perform. We encouraged them to start a more interactive relationship with their powder supplier, perform testing to gain a better understanding of their oven’s performance, and establish coating and curing routines that would get the best results with the powders they were using.
Conclusion
Although the coating equipment they bought had shortcomings, especially the powder curing oven and steam pretreatment unit, the company’s finish defects were primarily related to the way they were preparing their parts for powder coating. The lack of adequate technical assistance from their coating system and chemical provider contributed to the problems they encountered.
Based on our input, the company immediately revised their pretreatment practices, developed a plan to acquire better pretreatment equipment, and began reaching out to established brand-name chemical companies.
Contact Us
Need coating advice or seeking a finishing equipment solution that prioritizes professional quality results, affordability, versatility, and efficiency? The experts at Reliant Finishing Systems are ready to help! Give us a call at (256) 355-9000 and we’ll discuss your needs, provide a budgetary estimate, and promptly follow it up with a detailed plan that defines the equipment, layout, and costs associated with a new powder coating or industrial painting system.