Technology Interchange—Troubleshooting Pretreatment Issues
Posted on Tuesday, January 28, 2020
Five years ago, I wrote a Tough Talk column entitled, “Don’t Blame the Powder”. The thesis of that piece trumpeted the high quality of powder coatings (properly applied and cured, of course) and explained that most failures observed in the field were related to problems associated with improper pretreatment and substrate issues.
This installment of Technology Interchange focuses on how to ensure that your parts are properly cleaned and pretreated so the powder coating you apply will provide the performance you expect.
The vast majority of powder coatings are applied to some type of metal. The condition and nature of the metal substrate will dictate what should be used as a cleaner and pretreatment. The quality of the metal surface prior to the application of powder is paramount in achieving excellent adhesion and corrosion resistance in the field. Overall the process of preparing the metal is known as “pretreatment” and consists of a number of stages, including cleaning, rinsing, chemical conversion, more rinses and often a final seal.
If the metal surface is oxidized or covered in mill scale, media blasting may be required to remove these materials before chemical pretreatment. Blasting is typically accomplished by the impingement of a high quality abrasive. Depending on the type of metal and material to be removed, abrasives include relatively hard minerals such as aluminum oxide, silicon carbide, crushed glass or glass beads. For softer substrates (e.g., some aluminum alloys), non-mineral abrasives such as plastic media, steel shot or walnut shells can be considered.
Cleanliness of the blast media is of the utmost importance. Blasting dirty substrates requires frequent replenishment of media as it is essential to replace media before it becomes rife with contaminants. Dirty media will imbed contaminants into the substrate, which will be difficult to remove even with the best cleaner/pretreatment system.
Blasted parts shouldn’t sit around long as the freshly abraded surface is prone to oxidation. The parts should be introduced to the cleaning stage of the pretreatment system as soon as possible after the blasting process.
The removal of “soils and oils” is critical to preparing a substrate to be powder coated. “Soils” are defined as airborne particulates, dirt, metal fines, grinding dust, hard water residues, etc. “Oils” pertain to fingerprints, lubricants, metalworking compounds, rust preventatives, etc. The cleaning stage is typically comprised of an alkali solution followed by a rinse (or two), although acidic cleaners are sometimes used if the substrate is heavily oxidized or mill scale is present.
Rinsing is critical and it is necessary to avoid “drag out” of the high pH alkali cleaner into the pretreatment stage, which is usually acidic in nature. Therefore, many pretreatment systems use a series of two rinses after the cleaning stage to completely remove any residual alkaline on the surface of the parts. The second rinse is commonly overflowed into the first rinse to save on water consumption.
A sufficiently clean metal surface will pass the “water break” and “white cloth” tests. The water break test consists of applying fresh water to the metal surface after the cleaning stage. Complete removal of “oils” gives a continuous, even sheeting of water. Residual oil or lubricant will cause the water to break up. Wiping the surface with a clean, white cotton cloth will let you know if any residual dirt or dust is present on the metal surface. If the surface doesn’t pass either of these quick checks, you need to inspect your cleaner bath solution to see if it is outside the control parameters provided by your chemical supplier.
Chemical Conversion Coating
The key to achieving excellent adhesion to metal and optimum corrosion resistance is establishing a chemical anchor between the metal surface and the powder coating layer. This is exactly what a chemical conversion coating forms. The type of metal will dictate what chemical conversion coating should be employed. It is essential that chemistry selected for your pretreatment system is suitable for the substrates that will be coated.
Iron phosphate pretreatment is designed for ferrous metals. It is great for most general-purpose specifications that require a modicum of corrosion resistance. Zinc phosphate pretreatment is more complex and provides a much higher level of corrosion resistance on both ferrous and zinc alloy (e.g., galvanized) surfaces. It is important to note that iron phosphate pretreatment is not designed for zinc or aluminum alloys. Some “all metals” pretreatment systems that are based on iron phosphate incorporate a fluoride modifier, typically in the form of hydrofluoric acid, to etch the surface of non-ferrous metals for improved adhesion and corrosion resistance. In these cases, the iron phosphate in the bath does essentially nothing to the non-ferrous metal surface.
Aluminum alloys are commonly pretreated with a chromate conversion coating. The chromic acid used is based on hexavalent chromium, which is relatively toxic and therefore hazardous to handle. Alternatives based on trivalent chrome or zirconium oxide provide acceptable chemical pretreatment without the hazards associated with hexavalent chrome. Zirconium based pretreatments have the added advantage of working with all three of the most common substrates: ferrous, zinc and aluminum. In addition, it operates at lower bath temperatures and produces a fraction of the sludge generated by phosphate-based pretreatments.
Rinses and Final Seal
Residual chemical conversion coating on the metal surface needs to be rinsed with clean water. High performance pretreatment systems use either RO (reverse osmosis) or DI (deionized) water to avoid any hard water contaminates that could compromise corrosion resistance. Softened water should be avoided as it usually contains free sodium ions which decrease corrosion resistance.
Chemical pretreatments do not completely cover a metal substrate and leave micro-voids open to corrosive attack. It is therefore wise to use a final seal to fill these voids to achieve optimal corrosion resistance. Historically, chrome-based seals have been used; however, most final seals are now organic based and eliminate the need to handle chrome.
For a more thorough exposition of pretreatment processes, please refer to Powder Coating: The Complete Finisher’s Handbook (4th Edition). It can be found at: https://www.powdercoating.org/store.
Preventative Maintenance and Recordkeeping
A comprehensive preventative maintenance program will head off most coating problems caused by an out of control pretreatment process. Here are a few checks that will help keep you out of trouble:
Visually Inspect Mechanical Processes and Parts
- Conveyor Cleanliness – lubricant contamination, dirt and contaminates.
- Spray Patterns and Nozzles – look for clogged nozzles.
- Risers – ensure positions and integrity.
- Pumps and Pump Screens.
- Oil Skimmers.
- Float Valves.
- Heat Exchangers.
Schedule overhauls as prescribed by equipment manufacturers.
- Monitor pH and TDS (total dissolved solids) in rinses.
- Titrate once per shift.
- Maintain chlorides and sulfates below 50 ppm (tap water), if not use RO or DI water.
- Recharge baths before they become ineffective.
- Ensure that parts are hung and racked correctly for optimum draining and water run-off.
- Measure part temperatures to ensure adequate release of water.
- Make note of line speed changes and effect on dry-off process.
- Keep dry-off oven and conveyor clean.
Keep meticulous records to recognize trends and to alert when adjustments are required. Many sophisticated systems now employ continuous monitoring with automated bath solution adjustments. These are great; however, nothing can take the place of routine visual inspection, especially for hard-to-automate controls such as conveyor cleanliness, oil skimming, parts orientation, dry-off oven efficiency, etc.
A number of coating defects and failures can be directly attributed to problems associated with the pretreatment process. The following is an overview of most of them.
Please note that not every coating performance issue is related to a pretreatment problem. It is wise to inspect all finishing processes to identify the root cause of the problem. For example, incomplete powder coating coverage could be application related and may be due to poor grounding or powder gun delivery issues.
I still stand by my original thesis regarding powder coating performance issues in the field: “Don’t Blame the Powder.” All kidding aside, here is a list of things to keep in mind to avoid some of the major performance issues influenced by metal pretreatment.
- Select the correct pretreatment for your substrate(s) – keep in mind that not all chemical pretreatments work with all metal substrates.
- Ensure that the substrate is clean before the chemical conversion coating stage. A dirty substrate will not accept chemical pretreatment.
- Make sure the parts are exposed to each stage for enough time to work correctly. Be careful not to increase line speed and cause insufficient cleaning, rinsing, chemical pretreatment and sealing.
- Keep your bath temperatures within the specification provided by your chemical supplier. Chemicals are less efficient at low temperatures.
- Regularly inspect the nozzles and risers to make sure all are working correctly.
- Titrate your baths every shift.
- Run water break and white cloth tests after the cleaner rinse stages once per shift.
Use clean water in your rinses. Avoid soft water and regenerate your rinse tanks before they become contaminated.
- Visually inspect the entire pretreatment process every shift. This includes conveyors, hooks, racks, nozzles, risers, condition of baths, pumps, skimmers, dry-off oven, etc.
- Hang and rack parts to ensure optimum drainage.
- Ensure that the part temperature in the dry-off oven is adequate to eliminate any residual water. Measure part temperature, not just oven air temperature.
- Keep your dry-off oven in good working condition. Inspect burners, controls, and oven interior.
- If using blast media, keep the media clean, and replace media before it becomes contaminated.
- Keep meticulous records and review them regularly to observe trends.
- Train your operators accordingly – your chemical supplier can provide this.
Running a successful powder coating operation is a complicated task. Ensuring that your cleaning/pretreatment process is sufficient is vital to producing a high-quality finish that will provide the performance expected from your powder coating. The old adage, “Can a bad tree bear good fruit?” applies here. I think you know the answer.
Kevin Biller is technical editor of Powder Coated Tough and the president of The Powder Coating Research Group.