Troubleshooting Field Defects

By Kevin Biller

In a perfect world there would be no need to troubleshoot coating defects in the field. If every powder applicator selected the highest quality products, used precisely matched metal pretreatment chemicals, cured their coatings in a well maintained and balanced oven, and meticulously kept all of their processes in control, there would be no field defects or failures.

Unfortunately, on occasion, coating failures are experienced in the field. Field failures can be classified in three major areas: adhesion/chipping failures, premature corrosion, and fading/gloss loss. Identifying the root cause usually takes a bit of detective work. The following is a guide for what to look for, how to confirm root cause, and recommendations for corrective action.

Adhesion/Chipping Failures
Adhesion failures are one of the most common issues experienced in the field. Adhesion failure can occur due to rough handling of coated parts during packaging or unpacking, with post-assembly processes, and in the field due to environmental effects. A high-quality powder coating properly applied and cured over a clean, pretreated substrate should not suffer adhesion failures.

The first place to start your investigation is the type and quality of your metal substrate. The quality of steel has changed significantly since the turn of the century. Mills use a high concentration of scrap steel when creating a new batch of steel. Recycled or scrap content can vary from 25% upwards to 95% depending upon the type of mill and the end product. The quality and composition of the scrap used affects the final quality of the steel and can influence how the cleaning/pretreatment process performs. This contributes to the variability of the composition of the steel.

The biggest issue is the batch-to-batch inconsistency of the steel. If the impurities content varies (nitrogen, silicon, phosphorus, sulfur, and carbon) the pretreatment may not work as well. In addition, some steel is delivered with an oil sealant (e.g., HRPO – hot rolled pickled and oiled). This “oil” can vary significantly and definitively affect how the cleaning stage of the pretreatment process behaves. If the oil is not completely removed, the subsequent chemical pretreatment will not work well.

Specialty substrates, such as hot dipped galvanized steel, pose similar issues with inconsistency. The zinc layer deposited with galvanizing actually creates a moving target as the zinc slowly oxidizes over time. Pretreatment works differently depending upon the amount of zinc oxide present on the surface. If the pretreatment process does not align with the condition of the zinc/zinc oxide layer, then adhesion is often compromised.

It is generally difficult to achieve good coating adhesion on stainless steel and highly polished aluminum. Specialized pretreatments such as passivation (for stainless steel) and/or media blasting may be necessary to achieve good adhesion.

Use of the wrong pretreatment can also create adhesion failures. Iron phosphate pretreatments work well over ferrous substrates but using a traditional iron phosphate pretreatment on aluminum will not provide the same degree of powder coating adhesion to the metal. Some chemical suppliers recommend a fluoride etching additive to be added to your phosphate pretreatment process to provide coating adhesion to aluminum.

Chipping failures are similar to adhesive problems as the failure point is typically between the coating and the substrate. The aforementioned issues with metal quality and pretreatment apply here. In addition, excessively thick coatings can introduce chipping failures. Undercured powder coatings will also be prone to chipping. Recoats also present a mode of chipping failure. This is especially true if the initial coat has experienced an overbake condition. This chipping failure typically occurs at the interface of successive coats of powder.

This may be a good time to mention that it is extremely difficult to perform effective troubleshooting if you don’t keep good documentation of your processes and equipment maintenance. If you don’t have detailed documentation of maintenance schedules and customer jobs, it may be impossible to find the right corrective actions needed to solve coating issues.

Corrective Actions

• Inspect and have your vendor(s) certify the quality of your substrates. Look for oils and soils, corrosion, and other contaminants. Corrosion will have to be removed with media blasting or aggressive chemicals.
• Be very careful with galvanized steel. Ensure that it is fresh enough and does not have a high concentration of surface zinc oxide. If oxidized, sweep blast the surface before pretreating.
• Ensure that your pretreatment chemicals are suitable for the substrates that you coat. Be careful with coating mixed metal parts.
• Ensure that your pretreatment system is in-spec for temperature, concentration, pH, TDS (total dissolved solids), exposure time, impingement, etc. Make sure that your rinses are clean.
• Confirm that your coating is adequately cured.
• Follow the recommended bake conditions (time and temperature) for each specific powder. This can be found on the TDS (technical data sheet). Conduct random cure tests using a solvent rub test.
• Avoid excessively thick coatings. Follow film thickness guidelines per the product’s technical data sheet.
• Confirm recoat adhesion in an inconspicuous spot.

Premature Corrosion Failures
Corrosion failures in the field can be attributed to a few critical issues. Incomplete coverage due to thin films and/or poor coverage of edges can lead to early corrosion failures. In addition, poor metal preparation can cause these failures. The discussion above regarding substrate quality and pretreatment technique and its effect on adhesion applies here. Marginal or poor adhesion will lead to scissions or breaks in the coating, which will invite moisture and salt to accelerate corrosion.

Additionally, choice of powder chemistry affects corrosion resistance. Most low-cost powder coatings have higher levels of low-quality fillers. These low-cost fillers compromise many film properties, including corrosion resistance. Also, some powder chemistries are more corrosion resistant than others. In general, epoxies are the best, followed by hybrids (epoxy polyesters) and polyurethanes. Polyester-TGIC types are generally good, although polyester-HAA (non- TGIC) are typically less corrosion resistant than their TGIC counterparts. Highly durable types, such as superdurable polyesters, acrylics, and fluoropolymer-based powders may require primers to meet the expected durability requirements of the architectural and transportation markets.

Corrective Actions

• Ensure that the film thickness on all surfaces is at, or above, the minimum thickness recommended on the product’s TDS (technical data sheet).
• Use high quality powder coatings that are suitable for the required performance in the field.
• Use a primer/topcoat system if extremely high corrosion resistance performance is needed. An epoxy primer with a UV durable topcoat is recommended.
• Inspect and have your vendor(s) certify the quality of your substrates. Look for oils and soils, corrosion, and other contaminants. Corrosion will have to be removed with media blasting or aggressive chemicals.
• Ensure that your pretreatment chemicals are suitable for the substrates that you coat. Be careful with coating mixed metal parts.Ensure that your pretreatment system is in-spec for temperature, concentration, pH, TDS (total dissolved solids), exposure time, impingement, etc. Make sure that your rinses are clean.
• Make sure that your coating is adequately cured.
• Follow the recommended bake conditions (time and temperature) for each specific powder. This can be found on the TDS (technical data sheet).

Fading/Gloss Loss
The most common cause of fading and gloss loss from environmental exposure is usually attributed to the choice of powder coating. The UV durability of a powder coating is based on the chemistry of the binder system. That said, even the most durable binder systems can be compromised when formulated with low quality pigments and additives. Durability follows the old weakest link maxim. In my experience, unwitting formulators will sometimes ruin the durability of a great powder binder system by using low quality colorant pigments. Someone makes a decision based on formula cost (i.e., make it cheaper to make the sale) and nine months later the sales department gets a product performance complaint from the field.

In other cases, the coating user may not provide the outdoor durability required by their customer. Still other times, a lazy job shop operator will use what’s on the shelf instead of ordering the appropriate product to meet the customer’s need. I have a good friend who unknowingly had his high-priced alloy wheels coated with a fancy looking silver vein effect powder. Problem was, the silver vein powder was probably a hybrid (epoxy polyester) powder because his wheels lost all of their gloss in less than 12 months.

Undercured powder will also cause premature gloss loss on a powder coating finish. The coating needs to be adequately crosslinked to achieve full performance features.

One other not-so-obvious cause of fading/gloss loss is that, over a number of years, organic coatings eventually degrade and lose their luster. I have received complaints from the field where an end user was unhappy that his powder coated gates had faded. When I inquired about the age of the coating, I was told seven or eight years. Coating durability expectations need to be in line with the reality of the inherent durability of the coating. It’s not uncommon for a standard durable polyester powder coating to fade after a few years.

Corrective Actions

• Communicate with your powder coating supplier. Make sure they know the required performance to meet your customer’s expectations. The best tool is a well-thought-out, formal specification that details performance requirements.
• Ask your powder coating supplier for certification of the outdoor durability performance of the coatings you are using. A reputable vendor will associate known industry specifications for the products that they sell, such as AAMA specifications for architectural and automaker specifications for automotive applications.
• Make sure that your curing conditions and part temperatures reflect the recommendations on each specific product’s TDS.
• Read the powder coating TDS carefully before using a powder. Make sure that it will meet the durability needs of the intended use for the parts being coated.

Avoiding Field Defects
The best strategy to avoid experiencing powder coating field defects is a proactive approach to your finishing operation. First, establish relevant and comprehensive specifications. These should reflect the requirements of your customers and include substrates, cleaning/pretreatment processes, powder coatings, and curing equipment.

Process control is critical at all stages of the finishing process: pretreatment, application, and curing. Continuous improvement should be part of your operational culture.

Document routine maintenance procedures, document as much you can about each job that travels through your processes, and just as important, document corrective actions you perform and what you learned from each. This may help with future coating failures you have to investigate. And finally, hire good people and train them well and regularly.

Kevin Biller is technical editor of Powder Coated Tough and president of The Powder Coating Research Group.