Technology Interchange — Best Powder Coating Strategies for Corrosion Resistance
Posted on Thursday, May 30, 2019
Overall, powder coatings are known for their reasonably high durability, including resistance to corrosive environments. Corrosive environments are encountered in many situations.
For those parts of North America that receive any appreciable amount of snowfall, roads are probably treated with granular salt and/or brine to combat dangerously slick, ice-covered surfaces. For the coastal areas, corrosion is accelerated by the airborne salt emanating from the ocean. Even indoor applications can encounter corrosive conditions such as laundry equipment and regularly wet environments in various processing plants.
Corrosion Resistance Testing
Before we can discuss the relative performance of different powder coating types and their inherent corrosion resistance, it is helpful to consider the test methods used to characterize this property. Of course, real-world testing is by far the most relevant manner in which to gauge the corrosion resistance of a coating system and it is wise to place samples in the field to verify performance. However, it is requisite to have predictive accelerated exposure test methods to make relative comparisons between coatings and also to approximate expected durability under real world conditions.
ASTM B-117 Standard Practice for Operating Salt Spray (Fog) Apparatus is the most common technique to measure a coating’s resistance to corrosion. This test is comprised of a cabinet heated to 95 degrees Fahrenheit with a continuous aqueous five percent neutral salt fog. Coated test panels are typically scribed down to the substrate and then exposed for a determined number of hours. Exposed test panels are inspected usually every 250 hours for scribe creep, edge rusting and the presence of blisters. Two-hundred and fifty hour interval inspections are typical for powder coatings, whereas some other coating technologies are tested more often than that. Salt fog testing is not perfect, as it only measures resistance to a misted salt solution at a static temperature and no other environmental factors such as UV exposure, temperature variations and humidity fluctuations. Realizing this, the coatings industry has developed cyclic test methods that include these dynamic conditions.
One of the first cyclic corrosion tests was developed by General Motors in an effort to better replicate real world conditions. The GM-9540P test method consists of a series of salt mist applications at 25 degrees Celsius and 50 percent relative humidity (RH), each followed by a dry cycle. An eight-hour 100 percent RH cycle follows, which is then followed by an eight-hour dry cycle at 60 degrees Celsius and then eight hours at 25 degrees Celsius. Twenty cycles were typically run to assess corrosion resistance.
GM-9540P was superseded by the more streamlined GM-14872 test method, which consists of cycles of eight-hour salt mist applications at 25 degrees Celsius and 10 percent RH followed by eight hours 100 percent RH and then eight hours 60 degrees Celsius, 30 percent RH. Weekends and holidays are run at 25 degrees Celsius, 45 percent RH. The salt mist is lower than ASTM B-117 and is comprised of 0.9 percent NaCl, 0.1 percent CaCl and 0.075 percent NaHCO3.
A twist on the cyclic corrosion testing is SCAB (simulated corrosion atmospheric breakdown) corrosion testing. This was developed in the 1980s in response to the massive coating failures experienced on domestic cars and small trucks and the lack of correlation with ASTM B-117. ISO 11474 describes this test which employs a series of salt spray cycles followed by drying cycles. General Motors describes their requirements in GM-9511P, which calls for exposure at 60 degrees Celsius and 85 percent RH after daily immersion in a neutral salt solution and weekly cycles of -25 degrees Celsius thermal shock.
Filiform corrosion occurs when ions breach the interface of a coating and substrate and burrow worm-like trails of corrosion. ASTM D-2803 defines this method, which basically involves exposing a scribed coated test panel to ASTM B-117 salt spray for four to 24 hours then follows with 85 percent RH at 25 degrees Celsius. Panels are inspected every 168 hours for the evidence of filiform corrosion.
ASTM D-5894 Cyclic Salt Fog-UV Exposure combines a corrosive environment with UV exposure. Test specimens are exposed alternately to one week in QUV accelerated exposure and one week of salt fog exposure. These combined two weeks are counted as one cycle and testing typically is conducted for five or six cycles. This testing has the advantage of combining degradation due to UV exposure with a corrosive atmosphere.
Metal Cleaning and Pretreatment
Integral to achieving a corrosion resistant coating system is starting with a clean, well-prepared substrate. The old adage, “Can a bad tree bear good fruit?” applies here. If the substrate is contaminated with dirt, oil, metal processing fluids, oxides or mill scale, the coating will not perform well in a corrosive environment. It is therefore requisite to thoroughly remove all contaminants and oxides prior to applying the coating. This is usually accomplished with an alkaline cleaner followed by one or two clean rinses.
If tenacious metal oxides and/or mill scale are present, then mechanical cleaning may also be required. This can be accomplished with abrasion or blasting with a good clean media. Mechanical surface preparation should be followed by alkaline cleaning to ensure a pristine surface.
It is highly recommended that once the substrate surface is clean and rinsed that a chemical conversion process be used to provide a bond between the coating and the metal. Ferrous substrates are typically pretreated with either an iron phosphate or zinc phosphate process. Iron phosphate provides an excellent interface between the ferrous metal and powder, whereas zinc phosphate is even better. The phosphate process is followed by a rinse or two and then a chemical sealer for the best corrosion resistance. Zirconium-based pretreatments can also be used instead of the phosphate-based ones.
Aluminum alloys should also be thoroughly cleaned and rinsed using an alkaline cleaning process. For optimum corrosion resistance, either a chromate or a zirconium-based pretreatment should be used. It is best to consult a chemical supplier for recommendations for your metal cleaning and pretreatment needs. Keep in mind that the best pretreatment for ferrous substrates will not be the best for non-ferrous metals.
Corrosion Resistant Chemistries
Powder coatings provide relatively good corrosion resistance; however, some perform significantly better than others. Epoxy-based powder coatings are inherently the best chemistry for corrosion resistance.
The polymeric backbone of an epoxy resin wets out the surface of the substrate and also anchors itself well to the pretreatment and metal. In addition, epoxies can be formulated to exhibit excellent barrier properties to protect the metal substrate from moisture, salt and other environmental chemicals (e.g., acid rain). The corrosion resistance of epoxy powders can be further fortified with a healthy concentration of zinc. Zinc-filled epoxies provide outstanding corrosion resistance over ferrous substrates.
However, one drawback with using epoxy-based powder coatings is their poor UV durability. Sunlight will degrade an epoxy in a matter of months. Corrosion resistance will not wane immediately, but the coating will chalk and yellow and eventually erode. Consequently, a UV-resistant powder coating may be required if outdoor durability and the maintenance of appearance is required.
Polyurethane-based powder coatings can be formulated for very good corrosion resistance that meets most industrial specifications. Polyesters can also be formulated to provide relatively good corrosion resistance. Those based on triglycidyl isocyanurate (TGIC) generally outperform polyesters based on betahydroxy alkyl amide (HAA). HAA is commonly used in many “TGICFree” polyesters. Hybrid (aka epoxypolyester) powders can provide very good corrosion resistance if formulated specifically for this property. Hybrids fade and chalk similar to epoxy powders so outdoor applications are not recommended for this chemistry.
Choosing the best powder coating chemistry for a corrosion resistance specification should take into account the chemistry of the powder and also the quality of that specific powder coating product. Avoid low cost ‘bargains.’ Powders from unknown sources should be avoided as well as surplus or recycled powders. When in doubt, you can submit a powder coating to an independent testing laboratory for confirmation of performance. If possible, it is wise to provide sample customer parts that have gone through your cleaning/pretreatment process or a proposed process.
Purchasing the appropriate powder coating for your corrosion resistance requirements is the first step. However, adhering to the proper processing conditions is critical to obtain the best performance. Besides using a high performance cleaning/pretreatment and keeping it in control, it is necessary to apply and cure the powder coating per the powder supplier’s recommended conditions. You can find these on the Product Data Sheet (aka Technical Data Sheet). This will specify the recommended film thickness and curing conditions. It goes without saying that complete coverage and appropriate cure is necessary to achieve the performance expected.
For the highest level of corrosion resistance, a two-coat system should be considered. These are usually comprised of an epoxy-based primer and an outdoor durable topcoat. The outdoor durability requirements will influence what type of chemistry you select as a topcoat. Polyesters, polyurethanes, acrylics and fluoropolymer topcoats provide varying degrees of UV durability ranging from a couple years of Florida durability with standard polyesters and polyurethanes to five to 10 years with “superdurable” polyesters and acrylics. Fluoropolymer-based powders can exceed over 20 years durability in Florida.
When selecting a two-coat system, it is wise to obtain both the primer and topcoat from the same supplier. Marrying the two coats takes coordination of compatible chemistries and curing processes. Primers are commonly under-cured, followed by the application of a topcoat layer. The two layers are then cured completely with the final bake. This ensures a very strong chemical bond between the two coatings.
In conclusion, high performance powder coatings, when properly applied over a clean and properly pretreated substrate, can provide years of excellent performance in even the most extreme field conditions. It is always important to take a team approach when selecting your cleaning/ pretreatment chemicals, powder coating supply and application/curing equipment.
By: Kevin Biller, technical editor of Powder Coated Tough and president of The Powder Coating Research Group.