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sion method using the ASTM D2197 test procedure can also be used. Defined procedures, equipment, and pass/fail criteria are clearly described in each of these test methods. The test results are rated as different degrees of passing on a scale from perfect (no coating loss) to complete coating disbondment, as each application can require different degrees of coating bond. Testing for initial adhesion should occur after the part has been allowed to cool to room temperature. However, some companies state in their in-house quality documents that adhesion should be checked no less than 24 hours after the test part has been cured. Select the timeline that works best for your production operation. For instance, a high volume operation probably cannot wait for 24 hours to test adhesion be- cause failure of the test part may mean thousands of suspect production parts were produced. Coating adhesion testing is considered a destructive test, meaning the coating has been destroyed as part of the test. For this reason, most people test coating adhesion in hidden areas where the af- fected test site is not visible or subject to corrosion exposure. Test panels made from similar materials as run in the process are the most accepted specimen for testing coating adhesion. Just be sure that the metallurgy, mass, and soil loads of the test panels are the same as the pro- duction parts to ensure reliable test re- sults. What causes coating adhesion failures? Once you have determined that you are dealing with a pure coating adhesion failure and not an adhesion failure caused by some other condition, it be- comes necessary to look for solutions. The most obvious cause for adhesion failures are poor substrate cleaning, where soils not completely removed from the substrate prevent the coating from properly bonding to the substrate surface. This defect is most easily solved by improving cleaning techniques or chemistries to remove the offending soils or going upstream in the manufac- turing process to eliminate the soil en- tirely. 24 POWDER COATING, April 2016 Coating adhesion: A misunderstood defect Nick Liberto, P.E. Powder Coating Consultants, div. of Ninan, Inc. O f all the possible coating defects, adhesion failure is often the most misunderstood. This misunder- standing is prevalent because the lack of coating adhesion can be a defect of its own or can be caused by another coat- ing failure. In either case, the coating has disbonded from the substrate, resulting in a defective part. What is coating adhesion? Let's start with the definition of "pure" adhesion failure. The coating bond to the substrate is broken when sufficient force is applied and the coating be- comes dislodged from the surface. Pure adhesion failure occurs in the ab- sence of any other factors that may in- fluence the coating bond strength, such as corrosion, softening, impact, flexibility, abrasion, or chipping. This is where the misunderstanding often occurs because coatings can be- come dislodged from a surface as the re- sult of many conditions. If the coating bond is broken before it is subjected to any other force or condition, this is a pure adhesion failure. However, the coating bond can be broken after it has been subjected to corrosion, softening, bending, impact, abrasion, or chipping conditions. These are not pure adhesion failures, but rather they are loss of the coating-to-surface bond as a result of the condition that initially stressed the coating. Therefore, a coating that has been disbonded after corrosion testing is a corrosion failure, not an adhesion failure. Just as a coating that cracks and flakes off after impact or flexibility test- ing are impact or flexibility failures and not adhesion failures. The same applies to chipping and abrasion which may also result in coating loss to the sub- strate. How do you test for coating adhesion? There are several test procedures for judging coating adhesion. The most popular test is the crosshatch method using the ASTM D3359 test proce- dure. The less popular scraping adhe- Guest Column Nick's Niche