Test Coupons in the Protective Coatings Industry
What is inside this article: what test coupons are, their benefits, and how to obtain them.
Test coupons are metal plates with an applied coating system over them. Their purpose is to replace the structure during destructive testing and provide a portable test sample.
A test coupon is a cost-efficient, convenient, and proof for the future, alternative to destructive testing directly on an asset.
Preparing test coupons is another step that requires specific considerations, such as:
- How to obtain them
- What size should they be
- What material to choose
What are test coupons?
Test coupons are metal plates with a coating system that mirror an existing or new coating system. Also known as:
- Test panels
- Replica plates
A test coupon is a sample/replica of the substrate, coating, and application method. The purpose of such a replica is to replace the product during destructive testing. As the name suggests, destructive testing is a form of test that destroys or adversely affects coatings. (CIP level 2 Student Manual, Chapter 19, p 1)
Examples of such testing would be:
-
- Using portable adhesion testers (ASTM D 4541)
- Tape test (Method A and B) (ASTM D3359)
- Knife (ASTMD 6677)
- Hardness testing (CIP Level 2 Student Chapter 19, p 24)
-
- Pencil hardness test (ASTM D 3363)
- Indentation hardness test (ASTM D 2583)
- Solvent Sensitivity Testing (ASTM D 4752)(ASTM D 5402) ( CIP Level 2 Student Manual. Chapter 19, p 2)
Most coatings adequately applied to a well-prepared surface have good adhesion to the substrate; however, asset owners may choose to conduct spot adhesion tests to determine the quality of the coatings bond to the substrate. (CIP level 2 Student Manual. Chapter 19, p 1)
As shown in the image, this is a less than ideal condition for coating. That is why it is not recommended to use destructive tests or use destructive instruments on coatings.
Unless:
- The specification requires specific destructive testing
- The owner or owner’s representative requires or allows such testing
- Such tests are necessary for a failure analysis assignment.(CIP level 2 Student Manual, Chapter 19, p 1)
It is the exact reason why test coupons are so important, as they allow Coatings Inspectors to conduct such testing without damaging the main structure.
Benefits of Test Coupons
After clarifying what test coupons are and how they are used, we can describe their benefits.
Cost-efficient
As mentioned before, when a destructive test is conducted, it leaves a damaged area on the surface. The damaged area needs to be marked and repaired, which requires additional labour hours, equipment rentals, and coating material, making the project more expensive.
Unnecessary coating repair on a freshly painted structure
Unnecessary repairs might cause premature coating failure in the future. A failure can occur if repair work is not conducted in accordance with all the standards.
Convenient
Test panels can be in any shape or size. This makes them highly portable, which means that they can be tested on-site or be sent to the testing lab.
There is no benefit in damaging the film that has been inspected. Therefore, we always recommend test panels and coupons that mirror the coating system. This lets Inspectors play with it and take some extra measures to test it.
Future Proof
A test panel is a sample of the coating system applied to steel/concrete structures. It provides evidence for future testing should something happen to the coating or even possibly the substrate. Since the test panels are portable and can be made in multiple, they can be sent over to interested parties and be stored for future reference in the event the coating fails or has an adverse effect.
Coating Compatibility
Test panels can be used to determine if a coating or lining meets the environment it will be applied in. For example, a sample plate can be coated and tested before being introduced into the active environment for a while. Once the time has elapsed, the sample can be removed and re-tested against the original values. Suppose there has been any deterioration or delamination of the coating. In that case, a formal report can be produced either for or against the selected coating. This type of testing should be carried out with the manufacturer’s lab technicians to ensure that all parameters are being met.
How to obtain Test Coupons
Obtaining panels that can be used for tests should not be a problem. It is not uncommon to see leftovers during the construction, such as metal plates. Metal plates that once were a part of a structure can now be used to replicate the substrate and coating.
Having a test coupon that represents the product is essential as it should imitate the product’s coating as close as possible. Otherwise, the inspection might not be as accurate as it should be.
Preparing test coupons is another step that requires some considerations. Test coupons can be in different shapes and sizes. However, we at The Coating Inspector recommend having them no larger than 20×20 inches or 50×50 cm.
Coupons can be provided to the following:
- Contractor/Sub-Contractor
- General contractor, and
- The Engineering firm
Conclusion
To conclude, test coupons are metal plates with an applied coating system over them to replace the product during destructive testing.
A test coupon is a cost-efficient, convenient, provides future proof (if required), alternative to structural adhesion testing. Test coupons can be obtained from the construction leftovers. The optimal size for these coupons is recommended at 20×20 inches or 50x50cm. Test coupons represent the product’s coating, allowing owners to utilize all the benefits of an adhesion test without damaging the coating and costly repairs.
Test coupons are a useful tool for all your coating projects.
Contact us for a free consultation.
References:
ASTM Standard D 3359, 2002, “Standard Specification for Zinc (Hot-Dip Galvanized) Coating on Iron and Steel Products”, ASTM International, West Conshohocken, 2002 , astm.org
ASTM Standard D 4541, 2009, “Standard Test method for Pull-Off Strength of Coatings Using Portable Adhesion Testers”, ASTM International, West Conshohocken, 2009 , astm.org
ASTM Standard D 6677, 2018, “Standard Test Method for Evaluating Adhesion by Knife”, ASTM International, West Conshohocken, 2018 , astm.org
ASTM Standard D 3363, 2005, “Standard Test Method for Film Hardness by Pencil Test”, ASTM International, West Conshohocken, 2005 , astm.org
ASTM Standard D 2583, 2007, “Standard Test method for Indentation Hardness of Rigid Plastics by Means of a Barcol Impressor”, ASTM International, West Conshohocken, 2007 , astm.org
ASTM Standard D 4752, 2010, “Standard Practice for Measuring MEK Resistance of Ethyl Silicate (Inorganic) Zinc-Rich Primers by Solvent Rub”, ASTM International, West Conshohocken, 2010 , astm.org
ASTM Standard D 5402, 2019, “Standard Practice for Assessing the Solvent Resistance of Organic Coatings Using Solvent Rubs”, ASTM International, West Conshohocken, 2019 , astm.org
NACE International. (2012). Coating Inspector Program (CIP) Training & Certification Level 2 (1st ed.). NACE.
