Coatings adhere to a substrate by chemical or mechanical means. In order to check that this has been accomplished, adhesion tests can be conducted. Adhesion testing is also performed in failure analysis or when a customer is looking to “overcoat” a structure instead of completely removing a coating system.¬† Both of the test methods mentioned below are destructive tests and you should have the owner’s permission prior to conducting these tests.¬† ASTM¬† D 3359 “Standard Test Method for Measuring Adhesion by Tape Test” and ASTM D 6677 “Standard Test Method for Evaluating Adhesion by Knife” are two tests for measuring adhesion of a coating system in the field with little equipment costs.¬† They both require the use of a sharp utility knife and ASTM D 3359 requires pressure sensitive tape.
ASTM D 3359 was the first standard I learned in NACE CIP Level I training. The first item you need to determine when using this standard is: What is the overall dry film thickness of the coating or system you are dealing with? If the film thickness is over 5 mils, you will need to use Method A (also known as the “X” cut). For films less than 5.0 mils, you will need to use Method B (also known as the Cross-Hatch). Typically Method “A” is used in the field and Method B is used in the laboratory. After making the appropriate cuts and placing the proper pressure sensitive tape over the cuts, you must refer back to the standard to determine your end result. This standard should only be used on metallic substrates.
After doing field technical service work for more than 13 years, I found it next to impossible to keep the pressure sensitive tape in a properly controlled environment.¬† I found out that leaving the tape in the trunk of your car does not correlate to a properly controlled environment; half the time, the tape had no “tackiness” left. That is when I did some further research and discovered ASTM D 6677.
Have you ever just cut an “X” into the paint film on a job to check the adhesion? ASTM D 6677 “Standard Test Method for Evaluating Adhesion by Knife” is a standard we have all probably used in the field, without knowing we were actually conducting an ASTM test method. This method is very similar to ASTM D 3359 Method A (“X” cut) but without the tape. You simply cut an “X” into the film and then “lift” the edges to see how much paint can be removed. Based on the result, there is a table to rate your findings: from a 10 – “Coating is extremely difficult to remove; fragments no larger than approximately 0.8 (1/32″) mm by 0.8 mm (1/32″) removed with difficulty” to zero – “Coating can be easily peeled from the substrate to a length greater than 6.3 mm (1/4″)”. This standard can be used on any substrate.
Poor adhesion of a coating system can adversely affect a coating system’s performance.¬† If the adhesion between coats and/or the substrate is compromised, the life cycle of the coating system will decrease.