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Adhesion Testing for Protective Coatings
By Michael A. Oriol and
Manuel Najar, P.E.

When protective coatings are applied to water and wastewater structures there are several quality control methods used such as coating holiday testing, adhesion testing, and dry film thickness testing. Ultimately, the results of these tests will determine if a protective coating will be accepted by the owner or the third party inspector. Disputes arise when the type of testing equipment or the criteria for acceptance is not stated clearly in the specification. As a result, contractors are forced to recoat a surface or request a change order, both of which will leave the owner and the contractor dissatisfied. In some cases, coating application jobs are stopped while the interpretation of the specification is sorted out. Based on V&A’s experience with coating inspections and specifications, we have summarized some recommendations and provided some insight for installation.

Adhesion Test Background Information

There are several standards available that provide information for adhesion testing such as the following:

	ASTM C633	Standard Test Method for Adhesion or Cohesion Strength of Thermal Sprayed Coatings
	ASTM D3359	Standard Test Methods for Measuring Adhesion by Tape Test
	ASTM D4541	Standard Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers
	AWWA C203	Coal Tar Protective Coatings and Linings for Steel Water Pipelines

 
For protective coating applications in water and wastewater facilities, the most commonly used standards are ASTM D3359 and ASTM D4541. ASTM D3359 is commonly used for thin-filmed coatings on steel such as alkyds. ASTM D4541 is commonly used for thick-filmed coatings on concrete or steel. AWWA C203 is more commonly used to test the exterior coatings on steel water pipelines.

Writing Specifications

When writing protective coating specifications it is important to be clear, concise, complete, consistent and correct. V&A recommends the following tips when specifying adhesion tests:

For ASTM D4541 – Pull Tests

1. State which type (Type I, II, III, IV) of adhesion test equipment will be used to test the coating.
2. State the minimum pull strength for each type of testing instrument to determine how the coating will be accepted.
3. State how many dollies will be used and how often they will be used.
4. Require that the inspector include information about the type of adhesive used to secure the dolly to the coating.
5. Require the inspector to include information about the length of time allowed for the dolly to adhere to the coating before it is removed.
6. Require the inspector to report the type of failure (adhesive or cohesive) observed after the dolly is removed.

The following photos show the adhesion test procedure using a Type II instrument. The photos are not of the same testing area however they illustrate the general sequence of the adhesion test for a new coating application or existing coating system.

Photo 1. A metal dolly is attached to the coating using adhesive. .	Photo 2. A saw cut is made around the dolly.
Photo 3. The dolly is shown with the saw cut.	Photo 4. Type II adhesion tester is used to pull the dolly off the coated surface. .
Photo 5. Remaining hole after the dolly is removed. The hole is repaired before the structure is put back into service.	Photo 6. The type of failure can be verified by observing the surface of the dolly.

For ASTM D3359 – Tape Test

1. State which type of cut will be used, X-Cut or Cross-Hatch.
2. State the Classification 5 (A or B) to 0 (A or B) for the amount of coating removed to determine the minimum requirement for the acceptance of the protective coating.
3. Optional – State the type of knife (e.g., “Stanley Utility Knife Blade or Similar”) used to cut the coating.

Depending on whether the substrate is concrete or steel, the pull test results will vary significantly, therefore it is important that the required minimum pull strength is practical for the type of testing instrument used. Some adhesives used to secure the dolly to the coating will take longer to cure than others, therefore the pull test results may be misleading if the adhesive has not cured. Documenting type of failure is important because it will determine if there is a problem with the coating adhering to the substrate or to an existing coating. Stating the type of knife used to cut the coating for ASTM D3359 may be a good idea since a dull pocket knife will cut thicker lines than a sharp utility knife. The thickness and sharpness of the blade may determine how much coating will be removed by the tape test.

V&A’s Experience with Adhesion Testing

The best method for avoiding conflict during the coating application is to have a well written specification that will protect the owner and clearly state what is required from the contractor. V&A provides NACE-certified third party inspectors and assistance in writing protective coating specifications. Contact V&A at the number below if any further information or guidance is needed with respect to these services.

V&A Consulting Engineers
1999 Harrison Street, Suite 975, Oakland, CA 94612
Phone 510-903-6600     Fax 510-903-6601

One Riverway, Suite 1700, Houston, TX 94612
Phone 713-840-6490

www.vaengr.com
Copyright 2005