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Let’s Talk About In-Process and PostApplication Adhesion Testing
Leave a Comment / Adhesion / By Bill Corbett
KTA’s Certified Coating Inspector Forum Volume 2, Issue No. 9 – September 2023
William Corbett, COO
AMPP Senior Certified Coating Inspector & Certified Protective Coating Specialist
KTA’s Certified Coating Inspector Forum is designed to provide professional
development/continuing education on standards, inspection practices, new instruments, and other
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topics to help keep certified AMPP and FROSIO coating inspectors current. It represents the views
of the author and KTA-Tator, Inc. It may or may not represent the views of AMPP: The Association for
Materials Protection & Performance, even though SSPC, NACE, and AMPP standards are
frequently referenced in the content.
Introduction
I thought I’d begin this issue of the Certified Coating Inspector Forum on adhesion testing by
discussing various definitions of adhesion. When I Googled, “which is the best definition of adhesion
in science,” I got: “Adhesion is the ability of one thing to stick firmly to another.” Simple enough.
ASTM D3359, Standard Test Methods for Rating Adhesion by Tape Test defines it as “The
molecular attraction and mechanical bonds between a coating and its substrate,” which is bit more
technical. And although the testing we do is called “adhesion,” we are also assessing another
characteristic of the coating, which is cohesion. A Google definition of cohesion is “The sticking
together of particles of the same substance.” But I wanted something related to measuring adhesion
(and cohesion) simply stated, so I came up with: A determination of how well a coating is attached to
a substrate, how well coating layers are attached to one another, and the inner strength of any
individual coating layer. So now that we have a basis for what we are measuring, let’s dive into the
testing itself.
While some time is dedicated to learning about the basics of adhesion testing in AMPP CIP 2
training, I feel it is a topic worthy of more in-depth discussion, especially as it relates to in-process
adhesion testing of thermal spray coatings, since they are becoming more widely specified. This
issue of the Certified Coating Inspector Forum focuses on the following topics related to adhesion
testing:
1. Why Measure Coating Adhesion Properties?
2. Selecting an Adhesion Testing Method
3. Measuring Adhesion by the Tape Test
4. Evaluating Adhesion by the Knife Test
5. Measuring Pull-off Adhesion of Coatings on Steel
6. Measuring Pull-off Adhesion of Coatings on Concrete
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7. Reporting Tensile (pull-off) Adhesion Test Data
8. In-process Testing of Thermal Spray Coatings (TSC)
Why Measure Coating
Adhesion Properties?
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Given that adhesion testing of a coating or lining is typically a destructive test, why would we do it?
The primary reason is that a specification may require it as one way to help ensure the coating was
applied correctly to a properly prepared surface. But one could argue that if in-process QC and QA
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inspection was performed by knowledgeable, trained, and certified inspectors then there is little
need for this type of verification. Nonetheless it may be contractual. However, in the case of thermal
spray coatings (TSC) applied by metallizing, in-process tensile (pull-off) adhesion is frequently
required (in addition to companion coupon bend tests), since there are many application variables
that can influence the bond of the TSC to the prepared substrate. More on that a little later. Adhesion
testing is an excellent way to determine the integrity of an existing coating system that is being
considered for overcoating to extend its service life. In fact, overcoating an aged coating system
without evaluating the adhesive/cohesive properties of the existing system can be high-risk and can
result in catastrophic disbonding if the existing system has low adhesive/cohesive strength. Finally,
adhesion testing is often performed during a coating failure investigation, but that is outside the
scope of this article.
While adhesion gives us a “warm and fuzzy feeling” that the coating system is well-adhered, it is not
necessarily a predictor of long-term performance. That is, a coating system that displays a pull-off
value of 1,000 psi is not going to perform twice as long as a coating that displays a pull-off value of
only 500 psi. If the substrate is protected and the coating remains attached, the relative strength of
the coating bond to the substrate (and to underlying coating layers) may be somewhat moot. And
even if a coating possesses excellent adhesion, corrosion protection can be compromised for other
reasons, such as insufficient thickness.
Selecting an Adhesion
Testing Method
There are four common methods for testing the adhesion of protective coatings and linings, listed in
the table below.
ASTM Standard
Reference
Title of Method
D3359
Standard Test Methods for Rating Adhesion by Tape Test
D6677
Standard Test Method for Evaluating Adhesion by Knife
D4541
Standard Test Method for Pull-off Strength of Coatings Using Portable
Adhesion Testers
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D7234
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Standard Test Method for Pull-off Strength of Coatings on Concrete
Using Portable Adhesion Testers
The first two methods are used to evaluate the shear, peel-back, or peel strength of a coating, while
the latter two methods are used to evaluate the tensile or pull-off strength of a coating. Since the
testing mechanisms differ (peel-back versus pull-off) the data generated should not be compared.
That is, a coating can exhibit a high pull-off value, but a low peel-back rating, and vice-versa. If
adhesion testing of in-process work is to be performed, the specific test method(s) should be
specified. If testing is not specified but is mandated by the engineer or facility owner after-the-fact,
the purpose of the testing will help them to select the method(s) to be used. For example, if there is
a dispute on workmanship, or a question arises as to whether some of the existing coating on a
structure can remain and be overcoated, both peel-back and pull-off data may be beneficial.
Measuring Adhesion by
the Tape Test (ASTM
D3359)
The procedures described in ASTM D3359 are designed for use on metallic substrates. The
methods do require knowledge of the applied coating thickness, which can be measured
nondestructively (ASTM D70911). Method A (X-cut) is used on coatings/systems exceeding 5 mils,
while Method B (Crosscut) is used on coatings up to and including 5 mils. The distance between the
cuts creating the crosscut pattern is 2 mm for coatings between 2 and 5 mils in thickness, or 1 mm
for coatings less than 2 mils. Method B can be used on coatings thicker than 5 mils if the spacing
between the cuts is wider than 2 mm, but the specific spacing is not defined. Although not defined, a
spacing of 5 mm has been used for heavier coatings.
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The required equipment includes a straight edge, a razor knife or scalpel, a brush, tape with very
specific adhesive properties, and a magnifier. A cross-cut guide or X-cut guide can be used instead
of a straight edge, and a utility knife can be used in place of a razor knife/scalpel. A cross hatch
cutter is also available as an alternative to the straight edge or cross-cut guide for Method B. The
cross-hatch cutter makes all of the cuts at the same time, rather than individually as is the case with
the straight edge or cross-cut guide. The cross-cut guide is only available with 2mm spacing (for
testing coatings from 2-5 mils thick). If the coating is less than 2 mils thick, the cross-hatch cutter is
the best alternative to making individual cuts with 1mm spacing.
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To perform Method A (X-cut), make a 1.5” cut through the coating system in a single pass. If the cut
does not go all the way to the substrate, do not recut the incision. Move to a new location and make
a new cut. Make a second cut to form an “X” with a 30-45° angle at the intersection of the “X.”
Inspect the cuts with the magnifier to verify the incisions penetrate down to the metal surface.
Remove the cutting debris with the brush and remove 2 laps of tape (one time for each day of
testing) and discard. Apply a 3” piece of tape to the X-cut and rub it firmly on the surface to ensure
good contact.
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Remove the tape within 90 +/- 30 seconds 180° back across the X-cut (do not pull upwards). Repeat
the test at two more spots at each test location (triplicate testing). Evaluate the X-cuts for coating
detachment and rate the adhesion from 5A to 0A (rating chart shown below).
To perform Method B (crosscut), conduct the test in a similar fashion to that described for Method A,
except that a grid is cut into the coating. The grid consists of a series of equally spaced parallel cuts,
with a second series of parallel cuts made perpendicular to the first. The cuts are kept straight using
a straight edge, the cross-cut guide, or the cross-hatch cutter. Eleven cuts, 1 mm apart are made in
each direction for coatings up to 2 mils in thickness, and six cuts, 2 mm apart are made in each
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direction for coatings from 2 to 5 mils. Triplicate testing is required. Evaluate the crosscut for coating
detachment and rate the adhesion from 5B to 0B (rating chart shown below).
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The pressure-sensitive tape that is used for both Methods A and B must have an adhesive peel
strength between 58 and 64 oz./in. when tested according to ASTM D33302. This range is based on
the properties of Permacel P-99 tape (discontinued in 2009). The SEMicro CCT conforms to this
requirement (and comes with a Certificate of Conformance); other tapes can be used if the
contracting parties agree.
Here are a few things to remember when performing tape adhesion testing:
Dull knife blades/cutter bundles can produce erroneous results since the coating may chip
when the cuts are made.
Maintaining proper spacing between incisions when employing the cross-cut tape adhesion
method is critical.
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When employing the X-cut method, maintaining the proper angle at intersection of “X” is
critical. The legs of the “X” should be ~ 1” apart.
The tape should be stored properly. The tape typically has a one-year shelf life.
The name of the tape used should be included in the report.
The tape may not be effective in cold temperatures.
Evaluating Adhesion by
the Knife Test (ASTM
D6677)
The procedure described in ASTM D6677 is very similar to Method A in ASTM D3359 except that no
tape is used. Instead, the tip of the knife blade is used to probe at the intersection of the “X” and the
adhesion rated on a scale of 10 to 0 (rating chart shown below).
Rating
10
8
Description
Coating is extremely difficult to remove. Fragments no larger than 1/32” x 1/32”
removed with great difficulty
Coating is difficult to remove. Coating chips from 1/16” x 1/16” to 1/8” x 1/8” removed
with difficulty
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6
4
2
0
Let’s Talk About In-Process and Post-Application Adhesion Testing - KTA-Tator
Coating is somewhat difficult to remove. Coating chips from 1/8” x 1/8” to 1/4” x 1/4”
removed with slight difficulty
Coating is somewhat difficult to remove. Coating chips in excess of 1/4” x 1/4” removed
using light pressure with knife blade
Coating is easy to remove. Once started with the knife blade, the coating can be
grasped with fingers and easily peeled to a length of at least 1/4”
Coating is easy to remove. Once started with the knife blade, the coating can be
grasped with fingers and easily peeled to a length greater than 1/4”
Measuring Pull-off
Adhesion of Coatings
on Steel (ASTM
D4541)
Type II (fixed alignment)
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Type V (self-aligning hydraulic)
There are five different pull-off adhesion testers referenced in ASTM D4541, including one
mechanically operated tester (fixed alignment), one pneumatically operated tester (self-aligning),
and three hydraulically operated testers (self-aligning). Fixed alignment testers cannot compensate
for misalignment of the loading fixture (described below) to the coated surface, while self-aligning
testers can. Since different testers yield different pull-off values (due in part to fixed versus selfalignment, consistency of the applied loading rate, and other factors), it is important to decide which
type of tester will be used prior to testing. This point cannot be overemphasized. The results
obtained from each of the testers listed in ASTM D4541 will not be identical, so it is critical that the
specific test device be identified before the work begins. Information on the variation in pull-off
values of several coating types using the different testers (based on an interlaboratory study used to
prepare the precision and bias statement) is provided in ASTM D4541.
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To perform the testing, the coated surface and the base of the loading fixture (occasionally referred
to as a dolly) are roughened to help ensure a good bond. The loading fixture is the “plug” that is
attached to the coating and removed with the tester. The design and diameter of the loading fixtures
varies depending on the type of tester. Loading fixture diameters for coatings on steel can be as
small as 10mm (approx. 0.5-inch); typically, 20mm (approx. 1-inch) loading fixtures are used.
The loading fixture is bonded to the coated surface using an adhesive (glue). The glue type can vary
from epoxy to cyanoacrylate gel. After the glue cures, the adhesion testing device is attached to the
loading fixture and perpendicular force is applied (mechanically, pneumatically, or hydraulically) until
the loading fixture is detached, the specified load is applied without detachment of the fixture, or the
maximum load of the instrument is reached. The pressure loading rate cannot exceed 150
psi/second and scoring the coating to the substrate prior to applying the pressure load is not
performed unless agreed upon by the contracting parties. Note that certain adhesion testers either
monitor the pressure loading rate or control the pressure loading rate automatically (red-circled area
on display image). If not, the operator can use a stopwatch to verify the maximum rate is not
exceeded.
The pressure required to detach the loading fixture (or the pressure applied if the maximum load is
reached) is recorded, along with the location of break, which is defined as follows:
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Adhesive Break: A break between coating layers or between the substrate and first coating
layer
Cohesive Break: A break within an individual coating layer
Glue Break: Coating adhesive and/or cohesive strength exceeds bonding strength of the glue
used to attach the loading fixture.
If multiple locations of break occur, the percentage of each (e.g., 75% cohesive within primer; 25%
adhesive between primer and topcoat) is estimated.
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Measuring Pull-off
Adhesion of Coatings
on Concrete (ASTM
D7234)
Evaluating the tensile strength of coatings applied to concrete is similar to testing coatings applied
to steel, with the following testing variations.
Scoring the coating
Thick film coatings (>20 mils) have lateral bond strength that may produce misleading pull-off
values (false, high), so scoring the coating is required.
Scoring through the coating (down to the concrete surface) is typically performed prior to
attachment of loading fixture (except if the loading fixture is square)
Scoring coatings <20 mils may also be required by the specification, or as agreed upon by
the contracting parties.
When attaching the loading fixture, it is important to prevent the glue from flowing into the
groove created by scoring.
Loading Fixture Size and Loading Rate
Unlike steel, concrete is non-uniform and contains cement, aggregate, and additives,
therefore the loading fixture diameter should be larger – 50mm (approx. 2-inch) to ensure the
test is performed over a representative area.
Square loading fixtures are used with some testers.
The pressure loading rate should be 30 psi/second; the test should be completed in 5-30
seconds. Again, certain adhesion testers either monitor the pressure loading rate or control
the pressure loading rate automatically. If not, the operator can use a stopwatch to monitor
the rate of loading.
Compressive Vs. Tensile Strength
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“Typical” concrete has high compressive strength (e.g., 4000 psi), but low tensile strength
(e.g., 400 psi). High strength concrete has higher tensile strength.
Low pull-off values are common, and the plane of fracture is commonly within the concrete.
When fracture occurs in the concrete, the minimum coating tensile adhesion properties can
be reported; however, the actual coating tensile strength remains unknown.
Unlike ASTM D4541, the various instruments are not described in Annexes in ASTM D7234. An
interlaboratory study (ILS) published by ASTM involving two coatings applied to concrete and seven
instruments revealed insignificant variation in pull-off values between the various tester types.
Accordingly, the results between the instruments are more comparable.
Here are a few things to remember when performing tensile (pull-off) adhesion testing:
Determine which tester to use in advance. Different testers generate different pull-off values
on the same coating when applied to steel.
Verify the testing equipment has a current calibration certificate.
Perform testing in triplicate at each test location.
The rigidity of the substrate can affect the results. Testing on steel less than 1/8” thick can
result in lower pull-off values because the steel can flex.
Clean the coating surface and the loading fixtures to reduce the frequency of glue failures and
use care when abrading the surface to prevent the introduction of surface flaws.
Verify the glue is compatible with the coating.
Allow the glue to cure. Conducting pull tests before the glue is cured often results in glue
breaks.
Verify perpendicular pull.
Support the pull-off device when testing coatings on vertical surfaces.
Reporting Tensile (pulloff) Adhesion Test
Data
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The flow diagram shown below was excerpted from ASTM D4541 and contains the reporting
requirements based on two protocols. Protocol 1 (Test to Fracture) is used when the pull-off value is
unknown or unspecified and you want to know the pull-off strength; Protocol 2 (Pass/Fail) is used
when the pull-off value is known or specified. Note that for Protocol 1, glue failures exceeding ¼ of
the loading fixture contact surface are considered invalid tests and the results are discarded. For
Protocol 2, if the glue fails prior to achieving the predetermined pull-off value (i.e., passing value),
retesting is typically required.
In-process Testing of
Thermal Spray
Coatings (TSC)
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It is unusual to test the adhesion of coatings in the process of application since coating cure is
critical to achieving proper adhesion. However, in the case of thermal spray coatings (TSC) that are
neither liquid applied nor require cure time, in-process adhesion testing is frequently specified.
Surface profile depth, angularity, surface cleanliness, and the wire quality, amperage, voltage, spray
distance, and spray angle can all impact the adhesion of the applied TSC. Therefore, specifications
that invoke SSPC-CS 23.00/NACE No. 123 frequently require tensile (pull-off) adhesion testing on
the coated part or on companion panels using a self-aligning adhesion tester.
This requirement can be tricky since production is stopped until pull-off values are known and
acceptable. Waiting for glue to cure can be tedious. The manufacturer of the pneumatic pull-off
tester makes special loading fixtures with a transparent base. A light (UV) curable glue such as
Loctite AA 3554 is applied to the base of the loading fixture and a UV light source is used to cure the
glue in a few seconds, enabling the inspector to perform the adhesion testing with minimal
interruption to production. As indicated above, some specifications allow for the preparation of
companion panels at the same time the production work is being performed, with the coating on the
panels, rather than the coating on the structure, being tested.
Summary
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This edition of the Certified Coating Inspector Forum focused on post-application adhesion testing
of liquid-applied protective coating systems and in-process adhesion testing of thermal spray
coatings. Four methods of adhesion testing were described. Independent of the method used,
adhesion testing is destructive to the coating film (unless the loading fixture is not detached during
tensile [pull-off] testing and is allowed to remain attached) and should not be performed by an
inspector unless required by the project specification. In-process adhesion testing is commonly
required for TSC and may be performed on the actual coated steel or on companion panels
prepared to the same degree as the structural steel.
The Importance of Concrete Sealers on Concrete Surfaces
by Jay Helsel ● August 28, 2023
Let’s Talk About Using SSPC Visual Guide for Abrasive Blast Cleaning
to Create a Project-Specific Cleanliness Standard
by Bill Corbett ● August 14, 2023
Let’s Talk About Measuring Coating Thickness Destructively
by Bill Corbett ● July 10, 2023
[1] Standard Practice for Nondestructive Measurement of Dry Film Thickness of Nonmagnetic
Coatings Applied to Ferrous Metals and Nonmagnetic, Nonconductive Coatings Applied to NonFerrous Metals
[2] Standard Test Method for Peel Adhesion of Pressure-Sensitive Tape
[3] Specification for the Application of Thermal Spray Coatings (Metallizing) of Aluminum, Zinc, and
Their Alloys and Composites for the Corrosion Protection of Steel
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