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CM May 2001 - Page
21
ADHESION TESTING OF COATINGS ON STEEL
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INTRODUCTION
The initial measure of the successful application of any
protective coating is its adhesion to the substrate. If
there is one sight that epitomizes failing paint coatings,
it is the appearance of flaking paint.
One of the fundamentals of coating chemistry is to ensure
that the connection between to substrate and the coating
is secure. Incompatibility at the interface will ensure
coating failure. If manufacturers’ recommendations are adhered
to, this will not be an issue.
Adhesion then becomes a test of the quality of the application
from surface preparation through priming, top coating and
curing of whatever systems are applied.
Various methods of adhesion testing have been developed,
some simple and others requiring more sophisticated equipment.
A number of these test procedures have been included in
Australian, ISO and other international standards.
The problem with each method of adhesion testing is that
it is destructive. It also only measures the small section
being tested. This review is aimed at covering issues associated
with adhesion testing.
ADHESION CASE HISTORY
One of the divisions of the company for which I work won
a contract to supply all the lighting columns for the Sydney
Olympic Games site at Homebush Bay. Over 1000 poles of various
types were involved in the contract. Many were hot dip galvanized
while others were galvanized and then top-coated with a
decorative paint system.
Because of the tight time constraints on supply and installation,
the Olympic Co-ordination Authority (OCA) selected a paint
system based on its rapid curing characteristics, and Industrial
Galvanizers Pole Division was required to have this coating
applied by sub-contract, using the paint system nominated
by the OCA.
The paint did not perform to manufacturer's specifications,
taking much longer to cure than expected. In the course
of the accelerated installation program, a number of poles
were moved before the paint system was fully cured. Some
of the paint delaminated from the galvanized surface, and
the OCA questioned the integrity of the application.
A lengthy dispute ensued, based on claims that the paint
system's adhesion was ‘inadequate’. No subsequent coating
problems were experienced with the paint coating on any
of the poles. No delamination or other adhesion related
problems were identified.
The issues questioned by the OCA included the following:
1. Was the surface preparation (nominated as brush blasting)
adequate?
2. Was the primer coat put on to the correct thickness?
3. Was the topcoat put on to the correct thickness?
4. Did the adhesion meet the specification?
While these criteria seem straight forward, a review of
the practical issues and the contract documentation revealed
the following:
There was no clear definition or specification for ‘brush
blasting’ over galvanized surfaces in the coating specification.
Because the paint system was applied in multiple coats over
a hot dip galvanized surface, on structural steel, coating
thickness instruments measures only total coating thickness,
including the galvanized coating. As this was considerably
in excess of the minimum thickness required by the Australian
Standard (AS/NZS 4680 – 1999), there was no non-destructive
method of determining compliance with the paint specification
as the total coating thickness generally exceeded the specification.
There was no definition in the specification to nominate
adhesion values.
The issue was finally resolved in Industrial Galvanizers
Pole Division's favour, following some small scale testing
of adhesion values on the already coating damaged poles.
All these tests exceeded 250 psi.
Because the coating was applied for aesthetics only (the
galvanizing was deemed to be supplying the anti-corrosion
protection), the paint coating was eventually accepted without
further destructive testing because:
1. It was deemed to be fit for purpose (changing the
colour of the poles)
2. A 3-year performance guarantee on the system's adhesion
was offered by Industrial Galvanizers.
3. In the 9 months of negotiations, there had been no
coating problems on any of the Homebush Bay installations.
The moral of this story is that while adhesion is one of
the most critical factors in long-term coating performance,
it is one of the most difficult aspects of coatings to practically
define and cannot be tested without damaging the very thing
it is aimed at preserving.
ADHESION TESTING ON STEEL
Adhesion testing determines how well a coating is bonded
to the substrate (i.e., whether it is anchored to the substrate
itself or just to a thin layer of material on the surface
of the steel).
There are various reasons for wanting to evaluate adhesion.
The specifier and contractor would want to assess adhesion
when considering or performing overcoating work because
the new coating may act more like a paint stripper than
a protective coating, if adhesion of the existing coating
system is poor. Another reason for testing adhesion is for
bidding purposes.
Knowing the adhesion of the existing coating will tell
something about production rates that can be achieved. Adhesion
is sometimes measured during application, as required by
a specification.
In addition, adhesion tests are performed by coating manufacturers,
when qualifying coating materials.
The common adhesion tests are performed with a knife or
a specialised pull-off adhesion tester. All adhesion tests
are destructive, so the test area must be repaired. Therefore,
it is best to keep the tests to a minimum. However, when
adhesion testing is needed, it is important to know the
different ways to perform the tests, precautions for each
type of test, and what the results mean.
KNIFE TEST
The simplest method of evaluating adhesion is to pick at
the coating with a utility knife. This method has been used
for many years, though no formal procedure currently exists.
Two cuts are made in the coating to form the letter X. The
cuts are about 40 mm long, and the angle between them is
30-45 degrees. It is important to make sure the cuts go
all the way to the steel. The knife blade is then used to
pick at the intersection of the cuts with a vertical motion
to try and lift the coating. If more than about 5 mm of
material is removed, the adhesion is considered to be poor.
Another method is to insert the knife blade under the coating
at the intersection of the X and push forward with the blade.
When this is done with a sharp blade and a lot of pressure,
a well-adhered coating may slice, but it will not disbond
from the surface.
The knife adhesion test is subjective, and experience
is required to get consistent results. It is a good idea
to run this test on all sorts of coatings, especially the
ones that are going to be removed.
CROSS CUT ADHESION TEST
Australian Standard AS1580.804.4:1993 defines the requirements
for adhesion testing using the cross-cut, pull off tape
test. It is not recommended for coatings exceeding 125 microns
in thickness. In other jurisdictions, this may be called
the ‘Tape Test.’
A single bladed cutting tool such as razor blade or break-blade
knife can be used to make the cross-hatch cuts, or alternatively
a special multi-bladed cutter can provide a faster and more
uniform alternative, but may only be used on flat surfaces.
AS1580.408.4 details the preparation and testing procedure,
including the approved type of tape to be used. The classification
of test results is done by visual comparison with the Standard,
which defines the affected area of the test section in terms
of the percentage of coating detached from the surface by
the pull-off tape.
A rating of 0 through to 5 classifies the adhesion of
the test sample section from 100% pass (0) to fail (5) where
more than 65% of the test area delaminates from the surface.
PULL-OFF TESTS
Another method for evaluating adhesion is the pull-off
test. With this method, a loading fixture commonly called
a dolly or stud is glued to the surface. A special device
is then used to apply an increasing force until the coating
disbonds or the glue fails. The pull is perpendicular to
the surface, so tensile strength is being measured. This
is different from the tape tests where shear is being measured.
Therefore, the results obtained from the two different types
of tests are not comparable.
A portable adhesion tester, loading fixtures, and adhesive
are needed for this test. The first step is to prepare the
loading fixtures. These are supplied as smooth steel and
must be cleaned so the glue will stick. This usually involves
solvent cleaning. It is a good idea to roughen the bonding
surface of the loading fixture either with sandpaper or
light abrasive blasting. This will minimise the number of
glue failures that occur.
The coating surface must also be cleaned. Surface abrasions
can induce flaws, so only fine sandpaper (400 grit or finer)
should be used, if needed, to remove loose or weakly adherent
contaminants such as chalking or dirt that cannot be washed
off.
Epoxy or acrylic adhesives are used to glue the loading
fixture in place. The adhesive must cure for the amount
of time recommended by the manufacturer. This can be several
hours to a day, depending on the adhesive and the temperature.
It is important that constant contact pressure be maintained
as the adhesive sets and begins to cure. Magnetic or mechanical
clamps work best. For pull-off adhesion testing, at least
three test fixtures per test area are normally required.
The number of areas being tested will determine how many
fixtures and clamps are needed. The fixtures can be taped
in place with masking tape or duct tape if enough clamps
are not available.
There are two main types of commonly used adhesion testers.
One is a fixed-alignment, mechanical adhesion tester, the
other is a fixed-alignment, pneumatic adhesion tester. These
instruments come in different force ranges, so the proper
range instrument must be selected.
No matter what instrument is being used, the load to the
fixture should be increased continuously and smoothly at
a rate not to exceed 1 MPa/s (145 psi/s). The load is applied
until failure occurs (or until the maximum force has been
applied). Any test fixtures that do not detach with the
maximum load can be easily removed by tapping them on their
side. In fact, this is a good demonstration of how easily
the test fixtures can be removed in shear compared to the
tensile failure mode of the actual test.
Some instruments come supplied with a circular hole cutter
to score through the coating to the substrate around the
loading fixture before running the test.
Scoring around the fixture violates the fundamental criterion
of the test that an unaltered coating be tested. However,
the practice is a subject of debate among some adhesion
test experts. Therefore, if scoring is required for some
reason, be very careful to prevent micro-cracks in the coating
because they will give lower pull-off strengths.
More information about the pull-off test procedure can be
found in ASTM D4541, Standard Test Method for Pull-Off Strength
of Coatings Using Portable Adhesion Testers.
TEST RESULTS
Two pieces of information are obtained from a pull-off
test. The first is the pull-off strength of the coating.
This is the force attained at failure (or the maximum force
applied if the test fixture did not disbond). If required
by the manufacturer, a calibration conversion chart is used
to convert the force reading on the instrument to the actual
force applied. If the manufacturer does not require the
use of such a chart, then the actual applied force is read
directly from the instrument. In either case, the force
then must be divided by the surface area of the test fixture
to determine the pull-off strength. Some instruments use
only one size test fixture, so the instrument scale is already
in MPa (psi). Consult the operation manual for the instrument.
The other piece of information is where the separation occurs
in the coating system. Examination of the bottom of the
test fixture and the surface where the test was performed
will show if the failure could be an adhesive break, a cohesive
break, a combination of both, or a failure of the glue.
An adhesive failure is a break between coating layers or
between the coating and the substrate. If the break occurs
between coating layers of a multi-colour coating system,
one colour coating will be on the back of the test fixture
and another colour coating will be on the surface.
A cohesive failure is a break within one coating layer,
so the colours will be the same. Identification of where
these failures occurred in the coating system (i.e., adhesive
failure between the steel and primer; adhesive failure between
the primer and mid-coat; cohesive failure within the primer;
glue failure, etc.) are important. Information about the
location of the break in the coating system is just as important,
if not more important, than the pull-off strength. It identifies
the weakest area in the coating system.
When a glue failure occurs, the only information gained
is that the pull-off strength is greater than the value
obtained.
INTERPRETING TEST RESULTS
Interpreting the pull-off strength results must be done
with caution. There is a large variability in the repeatability
and reproducibility of the method, and different types of
instruments give different results.
Big differences in pull-off strengths also exist among instrument
types. Therefore, when comparing results, the type of instrument
used to perform the test must be considered when looking
at the tensile strength.
So, what is an adequate adhesion of a coating when using
a pull-off adhesion tester. That is a difficult question
to answer. It depends not only on the instrument used but
also on the generic type of coating. Because of its good
adhesive properties, an epoxy coating would be expected
to have an inherently higher pull-off strength than an alkyd,
for example. Information on acceptable pull-off strengths
(and the type of instrument to use) would have to be obtained
from the coating manufacturer.
If a specification or regulation calls for pull-off adhesion
testing, the minimum acceptable pull-off strength and the
type of instrument should be identified. Some manufacturers
report pull-off adhesion test values for their coatings.
Remember that these pull-off strengths generally refer to
laboratory testing and not field-testing.
This does not mean pull-off strength requirements do not
exist in some contracts or regulations as a performance
requirement. For example, a minimum pull-off strength of
1.4 MPa (200 psi) is required for coatings used in nuclear
power plants in the USA. This value was developed when only
the mechanical tester was available. Pull-off strength measurements
are also used for failure analysis, where information about
location of the break may be more important than the actual
strength and where continuity of adhesion on the structure
is being evaluated.
In Australian Standard AS 4361.1:1995 Guide to lead paint
management Part 1: Industrial applications, which is one
of the few paint standards to actually nominate an adhesion
value, a minimum adhesion of 2.0 Mpa is recommended as an
acceptance level for overcoating lead paints. This is deemed
to be equivalent to grade 3 in cross-hatch adhesion classification.
SUMMARY
The two main types of adhesion tests are cross-hatch tape
and pull-off tests. The cross-hatch tape test is easy to
run, requires a minimum of equipment, gives immediate results,
and is subjective. The pull-off test requires special equipment
and time for the adhesive to cure, and it is objective,
though there is a large variability among individual pulls
and among the types of instruments use.
The dilemma with this type of testing is that it evaluates
only a very small area in what may be a much larger area
of coating. Its value lies in its ability to measure coating
system performance rather than application quality.
The latter may be affected by localised faults, surface
contamination or other factors that are not a measure of
the ability of the coating to adhere to the substrate.
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