REPLICA TAPE
REPLICA
TAPE
Photo: Pamela Simmons
Unlocking Hidden Information
By David Beamish
DeFelsko Corporation
project. It is generally accepted that
the nature of these surfaces is predic-
All figures courtesy of the author
unless otherwise noted.
tive of long-term coating performance.
Characteristics of a blasted surface in-
S
clude peak height, peak density, devel-
teel surfaces are frequently
oped surface area, angularity, sharpness
cleaned by abrasive impact
and shape.
or by power tools prior to
the application of protective
monly measured. If this height is insuffi-
coatings. This process re-
cient, paint will not adhere. If too great,
Today, only peak height (H) is com-
moves previous coatings, mill scale, rust
more paint is required to fill the “valleys”
and contaminants. It also roughens the
and the high peaks may protrude through
surface to improve coating adhesion.
the paint to become foci for corrosion.
The resultant surface profile, or an-
While the importance of measuring
chor pattern, is composed of a complex
peak height is undeniable, one parameter
pattern of peaks and valleys which must
alone does not fully describe the dynam-
be accurately assessed to ensure com-
ics of a coating/substrate relationship.
pliance with job or contract specifica-
Peak density (Pd) is also an important in-
tions and ensure a successful coating
dicator of performance and can now be
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Surface Profile & Adhesion
measured thanks to recent developments in field instrumentation.
H = 50 µm
(2 mils)
Counting Peaks
2 mm
(80 mils)
In 1974, J.D. Kean, J.A. Bruno and
R.E.F. Weaver wrote in an article titled,
“Surface Profile for Anti-Corrosion
2 mm
(80 mils)
Fig. 1: Both surfaces have the same measured peak-to-valley height. A second important measurable parameter, peak density, helps explain why coatings bond differently to each surface.
Paints,” that a surface prepared for
painting via blast cleaning could not be
completely described by measuring
peak-to-valley distance (H) alone. Their
Roughness on Coating Performance,”
Stylus Profilometers
paper supported field experience
which reported that peak counts could
Stylus roughness instruments record the
which suggested that there was
be controlled and, like peak height,
up and down movements of an external
another important parameter besides
could affect coating performance. Their
stylus traversing across a surface (Fig.
H, namely, the number of peaks per
work resulted in the creation of ASTM
2). They measure a height parameter
unit length or peak count (Pc), or peaks
D7127, “Standard Test Method for
called Rt in compliance with ISO 4287,
per unit area or Pd. Besides increasing
Measurement of Surface Roughness of
“Geometrical Product Specifications
the bonding surface area, their paper
Abrasive Blast Cleaned Metal Surfaces
(GPS) — Surface texture: Profile method
explained that increasing the number of
Using a Portable Stylus Instrument.”
— Terms, definitions and surface
peaks in a defined area increased the
texture parameters,” which yields the
angularity of that area. That put more
that “the optimum steel profiles for
vertical distance between the highest
shear adhesion stress on the coating
a wide range of standard industrial
peak and lowest valley within any given
rather than tension (pull-off) stress. This
coatings that will completely wet the
evaluation length of 0.5 inches (12.5
increased coating bond strength for
surfaces are a 2 to 3 mil (50 to 75 micron)
mm). Five traces are made and Rt values
the applied coating as shear values are
profile height and a peak count between
averaged to obtain the average of the
always higher than tensile values. The
110 and 150 peaks per inch (40 and 60
maximum peak-to-valley distances.
applied coating, of course, must wet out
peaks per centimeter)”. The authors
100 percent of the surface (“wet out”
recommended that stylus roughness
study (Research Report RR:D01-1169)
meaning wet the surface thoroughly).
instruments, the best field instrument
determined peak-to-valley height Rt
Figure 1 is a simplified example
In a follow-up article they concluded
In 2011, an ASTM round-robin
available at the time, be used in the
as measured by stylus roughness
of why both peak height and peak
corrosion industry to provide both
instruments related closely to
density are important in understanding
critical pieces of profile information
H as measured by both depth
coating performance. The two surfaces
— peak height and peak density.
micrometers and replica tape. But if
have different geometries yet their
height measurements are the same.
To get a clearer picture of the surface
Direction of Travel
available for bonding, peak count
measurements must also be obtained.
Furthermore, both measured values
ip
us T
Styl
make it possible to investigate the
increase in surface area resulting
from the abrasive blasting process.
Highest Peak
There is little doubt that peak
Rt
density measurements are important
to the corrosion industry, but the
problem until now is that peak counts
have not been easy to determine.
Lowest Valley
Fig. 2: Stylus roughness instruments.
In June of 2005, JPCL published
a significant paper by Hugh J. Roper,
Weaver and Joseph H. Brandon titled
“The Effect of Peak Count on Surface
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“Field Measurement of Surface
Profile of Abrasive Blast-Cleaned
Steel Surfaces Using a Replica Tape.”
Compared to other methods, it has the
advantages of ruggedness, relatively
low start-up cost, good repeatability
and the ability to retain a physical
replica of the surface being evaluated.
Replica tape is made of a layer
of compressible foam affixed to an
incompressible polyester substrate
of a highly uniform thickness. When
pressed against a roughened steel
Fig. 3: Burnishing replica tape.
surface the foam collapses and forms
an impression of the surface. Placing
stylus roughness instruments could
or atomic force microscopy. But these
the compressed tape between the
generate measurements of both peak
powerful devices are also challenged
anvils of a micrometer thickness
height and the number of peaks
when measuring complex blasted sur-
gage and subtracting the contribution
encountered along a sampling
faces. They are costly to purchase, do
of the incompressible substrate,
length, why has the corrosion
not operate in the field, and require ex-
2 mils or 50.8 microns, gives a
industry been slow to adopt them?
tensive training to set-up myriad test pa-
measure of surface profile height.
rameters for topographic analysis.
Likely this reluctance is based on
Accessing New Information
economic and practical reasons. Stylus
profilometers are fragile in nature
affordable, robust field instrument de-
As common as this product is, it is
since they depend on a precisely
signed specifically for the corrosion in-
not widely known that these surface
calibrated stylus that often extends a
dustry that can provide both H and Pd
replicas contain far more information
distance from the body of the device
measurements to give inspectors a more
than just peak height as measured
itself. They can be complex to set
meaningful and functionally correla-
by a micrometer. Significant data is
up and to operate and they report
tive prediction of coating performance
available through digital imaging.
a number of roughness parameters
during surface preparation. The solution
that are of limited interest to the
lies with replica tape.
is to use a property of the tape that
coatings industry. All these factors
likely dissuade potential users.
A lesser known fact is that sty-
It is therefore desirable to have one
A new measurement approach
is related to, but different from, its
Measuring Peak Density:
A New Solution
capacity to replicate surfaces; that is,
the tape’s increase in transparency
lus-based roughness testers, although
Replica tape has been used to
where it is compressed. Using this
popular in metal machining industries,
characterize surfaces since the late
principle, a three-dimensional map of
are challenged by the complex patterns
1960s (Fig. 3). It is simple, relatively
the surface can be generated from an
generated by surface-cleaning opera-
inexpensive and is particularly useful
optical scan of the burnished replica
tions. They measure only a single line
on curved surfaces. Its operation is
tape. Peak counts can be determined
on a roughened surface and most of
described in a number of international
by simply counting bright spots on the
the features recorded as peaks are ac-
standards including ASTM D4417,
image taken by a digital image sensor.
tually peak “shoulders” where the sty-
“Standard Test Methods for Field
lus traced over the side of the peak
Measurement of Surface Profile
of tape reveals light areas of higher
rather than over the top of the peak.
of Blast Cleaned Steel”; ISO 8503-
compression (peaks) and dark areas of
A photograph of a back-lit piece
5, “Preparation of steel substrates
lower compression (valleys) (Fig. 4, p.
available? It is anecdotally believed
before application of paints and
57). A portable instrument can identify
within the corrosion industry that de-
related products — Surface roughness
peaks and determine areal peak density,
finitive measurement devices must
characteristics of blast-cleaned steel
that is, how many peaks are present per
use laboratory methods such as white
substrates — Part 5: Replica tape
square millimeter, or Pd, as defined by
light interferometry, focus variation mi-
method for the determination of the
ASME B46.1, “Surface Texture (Surface
croscopy, confocal laser microscopy
surface profile” and NACE RP0287,
Roughness, Waviness, and Lay).”
What other measuring solutions are
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Surface Profile & Adhesion
3-D Surface Mapping
adhesion and both H and Pd, an adhesion
tape imager. The samples were then
Going a step further, additional surface
study was carried out to determine
sprayed with three coatings: an epoxy,
parameters can be extracted once the
if 3-D replica tape imaging methods
a two-component acrylic and a polymer
thickness/transparency relationship has
gave similar results. Twenty-five steel
composite coating, and allowed to
been applied to the intensity image using
samples were prepared using a variety
cure. Three pull-off adhesion tests
3-D rendering software. The result is a
of blast media and measurements
were then performed on each sample
3-D map of the blasted steel surface at
were performed with the 3-D replica
following the test method described
a cost far less than interferometric or
in ASTM D4541, “Standard
confocal profiling devices. An example
Test Method for Pull-Off
of how the process works is shown in
Strength of Coatings Using
Figure 5 (p. 57) and Figure 7 (p. 59).
Portable Adhesion Testers”
using a Type V hydraulic
Correlating Replica Tape
Measurements to Established
Measurement Technique
pull-off adhesion tester
described in Annex A4 (Test
Method E) of that standard.
To validate 3-D replica tape
tape (H and Pd) were compared to
Correlation of
Adhesion with Surface
Profile Height
two established surface roughness
In the 2006 Roper et al. paper,
measurement methods: confocal
it was theorized that “the op-
microscopy and stylus profilometry.
timum steel profiles for a wide
range of standard industrial
measurements, the parameter
measurements obtained from the
The first step taken was to confirm
coatings that will completely
that light intensity imaging of replica tape yielded values comparable to
those obtained using known laboratory
methods. A study was carried out using three steel panels blasted with grit
50, garnet coarse, and shot 230/grit 40
Fig. 4: This figure depicts a 2-D
image derived from replica tape
(above) and digitally counting
bright spots or peaks (right).
media. The panels were sent to a university lab along with three sets of burnished replica tape for measurement
with a confocal microscope. The 3-D
replica tape measurements were found
to closely correlate with the laboratory
methods (Table 1, p. 58)
To compare measurements from 3-D
replica tape images with those determined from stylus roughness instruments, measurements of peak density
were taken on the five panels used in
the 2011 ASTM round-robin study. Twodimensional stylus profilometer measurements are not directly comparable
with the 3-D optical replica tape measurements. However, a direct correlation was observed.
Adhesion Testing
Given the previous research by Roper
Fig. 5: Replica tape embossed over a coin (center), a digital surface image
created from the tape and from a field instrument (right).
and others on the correlation between
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Adhesion Strength (psi)
6000
Table: 1: Comparison of Peak Density (Pd) Calculations
Between a Field and a Lab Instument
5000
Sample
4000
G50
Garnet Coarse
S230/G40
3000
2000
Field Instrument
RTR-P
(peaks/mm2)
Lab @ 1000 µm2
(peaks/mm2)
25
11.8
8.3
34
13.8
7.6
wet the surfaces are a 2- to 3-mil (50- to
1000
Insufficient Anchoring
75-micron) profile height”. The highest
Insufficient Wetting
adhesion strengths were observed in the
0
0
1
2
3
4
5
Peak-to-Valley Profile Height, H (mils)
2 to 3 mil range with adhesion values decreasing as profile heights increased
above 3 mils, likely because the coating failed to fully wet the substrate. The
Fig. 6: Observed peak-to-valley profile height versus adhesion strength for various
blast media types with hypothesized trend and justification.
observations in this study appear to affirm this hypothesis although no samples were taken with profile heights below 2 mils. It is theorized that adhesion
levels would begin to decrease below
this figure because of insufficient profile to anchor the coating. A hypothetical trend line, with Pd held constant, is
overlaid on this study’s data in Figure 6.
Correlation of Adhesion
with Peak Density (Pd)
There appears be a strong positive
correlation between peak density
and adhesion, reinforcing Roper’s
hypothesis that peak count is
relevant to coating performance.
Their paper asserted “the optimum
steel profiles for a wide range of
standard industrial coatings that will
completely wet the surfaces are …
a peak count between 110 and 150
peaks/in (40 and 60 peaks/cm).”
As stated earlier, a stylus
profilometer counts the number of
peaks in a straight line (expressed
in millimeters) whereas a replica tape
reader counts peaks in a unit of area
(expressed in square millimeters). Since
Fig. 7: 3-D images of blasted surfaces derived from replica tape
using a field instrument. In order from top: G50, Garnet, S230/G40
and hand-held bristle blasting tool. (Z-axis enhanced for clarity.)
those authors used a stylus profilometer
in their research, their numbers are not
directly comparable to the data in this
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Surface Profile & Adhesion
Adhesion Strength (psi)
6000
5000
4000
3000
2000
1000
Insufficient Anchoring
Insufficient Wetting
0
0
2
4
6
8
10
Peak Density, Pd (Peaks/mm2)
Fig. 8: Observed peak density versus adhesion strength with hypothesized trend and
justification.
an optimal 3-D peak density of around
parameters, H, is commonly measured
a hybrid parameter will be specified in
8 peaks per square millimeter was
today and is usually the only parameter
the future.
observed. It is hypothesized that the
reported. While its importance is
observed positive relationship between
undeniable, one parameter alone
About the Author
adhesion strength and peak density would
does not fully describe the dynamics
David Beamish is the president of
not persist at peak densities greater
of a coating/substrate relationship.
DeFelsko Corporation, a manufacturer of
than those observed in this study. This
Pd is also an important indicator of
hand-held coating test instruments sold
theorized relationship between adhesion
performance. While it also cannot be a
globally. He has a degree in civil engi-
and Pd with H held constant
sole measure like H has been for sever-
neering and more than 27 years of expe-
is shown in Figure 8.
Conclusion
It is generally accepted that the
The most important of these
abrasive material type and size. Perhaps
study. However, based on our study,
al decades, together with H it provides
rience in the design,
a better prediction of long-term coating
manufacture and
performance as measured by pull-off ad-
marketing of these
hesion testing.
instruments in a variety of international
nature of abrasive blast cleaned steel
industries including
surfaces is predictive of long-term
coating performance. The corrosion
This relationship helps to explain
industrial painting,
industry does not fully understand the
why H measurements alone have not al-
quality inspection,
dynamics of this complex problem but
ways been a reliable method for predict-
and manufacturing.
it has several measurable parameters
ing performance for all coating types. A
Beamish conducts training seminars and
available to it, including peak height,
coating may bond to a surface with low
is an active member of various organiza-
peak density, surface area, angularity,
H and high Pd just as well as to one with
tions including SSPC, NACE, ASTM and
sharpness and shape. Commonly
high H and low Pd. For this reason the
ISO. JPCL
held industry beliefs would suggest
corrosion industry should report both
that increasing several of these
values so that customers can determine
parameters will improve long term
the best ratio for their particular coat-
coating performance. Empirical data
ing application. Both parameters are
suggests it is not that simple.
controlled with the proper selection of
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