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8 QUALITY CONTROL AND INSPECTION
8.1 Introduction
Owners should make provisions to ensure the quality of the coating by providing detailed
specifications and competent inspection. Wire manufacturers need to have quality control
systems in place to ensure that the wire can be applied and will perform as intended.
Applicators must ensure that they have the correct experience and equipment and competent
applicators to prepare the structure and apply the coating.
Inspection is one of the most important aspects of coating. It provides a written record of the
details of the coating application, ensures that the coating specifications are met, and finds
and ensures the correction of inadequate coating areas before they become failure locations
in the future requiring costly correction.
Particular attention must be directed toward difficult-to-coat areas, such as the inside surfaces
of H-shapes and the interlock knuckles of sheet piling, because these are areas where optimum
nozzle/gun angles and distances will be difficult to attain.
8.2 Quality Assurance Functions for Owners
8.2.1 Informed Selection
An informed selection should be made of TSMCs taking into account planned use of the
coatings and the environment in which they are to be used.
8.2.2 Provide Definitive Specifications
Specifications should include, as a minimum and as an addition to contractual provisions,
the following:
· Scope of work, to include the structure to be coated and portions not to be coated;
· All applicable references;
· Provisions for payment;
· Definitions;
· A list of required submittals;
· Safety provisions;
· Requirements for delivery, storage, and handling of materials and supplies;
· Chemical composition, finish, coil weight, and preparation of metallizing wire;
· Requirements for sampling and testing thermally sprayed materials and the applied sealer;
· A job reference standard with a description of appearance and adhesion requirements;
· Requirements for surface preparation;
· Metallizing application;
· Workmanship;
· Atmospheric and surface conditions;
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· Sequence of operations;
· Approved methods of metallizing;
· Coverage and metallizing thickness;
· Progress of metallizing work;
· Sealing and painting instructions;
· Metallizing schedule; and
· Quality control requirements.
8.2.3 Coating Inspector
Provide a qualified coating inspector to provide full-time inspection services. This should
be a third-party inspector with the power and ability to work out problems with the applicator
to achieve the desired coating quality. Section 9.3 lists the necessary qualifications of an
inspector.
8.3 Quality Control
8.3.1 Documentation
The documentation of inspection activities provides a permanent record of the thermal spray
job. Thorough documentation provides a written record of the job in the event of a contract
dispute or litigation. Inspection records may also be used to help diagnose a premature
coating failure. Future maintenance activities may also be simplified by the existence of
complete inspection records. As a minimum, at least one full-time inspector should be used
on all thermal spray jobs to ensure adequate inspection and documentation. A qualified third-
party inspector from a reputable firm should perform the inspection. As a minimum, the
inspector should perform and document the inspection procedures described in this section.
Sample documentation forms for industrial coating activities are available through NACE
International and the Society for Protective Coatings.
The inspector should record the production and quality control information required by the
purchaser or the purchasing contract. Among the items that should be recorded are
· Information about the contractor and purchaser;
· Surface preparation and abrasive blasting media requirements;
· Flame or wire-arc spray equipment used;
· Spraying procedure and parameters used;
· TSMC requirements;
· Safety precautions followed;
· Environmental precautions;
· Test data taken, including
Nature of the test,
When conducted,
Where conducted,
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Results, and
Abnormalities and resolution; and
· Problems and resolution.
The inspector should keep records for the time period required for regulatory compliance
and required by the purchasing contract.
8.3.2 Testing Frequency
The required frequency of inspection procedures should be documented in the specification.
Inspection can be expensive, and care should be taken not to overspecify inspection
procedures. Conversely, inspection has an intrinsic value that is sometimes intangible. It is
difficult to measure the value added by inspection resulting from the conscientious
performance of the contract. Thermal spray can be quite sensitive to the quality of surface
preparation, thermal spray equipment setup, and application technique. Therefore, it is
important to specify an appropriate level of inspection. Table 8 presents recommended
frequencies for various inspection procedures.
8.3.3 Job Reference Standard and Material Samples
8.3.3.1 Material samples. Reference samples of each material used on a thermal spray job should
be collected, including clean, unused abrasive blast media; thermal spray wire; sealer; and
paint. Samples may be used to evaluate the conformance of materials to any applicable
specifications.
· A 2.2-lb (1-kg) sample of blast media should be collected at the start of the job. The
sample may be used to verify the cleanliness, media type, and particle size distribution
of the virgin blast media. A 12-in. (30-cm) sample of each lot of thermal spray wire
should be collected.
· The wire sample may be used to confirm that the manufactured wire conforms to the size
and compositional requirements of the contract.
· One-quart (1-liter) samples of all sealers and paints should be collected for compliance
testing.
TABLE 8 Recommended inspection frequencies for selected
procedures
Inspection Procedure Recommended Frequency per
Unit Area
Surface profile 3 per 500 ft2 (45 m2) or less
Thermal spray coating thickness 5 per 100 ft2 (9 m2) or less
Thermal spray adhesion 2 per 500 ft2 (45 m2) or less
Sealer thickness 2 per 500 ft2 (45 m2) or less
Paint thickness 2 per 500 ft2 (45 m2) or less
Soluble salts 1 per 1,000 ft2 (90 m2) or less
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8.3.3.2 Job reference standard. A thermal spray job reference standard (JRS) should be prepared.
The JRS may be used at the initiation of a thermal spray contract to qualify the surface
preparation, thermal spray application, and sealing processes. The JRS and the measured
values may be used as a visual reference or job standard for surface preparation, thermal
spray coating, sealing, and painting, in case of dispute.
8.3.3.3 Preparing the JRS. The JRS should be prepared prior to the onset of production work. To
prepare the JRS, a steel plate of the same alloy and thickness to be coated, measuring 2 × 2 ft
(60 × 60 cm) should be solvent and abrasive blast cleaned in accordance with the requirements
of the contract. The abrasive blast equipment and media used for the JRS should be the same
as those that will be used on the job.
One-quarter of the JRS plate should be masked using sheet metal, and the TSMC should be
applied to the unmasked portion of the plate. The TSMC should be applied using the same
equipment and spray parameters proposed for use on the job. The gun should be operated in
a manner substantially the same as the manner in which it will be used on the job. The
approximate traverse speed and standoff distance during spraying should be measured and
recorded.
Two-thirds of the thermal spraycoated portion of the JRS should be sealed in accordance
with the requirements of the contract. One-half of the sealed area should be painted in
accordance with the contract if applicable. The sealer and paint should be applied using the
same paint spray equipment that will be used for production.
The prepared JRS should be preserved and protected in such a manner that it remains dry
and free of contaminants for the duration of the contract. The preserved JRS should then be
archived for future reference in the event of a dispute or premature coating failure.
Once the JRS is qualified, the operating parameters should not be altered by the contractor,
except as necessitated by the requirements of the job.
Figure 6 depicts a representative JRS.
Figure 6. Job reference standard configuration (1
in. = 2.54 cm, 1 ft = 30.48 cm).
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8.3.3.4 Evaluating the JRS. The surface cleanliness; blast profile shape and depth; thermal spray
appearance, thickness, and adhesion; and sealer and paint thickness should be determined in
accordance with the contract requirements and recorded.
8.3.4 Testing Prior to Surface Preparation
8.3.4.1 Ambient conditions measurement. An assessment of the local atmospheric conditions should
be made before surface preparation and thermal spray application begins. Measurement of
ambient conditions includes substrate temperature, air temperature, dew point, and relative
humidity.
· A contact thermocouple or infrared pyrometer should be used to measure the substrate
temperature.
· Air temperature should be measured using a sling psychrometer, thermometer, or digital
measurement instrument.
· Dew point should be calculated using the appropriate psychrometric charts.
· Humidity should be determined in accordance with ASTM E337, "Test Method for
Measuring Humidity with a Psychrometer (The Measurement of Wet-Bulb and Dry-Bulb
Temperatures)."
8.3.4.2 Inspection preceding surface preparation. Prior to abrasive blasting, inspect the substrate
for the presence of contaminants including grease and oil, weld flux and spatter, heat-affected
zones, flame-cut edges, pitting, sharp edges, and soluble salts.
· Grease and oil. Painted surfaces and newly fabricated steel should be visibly inspected
for the presence of organic contaminants such as grease and oil, as required by the project
specification. Continue degreasing until all visual signs of contamination are removed.
Conduct the UV light test, qualitative solvent evaporation test, or the heat test to detect
the presence of grease and oil.
Use a UV lamp to confirm the absence of oil or grease contamination.
The solvent evaporation test should be made by applying several drops or a small
splash of a residual-less solvent, such as trichloromethane, on the areas suspected of
oil and grease retention (e.g., pitting and crevice corrosion areas and depressed areas,
especially those collecting contamination, etc.). An evaporation ring will form if oil
or grease contamination is present.
The heat test should be made by using a torch to heat the degreased metal to about
225oF (110oC). Residual oil/grease contamination should be drawn to the metal
surface and is visually apparent.
· Weld flux and spatter. A visual inspection for the presence of weld flux and spatter should
be performed, as required by the project specification. Weld flux should be removed prior
to abrasive blast cleaning using a suitable SSPC-SP 1 "Solvent Cleaning" method. Weld
spatter may be removed either before or after abrasive blasting using suitable impact or
grinding tools. Areas that are power-tool cleaned of weld spatter should be abrasive blast
cleaned.
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· Heat-affected zones caused by welding. Heat-affected zones should be identified and
marked prior to abrasive blasting as required by the specification. Extra care during surface
preparation and extra attention to profile inspection should be given to these areas.
· Flame-cut edges. Flame-cut edges should be identified and marked prior to abrasive
blasting as required by the specification. The demarcated areas should be ground using
power tools prior to abrasive blast cleaning.
· Pitting. Deep pits or pitted areas should be identified and marked prior to abrasive blast
cleaning as required by the specification. The demarcated areas should be ground using
power tools prior to abrasive blast cleaning.
· Sharp edges. Sharp edges should be identified and marked prior to abrasive blasting as
required by the specification. The demarcated edges should be prepared by grinding to a
minimum radius of 1/8 in. (3 mm) prior to blast cleaning.
· Soluble salts. When soluble salt contamination is suspected, the contract documents should
specify a method of retrieving and measuring the salt levels as well as acceptable levels of
cleanliness. Salt contamination is prevalent on structures exposed in marine environments
and on structures such as parking decks and bridges exposed to deicing salts. Structures that
are likely to have soluble salt contamination, including those in marine or severe industrial
atmospheres, bridges or other structures exposed to deicing salts, and seawater immersed
structures, should be tested. Soluble salt levels should be rechecked for compliance with
the specification after solvent cleaning and abrasive blasting have been completed.
Common methods for retrieving soluble salts from the substrate include cell retrieval
methods and swabbing or washing methods. Various methods are available for assessing
the quantity of salts retrieved, including conductivity, commercially available colorimetric
kits, and titration. The rate of salt retrieval is dependent on the retrieval method. The
retrieval and quantitative methods should be agreed upon in advance.
The recommended testing procedure employs the Bresle cell (ISO 8502-6) to extract
soluble salts from the substrate. Chloride ion concentration is readily measured in the field
using titration strips available from Quantab. The test strip analyzes the collected sample
and measures chloride ion concentration in parts per million. The unit area concentration
of chloride ions is calculated in micro-grams per centimeter. The lower detection limit for
the Bresle/Quantab method is about 2 µm/cm2. SSPC-SP-12/NACE #5 describes levels of
soluble salt contamination. It is recommended that surfaces cleaned to an SC-2 condition
be used for TSMCs. An SC-2 condition is described as having less than 7 µm/cm2 of
chloride contaminants, less than 10 µm/cm2 of soluble ferrous ions, and less than 17 µm/cm2
of sulfate contaminants. The number of tests per unit area (e.g., 1 per 1,000 ft2 [90 m2])
should be specified in the contract documents. Also refer to "SSPC Technology Update:
Field Methods for Retrieval and Analysis of Soluble Salts on Substrates," and SSPC-91-07.
8.3.5 Testing During Surface Preparation
8.3.5.1 Abrasive cleanliness. Abrasive blast media must be free of oil and salt to prevent
contamination of the substrate. Recycled steel grit abrasive should comply with requirements
of SSPC-AB-2, "Specification for Cleanliness of Recycled Ferrous Metallic Abrasives."
· Evaluating for salt in abrasives. Most abrasives used to prepare steel substrates for
thermal spraying are unlikely to contain appreciable amounts of soluble salts. However,
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slag abrasives used for strip blasting may sometimes contain measurable quantities of
salts. Slag abrasives should be evaluated in accordance with ASTM D4940, "Test
Method for Conductimetric Analysis of Water Soluble Ionic Contamination of Blasting
Abrasives."
· Testing for oil in abrasives. To test for oil in abrasives, a clear glass container should be
half filled with unused abrasive, and then distilled or deionized water should be added to
fill the container. The resulting slurry mixture should be stirred or shaken and allowed to
settle. The water should then be examined for the presence of an oil sheen. If a sheen is
present, the media should not be used, and the source of contamination should be identified
and corrected.
8.3.5.2 Air cleanliness. The following guidelines apply to air cleanliness.
· The compressed air used for abrasive blasting, thermal spraying, sealing, and painting
should be clean and dry. Oil or water in the blasting air supply may contaminate or corrode
the surface being cleaned. Oil or water in the thermal spray, sealing, or painting air supply
may result in poor coating quality or reduced adhesion. Compressed air cleanliness should
be checked in accordance with ASTM D4285, "Method for Indicating Water or Oil in
Compressed Air." The air compressor should be allowed to warm up, and air should be
discharged under normal operating conditions to allow accumulated moisture to be
purged. An absorbent clean white cloth should be held in the stream of compressed air not
more than 24 in. (60 cm) from the point of discharge for a minimum of 1 minute. The air
should be checked as near as possible to the point of use and always after the position of
the in-line oil and water separators. The cloth should then be inspected for moisture or
staining.
· If moisture or contamination is detected, the deficiency should be corrected before going
further.
8.3.5.3 Blast air pressure. The contractor should periodically measure and record the air pressure at
the blast nozzle. The measurement should be performed at least once per shift and should be
performed on each blast nozzle. Measurements should be repeated whenever work conditions
are altered such that the pressure may change. Pressures should be checked concurrently with
the operation of all blast nozzles. The method employs a hypodermic needle attached to a
pressure gauge. The needle is inserted into the blast hose at a 45-degree angle toward and as
close to the nozzle as possible. The blast pressure is read directly from the gauge.
8.3.5.4 Blast nozzle orifice. The contractor should visually inspect the blast nozzle periodically for
wear or other damage. Gauges are available that insert into the end of the nozzle and measure
the orifice diameter. Nozzles with visible damage or nozzles that have increased one size
should be replaced. Worn nozzles are inefficient and may not produce the desired blast
profile. Damaged nozzles may be dangerous.
8.3.5.5 Surface cleanliness. The following applies to surface cleanliness.
· Blast Cleanliness. The final appearance of the abrasive cleaned surface should be inspected
for conformance with the requirements of SSPC-SP-5. An SP-5 surface is defined as free
of all visible oil, grease, dirt, dust, mill scale, rust, paint, oxides, corrosion products, and
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other foreign matter. The appearance of SP-5 surfaces is dependent on the initial condition
of the steel being cleaned. SSPC-VIS-1 may be used to interpret the cleanliness of
various blast-cleaned substrates based on the initial condition of the steel and the type of
abrasive used. Initial conditions depicted include the following:
Rust Grade A--a steel surface completely covered with adherent mill scale with little
or no rust visible;
Rust Grade B--a steel surface covered with both mill scale and rust;
Rust Grade C--a steel surface completely covered with rust with little or no pitting; and
Rust Grade D--a steel surface completely covered with rust with visible pitting. The
inspector should determine the initial substrate condition or conditions.
The final appearance of the surfaces should then be compared with the appropriate
photograph. No stains should remain on the SP-5 surface. However, the appearance of
the surface may also vary somewhat, depending on the type of steel, presence of roller
or other fabrication marks, annealing, welds, and other differences in the original condition
of the steel. The job reference standard should be used as the basis for judging the surface
cleanliness.
· Dust. Abrasive blasting and overspray from painting or metallizing can leave a deposit of
dust on a cleaned substrate. The dust may interfere with the adhesion of the TSMC.
Residual dust may be detected by applying a strip of clear tape to the substrate. The tape
is removed and examined for adherent particles. Alternatively, a clean white cloth may be
wrapped around a finger and wiped across the surface. The cloth and substrate are then
examined for signs of dust. The preferred method of removing residual dust is by
vacuuming. Alternatively, the surface may be blown down with clean, dry compressed air.
8.3.5.6 Surface profile. The following applies to surface profile.
· ASTM D4417, "Test Methods for Field Measurements of Surface Profile on Blast Cleaned
Steel," provides test methods for surface profile measurement. Methods A and B use
either a needle depth micrometer to measure the depth of the valleys in the steel or
comparator charts. Method C is the recommended method for measuring the surface
profile depth. Methods A and B may provide unreliable measures of the blast profile.
· Method C employs replica tape and a spring gauge micrometer to measure the surface
profile. With the wax paper backing removed, the replica tape is placed face down against
the substrate, and a burnishing tool is used to rub the circular cutout until a uniform gray
appearance develops. The replica tape thickness (compressible foam plus plastic backing)
is then measured using the spring micrometer. The profile is determined by subtracting
the thickness of the plastic backing material, 0.002 in. (50 µm), from the measured
value. Three readings should be taken within a 16-in.2 (100-cm2) area, and the surface
profile at that location should be reported as the mean value of the readings. The
number of measurements per unit area (e.g., 3 per 500 ft2 [45 m2]) should be specified
in the contract document. Two types of replica tape are available, coarse (0.0008 to
0.002 in. [20 to 50 µm]) and X-coarse (0.0015 to 0.0045 in. [37.5 to 112.5 µm]). In most
cases, the X-coarse tape will be used to measure profile. It may be possible to measure
profiles as high as 0.006 in. (150 µm) using the X-coarse tape.
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The inspector must be aware that TSMCs create safety and health risks in the
form of hot surfaces, fumes, ultraviolet light, and noise. Precautions must be
taken to avoid these hazards--refer to Section 2.
8.3.6 Testing During and After Coating Application
8.3.6.1 Coating thickness. The following applies to coating thickness.
· Coating thickness is measured in accordance with SSPC-PA-2 using a Type 2 gauge.
· Calibrate the instrument using a calibration wedge that is close to the contract-specified
thickness placed over a representative sample of the contract-specified abrasive-blasted
steel or a prepared bend coupon, or both.
· Thickness readings should be made either in a straight line with individual readings taken
at 1-in. (2.5-cm) intervals or spaced randomly within a 2-in. (5-cm) diameter area. Line
measurements should be used for large flat areas, and area measurements should be used
on complex surface geometry and surface transitions such as corners. The average of five
readings constitutes one thickness measurement. A given number of measurements per
unit area (e.g., five per 100 ft2 [9 m2]) should be specified in the contract documents.
Figure 7 illustrates this method.
· Measure thickness according to ASTM D4138, "Test Methods for Measurement of Dry
Film Thickness of Protective Coating Systems by Destructive Means," Test Method A.
This method uses a tungsten carbide-tipped instrument to scribe through the sealer and
paint, leaving a V-shaped cut. A heavy dark-colored marking pen is first used to mark
the coated surface. The scribing instrument is then drawn across the mark. This process
sharply delineates the edges of the scribe. A reticle-equipped microscope is used to read
the film thickness. A total of three thickness readings should be performed in a 16-in.2
(100-cm2) area, with the average of the three tests reported as a single measurement. The
number of measurements per unit area (e.g., 1 per 500 ft2 [45 m2]) should be specified in
the contract documents.
Thickness testing using this method should be minimized because the test method
destroys the sealer and paint. Areas damaged by adhesion testing must be repaired by
5 in line at about 1 in. [2.5 cm]
5 in a spot of about 2 in. dia (5 cm)
Figure 7. Methods of taking coating thickness
measurements.
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touch-up with sealer or paint using a brush or spray gun. Thickness testing should be
performed in a small area (16 in.2 [100 cm2]) to limit the area that must be repaired.
8.3.6.2 Adhesion tests. The following applies to adhesion tests.
· Bend Test. The bend test (180-degree bend on a mandrel) is used as a qualitative test for
verifying proper surface preparation, equipment setup, and spray parameters. The bend
test puts the TSMC in tension. The mandrel diameter for the threshold of cracking depends
on substrate thickness and coating thickness.
Table 9 summarizes a very limited bend-test cracking threshold for arc-sprayed zinc
TSMC thickness versus mandrel diameter for steel coupons 0.050 in. (13 mm) thick.
Test panels--the test panels should be a cold-rolled steel measuring 3 × 6 × 0.05 in.
(7.5 × 15 × 1.25 cm). The panels should be cleaned and blasted in the same fashion
in which the panels will be cleaned and blasted for the job.
Application of thermal spray--the TSMC should be applied to five test panels using
the identical spray parameters and average specified thickness that will be used on
the job. The coating should be applied in a cross-hatch pattern using the same number
of overlapping spray passes as used to prepare the job reference standard. The coating
thickness should be measured to confirm that it is within the specified range.
Conduct bend test--test panels should be bent 180 degrees around a steel mandrel of
a specified diameter, as shown in Figure 5. Pneumatic and manual mechanical bend
test apparatus may be used to bend the test panels.
Examine bend test panels--test panels should be examined visually without
magnification. The bend test is acceptable if the coating shows no cracks or exhibits
only minor cracking with no lifting of the coating from the substrate. If the coating
cracks and lifts from the substrate, the results of the bend test are unacceptable.
TSMCs should not be applied if the bend test fails, and corrective measures must be
taken. Figure 5 depicts representative bend test results. A knife blade can be used to
facilitate the evaluation. Apply moderate pressure to the knife blade and if the coating
cannot be dislodged, the adhesion can be considered satisfactory.
· Tensile Adhesion.
Field Measurement--Evaluate the adhesion of the TSMC with the specification in
accordance with ASTM D4541, "Test Method for Pull-Off Strength of Coatings
Using Portable Adhesion Testers." A self-aligning Type IV tester, described in
Annex A4 of ASTM D4541, should be used. A total of three adhesion tests should
be performed in a 16-in2 (100-cm2) area, and the average of the three tests should be
TABLE 9 Bend-test mandrel diameter versus zinc thermal spray coating
thickness (for steel coupons 0.050 in. [13 mm] thick)
TSMC Thickness (mils) 10 (254 µm) 15 (381 µm) 25 (635 µm)
Mandrel Diameter 1/2 in. (1.27 cm) 5/8 in. (1.59 cm) <1 in. (2.54 cm)
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reported as a single measurement. Portable instruments with large-diameter test
specimens, for instance, 2-in. versus 1-in. (50-mm versus 25-mm) diameter, produce
better statistical results.
The number of measurements per unit area (e.g., 1 per 500 ft2 [45 m2]) should be
specified in the contract documents. Areas of deficient adhesion should be abrasive
blasted, and the coating should be reapplied. Additional testing will probably be
necessary to determine the extent of the area exhibiting poor adhesion. Adhesion testing
should be minimized because the test method destroys the coating. Areas damaged by
adhesion testing must be repaired by abrasive blasting and reapplication of the metallic
coating. Adhesion testing is performed in a small area (16 in2 [100 cm2]) to limit the
area that must be repaired.
As an alternative to testing adhesion to the failure point, the tests can be interrupted
when the minimum specified adhesion value is achieved. This method precludes the
need to repair coatings damaged by the test. The adherent pull stubs can then be
removed by heating to soften the glue or by firmly striking the side of the stub.
Table 10 lists the recommended adhesion requirements for field- or shop-applied
thermal spray coatings of zinc, aluminum, and 8515 wt% zinc/aluminum.
As a caution in performing this type of test, the inspector must be aware that since
the coating does contain some porosity, a low-viscosity adhesive might penetrate the
coating and reach the substrate. If this occurs, the measured adhesion value will be
influenced by the adhesion between the glue and the substrate. It is best to avoid low-
viscosity liquid adhesives in favor of high-viscosity pastes. If there is any doubt,
comparison tests should be performed to select the appropriate adhesive.
Laboratory Measurement--Tensile-bond test specimens should be carbon steel, 1 in.
(2.54 cm) in diameter and 1 in. (2.54 cm) in length, threaded per ASTM C 633,
"Adhesion or Cohesive Strength of Flame-Sprayed Coatings."
· Cut Test. The thermal spray coating cut test consists of a single cut, 1.5 in. [40 mm] long,
through the coating to the substrate without severely cutting into the substrate. All cuts
should be made using sharp-edge tools. The chisel cut should be made at a shallow angle.
The bond should be considered unsatisfactory if any part of the TSMC along the cut lifts
TABLE 10 Typical adhesion of field- and shop-applied
thermally sprayed metal coatings measured by pull-off
testing
Thermal Spray Tensile Adhesion
Material psi [MPa]
Zinc 500 [3.45]
Aluminum 1,000 [6.89]
85/15 Zinc-Aluminum 700 [4.83]
90/10 Aluminum Oxide 1,000 [6.89]
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from the substrate. The cutting tool used (knife, hammer and chisel, or other tool) should
be specified in the contract. Perform an adhesion test every 100 ft2 (9.3 m2). The tested
area and coated surfaces that have been rejected for poor adhesion shall be blast cleaned
and recoated.
8.3.7 Appearance
The coating should be free of blisters, cracks, chips or loosely adhering particles, oil, pits
exposing the substrate, and nodules. A very rough coating might indicate that the coating
was applied with the gun at too great an angle or too far from the surface. Evaluate coatings
that appear powdery or oxidized by scraping. If scraping does not produce a silvery metallic
appearance, the coating is defective and must be replaced.
8.3.8 Coating Morphology
Metallographic examination may be used for qualifying spraying parameters, but it is not
normally used for process control for corrosion control applications. Parameters included in
this examination include percent porosity, percent unmelted particles, percent oxides, and
the presence and amount of interface contamination.
