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5 TSMC APPLICATION
5.1 Equipment
5.1.1 Thermally Sprayed Metal Processes
Metals can be applied by thermal spray in a variety of ways that can be categorized as either
combustion or electric processes. Combustion processes include flame spraying, high-velocity
oxygen fuel (HVOF) spraying, and detonation-gun spraying. Electric processes include
wire-arc spraying and plasma spraying. This guide will address the flame and electric arc
processes for wire.
5.1.1.1 Flame process. The flame spray process can be used to apply a wide variety of feedstock
materials including metal wires, ceramic rods, and metallic and nonmetallic powders. In
flame spraying, the feedstock material is fed continuously into the tip of the spray gun or
torch, where it is then heated and melted in a fuel gas/oxygen flame and accelerated toward
the substrate being coated in a stream of atomizing gas. Common fuel gases used include
acetylene, propane, and methyl acetylene-propadiene (MAPP). Oxyacetylene flames are
used extensively for wire-flame spraying because of the degree of control and the higher
temperatures attainable with these gases. The lower-temperature oxygen/propane flame can
be used for melting metals such as aluminum and zinc, as well as polymer feedstock. The
basic components of a flame spray system include the flame spray gun or torch, the feedstock
material and a feeding mechanism, oxygen and fuel gases with flowmeters and pressure
regulators, and an air compressor and regulator.
With wire-flame spraying, the wire-flame spray gun or torch consists of a drive unit with
motor and drive rollers for feeding the wire and a gas head with valves, gas nozzle, and an
air cap that controls the flame and atomization air. Compared with wire-arc spraying, wire-
flame spraying is generally slower and more costly because of the relatively high cost of the
oxygen-fuel gas mixture compared with the cost of electricity. However, flame spraying
systems are generally simpler and less expensive than wire-arc spraying systems. Both flame
spraying and wire-arc spraying systems are field portable and may be used to apply quality
metal coatings for corrosion protection.
5.1.1.2 Wire-arc process. Due to its high deposition rates, excellent adhesion, and cost-effectiveness,
wire-arc spray is the preferred process for applying TSMCs to steel pilings. With the wire-
arc spray process, two consumable wire electrodes of the metal being sprayed are fed into a
gun such that they meet at a point located within an atomizing air (or other gas) stream. An
applied DC potential difference between the wires establishes an electric arc between the
wires that melts their tips. The atomizing air flow subsequently shears and atomizes the
molten droplets to generate a spray pattern of molten metal directed toward the substrate
being coated. Wire-arc spray is the only thermal spray process that directly heats the material
being sprayed, a factor contributing to its high energy efficiency.
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The wire-arc spray system consists of a wire-arc spray gun or torch, atomizing gas, flowmeter
or pressure gauge, a compressed air supply, DC power supply, wire guides/hoses, and a
wire feed control unit. Operation of this equipment must be in strict compliance with the
manufacturers' instructions and guidelines.
5.1.2 Thermal Spray Guns (Wire-Arc and Flame)
Figure 3 illustrates a typical wire-arc spray gun or torch and Figure 4 illustrates a typical
flame spray gun.
5.1.3 Air Compressors (Arc and Flame)
Compressed air should be free of oil and water. Accurate air regulation is necessary to
achieve uniform atomization. Under continuous use conditions, the actual atomization air
Arc Shield
Shroud Gas
Electric Arc
Wire Atomizing
Feed Gas
Shroud Cap Atomization
Zone
Figure 3. Schematic of a typical wire-arc spray gun.
Figure 4. Schematic of a typical flame-wire spray gun.
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pressure and volumetric flow rate should remain nearly constant and, ordinarily, should not
deviate from the set value by more than 5 percent.
5.1.4 Atomizing Gas Supply (Wire-Arc and Flame Spray)
Provisions shall be made to monitor and control, read clearly, and adjust (by means of
instruments), any deviations of the atomizing gas pressure and volumetric flow rates from the
set values during the spraying process. These values shall be recorded during acceptance
inspection. For the wire-arc spray process, the atomizing gas supply and control system shall
be designed and constructed to allow continuous operation at selected pressures and flow rates.
5.1.5 Air Dryers (Wire-Arc and Flame Spray)
An air dryer is necessary to provide clean, dry air (as per ASTM D4285) for surface
preparation and thermal spraying. Air dryers shall be inspected and tested regularly and
replaced as necessary to maintain the desired moisture content in the process air streams.
5.1.6 Oxygen and Fuel Gas (Flame Spray)
The use of oxygen and fuel gas flowmeters allows for the best control of the flame and thus
higher spray rates. Under continuous use conditions, the actual oxygen and fuel gas flow
rates and pressures should remain nearly constant and, ordinarily, should not deviate from
the set values by more than 5 percent. Flame spraying equipment shall permit spraying with
the combustible gases, atomizing gas (if any), and powder carrier gas (if any) for which it
was designed.
5.1.7 Gases for Flame Spraying
Gaseous oxygen equal or equivalent to Federal Specification BB-O-925 should be used for
thermal spraying. Acetylene equal or equivalent to Federal Specification BB-A 106 should
be used for thermal spraying. Other fuel gases (e.g., methyl acetylene-propadiene [MAPP]
stabilized, propane, or propylene) as specified by the thermal spray equipment manufacturer
may also be used.
5.1.8 Power Supply (Wire-Arc Spray)
In general, the higher the power output of the direct current (DC) power supply used, the
greater the possible production spray rate of the unit. Under continuous use conditions, the
actual current output should remain nearly constant and, ordinarily, should not deviate from
the set value by more than 5 percent. Power supplies that are adequately sealed may be
operated in dusty atmospheres and do not need to be located at a remote distance from the
thermal spray operation. DC power supplies rated up to 600 A are common. A lightweight
power supply mounted on pneumatic tires will have added portability. There is typically an
optimum amperage for each coating material that may further depend on wire diameter and
the particular equipment model.
The open-circuit voltage should be adjustable to accommodate different wire materials. The
voltage should be set slightly above the lowest level consistent with good arc stability. This
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will provide smooth dense coatings with superior deposition efficiency. Higher voltages tend
to increase droplet sizes, resulting in rougher coatings with lower densities. Under continuous
use conditions, the actual arc voltage should remain nearly constant and, ordinarily, should
not deviate from the set value by more than 5 percent.
5.1.9 Wire Feed Control (Wire-Arc and Flame Spray)
The wire feed and guide mechanisms should be designed to provide automatic alignment.
Manual alignment of the wires is both time consuming and inexact. The wire feed mechanism
must be capable of delivering wire to the arc tips at a rate commensurate with the power
generated in the arc. Under continuous use conditions, the actual wire feed rate should
remain nearly constant and, ordinarily, should not deviate from the set value by more than
5 percent.
For flame spray equipment, the spraying material feed unit shall comply with the following
conditions:
· The unit shall permit uniform and consistent processing of the consumables for which it
was designed.
· It shall enable adjustment of the feedstock material feed rate.
· The set-point values shall be constant and reproducible; a precondition of this is adequate
and constant gas pressures and flow rates, atomizing air pressures (where used), and
supply of electrical power, as appropriate.
5.1.10 Air Cap Selection (Wire-Arc Spray)
A range of different air caps is usually available for use with wire-arc spray equipment. Air
caps used in wire-arc spraying include fan (elliptical) and circular spray patterns. Some air caps
are adjustable. The nozzle system (contact tubes and air nozzle) shall permit a continuous and
stable arc to be maintained and provide atomization of the feedstock materials without
causing a buildup of deposits that may degrade gun operation.
5.1.11 Cable Length (Wire-Arc Spray)
Most manufacturers offer optional cable packages that allow operation of the spray gun or
torch up to 100 ft (30 m) from the power supply. Longer cables provide added flexibility
when thermal spraying in the field.
5.1.12 Arc-Shorting Control (Wire-Arc Spray)
Arc shorting is a phenomenon wherein the wires are fused or welded together, creating a
short circuit and cessation of melting and spraying. Shorting sometimes requires that the
wire ends be manually clipped before the arc can be restruck. This operation can be very
time consuming and must only be conducted with the power supply de-energized and by
appropriately trained personnel. Occasionally during arc shorting, lumps of unmelted wire
are shorn off and deposited on the substrate, resulting in poor coating quality. An added
feature available on some wire-arc spray equipment can control arc shorting.
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5.1.13 Wire Tips (Wire-Arc Spray)
Wire guide tips that hold and align the wires as they enter the arc zone are subject to wear.
Wear rate depends on the properties of the material being sprayed and the level of current
used, since these tips are also part of the means by which electrical current is transferred to
the wires. Properly designed equipment will allow cooler operating temperatures that will
prolong tip life and reduce maintenance time. Easy-to-change contact tips are also beneficial.
5.1.14 Nozzles (Flame Spray)
Processing of the feedstock materials shall be possible without any degrading deposit buildup
on the gun, air nozzle, or both.
5.2 Thermal Spray Equipment Setup and Validation
· The thermal spray equipment should be set up, calibrated, and operated as per the
manufacturer's instructions and technical manuals.
· Spray parameters should be set for spraying the specified feedstock material and, at a
minimum, be validated using the bend test.
· Validation of the TSMC procedure includes (1) successful surface preparation, (2) correct
application procedures for the specified TSMC, (3) achievement of the required thickness,
and (4) successful bend testing of at least one bend coupon at the beginning of each
work shift.
· If the bend test fails, the problem shall be identified and fixed before spraying continues.
· Results of all validation tests shall be clearly identified and documented.
5.2.1 Procedures for Acceptance Inspection
5.2.1.1 Electrical power and wire feed unit (wire-arc spray). Compliance with the requirements
specified for electrical power for continuous operation wire feed units shall be met by
(1) Spraying 8515 wt% zinc/aluminum wire at maximum capacity for 20 minutes (alternate
feedstock--for example, aluminum or zinc wire--may be specified by the purchaser).
(2) Measuring 5 percent deviations of the adjusted electrical values or other disturbances.
5.2.1.2 Atomizing gas (wire-arc spray). The equipment shall be deemed to comply with the
requirements if the atomizing gas supply gauge pressure does not deviate by more than
± 5 percent from the set value over a 20-minute period of spraying.
5.2.1.3 Nozzle system (wire-arc and flame spray). The nozzle system shall be deemed to comply
with the requirements if, after 20 minutes of spraying 8515 wt% zinc/aluminum wire at the
maximum spray rate, there are no degrading deposits of feedstock material visible on or
inside the nozzle.
5.2.1.4 Monitoring (wire-arc spray). The limits of error of the measuring instruments shall not
exceed ±5 percent for all set values and shall correspond to at least Class 2.5 instruments.
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5.2.1.5 Gases (flame spray). Flame spray equipment shall be deemed to comply with the requirements
if the values of supply gas pressure and gas flow volume meet the class deviations of the
following from the set values over a 10-minute period of spraying.
Class Deviations for Supply Gas Pressure and Flow Volume
Class A Class B
2% 5%
5.2.1.6 Validity of inspection report. The inspection report shall be deemed valid for as long as all
specifications of this guide are in compliance.
5.2.1.7 Retests. The guidelines listed below should be followed for retests.
(1) If the values obtained during acceptance inspection of a thermal spraying system are
altered by modification or repair work, retesting of the properties affected shall be
carried out.
(2) Retests shall be carried out in the same way as the initial tests described in this guide.
5.3 Coating Application
5.3.1 Thickness
Table 2 provides recommended thickness values for various coatings and environments.
5.4 Application and Feed Rates
5.4.1 Feed Rates and Spray Rates
Table 5 provides information on typical feed rates for flame spray and arc spray. Table 6
provides information on spray rates for flame spray and arc spray for different wire diameters.
TABLE 5 Nominal feedstock required per unit area/unit thickness (deposition
efficiency on a flat plate)
Feedstock Flame Spray Wire-Arc Spray
Material Deposit Material Deposition Material
(wire) Efficiency Required Efficiency Required
(%) kg/m2/µm lb/ft2/mil (%) kg/m2/µm lb/ft2/mil
Aluminum 8085 0.0027 0.014 7075 0.0029 0.017
(Al)
Zinc (Zn) 6570 0.0098 0.050 6065 0.011 0.054
85:15 Zn/Al 8590 0.0070 0.036 7075 0.0093 0.049
1 mil = 0.001 in.
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TABLE 6 Nominal wire feedstock spray rates and coverage
Flame Spray Wire-Arc Spray
(by wire diameter) (per 100 amps)
Feedstock Spray Rate Spray Rate
Material 2.4 mm 3.2 mm 4.8 mm 3/32 in. 1/8 in. 3/16 in. (Coverage) (Coverage)
Spray Rate, kg/hr Spray Rate, lb/hr kg/hr lb/hr
(Coverage, m2/hr/100µm) (Coverage, ft2/hr/mil) (m2/hr/100µm) (ft2/hr/mil)
Aluminum 2.5 5.4 7.3 5.5 12 16 2.7 6
(8.73) (18.9) (25.3) (370) (800) (1,070) (8.26) (350)
Zinc 9.1 20 30 20 45 65 18 25
(9.44) (21.2) (30.7) (400) (900) (1,300) (11.0) (465)
85:15 8.2 18 26 18 40 58 16 20
Zn/Al (11.8) (26.2) (38.0) (500) (1,110) (1,610) (9.68) (410)
1 mil = 0.001 in.
The values in these tables should be taken as approximate only. The wire feed rate should
be adjusted to properly optimize the dwell time in the flame. Excessive feed rates may result
in inadequate or partial heating and melting of the feedstock and may result in very rough
deposited coatings. Too slow a feed rate may cause the wire to be over-oxidized, resulting
in poor-quality coatings containing excessive levels of oxides and poor cohesion. Under
continuous use conditions, the actual wire feed rate should remain virtually constant and,
ordinarily, should not deviate from the set value by more than 5 percent.
5.4.2 Holding Period
5.4.2.1 Correct surface cleanliness and profile. TSMCs should always be applied to "white" metal
(SSPC-SP-5/NACE # 1). It is common practice in fieldwork to apply the TSMC during the
same work shift in which the final blast cleaning is performed. The logical end point of the
holding period is when the surface cleanliness degrades or a change on performance (as per
bend or tensile test) occurs. If the holding period is exceeded, the surface must be re-blasted
to establish the correct surface cleanliness and profile.
5.4.2.2 Duration of the holding period. Thermal spraying should be started as soon as possible after
the final anchor-tooth or brush blasting and completed within 6 hours for steel substrates
subject to the temperature to dew point and holding-period variations. In high-humidity and
damp environments, shorter holding periods should be used.
5.4.2.3 Extending the holding period--temperature/humidity. In low-humidity environments or
in controlled environments with enclosed structures using industrial dehumidification
equipment, it may be possible to retard the oxidation of the steel and hold the near-white-
metal finish for more than 6 hours. With the concurrence of the purchaser, a holding period
of greater than 6 hours can be validated by determining the acceptable temperature-humidity
envelope for the work enclosure by spraying and analyzing bend test coupons or tensile
adhesion coupons, or both. Should the sample fail the bend test, the work must be re-blasted
and re-tested.
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5.4.2.4 Extending the holding period--application of flash coat. When specified by the
purchasing contract, a flash coat of TSMC equal to or greater than 1 mil (25 µm) may be
applied within 6 hours of completing the surface preparation in order to extend the holding
period for up to 4 hours beyond the application of the flash coat. The final TSMC thickness,
however, should be sprayed within 4 hours of the application of the flash coat. This
procedure should be validated using a tensile adhesion test or bend test, or both, by spraying
a flash coat and waiting through the delay period before applying the final coating thickness.
5.4.2.5 Small and moveable parts. For small and movable parts, if more than 15 minutes is
expected to elapse between surface preparation and the start of thermal spraying or if the
part is moved to another location, the prepared surface should be protected from moisture,
contamination, and finger/hand marks. Wrapping with clean print-free paper is normally
adequate.
5.4.2.6 Rust bloom, blistering, or coating degradation. If rust bloom, blistering, or a degraded
coating appears at any time during the application of the TSMC, the following procedure
should be performed:
(1) Stop spraying.
(2) Mark off the satisfactorily sprayed area.
(3) Repair the unsatisfactory coating (i.e., remove the degraded coating and re-establish the
minimum "white metal" finish and anchor-tooth profile depth as per the maintenance and
repair procedure).
(4) Record the actions taken to resume the job in the job documentation.
(5) Call the coating inspector to observe and report the remedial action to the purchaser.
5.5 Overspray
5.5.1 Examples of Overspray
· TSMC material that is applied outside the authorized parameters, primarily the gun-to-
substrate standoff distance and spray angle (perpendicular ± 30 degrees).
· TSMC material that misses the target or bounces off of the substrate.
5.5.1.1 Foreign matter. Foreign matter such as paint overspray, dust and debris, and precipitation
should not be allowed to contact prepared surfaces prior to thermal spraying.
5.5.1.2 Masking. Cleaning, thermal spray application, and sealing should be scheduled so that dust,
overspray, and other contaminants from these operations are not deposited on surfaces readied
for TSMC or sealing. Surfaces that will not be thermally sprayed should be protected from the
effects of blast cleaning and thermal spray application through the use of removable masking
materials or other means. Mask all fit and function surfaces and surfaces and areas specified
by the purchaser not to be abrasive blasted or to be thermally sprayed. Mask on complex
geometries (e.g., pipe flanges, intersections of structural beams, and valve manifolds) to
eliminate or minimize overspray. Ensure that the covers and masking are securely attached
and will survive the blasting and thermal spraying operations. Masking should also be
designed to avoid "bridging," which can lead to debonding or edge lifting.
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5.5.1.3 Overspray-control area. For complex geometries where overspray cannot be eliminated,
an overspray-control area should be established. Clean, metal masks or clean, removable
masking materials should be used to prevent overspray from depositing on surfaces not
already sprayed to the specified thickness.
5.5.1.4 Dust, fumes, and particles. Special care should be taken to prevent the entry of abrasive
and thermally sprayed metal dusts and fumes into sensitive machinery and electrical
equipment. Painted surfaces adjacent to surfaces receiving TSMCs should be adequately
protected from damage by molten thermally sprayed metal particles.
5.5.1.5 High winds. High winds may affect the types of surface preparation and coating application
methods that are practical for a given job. High winds will tend to carry surface preparation
debris and paint overspray longer distances. This problem can be avoided by using methods
other than open abrasive blasting and spray application of paints.
5.6 Temperature
5.6.1 Ambient Temperature
Although there are no high or low temperature limitations when applying thermally sprayed
metal, it is often advisable to preheat the surface to 250°F (120°C) when first beginning to
flame spray to prevent water vapor in the flame from condensing on the substrate. Preheat
the initial 1- to 2-ft (0.1- to 0.2-m2) starting-spray area.
5.6.2 Metal Surface Temperature
The steel surface temperature must be at least 5°F (3°C) above the dew point in order to
prevent condensation on the surface that will adversely affect coating adhesion.
5.6.3 Low Ambient Temperature
Thermal spraying in low-temperature environments, below 40oF (5oC), must
(1) Meet the substrate surface temperature and cleanliness (Section 5.4.2.1) and holding
period (Section 5.4.2.2). Moisture condensation on the surface is not permissible during
thermal spraying.
(2) Be qualified using a bend test or a portable tensile-bond test, or both.
(3) Meet the substrate surface temperature (Section 5.4.2.3). Substrate heating may be
required to improve the TSMC tensile adhesion to the substrate and reduce internal
(residual) stresses because the TSMCs are mechanically bonded to the substrate.
5.7 Coating Thickness Build
5.7.1 Achieving Specified Coating Thickness
Manually applied TSMCs should be applied in a block pattern measuring approximately
24 in. (60 cm) on a side. Each spray pass should be applied parallel to and overlapping the
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previous pass by approximately 50 percent. Successive spray coats should be applied at right
angles to the previous coat until the desired coating thickness is achieved. Approximately
0.002 to 0.003 in. (50 to 75 µm) of coating should be applied per spray pass. In no case
should less than two spray coats applied at right angles be used to achieve the specified
coating thickness. Laying down an excessively thick spray pass increases the internal
stresses in the TSMC and will decrease the ultimate tensile-bond strength of the TSMC.
5.7.1.1 Minimizing thin spots--manual spraying. During manual spraying, use crossing passes
to minimize the thin spots in the coating.
5.7.1.2 Minimizing thin spots--robotic spraying. During robotic spraying, program overlapping
and crossing passes to eliminate thin spots and stay within the coating thickness specification.
Validate the automated spraying parameters and spraying program using tensile-bond or
metallographic analysis, or both.
5.7.1.3 Equipment. Use approved spray gun extensions, compressed-air deflectors, or similar
devices to reach into recessed spaces and areas.
5.7.2 Spray Angle, Width, and Standoff Distance
5.7.2.1 Gun-to-surface angle. The gun-to-surface angle is very important because of the generally
greater distances that the sprayed particles travel prior to striking the substrate, producing a
porous and oxidized coating with reduced cohesion and adhesion for similar reasons as those
described in Section 5.7.2.2. Porosity, oxide content, and adhesion are strongly affected
by spray angle. In some cases, it may be necessary for the applicator to spray at less than
90 degrees because of limited access to the surface. In no case should the applicator spray
at an angle of less than 45 degrees. Some degradation in performance might result even at the
45-degree angle. Spray gun extensions are available from some equipment manufacturers that
allow better access to difficult-to-spray areas. A good spray technique consists of the applicator
maintaining the spray gun perpendicular (at 90 degrees) or near perpendicular (90 ± 5 degrees)
to the substrate at all times. Maintain the gun as close to perpendicular as possible and within
± 30 degrees from perpendicular to the substrate.
5.7.2.2 Standoff distance. Standoff distance depends on the type and source of thermal spray
equipment used. Excessive standoff distance will result in porous and oxidized coatings with
reduced cohesion and adhesion. The higher porosity may be attributed to the greater degree
of cooling and the lower velocity that the thermal spray particles or droplets experience prior
to impact. Adhesion is directly proportional to the kinetic energy of the spray particles, and
the kinetic energy varies as the square of the particle velocity. Cooler, more slowly impacting
particles will not adhere as well to each other or to the substrate, resulting in weaker, less
adherent coatings. Excessive standoffs may occur because the applicator is not sufficiently
familiar with the requirements of the equipment or because of fatigue or carelessness. Increased
standoffs may also result from the applicator's arm or wrist arcing during application. It is
very important that the applicator's arm move parallel to the substrate in order to maintain
a consistent standoff distance. Holding the thermal spray gun too close to the surface may
result in poor coverage and variations in coating thickness because of the reduced size of the
spray pattern. Some degradation in performance might result at the higher standoff distance.
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Use the manufacturer's recommended standoff distance for the air cap installed. Table 7 lists
nominal standoff and spray pass width values.
5.7.3 Supplemental Surface Preparation
Surfaces that have become contaminated or that have exceeded the holding period must be
recleaned to establish the required degree of cleanliness and profile.
Small areas that have been damaged and require coating repair should be treated according
to Section 7, "Repair and Maintenance."
5.8 Inspection and Quality Control
5.8.1 Quality Control (QC) Equipment
Quality control (QC) equipment for thermal spray application should include the following:
· Substrate Temperature--Contact thermocouple or infrared pyrometer to measure substrate
temperatures.
· Air Temperature, Dew Point, and Humidity--Psychrometer or an equivalent digital
humidity measurement instrument.
· TSMC Thickness--Magnetic pull-off or electronic thickness gauge with secondary
thickness standards per SSPC-PA-2.
· TSMC Ductility--2 in. × 4 to 8 in. × 0.050 in. (50 mm × 100 to 200 mm × 13 mm) (ANSI-
SAE 10xx sheet) for bend test coupons and a mandrel of a diameter suitable for the
specified TSMC thickness.
· Bend Coupon, Companion Coupon, and Sample Collection--Sealable plastic bags to
encase bend coupons and other QC samples collected during the job.
5.8.2 Coating Thickness
The thickness of the TSMC should be evaluated for compliance with the specification.
Magnetic film thickness gauges such as those used to measure paint film thickness should
be used. Gauges should always be calibrated prior to use. Thickness readings should be
made either in a straight line with individual readings taken at 1-in. (25-mm) intervals or
spaced randomly within a 2-in.- (50-mm-) diameter area. Line measurements should be
used on 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
TABLE 7 Nominal flame-spray and arc-spray standoff distances and spray
widths
Thermal Spray Perpendicular Spray Pass Width, in. [mm]
Method Standoff Air Cap
in. [mm] Regular Fan
Wire-flame 57 [130180] 0.75 [20] Not Available
Wire-arc 68 [150200] 1.5 [40] 36 [75150]
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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. Areas of deficient coating
thickness should be corrected before sealing begins.
5.8.3 Adhesion
5.8.3.1 Bend test. Each day, or every time the thermal spray equipment is used, the inspector should
record and confirm that the operating parameters are the same as those used to prepare the
job reference standard (Section 8.3.3). The thermal spray applicator should then apply the
coating to prepared test panels and conduct a bend test. The bend test is a qualitative test
used to confirm that the equipment is in proper working condition. The test consists of bending
coated steel panels around a cylindrical mandrel and examining the coating for cracking.
Details of the test are described in Section 8. If the bend test fails, corrective actions must
be taken prior to the application of the TSMC. The results of the bend test should be recorded,
and the test panels should be labeled and saved.
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.
Bend test samples can also be used for metallographic evaluations of porosity, oxide content,
and interface contamination.
5.8.4 Appearance
5.8.4.1 Inspecting the coating. The applied TSMC should be inspected for obvious defects related
to poor thermal spray applicator technique and/or equipment problems. The coating should
Figure 5. 180-degree bend test illustrating pass and fail
appearance.
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be inspected for the presence of blisters, cracks, chips or loosely adherent particles, oil, pits
exposing the substrate, and nodules. A very rough coating may indicate that the coating was
not applied with the gun perpendicular to the surface or that the coating was applied at too
high of a standoff distance. Coatings that appear oxidized or powdery should be evaluated
by light scraping. If scraping fails to produce a silvery metallic appearance, the coating is
defective.
5.8.4.2 Coating appearance. The appearance of the coating should match that of the job reference
standard.
5.9 Handling, Storage, and Transportation of Thermally Sprayed Metal
Coated Piles
5.9.1 Aluminum, Zinc, and Zinc-Aluminum Coatings
Thermally sprayed metal surfaces are tough and are ready to be handled immediately after
the application of the coating. However, aluminum, zinc, and zinc-aluminum coatings are
softer than the steel substrate and are subject to scratching, gouging, and impact damage.
5.9.2 Handling Coated Piles
Coated piles should at all times be handled with equipment such as stout, wide belt slings
and wide padded skids designed to prevent damage to the coating. Bare cables, chains,
hooks, metal bars, or narrow skids shall not be permitted to come in contact with the coating.
All handling and hauling equipment should be approved before use.
5.9.3 Loading Piles for Shipping by Rail
When shipped by rail, all piles should be carefully loaded on properly padded saddles or
bolsters. All bearing surfaces and loading stakes shall be properly padded with approved
padding materials. Pile surfaces should be separated so that they do not bear against one
another, and the whole load must be securely fastened together to prevent movement in
transit.
5.9.4 Loading Piles for Shipping by Truck
When shipped by truck, the piles should be supported in wide cradles of suitably padded
timbers hollowed out on the supporting surface to fit the curvature of pipe, and all chains,
cables, or other equipment used for fastening the load should be carefully padded.
5.9.5 Storing Piles
Stored piles should be supported on wooden timbers above the ground.
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5.9.6 Hoisting Piles
Piles should be hoisted using wide belt slings. Chains, cables, tongs, or other equipment, no
matter how well padded, are likely to cause damage to the coating and should not be
permitted. Dragging or skidding the pile should not be permitted.
5.9.7 Repairing Damaged Coating
Damaged coating should be repaired in accordance with Section 7 of this guide.
