== General Description of Thermal Spraying ==
Thermal spraying is a group of processes wherein a feedstock material is heated and propelled as individual particles or droplets onto a surface. The thermal spray gun generates the necessary heat by using combustible gases or an electric arc. As the materials are heated, they are changed to a plastic or molten state and are confined and accelerated by a compressed gas stream to the substrate. The particles strike the substrate, flatten, and form thin platelets (splats) that conform and adhere to the irregularities of the prepared substrate and to each other. As the sprayed particles impinge upon the surface, they cool and build up, splat by splat, into a laminar structure forming the thermal spray coating. Figure 2-1 illustrates a typical coating cross section of the laminar structure of oxides and inclusions. The coating that is formed is not homogenous and typically contains a certain degree of porosity, and, in the case of sprayed metals, the coating will contain oxides of the metal. Feedstock material may be any substance that can be melted, including metals, metallic compounds, cements, oxides, glasses, and polymers. Feedstock materials can be sprayed as powders, wires, or rods. The bond between the substrate and the coating may be mechanical, chemical, or metallurgical or a combination of these. The properties of the applied coating are dependent on the feedstock material, the thermal spray process and application parameters, and post treatment of the applied coating.
Other thermal spray coating materials are used for special applications. Special metal alloy coatings are commonly used for hardfacing items such as wear surfaces of farm equipment, jet engine components, and machine tools. Ferrous metal alloys are often used for restoration or redimensioning of worn equipment. Special ferrous alloys are sometimes used for high temperature corrosion resistance. Inert ceramic coatings have been used on medical prosthetic devices and implants such as joint replacements. Conductive metal coatings are used for shielding sensitive electronic equipment against electric and magnetic fields. Ceramic coatings have also been used to produce very low-friction surfaces on near net shape components. These and other applications make thermal spray coatings a diverse industry.
== Thermal Spray Processes ==
Thermal spray processes may be categorized as either combustion or electric processes. Combustion processes include flame spraying, HVOC spraying, and detonation flame spraying. Electric processes include arc spraying and plasma spraying.
a. Combustion processes.
(1) Flame spraying. The oldest form of thermal spray, flame spraying, may 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 to the tip of the spray gun where it is melted in a fuel gas flame and propelled to the substrate in a stream of atomizing gas. Common fuel gases are acetylene, propane, and methyl acetylene-propadiene. Air is typically used as the atomization gas. Oxyacetylene flames are used extensively for wire flame spraying because of the degree of control and the high temperatures offered by these gases. By gauging its appearance, the flame can be easily adjusted to be an oxidizing, neutral, or reducing flame. The lower temperature propane flame can be used for lower melting metals such as aluminum and zinc as well as polymer feedstocks. The basic components of a flame spray system include the flame spray gun, feedstock material and feeding mechanism, oxygen and fuel gases with flowmeters and pressure regulators, and an air compressor and regulator.
(a) Wire flame spraying. Wire flame spray is the flame process of greatest interest to the Corps of Engineers. CEGS-09971 allows for the application of aluminum, zinc, and zinc/aluminum alloy coatings using the flame spray method. Figure 2-2 shows a schematic of a typical flame spray system. Figure 2-3 depicts a typical wire flame spray gun. The wire flame spray gun consists of a drive unit with a motor and drive rollers for feeding the wire and a gas head with valves, gas nozzle, and air cap that control the flame and atomization air. Compared with 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, at only one-third to one-half the cost of wire arc spray systems, are significantly cheaper. Flame spray systems are field portable and may be used to apply quality metal coatings for corrosion protection.
(b) Powder flame spraying. Powder flame operates in much the same way as wire flame spray except that a powder feedstock material is used rather than wire and there is no atomizing air stream. The melted coating material is atomized and propelled to the surface in the stream of burning fuel gas. The powder is stored in either a gravity type hopper attached to the top of a spray gun or a larger air or inert gas entrainment type detached hopper. Powder flame spray guns are lighter and smaller than other types of thermal spray guns. Production rates for powder flame spray are generally less than for wire flame spray or arc spray. Particle velocities are lower for flame spray, and the applied coatings are generally less dense and not as adherent as those applied by other thermal spray methods. USACE use of powder flame spray should be limited to repair of small areas of previously applied thermal spray coatings and galvanizing. Figure 2-4 illustrates a typical combustion powder gun installation, and Figure 2-5 shows a powder gun cross section.
(2) HVOF spraying. One of the newest methods of thermal spray, HVOF, utilizes oxygen and a fuel gas at high pressure. Typical fuel gases are propane, propylene, and hydrogen. The burning gas mixture is accelerated to supersonic speeds, and a powdered feedstock is injected into the flame. The process minimizes thermal input and maximizes particle kinetic energy to produce coatings that are very dense, with low porosity and high bond strength. HVOF systems are field portable but are primarily used in fabrication shops. HVOF has been used extensively to apply wear resistant coatings for applications such as jet engine components. The Corps has conducted an experimental evaluation of HVOF-applied metal alloy coatings for protection against cavitation wear in hydraulic turbines.
(3) Detonation flame spraying. In detonation flame spraying, a mixture of oxygen,
acetylene, and powdered feedstock material are detonated by sparks in a gun chamber several
times per second. The coating material is deposited at very high velocities to produce very dense
coatings. Typical applications include wear resistant ceramic coatings for high-temperature use.
Detonation flame spraying can only be performed in a fabrication shop. Detonation flame
spraying is not applicable for USACE projects.
b. Electric processes.
(1) Arc spraying. Arc spraying is generally the most economical thermal spray method
for applying corrosion resistant metal coatings, including zinc, aluminum, and their alloys as
described in CEGS-09971. Energy costs are lower and production rates are higher than they are
with competing methods such as wire flame spray. Arc spraying may be used to apply electrically
conductive materials including metals, alloys, and metal-metal oxide mixtures. In arc spraying, an
arc between two wires is used to melt the coating material. Compressed gas, usually air, is used to
atomize and propel the molten material to the substrate. The two wires are continuously fed to the
gun at a uniform speed. A low voltage (18 to 40 volts) direct current (DC) power supply is used,
with one wire serving as the cathode and the other as the anode. Figure 2-6 shows a typical arc spray system comprised of a DC power supply, insulated power Oxygen and fuel gas supply cables, a wire feed system, a compressed-air supply, controls, and an arc spray gun. Figure 2-7 shows the components of a typical arc spray gun, including wire guides, gun housing, and gas nozzle. Coating quality and properties can be controlled by varying the atomization pressure, air nozzle shape, power, wire feed rate, traverse speed, and standoff distance. Arc sprayed coatings exhibit excellent adhesive and cohesive strength.
(2) Plasma spraying. Plasma spraying is used to apply surfacing materials that melt at
very high temperatures. An arc is formed between an electrode and the spray nozzle, which acts
as the second electrode. A pressurized inert gas is passed between the electrodes where it is
heated to very high temperatures to form a plasma gas. Powdered feedstock material is then
introduced into the heated gas where it melts and is propelled to the substrate at a high velocity. A
plasma spray system consists of a power supply, gas source, gun, and powder feeding
mechanism. Plasma spraying is primarily performed in fabrication shops. The process may be
used to apply thermal barrier materials, such as zirconia and alumina, and wear resistant coatings
such as chromium oxide.
== Thermal Spray Uses ==
a. Thermal spray is used for a wide variety of applications. The primary use of thermal spray coatings by the Corps is for corrosion protection. Coatings of zinc, aluminum, and their alloys are anodic to steel and iron and will prevent corrosion in a variety of service environments including atmospheric, salt- and freshwater immersion, and high-temperature applications. Coatings of aluminum are frequently used in marine environments. The U.S. Navy uses aluminum coatings for corrosion protection of many ship components. Because these materials are anodic to steel, their porosity does not impair their ability to protect the ferrous metal substrate. Zinc and zinc-aluminum alloy coatings may corrode at an accelerated rate in severe industrial atmospheres or in chemical environments where the pH is either low or high. For this reason these materials are typically sealed and painted to improve their performance.
b. Cathodic coatings such as copper-nickel alloys and stainless steels can also be used to protect mild steel from corrosion. These materials must be sealed to prevent moisture migration through the coating. These metals are particularly hard and are often used for applications requiring both corrosion and wear resistance.
c. Aluminum coatings are often used for corrosion protection at temperatures as high as 660 qC (1220 qF).
d. Thermal spray deposits containing zinc and/or copper can be used to prevent both marine and freshwater fouling. Zinc and 85-15 zinc-aluminum alloy coatings have been shown to prevent the significant attachment and fouling by zebra mussels on steel substrates. Because these coatings are long lived and prevent corrosion, their use is recommended for Corps structures. Copper and brass coatings have also been shown to be effective antifoulants but should not be used on steel due to the galvanic reaction between the two.
e. Zinc thermal spray coatings are sometimes used to prevent the corrosion of reinforcing steel imbedded in concrete. For such applications, the zinc is deposited onto the concrete and is electrically connected to the steel.
f. Thermal spray coatings are frequently used to repair surfaces subject to wear. A common application is the redimensioning of rotating shafts. Metal is sprayed onto the part as it is rotated on a lathe. The rebuilt part can then be machined to the required diameter. Similarly, thermal spray deposits can be used to re-contour foundry molds or to repair holes.
g. Thermal spray coatings are also used for electrical applications. Conductive metals such as copper can be used for conductors. Ceramic materials may be used for electrical insulation. Conductive metals are also used to magnetically shield sensitive electronics.
h. Very hard and dense thermal spray deposits have been used on an experimental basis as cavitation resistant materials and in conjunction with weld overlays as a repair technique.
Thermal Spray Coating Cost and Service Life
This chapter contrasts paint and thermal spray coatings based on cost and expected service life. Both paint and thermal spray coatings may be used to provide corrosion protection for most civil works applications. The use of thermal spray coatings is preferred on the basis of fitness-for-purpose for a few specific applications, including corrosion protection in very turbulent ice- and debris-laden water, high-temperature applications, and zebra mussel resistance. Thermal spray coatings may also be selected because of restrictive air pollution regulations that do not allow the use of some paint coatings specified in CEGS-09965. For all other applications, the choice between thermal spray and paint should be based on life-cycle cost.
a. Whenever possible, coating selection should be based on life-cycle cost. In reality, the engineer must balance competing needs and may not always be able to specify the least expensive coating on a life-cycle cost basis. Because of their somewhat higher first cost, thermal spray coatings are often overlooked. To calculate life-cycle costs, the installed cost of the coating system and its expected service life must be known. Life- cycle costs for coating systems are readily compared by calculating the average equivalent annual cost (AEAC) for each system under consideration.
b. The basic installed cost of a thermal spray coating system is calculated by adding the costs for surface preparation, materials, consumables, and thermal spray application. The cost of surface preparation is well known. The cost of time, materials, and consumables may be calculated using the following stepwise procedure:
(1) Calculate the surface area (SA). (SA = length x width)
(2) Calculate the volume (V) of coating material needed to coat the area. (V = SA x coating thickness)
(3) Calculate the weight of the material to be deposited (Wd). The density (D) of the applied coating is less than that of the feedstock material. A good assumption is that the applied coating is about 90 percent of the density of the feedstock material. The densities of aluminum, zinc, and 85-15 zinc-aluminum wire are 2.61 g/cm3 (0.092 lb/in.3), 7.32 g/cm3 (0.258 lb/in.3), and 5.87 g/cm3 (0.207 lb/in.3), respectively. (Wd = V x 0.9 D)
(4) Calculate the weight (W) of material used. Estimates of deposition efficiency (DE) for various materials and thermal spray processes are given in Chapter 7, Table 7-2. (W = Wd/DE)
(5) Calculate the spray time (T). Spray rates (SR) for various materials and thermal spray processes are given in Chapter 7, Table 7-4. (T = W/SR)
(6) Calculate electricity or oxygen and fuel gas consumption (C). Typical consumption rates (CR) for electricity, fuel gas, and oxygen are available from equipment manufacturers. (C = CR x T)
(7) Calculate cost of materials (CM). (CM = W x cost per unit weight)
(8) Calculate cost of application (CA). (CA = T x unit labour cost)
(9) Calculate cost of consumables (CC). (CC = T x unit cost of consumable)
(10) Calculate total cost (TC) of thermal spray coating. (TC = CM + CA + CC)
c. Other factors that increase the cost of thermal spray and other coating jobs include the costs of containment, inspection, rigging, mobilization, waste storage, and worker health and safety.
d. The Federal Highway Administration (FHWA) (1997) compared the performance of a number of coating systems, including paints and thermal spray. Coating life expectancies were estimated based on performance in an aggressive marine atmospheric exposure and a mildly corrosive environment. Installed and life-cycle costs were calculated for each coating system for each exposure. Average equivalent annual costs were calculated based on a 60-year structure life. For the more severe marine atmospheric exposure, thermal spray coatings of aluminum, zinc, and 85-15 zinc-aluminum alloy were the most cost-effective coatings. For the less severe mildly corrosive atmospheric exposure, thermal spray was no more or less cost effective than other coating options.
3. Service Life
There are many documented examples of thermal spray coatings of zinc and aluminum with very long service lives. Service life depends on thermal spray coating thickness and the exposure environment. There does not appear to be a significant difference in the long-term performance of thermal spray coatings applied by different processes, including arc, wire flame, and powder flame spray. Thermal spray zinc coatings applied at thicknesses of 250 Pm (0.10 in.) have performed for more than 40 years in atmospheric exposures. Zinc thermal spray coatings in potable water tanks have lasted longer than 30 years. The FHWA (1997) report estimates a service life of 30 and 60 years for 85-15 zinc-aluminum alloy coating (150 Pm (0.006 in.)) in severe marine and mildly corrosive atmospheres, respectively. USACE has experience with 85-15 zinc-aluminum alloy coatings (400 Pm (0.016 in.)) providing 10 years of service in very turbulent ice- and debris-laden water. Table 4-1 provides typical service lives of paint coatings and predicted service life of thermal spray coatings for selected USACE applications. The tabulated service lives are given as the time to first maintenance.
== Thermal Spray Coating Selection ==
A systematic approach to coating selection for new construction and maintenance thermal spraying is described in this chapter. Paragraph 5-2 discusses criteria important to the selection of thermal spray coatings including the service environment, expected longevity, ease of application, and maintainability. Paragraph 5-3 discusses the relative merits of paint coatings and thermal spray coatings including durability and environmental considerations. Subsequent paragraphs discuss thermal spray coating systems for specific USACE applications.
5-2. Service Environments
Foreknowledge of the environmental stresses to which the protective coating system will be exposed is critical for proper selection of the coating system. This is true of both paint and thermal spray coating systems. Exposure environments typically encompass one or more of the following environmental stresses: extremes of temperature, high humidity, immersion, extremes of pH, solvent exposure, wet/dry cycling, thermal cycling, ultraviolet exposure, impact and abrasion, cavitation/erosion, and special exposures. The service environment is the single most important consideration in the selection of a coating system.
a. Extremes of temperature. Most exposure environments show some variability in temperature. Normal atmospheric service temperatures in northern latitudes of the continental United States vary from -23 to 38°C (-10 to 100°F). Temperatures for immersion exposure show somewhat less variation and typically range from about -1 to 27°C (30 to 80°F). These normal variations in temperature are relatively insignificant to the performance of thermal spray coatings of zinc, aluminum, and their alloys. Paint coating performance is generally more sensitive to these normal extremes of temperature. Some components, such as the stacks of floating plants, may be subject to higher than normal atmospheric temperatures. With some exceptions, most paint coatings will not perform well at these elevated service temperatures. Most alkyd paints such as CID A-A-2962 will tolerate temperatures up to only about 120°C (250°F). Special black bituminous coatings such as CID A-A-3054 will withstand temperatures up to 204°C (400°F). Aluminum and carbon black pigmented silicone coatings may perform at temperatures as high as 650°C (1200°F). Special ceramic frit coatings may perform at temperatures of 760°C (1400°F). SSPC Paint 20 Type I-B or I-C inorganic zinc-rich coatings can usually perform at temperatures up to 400°C (750°F). Thermal spray coatings of aluminum, zinc, and their alloys will provide long-term performance superior to paint coatings at temperatures approaching their respective melting points of 660°C (1220°F) and 420°C (788°F). Because of its excellent temperature resistance and corrosion protection, the aluminum thermal spray system 8-A from CEGS-09971 is recommended for applications where temperatures will exceed 400°C (750°F). Below this temperature, the specifier may elect to use paint system number 10 from CEGS-09965 which consists of two coats of SSPC Paint 20 Types I-B or I-C.
b. High humidity. High humidity is often accompanied by condensation, which is considered to approximate the severity of freshwater immersion. All of the thermal spray systems described in CEGS-09971 will also perform well in high-humidity condensate exposures. System 5-Z-A is the recommended thermal spray system for high-humidity condensate environments. Typically, high-performance paint systems such as the epoxy and vinyl systems described in CEGS-09965 are specified for high-humidity applications. Because paint systems are generallyless costly to apply, they are more likely to be used for these types of exposures. However, thermal spray system 5-Z-A should have a longer service life than paint coatings for this application.
c. Immersion. Immersion exposures range from immersion in deionized water to immersion in natural waters, including fresh water and seawater. Ionic content and pH contribute to the corrosivity of immersion environments. Typical sealers and topcoats are vinyl paints V-766e, V-102e, V-103c, and V-106d and coal tar epoxy coating C-200A. Several of the epoxy systems and all of the vinyl systems described in CEGS-09965 are appropriate for various immersion exposures depending on whether the water is fresh or salt and the degree of impact and abrasion. The epoxy systems are preferred for saltwater exposures, while the vinyl systems are generally preferred for freshwater exposures, especially where the level of impact and abrasion is significant.
(1) Seawater. Aluminum thermal spray system 8-A described in CEGS-09971 is recommended for seawater immersion. Aluminum thermal spray has been used extensively by the offshore oil industry to protect immersed and splash zone platform components from corrosion. Aluminum thermal spray is thought to perform better in seawater immersion without an organic sealer and paint topcoat.
(2) Fresh water. Thermal spray systems 5-Z-A and 6-Z-A are recommended for freshwater immersion, with 6-Z-A being the preferred choice for more severe exposures. These systems can be used either with or without sealers and topcoats.
d. Extremes of pH. Extremes of pH, such as strongly acidic or alkaline environments can greatly affect coating performance. The coating must be relatively impermeable to prevent migration of the acidic or alkaline aqueous media to the substrate, and the coating material itself must be resistant to chemical attack. Thermal spray coatings of aluminum, zinc, and their alloys may perform poorly in both high and low pH environments. Both metals show increased solubility as pH increases or decreases from the neutral pH of 7. Thermal spray aluminum and zinc may be used in acidic or alkaline environments provided that they are sealed and top-coated with vinyl or epoxy coatings. Unsealed zinc thermal spray coatings are suitable for pHs of 6 to 12 and aluminum thermal spray coatings for pHs of 4 to 8.5. Thermal spray coatings containing zinc or aluminum should not be used in chemical environments where they may be exposed to strong acids such as battery acids. Alkyd paints generally have poor resistance in alkaline environments. The epoxy and vinyl systems described in CEGS-09965 perform well in mildly acidic and alkaline environments. Topcoats with aluminum pigmentation should not generally be used in these exposures. Organic coatings and linings, as well as special inorganic building materials, should be used in highly alkaline or acidic environments.
e. Solvent exposure. Solvent exposure covers a wide variety of solvent types. Thermal spray metal coatings are essentially unaffected by solvent exposure and are good candidates for service in such environments. Some owners exclude the use of all zinc-containing coatings from use in aviation fuel storage tanks because metal contamination may affect the performance of the fuel. A solvent exposure that may be harsher on thermal spray coatings than anticipated is petroleum storage tanks where a layer of corrosive water can collect on the inside bottom of the tank. The water results in a much more corrosive environment than would be assumed if only the petroleum product was present. Some blends of organic solvents or natural petroleum products may also be acidic, which may affect thermal spray coating performance. Normal exposures to organic products such as cleaning solvents, lubricants, and hydraulic fluids should not preclude the use of thermal spray coatings of aluminum, zinc, and their alloys at USACE projects. Theperformance of paint coatings in solvent exposures depends on the coating type and the solvent species. Specific paint types, such as epoxies, are more solvent resistant than others. Some solvent types are more aggressive than others, independent of coating type.
f. Wet/dry cycling. Alternating wet and dry conditions are normal for most atmospheric exposures and, as such, most coating systems will provide adequate protection under such conditions. Thermal spray metal coatings will provide excellent performance under normal atmospheric conditions. Sealing and top-coating of the thermal spray coating is not generally necessary for such simple exposures. Generally, coating system selection will depend more on other stresses in the environment than on simple wet/dry cycling.
g. Thermal cycling. Thermal cycling may result from normal diurnal temperature variations as well as temperature changes found in operating machinery and process vessels. Thermal cycling induces stresses within the coating. Thermal sprayed metal coatings are more apt to have coefficients of expansion similar to the substrate; therefore, their relative inflexibility does not cause them to fail under normal conditions of thermal cycling.
h. Ultraviolet exposure. Resistance to ultraviolet (UV) radiation induced degradation is an important aspect of coating performance. All thermal sprayed metallic coatings are essentially unaffected by UV radiation. Organic sealers and topcoats used over thermal spray coatings will be affected the same as any other paint material of the same type. Organic paint coatings are affected by UV radiation to varying degrees. Depending on the coating resin and pigmentation types, UV degradation may result in loss of gloss, color fading, film embrittlement, and chalking. Certain paints, including silicone and aliphatic polyurethane coatings, exhibit superior UV resistance. Some coatings, including most epoxies and alkyds, have fairly poor UV resistance.
i. Impact and abrasion. Impact and abrasion are significant environmental stresses for any coating system. Abrasion is primarily a wear-induced failure caused by contact of a solid material with the coating. Examples include foot and vehicular traffic on floor coatings, ropes attached to mooring bitts, sand suspended in water, and floating ice. When objects of significant mass and velocity move in a direction normal to the surface as opposed to parallel, as in the case of abrasion, the stress is considered impact. Abrasion damage occurs over a period of time while impact damage is typically immediate and discrete. Many coating properties are important to the resistance of impact and abrasion including good adhesion, toughness, flexibility, and hardness. Thermal spray coatings of zinc, aluminum, and their alloys are very impact resistant. Zinc metallizing has only fair abrasion resistance in immersion applications because the coating forms a weakly adherent layer of zinc oxide. This layer is readily abraded, which exposes more zinc, which in turn oxidizes and is abraded. Thermal spray coating system 6-Z-A described in CEGS-09971 is considered to be the most impact/abrasion resistant of all of the Corps' coating systems. Application of this system to tainter gates in very harsh environments has been shown to be highly effective. The vinyl paint systems described in CEGS-09965 are particularly resistant to impact damage caused by ice and floating debris, but are less resistant than metallizing. The epoxy systems are somewhat brittle and are not nearly as resistant to impact damage as are the vinyls.
j. Cavitation/Erosion. More severe than impact and abrasion environments are exposures involving cavitation and erosion. Cavitation results when very-high-pressure air bubbles implode or collapse on a surface. The pressures involved can be very high (413,000 to 1,960,000 kPa (60,000 to 285,000 psi)) and destructive. Metallic components of hydraulic equipment such as hydroelectric turbines, valves and fittings, flow meters, hydrofoils, pumps, and ship propellers are particularly susceptible to cavitation damage. Low, medium, and high severity of cavitation havebeen defined based on an 8000-hr operating year. Low cavitation is defined as loss of carbon steel in the range 1.6 to 3.2 mm (1/16 to 1/8 in.) over a 2-year period. Medium cavitation is loss of austenitic stainless steel at greater than 1.6 mm (1/16 in.) per year. High cavitation is loss of austenitic stainless steel greater greater than 3.2 mm (1/8 in.) in a 6-month period. The standard method of repairing cavitation damage is to remove corrosion products by gouging with an electric arc and then grinding the damaged area. The cleaned area is then filled with weld metal and redimensioned by grinding. This method is very time consuming and expensive. Very hard and dense thermal spray deposits applied by HVOF spray have been used on an experimental basis as cavitation resistant materials and in conjunction with weld overlays as a repair technique.
k. Special exposures. Special exposures may include the coating of surfaces governed by the Food and Drug Administration (FDA) and National Sanitation Foundation (NSF) for food and potable water contact, respectively. Guide specifications CEGS-09965 and CEGS-09971 do not address either of these applications. Another special exposure is the use of coatings to prevent macrofouling caused by either marine fouling organisms or zebra mussels. Many of the coatings used to control fouling contain a toxin, which must be registered with the Environmental Protection Agency under the requirements of the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA). Corps guidance documents do not address the use of coatings to control fouling organisms, however thermal spray systems containing zinc and/or copper are known to be effective zebra mussel deterrents. Thermal spray coating systems 6-Z-A and 3-Z described in CEGS-09971 are recommended control coatings for zebra mussels on immersed steel and concrete surfaces. Neither material requires FIFRA registration.
== Surface Preparation ==
Thermal spray coatings require a very clean surface that is free of oil, grease, dirt, and soluble salts. Surface contaminants must be cleaned with solvents prior to removal of mill scale, corrosion products, and old paint by abrasive blasting.
a. Surface preparation is the single most important factor in determining the success of the corrosion protective thermal spray coating system. Abrasive blasting or abrasive blasting combined with other surface preparation techniques is used to create the necessary degree of surface cleanliness and roughness.
b. The principal objective of surface preparation is to achieve proper adhesion of the thermal spray coating to the steel substrate. Adhesion is the key to the success of the thermal spray coating.
c. The purpose of surface preparation is to roughen the surface, creating increased surface area for mechanical bonding of the thermal spray coating to the steel substrate. The roughening is typically referred to as the anchor pattern or profile. The profile is a pattern of peaks and valleys that is etched onto the steel when high-velocity abrasive blast particles impinge upon the surface.
d. Surface cleanliness is essential for proper adhesion of the thermal spray coating to the substrate. Thermal spray coatings applied over rust, dirt, grease, or oil will have poor adhesion. Premature failure of the thermal spray coating may result from application to contaminated substrates.
== Abrasive Blast Cleaning ==
Abrasive blasting is performed in preparation for thermal spray after the removal of surface contaminants by solvent cleaning. Abrasive blasting is conducted to remove mill scale, rust, and old coatings, as well as to provide the surface profile necessary for good adhesion of the thermal spray coating to the substrate. Conventional abrasive blast cleaning is accomplished through the high-velocity (724 km/h (450 mph)) propulsion of a blast media in a stream of compressed air (620 to 698 kPa (90 to 100 psi)) against the substrate. The particles' mass and high velocity combine to produce kinetic energy sufficient to remove rust, mill scale, and old coatings from the substrate while simultaneously producing a roughened surface. The Society for Protective Coatings (SSPC) and the National Association of Corrosion Engineers (NACE) have published standards for surface cleanliness. These standards and an SSPC supplemental pictorial guide provide guidelines for various degrees of surface cleanliness. Only the highest degree of cleanliness, SSPC-SP-5 "White Metal Blast Cleaning" or NACE #1, is considered acceptable for thermal spray coatings. Paragraph 6-7 discusses these standards in greater detail. Abrasive blast cleaning may be broadly categorized into centrifugal blast cleaning and air abrasive blast cleaning. Air abrasive blast cleaning may be further subdivided to include open nozzle, water blast with abrasive injection, open nozzle with a water collar, automated blast cleaning, and vacuum blast cleaning. Open nozzle blasting is the method most applicable to preparation for thermal spray coating.