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23 4 SURFACE PREPARATION 4.1 Introduction TSMCs require a very clean, rough surface that is free of oil, grease, dirt, scale, and soluble salts. Surface contaminants must be removed with solvents prior to removal of mill scale, corrosion products, and old paint by abrasive blasting. 4.1.1 Role of Surface Preparation Surface preparation is the single most important factor in determining the success of a corrosion-protective TSMC system. Abrasive blasting or abrasive blasting combined with other surface preparation techniques is used to create the necessary degree of surface cleanliness and roughness. 4.1.2 Objective of Surface Preparation The principal objective of surface preparation is to provide proper adhesion of the TSMC to the substrate being coated. Adhesion is the key to the success of the TSMC. 4.1.3 Purpose of Surface Preparation The purpose of surface preparation is to roughen the surface, creating angular asperities and increased surface area for mechanical bonding of the TSMC 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 are cut into the substrate surface when high-velocity abrasive grit blast particles impact on the surface. 4.1.4 Surface Cleanliness Surface cleanliness is essential for proper adhesion of the TSMC to the substrate. TSMCs applied over rust, dirt, grease, or oil will have poor adhesion. Premature failure of the TSMC may result from coating application to contaminated substrates. 4.2 Solvent Cleaning (SSPC-SP-1) Solvent cleaning (SSPC-SP-1) is a procedure for removing surface contaminants, including oil, grease, dirt, drawing and cutting compounds, and soluble salts, from steel surfaces by means of solvents, water, detergents, emulsifying agents, and steam. Solvent cleaning is not designed to remove mill scale, rust, or old coatings and precedes the use of abrasive blast cleaning. Ineffective use of the solvent cleaning technique may spread or incompletely remove surface contaminants. Three common methods of solvent cleaning are water washing, steam cleaning, and cleaning with hydrocarbon (organic) solvents.

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24 4.2.1 Hydrocarbon Solvent Cleaning Hydrocarbon solvents used to remove grease and oil are typically petroleum-based distillates, as described by ASTM D235, "Standard Specification for Mineral Spirits (Petroleum Spirits) (Hydrocarbon Dry Cleaning Solvent)." Type I--Regular (Stoddard Solvent) with a minimum flash point of 100F (38C) can be used when ambient temperatures are below 95F (35C). Type II--High Flash Point mineral spirits with a minimum flash point of 140F (60C) should be used when ambient temperatures exceed 95F (35C). Aromatic solvents such as xylene and high flash aromatic naphtha 100 or 150 (ASTM D 3734 Types I and II) are sometimes used when a stronger solvent is needed. The use of aromatic hydrocarbons (e.g., benzene) should be limited because of their generally greater toxicity. Solvent cleaning using hydrocarbon solvents is typically accomplished by wiping the surface with solvent- soaked rags. Rags should be changed frequently to afford better removal and to prevent spreading and depositing a thin layer of grease or oil on the surface. 4.2.2 Water Washing Low-pressure water cleaning, up to 5,000 psi (34 MPa), and high-pressure water cleaning from 5,000 to 10,000 psi (34 MPa to 70 MPa) are effective means of removing dirt and soluble salt contamination. When used with a detergent or emulsifying agent, the method can also be used to remove organic contaminants such as grease and oil. Thorough rinsing with clean water is necessary to ensure complete removal of the cleaning agent. If an alkaline cleaner is used, the pH of the cleaned surface should be checked after the final rinse to ensure that the cleaning agent has been completely removed. 4.2.3 Steam Cleaning Steam cleaning is an effective means of removing dirt, salt, oil, and grease from both coated and uncoated substrates. The method employs a combination of detergent action and high- pressure heated water (280F [138C] to 300F [149C] at 3 to 5 gpm [11.3 to 18.9 l/min]). Thorough rinsing with steam or water should be used to remove any deposited detergent. 4.3 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 roughness profile necessary to ensure good adhesion of the thermally sprayed coating to the substrate. Conventional abrasive blast cleaning is accomplished through the high-velocity (450 mph [724 km/h]) propulsion of a blasting media in a stream of compressed air (90 to 100 psi [620 to 698 kPa]) against the substrate. The particle 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 NACE International 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

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25 cleanliness, SSPC-SP-5 "White Metal Blast Cleaning," or NACE #1, is considered acceptable for TSMCs. SSPC and NACE have developed blast-cleaning standards and specifications for steel surfaces. SSPC-SP-5 and NACE #1 describe the condition of the blast-cleaned surface when viewed without magnification as free of all visible oil, grease, dust, dirt, mill scale, rust, coating, oxides, corrosion products, and other foreign matter. SSPC-SP-VIS 1-89 supplements the written blast standards with a series of photographs depicting the appearance of four grades of blast cleaning over four initial grades of mill scale and rust. The last two pages of the standard depict a white metal blast-cleaned substrate achieved using three different types of metallic abrasives and three types of nonmetallic abrasives. The resulting surfaces have slight color and hue differences caused by the type of media used. 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 (suction) blast cleaning. Open nozzle blasting is the method most applicable to preparation for TSMC. 4.3.1 Equipment An open nozzle abrasive blast-cleaning apparatus consists of an air compressor, air hose, moisture and oil separators/air coolers and dryers, blast pot, blast hose, nozzle, and associated safety equipment. 4.3.1.1 Air compressor. The air compressor supplies air to the system to carry the abrasive. Production rate depends on the volume of air that the compressor can deliver. A larger compressor can supply more air and can therefore sustain operation of more blast nozzles or larger blast nozzle diameters. 4.3.1.2 Air hose. The air hose supplies air from the compressor to the blast pot. The air hose should be as short as possible, with as few couplings and as large a diameter as possible to optimize efficiency. The minimum inside diameter (ID) should be 1.25 in. (31.75 mm) with measurements of 2 to 4 in. (50.8 to 101.6 mm) ID being common. 4.3.1.3 Moisture and oil separators/air coolers and dryers. If not removed, moisture from the air and oil mists from the compressor lubricants may contaminate the abrasive in the blast pot and, subsequently, the surface being cleaned. Oil/moisture separators are used to alleviate this problem. The devices should be placed at the end of the air hose as close to the blast pot as possible. Separators are typically of the cyclone type with expansion air chambers and micron air filters. Air coolers/dryers are commonly used to treat the air produced by the compressor. 4.3.1.4 Blast pot. Most blast pots used for large blasting projects are of the gravity-flow type. These machines maintain equal pressure on top of and beneath the abrasive. The typical blast pot consists of air inlet and outlet regulator valves, a filling head, a metering valve for regulating the abrasive flow, and a hand hole for removing foreign objects from the pot chamber. For large jobs, the pot should hold enough media to blast for 30 to 40 minutes. For continuous production, a two-pot unit can be used, allowing one pot to be filled while the other operates.

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26 4.3.1.5 Blast hose. The blast hose carries the air-media mixture from the blast pot to the nozzle. A rugged multi-ply hose with a minimum 1.25-in. (31.75-mm) ID is common. A lighter, more flexible length of hose called a whip is sometimes used for added mobility at the nozzle end of the blast hose. Maximum blast efficiency is attained with the shortest, straightest blast hoses. Blast hoses should be coupled with external quick-connect couplings. 4.3.1.6 Blast nozzle. Blast nozzles are characterized by their diameter, material, length, and shape. Nozzle sizes are designated by the inside diameter of the orifice and are measured in sixteenths of an inch. A 3/16-in. diameter orifice is designated as a No. 3 nozzle. The nozzle diameter must be properly sized to match the volume of air available. Too large an orifice will cause pressure to drop and productivity to decrease. Too small an orifice will not fully utilize the available air volume. The nozzle size should be as large as possible while still maintaining an air pressure of 90 to 100 psi (620 to 689 kPa) at the nozzle. Blast nozzles may be lined with a variety of different materials distinguished by their relative hardness and resistance to wear. Ceramic- and cast ironlined nozzles have the shortest life. Tungsten and boron carbide are long-lived nozzles. Nozzles may be either straight bore or of the venturi type. The venturi nozzle is tapered in the middle, resulting in much higher particle velocities. Venturi nozzles have production rates 30 to 50 percent higher than straight bore nozzles. Long nozzles, 5 to 8 in. (127 to 203 mm), will more readily remove tightly adherent rust and mill scale and increase production rates. Worn nozzles can greatly decrease productivity and should be replaced as soon as they increase by one size (1/16 in. [1.6 mm]). 4.3.2 Blast-Cleaning Techniques A proper blasting technique is important in order to accomplish the work efficiently with a high degree of quality. The blast operator must maintain the optimal standoff distance, nozzle angle, and abrasive flow rate. The best combination of these parameters is determined by an experienced blaster on a job-to-job basis. 4.3.2.1 Balance of abrasive and air flows. The blaster should balance the abrasive and air flows to produce a "bluish" colored abrasive airstream at the nozzle, which signals the optimum mix. Blasters often use too much abrasive in the mix, which results in reduced efficiency. The mix is adjusted using the valve at the base of the blast pot. 4.3.2.2 Nozzle-to-surface angle. The nozzle-to-surface angle should be varied to achieve the optimal blast performance for the given conditions. Rust and mill scale are best removed by maintaining a nozzle-to-surface angle of 80 to 90 degrees. A slight downward angle will direct dust away from the operator and improve visibility. The best nozzle-to-surface angles for removing old paint are 45 to 70 degrees. The final blast profile should always be achieved with a nozzle-to-surface angle of 80 to 90 degrees. Inside corners--for example, the inside flange surface of a narrow H-pile--require special attention to achieve angles as close to the optimum angles as possible. 4.3.2.3 Standoff. Standoff, or nozzle-to-surface distance, will also affect the quality and speed of blast cleaning. The lower the standoff distance, the smaller the blast pattern will be, and the longer it will take to cover a given area. However, close standoff distances allow more

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27 kinetic energy to be imparted to the surface, allowing for the removal of more tenacious deposits, such as mill scale. A standoff distance of as little as 6 in. (153 mm) may be necessary for the removal of tight mill scale and heavy rust deposits. Greater standoff distances, on the order of 18 in. (457 mm), are more efficient for the removal of old, loosely adherent coatings. 4.3.3 Abrasive Media The selection of the proper abrasive blast media type and size is critical to the performance of the TSMC. Blast media that produce very dense and angular blast profiles of the appropriate depth must be used. 4.3.3.1 Mix. Steel shot and slag abrasives composed of all rounded or mixed angular, irregular, and rounded particles should never be used to profile steel surfaces for thermal spraying. Mixed abrasives and lower-cost abrasives (e.g., mineral slag and garnet) may be used to initially clean the surface, but the final profile must be obtained with completely angular abrasives. 4.3.3.2 Type/size. New steel grit should conform to the requirements of SSPC-AB-3, "Newly Manufactured or Re-Manufactured Steel Abrasives." Various hardnesses of steel grit are available, but generally grit with Rockwell C hardness in the range of 50 to 60 is used. Harder steel grit (Rockwell C 60 to 66) may also be used, provided that the proper surface profile is obtained. Table 3 shows the recommended blast media types as a function of the thermal spray process and coating material. 4.3.3.3 Angularity. An angular blast media must always be used. Rounded media such as steel shot, or mixtures of round and angular media, will not produce the appropriate degree of angularity and roughness in the blast profile. The adhesion of TSMCs can vary by an order of magnitude as a function of surface roughness profile shape and depth. TSMCs adhere poorly to substrates prepared with rounded media and may fail in service by spontaneous delamination. Hard, dense, angular blast media such as aluminum oxide, silicon carbide, iron oxide, and angular steel grit are needed to achieve the depth and shape of blast profile necessary for good TSMC adhesion. Steel grit should be manufactured from crushed steel shot conforming to SAE J827. Steel grit media composed of irregularly shaped particles or mixtures of irregular and angular particles should never be used. Steel grit having a classification of "very angular," "angular," or "subangular," as classified by the American Geological Institute, should be used (also found in J. D. Hansink, "Maintenance Tips," Journal of Protective Coatings and Linings, Vol. 11, No. 3, March 1994, p. 66). TABLE 3 Recommended blast media for thermal spray surface preparation Thermal Spray Material Spray Process Blasting Media Aluminum, zinc, 85:15 zinc-aluminum Wire flame spray Aluminum oxide Angular steel grit Aluminum, zinc, 85:15 zinc-aluminum Arc spray Aluminum oxide Angular steel grit Angular iron oxide

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28 4.3.4 Blast Profile TSMCs are generally more highly stressed than paint coatings and as such require a deeper blast profile to dissipate the tensile forces within the coating. In general, the greater the thickness of TSMC being applied, the deeper the blast profile that is required. The minimum recommended blast profile for the thinnest coatings of zinc and 8515 wt% zinc/aluminum (0.004 to 0.006 in. [100 to 150 m]) is 0.002 in. (50 m). Thicker coatings of zinc and 8515 wt% zinc/aluminum, 0.010 in. (250 m) or greater, require a minimum 0.003-in. (75-m) profile. A 0.005-in. (125-m) aluminum coating requires a minimum surface profile of 0.002 in. (50 m), and a 0.010-in. (250-m) aluminum coating requires a minimum 0.0025-in. (62.5-m) profile. The specifier should specify the maximum and minimum surface profile required for the TSMC. The maximum profile for thicker TSMCs should not exceed approximately a third of the total average coating thickness. As a general rule, the maximum blast profile should be 0.001 in. (25 m) greater than the specified minimum profile depth. Table 4 shows average profiles for various abrasive sizes. 4.3.5 Centrifugal Blast Cleaning Centrifugal blast cleaning is commonly used in fabrication shops. The method is generally faster and more economical than open abrasive blasting. The method involves conveying the steel through a blast cabinet or enclosure where high-speed rotating wheels fitted with blades propel abrasive particles at the steel. The blasting debris falls to the bottom of the chamber, where it is reclaimed, cleaned, and then recycled. The degree of cleanliness achieved is determined by the abrasive velocity and the conveyor speed. Steel shot is usually used in centrifugal blast machines. For TSMCs, a subsequent profiling blast using angular media is required to achieve the desired blast profile depth and angularity. Centrifugal blast-cleaning machines are now available for fieldwork as well, but their use is not widespread. 4.3.6 Cleaning After Blasting Cleanliness after abrasive blasting is important. Any remaining traces of spent abrasive or other debris must be blown, swept, or vacuumed from the surface prior to thermal spray application. A hard-to-see layer of abrasive dust may adhere to the substrate by static electric charge and must be removed. The thermal spray applicator may accomplish this by triggering just the compressed air from the flame or arc gun. Scaffolding, staging, or support steel above the thermal spray coating area must also be cleaned prior to application to prevent debris from falling onto the surfaces to be coated. Blasting and thermal spraying should not occur simultaneously unless the two operations can be adequately isolated to prevent contamination of the thermal spray surfaces. TABLE 4 Average surface profiles for selected abrasive sizes Abrasive Size Profile, mils (m) Steel grit G40 2.4 0.5 (61 12.7) Steel grit G25 3.1 0.7 (79 17.8) Steel grit GL16 4.0 + (102) Steel grit G14 5.1 0.9 (130 22.8) Aluminum oxide 16 4.0 + (102)

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29 4.3.7 Time Between Blasting and Thermal Spraying After completion and inspection of the final profiling blast, the steel substrate should be coated as soon as possible. The TSMC should be applied within the same work shift in which the final surface preparation is completed. A maximum holding period of 6 hours should be allowed to elapse between the completion of blast cleaning and thermal spraying. Shorter holding periods should be used under humid or damp conditions or when it is clear that the quality of the blast or coating is degraded. This period should allow adequate time for the changeover from blasting to thermal spraying. Thermal spray should commence prior to the appearance of any visible rust bloom on the surface. Foreign matter such as paint overspray, dust and debris, and precipitation should not be allowed to contact the prepared surfaces prior to thermal spraying. Under no circumstances should the application of thermal spray be allowed on re-rusted or contaminated surfaces. In some cases, it may be possible to apply only a single spray pass or some other fraction of the total thermal spray system within 6 (maximum) hours of blasting. This single layer must cover the peaks of the surface profile. The partial coating is intended to temporarily preserve the surface preparation. Before applying additional sprayed metal to the specified thickness, the first layer of coating should be visually inspected to verify that the coating surface has not been contaminated. Any contamination between coats should be removed before any additional material is applied. The remaining coating should be sprayed to achieve the specified thickness as soon as possible. In some cases, it may be possible to hold the surface preparation for extended periods using specially designed dehumidification (DH) systems. These systems supply dry air to a blast enclosure or other contained air space. The dry air prevents the reappearance of rust for extended periods of time and allows for thermal spray jobs to be staged in a different fashion. Dehumidification systems may be particularly useful for jobs in very humid environments, which are typical of many locations during the spring through fall maintenance season. These areas typically have dense morning fog and hot humid afternoons. Holding the quality of blast needed for TSMCs would be difficult under such conditions without the use of dehumidification. 4.3.8 Pitted Steel Heavily corroded, deeply pitted surfaces are difficult to prepare for TSMC. Wide, shallow pits do not pose any particular problem, but deep and irregularly shaped pits can pose a problem. Pits with an aspect ratio of greater than unity (i.e., as deep as they are wide) should be ground with an abrasive disk or other tool prior to blasting. Pits with sharp edges, undercut pits, and pits with an irregular horizontal or vertical orientation must be ground smooth prior to abrasive blasting. Grinding does not need to level or blend the pit with the surrounding steel, but it should smooth all the rough and irregular surfaces to the extent necessary to allow the entire surface of the pit to be blasted and coated. Nozzle-to-surface angles of 80 to 90 degrees are optimal for cleaning pits. Heavily pitted steel on bridges or in other environments where soluble salt contamination is likely should be cleaned with high- pressure water after grinding to ensure that salt contaminants are removed from the pits.

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30 4.3.9 Edges and Welds 4.3.9.1 Sharp edges. These present problems in achieving adequate surface preparation and coating. As a general rule, all sharp edges should be ground prior to blasting to a uniform minimum radius of 1/8 in. (3 mm). A radius of 1/4 in. (6 mm) is preferred. 4.3.9.2 Flame-cut edges. Flame cutting results in localized hardening of the steel on the cut edge. This will degrade the ability of abrasive blasting to provide an adequate surface profile in these areas because the hardened steel can be harder than the abrasive. The result will be poor coating adhesion on the hardened edge. The hardened edge must be removed either with a grinder or belt-driven abrasive, followed by abrasive blasting. Abrasive that is harder than the flame-hardened edge, such as alumina, can also be used. 4.3.9.3 Welded areas. Rough welds shall be ground to remove sharp edges, undercuts, pinholes, and other irregularities. Remove weld spatter. Welds can also result in locally hardened areas on the steel in the heat-affected zone, on which it could be difficult to achieve an adequate surface profile compared with the unaffected steel surface. Particular care should be taken in these areas to ensure that adequate surface profile is achieved. Surface profile testing should be conducted in weld heat-affected zones to develop the correct blasting procedure for that piece. 4.4 Water Jetting High-pressure water jetting from 10,000 to 25,000 psi (70 MPa to 170 MPa) and ultra-high- pressure jetting above 25,000 psi (170 MPa) are used to prepare a surface for recoating. These methods will not produce a surface profile on the metal that is sufficient for the adhesion of TSMCs unless that profile already exists on the metal surface from prior abrasive blasting. Water jetting will also not remove mill scale. The flash rusting that can occur on a water-jetted surface can interfere with the adhesion of TSMCs. However, water jetting can be used to remove existing coatings as a preliminary step in preparing the surface prior to abrasive blasting. The use of high-pressure water jetting can result in savings in abrasive volume and reduced costs in disposal of wastes. More information is available in the joint NACE International/SSPC Standard, NACE #5/SSPC-SP-12, "Surface Preparation and Cleaning of Steel and Other Hard Materials by High- and Ultrahigh-Pressure Water Jetting Prior to Recoating." 4.5 Surface Contamination Surface contamination from chlorides (deicing salts and sea salts) prior to applying the TSMC can lead to loss of coating adhesion, particularly with aluminum TSMC. Surface contamination can be removed by detergent washing, power washing, water jetting, or wet abrasive blasting.