Metal Painting and Coating Operations
Table of Contents Background
Regulatory Overview Planning P2 Programs
Overview of P2 Surface Preparation
This chapter covers a variety of surface preparation methods along with technology-specific suggestions for optimizing processes in order to reduce waste. Detailed descriptions of techniques for optimizing traditional cleaning methods and alternative cleaning methods, eliminating pollutants from conversion coatings, and modifying or replacing traditional stripping operations are provided below. For an overview of alternative surface preparation technologies, refer to table 8.
Methods for surface preparation vary depending on the material to be painted, the paint to be used and the desired properties of the resulting finish (IHWRIC, p. 33). Many products require a preparation step prior to painting. This step is commonly called pretreatment for new products, or paint stripping for products that need to be reworked (Ohio EPA, p. 1). Pretreatment of a metal surface can include chemical-assisted cleaning, mechanical cleaning, and chemical or abrasive blasting, application of conversion coatings or stripping methods.
Halogenated solvents have traditionally been used as cleaning and stripping agents. Conventional surface preparation generally involves applying some form of a solvent. However, environmental problems with air emissions often arise from solvent use. In addition, after surface preparation, a waste stream composed of the solvent combined with oil, debris and other contaminants is left for disposal (EPAi, p. 1). Fortunately, a number of alternative methods are now widely available.
These are discussed in the cleaning section of this chapter. Surface preparation can consist of a variety of processes including several cleaning steps, conversion coatings, and a stripping operation.
Surface preparation can generate a number of wastes, including spent abrasives, solvents and/or aqueous cleaning baths, and surface treatment baths; air emissions from abrasives and solvents; rinsewaters following aqueous processing steps; and solvent-soaked rags used for wiping parts before painting. Depending on the complexity of the operation and the nature of the chemicals used, the volume and toxicity of wastes generated can vary widely (Freeman, p. 484-485).
Removing old paints that contain lead, for example, can be particularly problematic, as abrasive stripping of these paints generates a fine lead dust that is highly toxic to workers. The use of sand and other silica-containing materials in stripping processes also has been associated with lung disease in workers (IHWRIC, p. 48).
Usually, the first step in the surface preparation process is to mechanically remove rust or debris from the substrate. Wiping loose dust and dirt off the part is an example of mechanical cleaning. Typically, though, more aggressive mechanical action is needed to remove rust or other contaminates. Rust and metal scale can be removed mechanically by sanding, brushing with a wire brush or plastic "wool" pads, or by using abrasive blasting techniques (KSBEAP, p. 1-2). Abrasive blasting can also be used for removing old paint from products; solvent-based chemical stripping is another option. Environmental concerns and rising chemical prices have pushed more companies into using mechanical cleaning to accomplish a larger portion of the cleaning process (KSBEAP, p. 1).
Traditionally, solvents have been used for removing contaminates such as oils and greases. Companies use various solvent-based methods to clean a workpiece. For example, metal parts can be immersed in a solvent tank (i.e., cold cleaning). Solvents also can be wiped or sprayed onto the parts, or solvent vapor degreasing units can be used. There are environmental problems associated with all of these cleaning methods. Dip tanks get dirty as they are used. Spraying can be wasteful if too much solvent is used. Wiping is labor intensive. Vapor degreasers are regulated under the Clean Air Act and OSHA and pose health hazards. Often a combination of techniques can be used to reduce solvent use and still obtain a properly cleaned workpiece. For example, a dip tank can be used followed by wiping or confined spraying. The key to solvent cleaning is to have the part as clean as possible before it enters the solvent cleaning process (KSBEAP, p.2). Optimizing solvent cleaning systems and alternatives to solvent cleaning are discussed in greater detail in the cleaning section of this chapter.
A conversion coating may be applied to the workpiece prior to painting to improve adhesion, corrosion resistance, and thermal capability. Conversion coatings chemically react with the metal surface to create a physical surface that allows for better paint adhesion. In addition, conversion coatings act as a buffer between the coating and the substrate, reducing the effects of sudden temperature changes. Phosphate and aluminum conversion coatings are usually confined to large operations with elaborate waste treatment facilities because of the extensive regulations controlling the disposal of rinse waters and sludges containing heavy metals. For more information on conversion coatings, refer to the section on conversion coatings in this chapter.
When a part needs repainting, the old paint usually must be removed before a new coating can be applied. The first thing a technical assistance provider should do is determine why the piece needs to be reworked. Reducing reject rates can greatly reduce the amount of waste generated from these processes. Once the need for rework has been reduced, alternative stripping methods can be examined.
This section covers general methods to improve the efficiency of the surface preparation process and to reduce the pollution generated during the surface preparation processes. Detailed information on alternative technologies/processes is discussed.
A cost-effective method for reducing these wastes is to minimize the need for surface preparation by (1) improving current operating practices and (2) setting standards for cleaning and stripping. If the need for surface preparation cannot be reduced by these methods, alternative technologies must be assessed (MnTAP, p. 1). Maximizing the cleaning capacity of current methods also can help reduce wastes (KSBEAP, p. 2). Each of these options is discussed below.
To reduce the need for cleaning, technical assistance providers can help companies examine the sources of workpiece contamination. Technical assistance providers should determine how contaminants such as lubricants from machining, dirt from the manufacturing environment, and finger oil from handling by shop personnel are contaminating the workpieces. Once the contamination sources are identified, technical assistance providers can help determine whether some or all contamination sources can be eliminated by improving current operating practices. For example, proper storage of materials and just-in-time delivery of parts can keep contaminants from becoming a problem (KSBEAP, p. 1). To eliminate finger oil contamination, gloves can be used in areas of parts handling; gloves can be made of lint-free material, or lint can be removed with a dry cloth (OH EPAe, p. 1).
In the case of paint stripping, technical assistance providers can help firms examine what causes the need for paint stripping. Possibilities include: inadequate initial part preparation, defects in coating application, improper time/temperature cycle for the curing oven, and equipment problems or coating damage due to improper handling. While no process is perfect, reducing the need for repainting can greatly reduce the volume of waste generated from paint removal (MnTAP, p. 1-2).
Next, companies should determine the cleanliness level or cleanliness standard that is needed. Cleaning requirements are generally based on two factors: process specifications and customer requirements. A system to measure cleanliness should be used to prevent over-cleaning and ensure efficient use of cleaning agents (MnTAPe).1
1 For more information on setting cleanliness standards, see Is it Clean? Testing for Cleanliness of Metal Surfaces by Anselm Kuhn in the September 1993 issue of Metal Finishing.
In the case of abrasive stripping, standards should be set to avoid blasting a surface longer than necessary, creating excess waste and reducing productivity. Measuring devices can be used to define the level of surface scratching or "profile" desired. Most standards use Structural Steel Painting Council (SSPC) classifications for surface cleanliness. There are two types of standards available: visual disk and photographic. A surface profiler instrument also can be used (Freeman, p. 490-491).
Pollution prevention approaches tends to favor mechanical or aqueous cleaning methods, but solvent vapor degreasing can be more economical and suitable for certain types of parts (e.g., parts that slide into each other to form a close fit, preventing some surfaces from being exposed) (MnTAP, p. 2). Advanced technologies have made both of these processes more effective and less harmful to the environment (Freeman, p. 469). More information on this topic is found in the cleaning section of this chapter.
2 For more information on extending the life of aqueous cleaning solutions, see Extending the Life of Aqueous Cleaning Solutions a fact sheet developed by the Office of Pollution Prevention, Ohio Environmental Protection Agency.
The following practices should be implemented where possible to maximize the cleaning capacity of aqueous or solvent cleaners:
The following sections provide more detail on specific surface preparation processes including solvent vapor degreasing, aqueous cleaning, alternative solvents, phosphatizing, anodizing, stripping, and abrasive blasting.
This section provides information on a variety of conventional and alternative P2 technologies typically used for cleaning and degreasing metal parts prior to coating.
The conventional method used for cleaning most metal parts is vapor degreasing using a variety of halogenated solvents. In vapor degreasing, parts are usually suspended over a solvent tank. The solvents are then heated to their boiling point, which creates a vapor that condenses on the parts and dissolves contaminants. The condensate drips back into the tank along with the contaminants. However, because the contaminants usually have higher boiling points than the solvent, the vapor itself remains relatively pure. The cleaning process is complete when the parts reach the temperature of the vapor, and no more condensate is generated (EPAh, p. 2).
Advantages and Disadvantages
Unlike other cleaning processes involving water, solvent vapor degreasing does not require down stream drying because the solvent vaporizes from the parts over time. However, solvents such as TCE vaporize resulting in significant VOC emissions and solvent losses (Freeman, p. 468). Other common solvents are either toxic, HAPs, and/or ozone depleters. In fact, conventional vapor degreasing units commonly lose 60% of their solvents through evaporation (SHWEC, p. 1).
Solvent Vapor Degreasing Processes
Conventional vapor degreasing units or open-top vapor cleaners (OTVC) use an open tank where a layer of solvent vapor is maintained. Air emissions from an OTVC occur during startup, shutdown, working, idling, and downtime. However, movement of the work load in and out of the vapor degreaser is the main cause of air emissions (EPAi, p. 7). During startup, losses occur as the solvent in the sump is heated and a vapor layer is established in the open tank. Shutdown losses occur when the unit is switched off and this vapor layer subsides. Downtime losses occur due to normal evaporation of the solvent when the OTVC is not in use. Idling losses occur by diffusion from the vapor layer in the period between loads. Completely enclosed vapor cleaners (CEVC) are available, although use is generally confined to Europe (Freeman, p. 468-474).
A number of equipment-related and operational changes can reduce solvent emissions from traditional OTVCs by as much as 50%. Many of these practices are required under the MACT standard since solvent degreasers are regulated under the NESHAP. These include:
This method of cleaning uses traditional solvents in their liquid form rather than their vapor form to clean the workpiece. This is a common practice in painting operations. Typically, solvents such as methyl isobutyl ketone (MIBK), methyl ethyl ketone (MEK), or 1,1,1 trichloroethane are used. The primary advantage of this method is its versatility. Liquid solvents can be used to clean an entire part by spraying or immersing the part in the solvent, or by wiping with a rag. Typically, this process is used to clean small workpieces rather than parts that are large or have complex geometries.
Like vapor degreasing, capital costs for cold-solvent degreasing generally are low, and the system requires minimal equipment, floor space, and training. Also, spent solvent can be distilled and recycled onsite. In states where the solvent is regulated as hazardous material, however, most facilities send exhausted cleaning solution offsite to commercial recycling operations. Assistance providers should be aware that special safety equipment is required by OSHA for distillation systems.
As with vapor degreasing, the principal limitation of cold solvent cleaning is that emissions from the solvents can be damaging to the environment, and may pose a threat to human health. Other limitations include:
Best Management Practices for Cold Cleaning
Best management practices for enhancing efficiency in the cold cleaning process include the following:
For many facilities, the most effective way to reduce waste from cleaning operations is to invest in a new cleaning method. The following section provides information on alternatives to solvent degreasing.
Aqueous cleaning involves the use of solutions which are largely made up of water, detergents, and acidic or alkaline chemicals rather than solvents. Typically, aqueous cleaning solutions contain at least 95% water. Solutions that include larger percentages of other compounds, including terpenes and other solvents, typically are called semiaqueous (Freeman, p. 707).
Both aqueous cleaning and semiaqueous cleaning are usually more environmentally friendly than traditional solvent cleaning and adapt to a wide variety of cleaning needs. Aqueous cleaning is usually used after mechanical cleaning. A spray, dip, or a combination of both is typically used, depending on the workpiece. The particular solution selected depends on both the type of contaminant and the type of process equipment used (EPAh, p. 13). Elevating the temperature of the solution can make it more effective in removing greases and oils, which have increased mobility at higher temperatures (KSBEAP, p. 2). However, solutions that have too high a temperature may set some soils and make them more difficult to remove.
Advantages and Disadvantages
Aqueous cleaning can be used on a wide range of substrates and is less toxic than solvent processes. Some disadvantages include a high rate of water consumption and hazardous wastewater discharge (Freeman, p. 707). In addition, some acids used in aqueous cleaning can cause hydrogen embrittlement, reducing the strength of metal substrates (KSBEAP, p. 2). Ferrous parts need to be dried rapidly to avoid rusting.
Aqueous Cleaning Processes
The conventional aqueous cleaning processes are vibratory deburring and hand-aqueous washing, although automated and power washing processes are available.
In vibratory deburring, soiled parts are placed in an open vessel with an aqueous cleaning solution. The vessel is then rotated, which tumbles the parts. The cleaning solution is removed and clean tap water is added to rinse the parts (Freeman, p. 470-472). In this method, the part is simultaneously cleaned and deburred.
In hand-aqueous washing, parts are dipped by hand into a series of tanks containing surfactant solutions and rinsewater. A continuous clean water flow must be maintained in the final rinse tanks, but the surfactant and other rinse tanks (also known as drag-out tanks) can be used for an entire day without changing the solutions (Freeman, p. 470-472).
The most common automated aqueous washer used in coating operations is a 3- to 7-stage spray washer which uses an overhead conveyor and racks to move the parts. Belted conveyor spray washers are also common, as are multistage agitated immersion washers of various types. Centrifugal washers can be part of an automated aqueous system, but they are uncommon in coating pretreatment systems (Callahan, 1997).
A number of other processes used as part of an aqueous cleaning system can enhance cleaning effectiveness. These include high-pressure sprays, mechanical agitation, and ultrasonic methods. In fact, in the manufacturing environment, many aqueous cleaning systems are multistaged and include several different processes (Levitan et al., p. 54).
Ultrasonic cleaning uses high-frequency sound waves to improve the efficiency of aqueous and semiaqueous cleaners. By generating zones of high and low pressures in the liquid, the sound waves create microscopic vacuum bubbles that implode when the sound waves move and the zone changes from negative to positive pressure. This process, called cavitation, exerts enormous localized pressures (approximately 10,000 psi) and temperatures (approximately 20,000°F on a microscopic scale) that loosen contaminants and actually scrub the workpiece (Freeman, p. 472). A typical ultrasonic system moves the pieces through three stages: an ultrasonic cleaning tank containing a water-based detergent; two rinse tanks; and a drying stage (Levitan et al., p. 57). Ultrasonic cleaning can be used on ceramics, aluminum, plastic, and glass, as well as electronic parts, wire, cables, rods, and detailed items that might be difficult to clean by other processes (Freeman, p. 472).
The methods described below are not widely used to clean metal parts. However, they can be used as substitutes for conventional solvent vapor degreasing.
Vacuum De-oiling. This method uses a vacuum furnace and heat to vaporize oils from parts. Vacuum furnace de-oiling can be applied where vapor degreasing typically is used to clean metal parts. It also can remove oil from nonmetallic parts. Although capital costs for vacuum de-oiling are high, the operating costs are low. Unlike other clean technologies, vacuum de-oiling does not leave the cleaned parts water soaked, so they do not need to be dried. Because the time and temperature of the de-oiling process depends on the material to be cleaned and the oil to be removed, adjustments might be needed for each new material, oil, or combination. Also, the parts must be able to withstand the required temperature and vacuum pressure (Freeman, p. 478-479).
Laser Ablation. In this method, short pulses of high-peak-power laser radiation are used to rapidly heat and vaporize thin layers of material surfaces. Laser ablation can perform localized cleaning in small areas without affecting the entire part. Laser ablation does not use solvents or aqueous solutions and therefore generates little hazardous waste. The only waste generated is the small amount of material removed from the surface of the item being cleaned (Freeman, p. 479). Laser ablation has been used to strip paint from aircraft. At the other extreme, it has been used to remove sub-micron particles and thin fluid films from semiconductor components (SHWEC, p. 16).
Supercritical Fluid Cleaning. This process involves the application of fluids at temperatures and pressures above their critical point to remove contaminants from parts. CO2 is the most commonly used fluid in this process because it is widely available and considered to be nontoxic. Supercritical fluid cleaning is compatible with stainless steel, copper, silver, porous metals, and silica. It leaves no solvent residue after cleaning and has low operating costs. However, capital costs are high (e.g., $100,000 for small-capacity equipment) (Freeman, p. 708-709). Therefore, supercritical fluid cleaning has been used mainly in precision cleaning (EPAh, p. 27).
With the phase out of chlorofluorocarbon (CFC)-based cleaners, there has been an increased interest in investigating alternatives to these chemicals. Table 8 lists typical soils and alternative cleaning methods that are effective in reducing the use of chlorinated solvents.
Many alternatives to methylchlorofluorocarbons (MCF) and CFC-113 are available for use in cold cleaning and vapor degreasing applications such as wipe cleaning, dip cleaning, immersion soaking, pressure washing, and vapor degreasing. Some solvents are recommended only for specific applications while others are used for many applications. In general, the following properties are desirable when considering solvent alternatives: low surface tension to penetrate small spaces, high density to remove small particles, high volatility to provide rapid drying, non-VOC, good solvency to readily improve organic soils, low cost, low toxicity, nonflammable, little residue, and easy cleanup and disposal (NFESC).
Drop-in solvent replacements for traditional solvents such as MCF and CFC-113 usually are not possible. However, because vapor degreasing is effective in cleaning delicate parts, some facilities might want to consider maintaining the process with a substitute solvent. Some possible CFC-free alternatives include:
Some companies have begun using other HCFC solvents such as trichloroethylene, perchloroethylene, and methylene chloride. These solvents have been used often in vapor degreasing because of their similarity to CFC solvents in both physical properties and cleaning effectiveness. However, using these alternatives has significant disadvantages for the facility. All of the above three alternatives have been classified as Hazardous Air Pollutants (HAPs) by EPA and are targeted by the Emergency Planning and Community Right-to-Know Act as well. Furthermore, these spent solvents are classified as a hazardous waste. As a result, handling and disposal of these solvents is complicated and more expensive.
Once a part has been cleaned, it can receive a conversion coating prior to the painting process. The next section provides information on conversion coatings and techniques to reduce waste from these processes.
Chemical and electrochemical conversion treatments provide a coating on metal surfaces to prepare the surfaces for painting. These conversion treatments include anodizing and phosphating. Conversion coatings are usually confined to large operations with elaborate waste-treatment facilities because of extensive regulations controlling disposal of rinse water and sludges containing heavy metals.
Anodizing is a specialized electrolytic surface finish for aluminum that imparts hardness and corrosion resistance, increases paint adhesion, provides electrical insulation, imparts decorative characteristics, and aids in the detection of surface flaws on the aluminum. This process employs electrochemical means to develop a surface oxide film on the workpiece, enhancing corrosion resistance. Anodizing is a similar process to electroplating but it differs in two ways. First, the workpiece is the anode rather than the cathode as in electroplating. Second, rather than adding another layer of metal to the substrate, anodizing converts the surface of the metal to form an oxide that is integral to the substrate (SME, 1985).
Industry uses three principal types of anodizing: chromic-acid anodizing (called Type I anodizing), sulfuric-acid anodizing (called Type II anodizing), and hard-coat anodizing, which is a combination of sulfuric acids with an organic acid such as oxalic acids (called Type III anodizing). Because of the structure, the anodized surface can be dyed easily. These dyes include organic or organometallic dyes and often contain chrome in the trivalent state. Whether the pieces are dyed, they need to be sealed. Sealing can be performed with hot water, nickel acetate, or sodium dichromate, depending on the required properties (SME, 1985).
Various methods are used to treat wastes generated from anodizing bath solutions. Technologies that have been employed successfully include: evaporation systems operating under reduced pressure, sedimentation, reverse osmosis, filtration, and anion and cation exchangers.
Substituting Type I Chromic-Acid Anodizing with Type II Sulfuric-Acid Anodizing
Because of federal and state mandates imposed on operations using hexavalent chrome, researchers have investigated the feasibility of substituting Type I anodizing with Type II sulfuric-acid anodizing. A NASA study found that in applications where anodizing is used to impart corrosion protection on aluminum, Type II sulfuric-acid anodizing is superior to Type I chromic-acid anodizing (Danford, 1992).
According to suppliers, conversion from chromic-acid to sulfuric-acid anodizing is not a simple chemical substitution. The conversion requires a complete changeover of anodizing equipment and partial modifications to downstream waste- treatment facilities. Replacement of the anodizing tank often is required because of the differences in material compatibility between the tank (and tank liner) and sulfuric acid and chromic acid. Sulfuric-acid anodizing processes also have different voltage and amperage requirements, necessitating replacement of the rectifier. The operating temperature of the electrolytic bath also is different for the two processes. The chromic process is usually maintained by steam heat at an operating temperature of 90 to 100°F whereas the sulfuric acid process must be chilled using cooling water to an operating temperature of 45 to 70°F.
Operation and maintenance costs are typically much lower for sulfuric-acid anodizing than for chromic-acid anodizing because of lower energy requirements. Wastewater treatment costs are lower as well because sulfuric acid only requires removal of copper whereas chromic acid requires more complex chrome reduction techniques. The change in materials also means that the cost of sludge disposal is greatly reduced.
Sulfuric-Acid Anodize Regeneration with Ion Exchange
Traditionally, facilities use ion exchange to remove metallic contaminants from wastewater streams. However, ion exchange resins also remove the hydrogen and sulfate components of the sulfuric acid/aluminum anodizing solution. As the solution passes through the columns, the acid is removed. Then the waste stream, which consists of a small amount of acid plus all the aluminum from the anodizing solution, flows to the wastewater treatment system. To recover the acid, platers use water to flush the acid components from the resin. This forms a sulfuric acid solution that is low in dissolved aluminum and can be used again in the anodizing process (Ford, 1994).
Sulfuric-Acid Anodize Regeneration with Electrodialysis
Electrodialysis removes metal ions (cations) from solutions using a selective membrane, an electrical current, and electrodes. This technology uses a chemical mixture (catholyte) as a capture and transport media for metal ions. This catholyte forms a metal sludge and requires periodic change-outs. The recovered sludge is hazardous, and companies might want to work with an outside firm to recover the metal in the sludge. Using electrodialysis, facilities can remove all the metal impurities from the anodizing bath, maintaining the bath indefinitely. By keeping the concentration of contaminants in the process bath low, the rinsewater potentially can be recycled back to the bath, closing the loop on the process. The cost to operate this system depends on the size of the acid anodizing bath, the level of metal concentration, the metal removal capacity of the electrodialysis unit, and the company's ability to reclaim metals in the sludge.
Alodine is a nonelectrolytic process used to create a chrome oxide film similar to anodizing. It is widely used in military and aerospace applications.
Phosphating is used to treat various metals (mainly steel and iron) to impart corrosion resistance and to promote the adhesion of finishes such as paint and lacquers. Phosphating treatments provide a coating of insoluble metal-phosphate crystals that adhere strongly to the base metal. Generally, phosphating solutions are prepared from liquid concentrations containing one or more divalent metals, free phosphoric acid, and an accelerator (Ford, 1994).
The phosphating process consists of a series of application and rinse stages typically involving the application of either an iron, manganese, or zinc phosphate solution to a substrate. A simple iron phosphating system is composed of two stages: an iron phosphate bath that both cleans the part and applies the conversion coating followed by a rinse bath to remove dissolved salts from the treated surface. An advanced zinc phosphating line might feature seven stages of spray/dip and rinse baths. In addition, a final rinse in a low-concentrate acidic chromate or an organic nonchromate solution is often used to further enhance corrosion resistance and seal the coating. Following the conversion application, the parts are dried to prevent flash rusting (Ford, 1994).
Iron and zinc phosphate coatings often are used as paint bases, and manganese phosphate coatings are applied chiefly to ferrous parts for break-in and galling (e.g., to engine parts). The choice of iron or zinc phosphate coating depends on product specifications. In general, the more extensive multistage zinc phosphate processes provide better paint adhesion, corrosion protection, and rust protection than iron phosphate processes. Zinc phosphate baths, however, tend to be more expensive, require more maintenance, and often result in more sludge disposal (SME, 1985).
Iron or zinc phosphate coatings are usually used for steel. In the phosphating process, acid attacks the metal surface, forming a protective coating of iron or zinc phosphate salts. Zinc phosphate forms finer, denser crystals than iron phosphate and has better corrosion resistance and paint adhesion. Accelerators and oxidizers are added to the phosphating solution to improve its effectiveness. Molybdic acid, added for corrosion inhibition, gives a purple cast to iron phosphate coatings. A clean surface is critical to successful application of the phosphate coating (KSBEAP, p.3).
Process time, temperature, and chemical concentration affect the acid's reaction with the steel part. Process time is usually fixed because the line must run at a certain speed, however, temperature can have a great effect on the phosphating process. In order for the process to run at optimum efficiency, the temperature preceding the phosphating process should be higher than the temperature required for phosphating. This allows the part to become heated prior to entering the phosphating process. If the part is not heated prior to phosphating, process efficiency is reduced. For example, if deposition efficiency is reduced, additional chemicals may be required, and more sludge could be generated. Iron phosphating solutions typically operate between 120 and 140°F, but can also be operated at room temperature.
Cleaning and iron phosphating can be combined in a single solution, however, this is usually successful only when the parts are lightly soiled. It is not possible to use a combination process with zinc phosphating (KSBEAP, p. 3).
Iron and zinc phosphate coatings are used on aluminum parts or products. The choice of solution largely depends on the volume of aluminum in the process. When a company is processing a small amount of aluminum, the same phosphating solution is typically used for all metals that are processed. For instance, if a company processes mainly steel and a small volume of aluminum, iron phosphating will be the only process used.
Iron phosphating solutions can effectively clean the surface of aluminum and improve paint adhesion. However, they leave little or no coating on the substrate. In order to etch the aluminum, a fluoroborate or fluoride additive is required.
Companies often use chromium phosphate coating for small volumes of aluminum. Often, no-rinse chromium phosphate solutions are used because they have the advantage of not being classified as a hazardous waste. However, they typically provide less corrosion resistance due to incomplete coverage. Chromic acid sealers can be used but they contain hexavalent chromium (KSBEAP, p.3).
The most common problems associated with chemical pretreatment systems are poor adhesion and premature corrosion failure. Frequently these problems are caused by the following:
Residual soils: These soils may be caused by (1) conveyer line speed that exceeds the design limits of the cleaning system, causing low dwell time, (2) inappropriate cleaner for the soils present, and (3) incorrect temperature for the cleaner being used. Generally, high temperatures, 120 to 130°F, are best for good cleaning unless the facility is using a low-temperature cleaner. In that case, high temperatures can be detrimental. To determine the cleaning temperature that removes the soil from the parts, the operator can immerse an uncleaned part into a container of water and begin heating it. The operator should use a thermometer to watch the temperature rise while keeping an eye on the point where the water line touches the part. At some point, the water will become hot enough to visibly loosen the soils, causing globules to float to the surface (CAGE).
Flash rust: This can be caused by (1) excessive line speeds that prevent adequate exposure to the sealer in the final rinse, (2) line stops that overexpose parts to chemicals or allow them to dry off between stages, and (3) lack of sealer in the final rinse. When using a solvent-type cleaning system or an iron phosphate conversion process, wiping with a clean, white cloth is an ideal way to check a part's cleanliness before coating (CAGE).
Aluminum oxide: A natural oxide is present on the surface of aluminum parts. This oxide interferes with adhesion if it is not removed. If a facility is using a combination of iron phosphate and cleaner to remove this oxide, they should be certain that the combination is made for steel and aluminum. Their chemical supplier can discuss this with them in more detail (CAGE).
Inadequate rinsing: This is one of the most common mistakes made in metal cleaning. It is caused by both increased line speeds that reduce rinse-stage dwell time and inadequate rinsewater overflow. Simple tests for inadequate rinsing can include slowing down the production line or hand-rinsing parts in deionized water. If the technical assistance provider suspects that the company's surface preparation system is causing a problem, they should suggest that the company clean test parts with clean rags dipped in a solvent, instead of running the parts through their normal cleaning process. If this fixes the problem, the firm should focus their investigation on the surface preparation system. If they are getting premature and massive lifting of the coating after exposure to water due to exterior weather elements, or after slat-fog tests, this can indicate inadequate rinsing. Water-soluble crystals (salts) are probably present at the coating and metal interface. Moisture can dissolve these salts quickly. When this happens, rapid undercutting of the film occurs and significant rust forms (CAGE).
Reduced water use is the primary waste reduction option for phosphatizing. The water added to maintain the solution in the phosphatizing bath can be reduced by analyzing and controlling the solution's temperature, chemical concentration, and pH level in each step, and recirculating solution or rinse water from one bath to others when possible. This option also reduces chemical use (Ohio EPA, p.1). A facility should analyze incoming water quality. City water can bring in considerable amounts of dissolved solids, and these contaminants can vary seasonally. The contaminant can have a damaging effect on control regimes. Determining control set points, and treating and conditioning incoming water is a good idea.
Properly matching the phosphating chemicals with the metal substrate is another key issue in minimizing waste from phosphating operations. This can significantly minimize sludge generation. For example, processing galvanized steel in an iron phosphate solution results in excess generation of zinc sludge because the acid reacts with the zinc in the substrate.
Ultrafiltration to Maintain Phosphating Baths
Precipitates continuously form in phosphating operations, primarily on the heating coils in the tanks. This presents challenges in maintaining the baths and often results in dumping of the solution.
When the solution is removed from the tank, this accumulation of sludge must be manually removed. The solution should be decanted back into the tank to minimize waste, but because this requires space and time, it is rarely done. A more efficient system involves the use of a continuous recirculation system through a clarifier with gentle agitation in the sludge blanket zone. This allows for indefinite use of the solution and easy removal of dewatered sludge from the bottom of the clarifier (Steward, 1985).
Various methods are available for removing old paint from metal substrates. In some cases, stripping also functions as a cleaning method to remove oils, greases, or other contaminants. Chemical stripping has been used in a number of applications, but there are alternative methods that are less toxic and less costly. Alternatives to chemical stripping include plastic media, sodium bicarbonate, wheat-starch, and carbon-dioxide blasting, as well as high-pressure water, high-energy light, mechanical, cryogenic, and high-temperature thermal stripping. Key factors that must be considered when selecting a paint-stripping method include: the characteristics of the substrate to be stripped; the type of paint to be removed; and the volume and type of waste produced. Waste type and volume can have a major impact on the cost and benefits associated with a change (MnTAP, p. 2). The following section describes conventional chemical stripping and the alternatives.
The conventional method for removing paints from metal surfaces is chemical stripping. This process may involve applying solvents by hand directly to a coated surface. The solvents soften or dissolve the coatings and are usually scraped away or otherwise mechanically removed (Freeman, p. 704-705). Facilities often use a water rinse for final cleaning of the part (EPAg, p. 2). Disassembled parts may be stripped in an immersion tank. Immersion strippers are advantageous because they can strip paint from recessed and hidden areas. This is not possible with abrasive blasting methods.
Chemical-based paint strippers are either hot (i.e., heated) or cold. Many hot strippers use sodium hydroxide and other organic additives. Most cold strippers are formulated with methylene chloride and other additives such as phenolic acids, cosolvents, water-soluble solvents, thickeners, and sealants. Handling and disposal of spent baths and rinses is a major problem for facilities employing both types of strippers (Freeman, p. 704-705).
Many new stripping formulations have been developed including strippers based on formulations of N-methyl-2-pyrollidone (NMP) and dibasic esters (DBE). Although these new strippers are used in the consumer market, they have not been accepted for use in industrial stripping operations because their effectiveness varies from paint to paint. Compared to the stripping achieved with formulations containing methylene chloride and phenol, many of the substitutes suffer from one or more of the following disadvantages: effectiveness varies with type of paint and extent of cure; elevated temperature is required; and increased stripping time is required. In selecting an alternative, technical assistance providers should make sure that the stripper does not attack the substrate or react with the substrate (i.e., is flammable, combustible, or photochemically reactive) (Freeman, p. 491-492).
Many facilities have reduced their reliance on chemical-based strippers by converting to abrasive blasting. Abrasive blasting uses mechanical energy to hurl particles at high speed, removing paints and other organic coatings from metallic and nonmetallic surfaces (Freeman, p. 704). Abrasives commonly used for stripping include steel grit, alumina, garnet, and glass beads. Steel grit creates a rough surface profile on the substrate which aids coating adhesion. Because it is so hard and durable, steel grit can be reused repeatedly, and it generates the least amount of waste per unit of surface area stripped. To maximize the reuse of steel grit, companies must keep the blast media dry to avoid rusting. Alumina is considered to be a multipurpose material that is less aggressive and less durable than steel grit, and it results in a smoother surface profile and less removal of substrate material. Garnet and glass beads are the least aggressive abrasive and often are used in a single-pass operation (i.e., the abrasive is not recycled). Use of garnet and glass beads is most suitable for preparation of soft materials that are easily damaged, and for maintenance of the dimensional tolerance of the part (Freeman, p. 490-491).
Types of Abrasive Blasting
Companies can use abrasive blasting to remove paint from larger metal structures in the field (field stripping) or from smaller metal structures in a hanger, booth, or blasting cabinet.
Field stripping can be performed in an open area. Operators must wear self-contained breathing equipment in order to be protected from the stripping dust. After blasting, the used abrasive can be shoveled or vacuumed from the area and processed through the reclaimer. Some systems combine dust control and abrasive recovery by including a vacuum collection pickup device with the blasting nozzle (Freeman, p. 490-491).
Blast stripping in cabinets is often performed using manual blast cabinets and automated blasting chambers to remove paint from parts. The abrasive is fed into the cabinet or chamber and directed against the part being stripped. Used abrasive and removed paint are then pneumatically conveyed to a reclaimer. Reusable abrasive is separated from the waste and fines (broken-down abrasives and paint chips) are collected in a dust collector (Freeman, p. 490-491).
Because the main advantage of chemical-based strippers is their inability to scratch or damage the substrate, most of the abrasives that companies consider as feasible substitutes are relatively soft materials. Glass-bead blasting has become popular because it is the least aggressive of the commonly used abrasives. New alternatives include plastic media, wheat starch, ice crystals, carbon dioxide pellets and sodium bicarbonate slurry (Freeman, p. 490-491). The major disadvantage with these processes is that they can only be used for line-of- sight stripping.
Plastic media blasting
Plastic media blasting (PMB) is an abrasive blasting process designed to replace chemical paint-stripping operations and conventional sand blasting. This process uses soft, angular plastic particles as the blasting medium. PMB is performed in ventilated enclosures such as small cabinets (a glove box), a walk-in booth, a large room, or airplane hangers. The PMB process blasts the plastic media at a much lower pressure (less than 40 psi) than conventional blasting. PMB is well suited for stripping paints, because the low pressure and relatively soft plastic medium have a minimal effect on the surfaces beneath the paint (TSPPO).
Plastic media are manufactured in 6 types and a variety of sizes and hardness. Military specifications (MIL-P-85891) have been developed for plastic media. The specifications provide general information on the types and characteristics of plastic media. The plastic media types are:
Type I Polyester (Thermoset)
Facilities typically use a single type of plastic media for all of their PMB work. The majority of DOD PMB facilities use either Type II or Type V media. Type V media is not as hard as Type II media and is gentler on substrates. Type V media is more commonly used on aircraft. Type II is better suited for steel-only surfaces (TSPPO).
After blasting, the PMB media is passed through a reclamation system that consists of a cyclone centrifuge, a dual adjustable air wash, multiple vibrating classifier screen decks, and a magnetic separator. In addition, some manufacturers provide dense particle separators as a reclamation system. The denser particles, such as paint chips, are separated from the reusable blast media, and the reusable media is returned to the blast pot. Typically, media can be recycled 10 to 12 times before becoming too small to remove paint effectively (TSPPO).
Waste material consists of blasting media and paint chips. The waste material may be classified as a RCRA hazardous waste because of the presence of certain metals (primarily lead and chrome from paint pigments). An alternative solution to handling the potential hazardous waste is to recycle the media to recapture the metals.
Reusing the plastic blasting media greatly reduces the volume of spent media generated as compared to that generated in sand blasting. When compared to chemical paint stripping, this technology eliminates the generation of waste solvent. PMB is also cheaper and quicker than chemical stripping. The U.S. Air Force and airlines have found PMB effective for field stripping of aircrafts, but PMB could also be used to strip vehicles, ships, and engine parts (IHWRICf). However, PMB can cover fatigue cracks at high blast pressures and prevent their detection.
As with any blasting operations, airborne dust is a safety and health concern with PMB. Proper precautions should be taken to ensure that personnel do not inhale dust and particulate matter. Additional protective measures should be taken when stripping lead chromate- or zinc chromate-based paints, as these compounds may be hazardous. Inhalation of lead and zinc compounds can irritate the respiratory tract, and other paint compounds are known to be carcinogenic. Inhalation of paint solvents can irritate the lungs and mucous membranes. Prolonged exposure can affect respiration and the central nervous system. Operators must wear continuous-flow airline respirators when blasting operations are in progress in accordance with OSHA requirements as specified in 29 CFR 1910.94 (TSPPO).
PMB systems can range in cost from $7,000 for a small portable unit to $1,400,000 for a major facility for aircraft stripping.
Vacuum Sanding Systems
A vacuum sanding system is essentially a dry-abrasive blasting process (e.g., sand blasting or plastic media blasting) with a vacuum system attached to the blast head that collects the blast media and the removed coating material (paint or rust). The unit then separates the used blast media from the removed coating material. The remaining blast material is recycled for further use, and the coating material is disposed.
This system is designed to replace chemical paint stripping, and has three added advantages. The first advantage is its collection of both the blasting media (sand, PMB, or other media) and its collection of the waste coating material being removed. The second advantage is that it separates the media from the waste material by a reverse pulse filter, and the media is reused in the system, thereby minimizing the quantity of media required. The third advantage is that, due to the confinement of the blast material, this technology may be used when it is impractical to use traditional sand blasting or chemical stripping (TSSOP).
Vacuum sanding is a stand-alone system, including the air compressor to drive the system. The units are portable (skid mounted) and can be moved by a forklift. The air compressor is a trailer unit (2-wheeled). The waste material may be classified as a RCRA hazardous waste because of the presence of metals in the waste (TSSOP).
This technology reduces pollution because the portable vacuum sander removes coatings and corrosion from composite or metal structures while capturing the media and solid waste. Vacuum sanding eliminates airborne particulate matter and potential lead-dust exposure hazards. When compared to chemical paint stripping, this technology eliminates the generation of waste solvent (TSSOP).
Storage and handling of sand or plastic media and blast waste associated with vacuum sanding pose no compatibility problems. Collection systems should not mix different types of waste, and should ensure that the most economic disposal method can be obtained for each. Prior to using plastic media for de-painting operations, personnel should check applicable military specifications [such as (MIL-P-85891)] and operations manuals for the PMB systems. Some military specifications do not allow PMB for de-painting certain types of materials (e.g., fiberglass, certain composites, honeycomb sandwich structures, and some applications with thin-skinned aircraft components). In certain cases, PMB can inhibit crack detection on softer alloys used for aircraft components (e.g., magnesium) (TSSOP).
Airborne dust, which is an important safety and health concern with any blasting operation, is essentially eliminated using the vacuum blasting system. However, in order for the vacuum system to be effective, the vacuum and blasting head must be kept in contact with the material being stripped of paint or corrosion. Therefore, training operators in the proper use of the equipment is essential. In addition, eye protection and hearing protection are recommended (TSSOP).
Vacuum sanding systems can range in cost from $17,000 to $40,000, excluding the portable generator to operate the system.
Sodium bicarbonate is another media that companies can use to remove paint. The process that uses sodium bicarbonate can be used with or without water. However, it is most frequently applied with water, which acts as a dust suppressant. The water-based process uses a compressed air delivery system that transfers the sodium bicarbonate from a pressure pot to a nozzle, where the sodium bicarbonate mixes with a stream of water. The soda/water mixture impacts the coated surface and removes old coatings from the substrate. The water dissipates the heat generated by the abrasive process, reduces the amount of dust in the air, and assists in paint removal through hydraulic action. Workers do not have to prewash or mask the surface of the material being stripped. The solid residue from the wastewater generated can be separated by filtration or settling (NFESC).
The effectiveness of sodium bicarbonate stripping depends on optimizing a number of operating parameters such as nozzle pressure, standoff distance, angle of impingement, flow rate, water pressure, and traverse speed. In general, sodium bicarbonate stripping systems remove paint more slowly than chemical stripping. The type of equipment used may also bring about significantly different results.
Use of sodium bicarbonate in its dry form (or when it is not fully mixed with water) can create a cloud of dust that requires monitoring and may require containment to meet air-quality standards. The dust is not an explosive hazard nor is it toxic, but air particulates generated from stripping operations can contain toxic elements. This process should be conducted in areas where exhaust particulates can be contained and/or vented to ventilation systems to remove hazardous airborne particulates.
Approximately 150 to 200 pounds of bicarbonate is needed per hour, while PMB requires 800 pounds. In the end, bicarbonate is cheaper than PMB because it neither generates large amounts of waste nor damages the metal. Nevertheless, sodium bicarbonate can have long-term corrosive effects because alkaline compounds that remain on the metal can foster corrosion or interfere with the paint bonding. Corrosion inhibitors can be added; however, the waste might then become hazardous, depending on the type of inhibitor used (IHWRICf).
Wastewater disposal methods and sodium bicarbonate waste disposal methods will depend on the toxicity of the coatings and pigments that are removed in the stripping process. The waste generated from bicarbonate of soda stripping systems in the wet form is a slurry consisting of sodium bicarbonate media, water, paint chips, and residues such as grease and oil. Some facilities are using centrifuges to separate the water from the contaminated waste stream, reducing the amount of hazardous waste. Filtered wastewater containing dissolved sodium bicarbonate may be treated at industrial wastewater treatment plants. In its dry form, the waste includes nuisance dust, paint chips, and residues of grease and oil. This waste may be disposed of in a solid waste landfill; however, due to the possibility of toxics in the paints and the presence of oils, the material should be tested prior to landfill disposal (NFESCa, p.4).
Wheat starch blasting
Wheat starch blasting is a user-friendly blasting process where wheat starch is used in systems designed for plastic media blasting, as well as systems specifically designed for wheat starch blasting. The wheat starch abrasive media is a crystallized form of wheat starch that is nontoxic, biodegradable, and made from renewable resources. The media is similar in appearance to plastic media, but it is softer (TSSPO).
The wheat starch blasting process propels the media at less than a 35 psi nozzle pressure for most applications. The low pressure and relatively soft media have minimal effects on the surfaces beneath the paint. For this reason, wheat starch is well suited for stripping paints without risking damage to the substrate. Examples of suitable applications include removing paint from aluminum alloys and composites like graphite and fiberglass (Kevlar).
The wheat starch blasting process can remove a variety of coatings. Coating types range from resilient rain erosion-resistant coatings found on radar absorbing materials to the tougher polyurethane and epoxy paint systems. The wheat starch system has been shown to be effective in removing bonding adhesive flash (leaving the metal-to-metal bond primer intact), vinyl coatings, and sealants. It has also been found to be effective in removing the paint from cadmium parts, while leaving the cadmium plating intact (TSSPO). Wheat starch blasting is mainly known for its gentle stripping action and is particularly suited for stripping operations on soft substrates, such as aluminum, very soft alloys, anodized surfaces, or sensitive composites.
There are several important components in wheat starch systems. First, a moisture control system is needed to control the storage conditions of the medium. This is especially important when the system is shut down for extended periods of time. Second, to remove contaminants from the wheat starch media, the spent wheat starch residue is dissolved in water and then either filtered or separated in a dense particle separator/centrifuge. The wheat starch media is recycled in the system and may be used for up to 15 to 20 cycles. Low levels of dense particle contamination in the media may result in a rough surface finish on delicate substrates. The waste stream produced from this process consists of sludge generated from the wheat starch recycling system. This system produces approximately 85% less waste sludge compared to the waste sludge produced in chemical stripping (TSSPO).
Wheat starch blasting can be used on metal and composite surfaces. Direct contact of wheat starch with water must be avoided to maintain the integrity of the blast media. Wheat starch blasting requires explosion protection. If conditions are right, a static electrical charge developed by a high velocity wheat starch particle in the air could ignite the material. Preventive measures must be taken.
As with other blasting procedures, airborne dust is a safety and health concern. Proper precautions should be taken to ensure that personnel do not inhale dust and particulate matter. Additional protective measures should be taken when stripping lead, chromate, zinc chromate, or solvent-based paints, as these components may be hazardous. Inhalation of lead and zinc compounds can irritate the respiratory system and some compounds are known to be carcinogenic. Inhalation of paint solvents can irritate the lungs and mucous membranes. Prolonged exposure to these emissions can affect respiration and the central nervous system. Proper personal protective equipment should be used (TSSPO).
Capital costs for wheat starch blasting systems vary depending upon the application. A PMB system for a small application can be modified for a cost of approximately $10,000. An automated, closed, dust-free system for a large application (e.g., aircraft) can cost up to $1.5 million. The operating costs for wheat starch blasting systems have been estimated to be 50% less than those for chemical paint stripping (such as methylene chloride).
Carbon dioxide (CO2) blasting is an alternative process to chemical cleaning and stripping. The obvious advantage of CO2 blasting over chemical stripping is the introduction of inert media that dissipates, in this case CO2. There are two basic types of CO2 blasting systems: pellet blasting for heavy cleaning and snow blasting for precision cleaning.
CO2 Pellet Blasting
CO2 pellets are uniform in shape and the effectiveness of the pellets as a blast medium is similar to abrasive blasting. However, the pellets do not affect the substrate; therefore, CO2 pellet blasting is technically not an abrasive operation. This process can be used for cleaning, degreasing, some de-painting applications, surface preparation, and de-flashing (flashing is the excess material formed on the edges of molded parts).
The process starts with liquid CO2 stored under pressure (~850 psig). The liquid CO2 is fed to a pelletizer, which converts the liquid into solid CO2 snow (dry ice flakes), and then compresses the dry ice flakes into pellets at about -110°F. The pellets are metered into a compressed air stream and applied to a surface by manual or automated cleaning equipment with specially-designed blasting nozzles. The CO2 pellets are projected onto the target surface at high speed. As the dry ice pellets strike the surface, they induce an extreme difference in temperature (thermal shock) between the coating or contaminant and the underlying substrate, weakening the chemical and physical bonds between the surface materials and the substrate. Immediately after impact, the pellets begin to sublimate (i.e., vaporize directly from the solid phase to a gas), releasing CO2 gas at a high velocity along the surface to be cleaned. The high velocity is caused by the extreme difference in density between the gas and solid phases. This kinetic energy dislodges the contaminants (e.g., coating systems and flash), resulting in a clean surface. Variables that facilitate process optimization include the following: pellet density, mass flow, pellet velocity, and propellant stream temperature.
CO2 pellet blasting is effective in removing some paints, sealants, carbon and corrosion deposits, grease, oil, and adhesives, as well as solder and flux from printed circuit board assemblies. Furthermore, because CO2 pellet blasting is not an abrasive operation, it is excellent for components with tight tolerances. This process also provides excellent surface preparation prior to application of coatings or adhesive and is suitable for most metals and some composite materials. However, thin materials may be adversely affected. Blasting efficiency is approximately equal to that of other blasting operations. CO2 blasting can be done at various velocities: subsonic, sonic, and even supersonic. Therefore, equipment noise levels are high (between 95 and 130 dB). This operation always requires hearing protection.
Waste cleanup and disposal are minimized because only the coating or contaminated residue remains after blasting. No liquid waste is created because CO2 pellets sublimate to CO2 gas. They pass from a liquid to a gaseous state, leaving no spent media residue. With regard to air pollution control, small quantities of coating particles are emitted to the air. A standard air filtration system should be utilized.
CO2 Snow Blasting
In contrast to CO2 pellet blasting, CO2 snow blasting is a low impact process. This process applies primarily to precision cleaning. A typical precision cleaning operation must clean small contaminant particles that, due to electrostatic attraction, attach to surfaces and/or surface layers of adsorbed moisture or soil. These particles are so small that a large fraction of their surface area attaches to the surface layers. CO2 snow blasting is most effective in breaking the adhesive forces and dislodging particles from the substrate surface. Small flakes of dry ice transfer their kinetic energy to submicron particulate contaminants and then sublimate, lifting the particulate matter from the substrate surface as the adhesive bonds are broken. This process is often used as a final cleaning process for submicron particulate removal and light soils removal.
CO2 snow is generated from liquid CO2, and is discharged directly from the nozzle of the blasting device. The liquid CO2 is partially vaporized as it passes through the nozzle, while the rest of the stream solidifies as pressure is reduced. The fine particles of "snow" are propelled by the fraction of CO2 that vaporizes. No compressed air or other inert gas is needed to propel the snow.
Many of the blasting media described in the previous sections cannot be used in precision cleaning because either they are too aggressive, or they contaminate the component with media residue. CO2 snow, however, is ideal for this application because it is relatively gentle in application, leaves no media residue, is highly purified, and does not introduce new contaminants. CO2 snow blasting is often done in a clean room or cabinet purged with nitrogen to provide a dry atmosphere, minimizing moisture buildup on the component (TSSOP).
As a completely oxidized compound, CO2 is a nonreactive gas, and thus is compatible with most metals and nonmetals. Dry ice processes are cold and can cause thermal fracture of a component. In addition, prolonged use in one spot will cause condensation and ice buildup. However, this is rarely a problem for CO2 blasting because it is a fast-acting, nonstationary process. Particulate and organic contamination is either quickly removed or unable to be removed by continued blasting. Therefore, the component temperature does not change much, because contact time is short. Nevertheless, should component temperature drop below the dew point of the surrounding atmosphere, moisture will accumulate on the component. This problem can be mitigated by heating the component in some manner so that its temperature remains above the surrounding atmosphere's dew point after blasting. If components cannot take heat, then blasting can be done in an enclosed space purged with a dry gas to lower or eliminate the dew point problem (TSSOP).
CO2 does not support combustion and it is nontoxic; however, it is an asphyxiant. CO2 will displace air because its density is greater than that of air, causing CO2 to accumulate at the lower level of enclosed spaces. When blasting with CO2 pellets, additional ventilation should be provided for workers in enclosed spaces. Companies should also require use of personal protection equipment (PPE) when blasting (TSSOP).
Static energy can build up if grounding is not provided. CO2 blasting should not be done in flammable or explosive atmospheres. High-pressure gases should be handled with great care. Companies should always chain or secure high-pressure cylinders to a stationary support such as a column.
Sponge blasting systems are a class of abrasive blasting that uses (1) grit-impregnated foam and (2) nonabrasive blasting media using foam without grit. These systems incorporate various grades of water-based urethane-foam cleaning media. Firms use the nonabrasive media grades to clean delicate substrates. The abrasive media grades are used to remove surface contaminants, paints, protective coatings, and rust from a variety of surfaces. In addition, the abrasive grades can be used to roughen concrete and metallic surfaces. A variety of grit types are used in abrasive media including aluminum oxide, steel, plastic, or garnet (TSPPO).
The foam cleaning medium is absorptive and can be used either dry or wet with various cleaning agents and surfactants to capture, absorb, and remove a variety of surface contaminants such as oils, greases, lead compounds, chemicals, and radionuclides. The capability of using the foam cleaning medium in a wet form provides for dust control without excessive dampening of the surface being cleaned. The equipment consists of three transportable modules, which include the feed unit, the classifier unit, and the wash unit (TSPPO).
The feed unit is pneumatically powered for propelling the foam cleaning medium. The unit is portable and produced in several sizes. A hopper, mounted at the top of the unit, holds the foam medium. The medium is fed into a metering chamber that mixes the foam cleaning medium with compressed air. By varying the feed-unit air pressure and type of cleaning medium used, sponge blasting can remove a range of coatings from soot on wallpaper to high-performance protective coatings on steel and concrete surfaces (TSPPO).
The classifier unit removes large debris and powdery residues from the foam medium after each use. The used medium is collected and placed into an electrically-powered sifter. The vibrating sifter classifies the used medium with a stack of progressively finer screens. Coarse contaminants, such as paint flakes and rust particles, are collected on the coarse screens. The reusable foam medium is collected on the corresponding screen size. The dust and finer particles fall through the sifter and are collected for disposal. After classifying, the reclaimed foam medium can be reused immediately in the feed unit. The abrasive medium can be recycled approximately six times and the nonabrasive medium can be recycled approximately 12 times (TSPPO).
During degreasing applications, the foam medium must be washed every 3 to 5 cycles. The washing of the foam medium takes place in the wash unit, which is a portable centrifuge, closed-cycle device. The contaminated wash water is collected, filtered, and reused within the wash unit (TSPPO).
This system removes paint, surface coatings, and surface contaminants from a variety of surfaces. Waste streams produced from this system include: coarse contaminants, such as paint flakes and rust particles; dust and finer particles; and the concentrated residue from the bottom of the wash unit.
This technology helps prevent pollution for two reasons: the stripping media can be recycled (i.e., every 10 to 15 events), and the quantity of wastewater that is typically generated using conventional methods (e.g., chemical stripping) is greatly reduced. Sponge blasting systems are compatible in most situations where other types of blasting media have been used.
As with any blasting operations, airborne dust is a safety and health concern. Proper precautions should be taken to ensure that inhalation of dust and particulate matter is avoided. Additional protective measures should be taken when stripping lead chromate- or zinc chromate-based paints, as these compounds may be hazardous. Inhalation of lead and zinc compounds can irritate the respiratory tract, and some compounds are known to be carcinogenic. Proper personal protective equipment should be used.
High- and Medium-Pressure Water Stripping
High- and medium-pressure water blast systems are used for paint stripping surfaces with low-volume water streams at pressures ranging from 3,000 to 15,000 psi (medium-pressure operations), and 15,001 to 55,000 psi (high-pressure operations). These systems remove paint by spraying a stream of high-pressure water at the surface of the part. The advantages of this process include a readily available medium (water), an easily treatable waste stream, and an absence of fume and hazardous waste production. A disadvantage of this process is the necessity for an automated system that usually uses robotics. Robotics is required for application due to the extremely high pressure of the water stream (Freeman, p. 491).
Medium-pressure systems may be augmented. For example, sodium bicarbonate may be added to the water stream, or environmentally compliant chemicals may be applied to painted surfaces prior to water blasting. High-pressure systems typically use pure water streams. With both medium- and high-pressure water systems, specialized nozzles can be used to achieve varying effects. A relatively gentle, layer-by-layer process may be used for removal of organic paints versus the use of a different nozzle for the removal of metal flame spray coating and other tough, tightly adherent coatings. The process water, paint, and residue are collected by an effluent-recovery system that filters the paint and residue. The recovery system removes leached ions (e.g., copper, cadmium, and lead), microparticulates, chlorides, sulfates, nitrates, and other contaminants from the water. The water is then passed through a coalescing tank for removal of oils and film, then through charcoal filters, microfilters, and finally, a deionization system to ensure that the water is Grade A deionized water. The recovered deionized water is recycled back into the process (TSSOP).
No material compatibility problems have been documented for use of high- and medium-pressure water processes to de-paint metallic surfaces. The use of specific chemicals to augment medium- pressure water processes must be evaluated on a case-by-case basis. The automotive industry currently uses high-pressure water jets to remove paint from the floor of painting booths (IHWRICf).
The capital costs for high- and medium-pressure water processes vary considerably depending on the process and its application. Capital costs for medium-pressure systems range from $40,000 to $70,000, and capital costs for high-pressure systems range from $850,000 to $1,500,000.
Fluidized Bed Stripping
The fluidized bed paint removal process is an alternative method to chemical paint stripping and degreasing of nonaluminum and nonheat sensitive metal parts. In fluidized bed stripping, an air stream is pumped into a tank of quartz sand or aluminum oxide, making it a fluid. Natural gas is mixed with the air and ignited above the tank, creating temperatures of approximately 800°F. Objects to be stripped are lowered in a basket into the tank. The paint is vaporized, and the gases and unburned natural gas are burned in a postcombustion chamber above the tank. A wet scrubber removes the solids from the final exhaust before it is vented into the air. The most notable advantage of this process is that it produces no solvent wastes. This method works for steel parts but not for aluminum parts (IHWRICf). Technical assistance providers should not recommend the fluidized bed paint stripping (FBPS) process for use with aluminum and aluminum alloy parts because these materials lose essentially all of their hardness or temper when exposed to the 700 to 800°F process temperatures (TSSOP).
The FBPS process typically consists of the following four components: 1) fluidized bed furnace or retort, 2) fluidized bed cooling system, 3) off-gas treatment system consisting of a cyclone, afterburner and scrubber, and 4) low energy shot-blast unit. The fluidized bed furnace or hot bed is where pyrolysis of the coatings takes place. A granular material, aluminum oxide (alumina) in most cases, is used as a heat-transfer medium. Air passing through the bed keeps the medium fluidized. Parts to be cleaned are lowered into the fluidized bed, which quickly heats the part and its surface coatings (e.g., paint, grease, and oil) to a temperature at which organic components of the surface material pyrolyze into carbon oxides, other gaseous combustion products, and char. The fluidized bed cooling system, or cold bed, is used to cool the parts after the organics have been pyrolyzed. Carbon monoxide and volatile organic compounds (VOCs) generated during pyrolysis are burned in the afterburner. The thermal decomposition of paint leaves some carbon and inorganic char on the part. Most of the char may be removed in the fluidized bed; however, most parts require further cleaning before they can be repainted. The shot-blast unit is used to remove the inorganic coatings and char to prepare the parts for repainting (TSSOP).
This process removes and destroys paint and grease from nonaluminum or nonheat sensitive materials. Waste streams from this process include spent heat-transfer medium, spent blast media, exhaust air from the afterburner and scrubber, water discharge from the scrubber, and dust from the cyclone separator. The heat-transfer medium, blast media, and cyclone dust contain metals from the stripped paint.
Assistance providers should also inform facilities that this blasting method requires employees to wear equipment to protect them from toxics in the paint. For example, inhalation of lead and zinc chromate paints can lead to irritation of the respiratory tract; some lead compounds are carcinogenic; solvent-based paints can irritate the lungs and mucous membranes; and prolonged exposure can affect respiration and the central nervous system.
Costs for fluidized bed paint strippers can range from $7,000 for a small parts stripper to $800,000 for an industrial scale stripper.
The methods that follow can also be used to strip old paints from metal parts.
High-energy Light. High-energy light uses optically directed beams of photon energy emitted by lasers or flash lamps (typically xenon lamps) to ablate the paint. Using high-energy light to remove paint decreases operating cost, minimizes the waste stream, and lowers the possibility of material damage. The disadvantages of this process are its high capital cost and precision robotics requirements (Freeman, p. 490)
Cryogenic Methods. Cryogenic methods generally use liquid nitrogen immersion at approximately -200°F, which causes the paint to contract, breaking the adhesive bond with the substrate. For small components, a tumbler design normally is used, where the parts can impact and abrade each other to assist in removing the paint. If the parts have complex shapes, tumbling media might be added (Freeman, p. 491). Cryogenic stripping has a harder time removing epoxy and urethane coatings than other coatings. Also, this stripping method removes thick coatings more efficiently than thin coatings. In addition, this method may damage or distort parts because of the extreme temperatures needed in the process (IHWIRCf).
High-temperature Thermal Methods. High-temperature thermal methods, such as burnoff ovens and molten salt baths, are sometimes used to strip paint. In general, these methods are labor intensive, and result in emissions of burned paint and metal surfaces that are fouled by heat scale. This heat scale, subsequently, must be removed by abrasive methods, such as sanding or wire brushing. Most thermal methods are limited to heavy metal parts that will not warp because of thermal expansion and distortion (Freeman, p. 490). In burnoff ovens, the ovens simply burn off the paints. This method is limited to steel parts (IHWRICf).
Molten salt baths remove paint easily from metal. Baths that are only 500 to 700°F significantly reduce any problems with heat distortion. Objects to be stripped are lowered into the salt bath, removed, rinsed with water, lowered in dilute acid, and immersed in water again. Care must be taken in stripping aluminum parts; leaving the parts in the bath for more than 60 seconds could soften the metal and make the parts unusable. Also, salt can solidify or get trapped in an area that cannot be thoroughly rinsed, causing corrosion at a later time (IHWRICf).
Burnoff ovens and molten salt baths often are used to remove paint overspray from hooks, racks, grates, and body carriers used in automotive plants. Stripped parts are left with a residue of ash, which can be removed by rinsing (Freeman, p. 491).
Table 17. Overview of Alternative Surface Preparation Technologies (EPAh, p. 7- 10, EPAi, p. 5-6, 24, EPAg, p. 7-9 and IHWRICf)