General Discussion

Degreasing with Solvents
This method of cleaning uses traditional solvents in their liquid form rather than their vapor form to clean the substrate. This is a common practice in painting operations. Typically, solvents such as methyl ethyl ketone (MEK), or 1,1,1 trichloroethane were 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 work pieces 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 OSHA requires special safety equipment 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:

• Moisture can form on the work piece and cause rusting problems when solvent evaporates too quickly.
• Some solvents can leave a residue that reduces the adhesion of the coating.
• Solvents with low flashpoints can be fire hazards.

Aqueous Cleaning
Aqueous cleaning involves the use of solutions that 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 semi-aqueous. Both aqueous cleaning and semi-aqueous 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 work piece. The particular solution selected depends on both the type of contaminant and the type of process equipment used. Elevating the temperature of the solution can make it more effective in removing greases and oils, which have increased mobility at higher temperatures.

Paint Removers & Cleaners
Solvent cleaning is a typical procedure for removing foreign matter such as oil, grease, soil, drawing & cutting fluid, and other contaminants from steel before abrasive blasting. Failure to perform this procedure can result in complete contamination of the prepared substrate. Poor solvent cleaning technique spreads contamination to an otherwise clean surface because the oil or grease becomes dissolved in the solvent. As the solvent evaporates it leaves behind the contamination. Solvent cleaning is a practical method of cleaning for all types of surface preparation methods.

Paint removers or strippers are generally used for small areas. There are many different types of strippers that are selected according to the condition and type of old coating as well as the type of substrate. Paint removers can be found in flammable and non-flammable forms and are available in liquid or paste.

Generally, paint removers only soften or loosen old coatings so some scrapping or use steel wool is required. There are, however, some removers available that allow the softened coating to be flushed away with hot water or steam.

One of a couple of considerations required when using paint removers is that many of them contain paraffin wax. This can result in an adherent waxy residue on the surface after paint removal. This residue must be completely removed before coating application or proper adhesion to the substrate will not occur. The manufacturers label should provide instructions for cleaning the surface of any residue. Another consideration when using paint removers is safety. Strippers are generally toxic and many release fumes so proper safety attire is required for personnel.

Alkaline Cleaning
Alkaline cleaners consist of highly alkaline salts such as sodium hydroxide, carbonates, and silicates along with surfactants, inhibitors, wetting agents and/or soaps. The most common alkaline cleaners are trisodium phosphate, caustic soda, and silicated alkalies. They work by attacking, emulsifying, and dispersing oils, greases, and other surface contaminants. Dirt and grease is converted into a soapy residue, which can be rinsed away with water.

These types of cleaners are dissolved in water and applied to the surface at relatively high temperatures to increase cleaning efficiency. Temperature of application is around 150°F. Brushing, spraying, scrubbing, or immersion in large tanks are methods used to apply alkaline cleaners. The soapy residue remaining after application of the alkaline material must be rinse thoroughly to prevent damage to applied paint. If rinsing is not fully completed this type of cleaning can actually create more problems than it solves. The rinse water is preferably applied under pressure and should be hot.

Alkaline cleaners are not used on aluminum, magnesium stainless, or galvanized steel.

Pickling
Pickling is a method of surface cleaning and preparation that is achieved through immersion of metals in dilute acids. Pickling is used in metal working industries to remove scales, tarnish, or oxides. Pickling can be accomplished with a variety of different acids. Acids used for pickling include sulfuric, hydrochloric, nitric, phosphoric and various other mixtures of these.

Pickling is usually carried out by immersing metals into large baths of acid solution but pickling can also involve spraying or flowing the solution over the specimen. The overall goal of pickling is to remove scale from the base metal without loss from dissolving the metal itself. This is usually accomplished through the use of inhibitors in the pickling solution. If the scale could somehow be removed from the surface uniformly acid attack of the base metal wouldn't be a problem as the sample could be removed from the solution as soon as the scale was removed. Since scale is removed faster from some areas an uninhibited acid solution would begin dissolving base metal in those areas before all of the scale is removed.

The pickling process breaks down in the following way. First the metal must be cleaned and prepared for the pickling process. This step includes removing any material from the metal that would prevent contact of the pickling acid with the surface. Generally this involves removing any oils on the surface. This is accomplished by either solvent or alkali cleaning. The next step is the actual pickling process. After pickling the metal is treated in preparation for coating. After pickling the metal is run through a cold clean water rinse to remove the film of pickling acids and salts that cling to the surface. These actually cause corrosion even under coatings if they are not removed. The film can be difficult to remove if it dries so rinsing must occur immediately. After the cold water rinse the metal undergoes a final hot rinse in boiling weak alkaline solution. This creates an alkaline surface that does not rust rapidly. Prior to painting the surface pH must be adjusted to ensure proper adhesion of the coating to the surface. This is achieved by a phosphoric or chromic acid solution.

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 starches that is nontoxic, biodegradable, and made from renewable resources. The media is similar in appearance to plastic media, but it is softer. 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.

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. 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. 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. 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).

References
M.H. Sandler and S. Spring, "Chemical Cleaning", Good Painting Practice: SSPC Painting Manual Volume 1, pp. 90-97, 2000.
D.W. Christofferson, "Pickling of Steel Surfaces", Good Painting Practice: SSPC Painting Manual Volume 1, pp. 104-116, 2000.
"Paint Removers", Naval Ship's Technical Manual: Chapter 631, Vol. 2, pg. 5-13.
"Alkaline Cleaning", Naval Ships' Technical Manual: Chapter 631, Vol. 2, pg. 5-18.