Metals cleaning

This is the removal of oil, grease or other contaminants from the surface of metals, and increasingly plastic components and assemblies. Substrates cleaned include mild steel, brass, copper, polypropylene and printed circuit boards, the level of cleanliness required depending on the application. 

A continued demand for significant volumes of solvents is expected in this sector despite the uptake of aqueous detergent systems. This is due to recognition that, properly managed, solvents allow retention of the performance advantages to which the user has been accustomed without adverse environmental impact. 

Acid pickling and descaling can be carried out with phosphoric, hydrochloric or sulphuric acids, each at about 20% concentration. Unlike phosphoric acid, traces of the other acids must afterwards be removed to prevent corrosive effects, but the action of phosphoric acid is somewhat slower. These acids, or their mixtures, usually have inhibitors added to reduce the attack on the bare metal once the oxide films have been dissolved. The major component of scale is usually FeO, which readily forms the soluble acid phosphate (12.24). 

Phosphoric acid is superior to hydrochloric acid for cleaning boilers. A hot 5% solution will remove rust from new boilers, and scales of calcium and magnesium carbonates from old boilers. 

Among the newer uses of phosphoric acid are as a metal cleaner in printed circuit board manufacture, and as a metal etchant in the production of semiconductor materials. A 1% solution of hexafluorophosphoric acid, HPFg, can be used to clean aluminium, prior to anodising [23]. 

Equations such as (12.25) through (12.27) have been used to represent what may be happening to aluminium surfaces, under some conditions, with alkaline cleaners. 

A1 + Na4P207 + 3H2O NaA102 + 3NaOH + l.SHj A1 + Na3P04 + H2O AIPO4 + 3NaOH + l.SHj A1 + NaOH + H2O NaAlOj + I.5H2 

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Other uses of oxyacetylene flames in mill operations are in building up or hardfacing metal, lancing (piercing a hole in a metal mass), and a variety of metal cleaning procedures. A minor but interesting fuel use of acetylene is in flame spectrophotometry where oxygen and nitrous oxide are used as oxidants in procedures for a wide variety of the elements. 

Hydrogen chloride and the aqueous solution, muriatic acid, find appHcation in many industries. In general, anhydrous HCl is consumed for its chlorine value, whereas aqueous hydrochloric acid is often utilized as a nonoxidizing acid. The latter is used in metal cleaning operations, chemical manufacturing, petroleum well activation, and in the production of food and synthetic mbber. 

Metal Cleaning. About 204 thousand metric tons of HCl (100% basis) was consumed in 1993 for steel pickling, wherein the hydrochloric acid readily dissolves all of the various oxides present in the scale formed during the hot rolling process. Using suitable inhibitors such as alkyl pyridines, HCl reacts very slowly with the base metal rendering the surface so clean that it must be passivated with a mild alkaline rinse. 

MiscelUneous. Mahc acid is used in pharmaceuticals (qv), cosmetics (qv), dentifrices (qv), metal cleaning, electroless plating (46), wash-and-wear textile finishing (47—49), for stabilization of heat-sensitive copying paper (50), as an inhibitor of gelation, livering, and agglomeration in cellulose nitrate Hqueurs, and in many other appHcations. 

Cleaners. Phosphoric acid is used ia several acidic hard-surface (tile, porcelain, metal) cleaning and sanitising formulations, as well as an acid cleaner for food processing equipment. 

About 264,000 metric tons of elemental capacity is available in North America, plus another 79,000 t (P equivalent) of purified wet phosphoric acid (14). About 85% of the elemental P is burned to P2 5 hydrated to phosphoric acid. Part of the acid (ca 21%) is used direcdy, but the biggest part is converted to phosphate compounds. Sodium phosphates account for 47% calcium, potassium, and ammonium phosphates account for 17%. Pinal apphcations include home laundry and automatic dishwasher detergents, industrial and institutional cleaners, food and beverages, metal cleaning and treatment, potable water and wastewater treatment, antifree2e, and electronics. The purified wet acid serves the same markets. 

Alkylphenol ethoxylates are chemically stable and highly versatile surfactants that find appHcation in a large variety of industrial products including acid and alkaline metal cleaning formulations, hospital cleaners, herbicides (qv) and insecticides, oil-weU drilling fluids, synthetic latices, and many others (see Disinfectants AND antiseptics Elastop rs, synthetic Insect control technology Metal surface treati nts Pesticides Petroleum, drilling fluids). 

Commercial use of many chlorinated derivatives imposes stress on the stabHity of the solvent. Inhibitors classified as antioxidants (qv), acid acceptors, and metal stabilizers are added to minimize these stresses. AH the chloriaated derivatives hydrolyze at a slow but finite rate when dissolved ia water. Hydrolysis of chloriaated solvents typicaHy Hberates hydrogen chloride that can corrode storage containers and commercial metal-cleaning equipment. The Hberated hydrogen chloride can be neutralized by an appropriate epoxide to form noncorrosive chlorohydrins (qv). 

AH volatile organic solvents are toxic to some degree. Excessive vapor inhalation of the volatile chloriaated solveats, and the central nervous system depression that results, is the greatest hazard for iadustrial use of these solvents. Proper protective equipment and operating procedures permit safe use of solvents such as methylene chloride, 1,1,1-trichloroethane, trichloroethylene, and tetrachloroethylene ia both cold and hot metal-cleaning operations. The toxicity of a solvent cannot be predicted from its chlorine content or chemical stmcture. For example, 1,1,1-trichloroethane is one of the least toxic metal-cleaning solvents and has a recommended threshold limit value (TLV) of 350 ppm. However, the 1,1,2-trichloroethane isomer is one of the more toxic chloriaated hydrocarboas, with a TLV of only 10 ppm. 

Trichloroethane and other chlorinated solvents are used for vapor degreasing (84—90). Other uses include cold metal cleaning, printed ckcuit board cleaning, and as a solvent for inks, coatings, adhesives, and aerosols. 1,1,1-Trichloroethane is an excellent solvent for development of photoresist polymers used in printed ckcuit board manufacture (see Integrated circuits Photoconductivepolymers). 

Tetrachloroethylene [127-18-4] perchloroethylene, CCl2=CCl2, is commonly referred to as "perc" and sold under a variety of trade names. It is the most stable of the chloriaated ethylenes and ethanes, having no flash poiat and requiring only minor amounts of stabilizers. These two properties combiaed with its excellent solvent properties account for its dominant use ia the dry-cleaning iadustry as well as its appHcation ia metal cleaning and vapor degreasiag. 

Tetrachloroethylene was first prepared ia 1821 by Faraday by thermal decomposition of hexachloroethane. Tetrachloroethylene is typically produced as a coproduct with either trichloroethylene or carbon tetrachloride from hydrocarbons, partially chloriaated hydrocarbons, and chlorine. Although production of tetrachloroethylene and trichloroethylene from acetylene was once the dominant process, it is now obsolete because of the high cost of acetylene. Demand for tetrachloroethylene peaked ia the 1980s. The decline ia demand can be attributed to use of tighter equipment and solvent recovery ia the dry-cleaning and metal cleaning iadustries and the phaseout of CFG 113 (trichlorotrifluoroethane) under the Montreal Protocol. 

Approximately 50% of the demand for tetrachloroethylene is in the dry-cleaning industry where about 80% of all dry cleaners use it as their primary cleaning agent. Use as a feedstock for chlorofluorocarbon production accounts for 30% of current demand. Metal cleaning and miscellaneous appHcations represent 12 and 8% of demand, respectively. The miscellaneous appHcations include such varied uses as transformer insulating fluid, chemical maskant formulations, and as a process solvent for desulfurizing coal. 

Chlorotoluene isomer mixtures, especially those containing a relatively high amount of o-chlorotoluene, are widely used as solvents in industry for such purposes as metal-cleaning formulations, railroad industrial cleaners, diesel fuel additives, carbon removal procedures, paint thinners, and agricultural chemicals. Halso 99 andHalso 125 are examples of such solvents. 

Citric acid is manufactured in over 20 countries with 1990 worldwide production estimated at approximately 550,000 t, distributed as shown in Figure 5. Most of this production is used for foods and beverages however, industrial appHcations, eg, detergents, metal cleaning, of citric acid are becoming more important on a worldwide basis. 

Metal Cleaning. Citric acid, partially neutralized to - pH 3.5 with ammonia or triethanolamine, is used to clean metal oxides from the water side of steam boilers and nuclear reactors with a two-step single fill operation (104—122). The resulting surface is clean and passivated. This process has a low corrosion rate and is used for both pre-operational mill scale removal and operational cleaning to restore heat-transfer efficiency. 

In specific cases of metal cleaning where small amounts of residual sod must be detected and are difficult to measure by conventional means. 

Hydrochloric add (HCI) Chemical manufactu well activation re, chlorine, food and rubber production, metal cleaning, petroleum... 

Control Technique Guidelines (CTG) EPA documents designed to assist state and local pollution authorities to achieve and maintain air quality standards for certain sources (e.g., organic emissions from solvent metal cleaning known as degreasing) through reasonably available control technologies (RACT). 

Aqueous solutions of acids as used in metal cleaning processes such as pickling for the removal of rust or rolling scale during the production and fabrication of metals, or in the post-service cleaning of metal surfaces. 

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