Cleaning soil (oil and dirt) from metal surfaces is an essential step preceding many of the unit operations involved in metal finishing. Oil and dirt may be organic or inorganic in nature. Organic materials include saponifiable oils of animal and vegetable origin, mineral oils and waxes, and other organic contaminants such as inhibitors. Metal oxides, residues from the operations such as polishing, abrading, fluxing, and dust are the main inorganic materials. Cleaning operations consume large amounts of water and involve the use of a variety of chemicals. Some processes such as electroplating and electroless plating require a high degree of cleanliness, while others may not require the same degree of cleanliness. On the other hand, the composition and physical properties of the material being cleaned are important for the cleaning processes. As a result of these varying requirements, many types of cleaning processes have been developed. However, considering the basic character of the cleaning solution, four main cleaning groups can be defined: solvent cleaning, alkaline cleaning, electrocleaning, and acid cleaning. Solvent cleaning is described under solvent degreasing. Alkaline cleaning involves the use of builders such as sodium or potassium salts of phosphates, carbonates, silicates and hydroxides, surfactants, and, sometimes, antioxidants or inhibitors, complex formers, stabilizers, and small amount of solvents. Newly introduced nonemulsifying surfactants are very effective in separating the soil from surfaces. Strong alkaline cleaners may also contain cyanide. Alkaline cleaning is more effective for removing soil from surfaces.
Electrocleaning uses a strong alkaline with an electric current either reverse, periodic reverse, or direct, to remove soils and activate the surface. It is applied, generally, as a last step of cleaning. An electric current electrolyzes the water, evolving hydrogen and oxygen gases. Oxygen exerts a scrubbing effect on the surfaces. In reverse cleaning, the workpiece functions as the anode and the evolved oxygen assists in the removal of soil. In direct cleaning, the workpiece is the cathode and liberated hydrogen at the surface facilitates the scrubbing action. In the periodic reverse system, the workpiece is made alternately anodic and cathodic.
The three most common application modes of aqueous cleaning are immersion with mechanical agitation, immersion with ultrasonic agitation, and spray washing. Ultrasonic cleaning is a highly effective method. The method uses high-frequency sound waves, which locally exert high pressure and temperatures to loosen and remove the contaminants.
Sometimes semi-aqueous methods of cleaning can be used. Emulsion cleaning uses common organic solvents such as kerosene, mineral oil, and benzene, dispersed in an aqueous medium. Diphase cleaning is a two-layer system of water-soluble and water-insoluble organic solvents.
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