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Alkaline cleaning

Dr. Dmitri Kopeliovich

Alkaline cleaning is a Surface preparation operation utilizing an immersion of the workpiece into an alkaline aqueous solution (or spraying of the solution on the workpiece) where the contaminants such as mineral oils and greases, vegetable oils, animal fats, Cutting fluids (coolants), residual soils, metal chips, dust, etc. are removed from the workpiece surface as a result of a combination of chemical and physical actions of the cleaning solution.

Advantages of alkaline cleaning

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Composition of alkaline cleaners

Alkaline cleaners are formulated as blends of the two major components:

  • Alkaline builder - a chemical compound producing and maintaining an alkalinity (increased level of PH) for the cleaner. The builders provide saponification of the vegetable oils and animal fats. They also neutralize the acidic contaminants. The most popular alkaline builders are:
    • Caustic soda (sodium hydroxide, NaOH) and caustic potash (potassium hydroxide, KOH). Caustics provide high alkalinity (PH 12-14)and saponify vegetable oils and animal fats. Caustics are strong alkaline builders and they are not applicable for cleaning Aluminum alloys and some other soft metals.
    • Sodium bicarbonate (NaHCO3). Alkalinity of sodium bicarbonate solutions is mild (PH 9-9.5). It does not attack aluminum alloys. Carbonates help to maintain alkalinity of strong builders.
    • Sodium phosphates such as trisodium phosphate (Na3PO4*12H2O) or sodium hexametaphosphate (Na6P6O18). Sodium phosphates provide medium alkalinity (PH 9.5-11.5). They are saponifiers. Phosphates soften hard water. Their solutions do not attack aluminum alloys.
    • Silicates (sodium metalicate, sodium orthosilicate). The alkalinity of silicates is medium (PH 11-12.5). Silicates protect aluminum alloys from corrosion attack. Silicates may remain on the poorly rinsed parts.
  • Surfactant (Surface active agent,detergent). Surfactant is a substance reducing the interfacial energy of the solution in contact with contaminants. Surfactants provide emulsification of oils and suspension of solid residuals in the alkaline cleaners.

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Actions of alkaline cleaners

Alkaline cleaning comprises several chemical and physical actions:

  • Saponification - a chemical reaction (hydrolysis) between the acidic substances (vegetable oils and animal fats) and a basic substance (alkaline solution). As a result of hydrolysis triglyceride contained in the natural oils and fats is converted into glycerol and fatty acids. The latter react with the the base (alkaline solution) forming a water soluble salts of fatty acids (soap). Due to their miscibility in water soaps are easily removed from the workpiece surface by Rinsing.
  • Detergency - displacement of solid or liquid soil from the workpiece surface. The detergency action is enabled by the presence of Surfactants (detergents), which reduce the energy of the contaminant-water interface. Displaced contaminants form either solid or liquid dispersions in the solution and may be rinsed away by water. Agitation (either solution or the part) helps the soil particles removal. The most popular methods of mechanical agitation used in alkaline cleaning are: application of ultrasonic waves and electrocleaning (anodic, cathodic or periodic-reverse).
  • Emulsification - dispersing oils and fats in form of fine globules suspended in the aqueous alkaline solution. Emulsification is promoted by the surfactants lowering the interfacial tension between the oil and water phases. Surfactants also stabilize the emulsion preventing its coagulation. Emulsified oils and fats are rinsed away.
  • Solubilization - dissolution of some soils in the alkaline solution. Some surfactants are able to blend the oily contaminants into true solutions (molecular dispersions).

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alkaline_cleaning.txt · Last modified: 2014/05/03 by dmitri_kopeliovich
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