Chemical Cleaning


A typical surface is covered with various films, oils, dirt, and other contaminants. Although some of these substances may operate in a beneficial way (such as the oxide film on aluminum), it is usually desirable to remove contaminants from the surface.

Some of the important reasons why manufactured parts (and products) must be cleaned are:
  • to prepare the surface for subsequent industrial processing, such as a coating application or adhesive bonding;
  • to improve hygiene conditions for workers and customers;
  • to remove contaminants that might chemically react with the surface; and
  • to enhance appearance and performance of the product.


    General Considerations

    There is no single cleaning method that can be used for all cleaning tasks. Just as various soaps and detergents are required for different household jobs, various cleaning methods are also needed to solve different cleaning problems in industry. Important factors in selecting a cleaning method are:
  • the comtaminant to be removed,
  • degree of cleanliness required,
  • substrate material to be cleaned,
  • purpose of the cleaning,
  • environmental and safety factors,
  • size and geometry of the part, and
  • production and cost requirements.

    Various kinds of contaminants build up on part surfaces, either due to previous processing or the factory environment. To select the best cleaning method, one must first identify what must be cleaned. Surface contaminants found in the factory usually divide into one of the following categories:
  • oil and grease, which includes lubricants used in metalworking;
  • solid particles such as metal chips, abrasive grits, shop dirt, dust, and similar materials;
  • buffing and polishing compounds; and
  • oxide films, rust, and scale.

    Degree of cleanliness refers to the amount of contaminant remaining after a given cleaning operation. Parts being prepared to accept a coating (e.g., paint, metallic film) or adhesive must be very clean; otherwise, adhesion of the coated material is jeopardized. In other cases, it may be desirable for the cleaning operation to leave a residue on the part surface for corrosion protection during storage, in effect replacing one contaminant on the surface by another that is beneficial. Degree of cleanliness is often difficult to measure in a quantifiable way. A simple test is a wiping method, in which the surface is wiped with a clean white cloth, and the amount of soil absorbed by the cloth is observed. It is a nonquantitative but easy test to use.

    The substrate material must be considered in selecting a cleaning method, so that damaging reactions are not caused by the cleaning chemicals. To cite several examples: aluminum is dissolved by most acids and alkalis; magnesium is attacked by many acids; copper is attacked by oxidizing acids (e.g., nitric acid); steels are resistant to alkalis but react with virtually all acids.

    Some cleaning methods are appropriate to prepare the surface for painting, while others are better for plating. Environmental protection and worker safety are becoming increasingly important in industrial processes. Cleaning methods and the associated chemicals should be selected to avoid pollution and health hazards.


    Processes

    Chemical cleaning uses various types of chemicals to effect contaminant removal from the surface. The major chemical cleaning methods are:
  • alkaline cleaning
  • emulsion cleaning
  • solvent cleaning
  • acid cleaning
  • ultrasonic cleaning

    In some cases, chemical action is augmented by other energy forms; for example, ultrasonic cleaning uses high-frequency mechanical vibrations combined with chemical cleaning.