The first step in the chemical pretreatment process is the removal of oils, dirt, and other soils that will interfere with the development of a good quality phosphate coating, good coating adhesion, or cause surface defects. Chemical cleaning can be accomplished by subjecting a part to an aqueous spray or dip cleaner. The cleaner may be alkaline, acidic, neutral, solvent, or emulsion depends upon the type of surface to be treated. The particular cleaner used will depend on the soils to be removed, the size and type of part, the type of coating to be applied, and the substrate material. We are using different processes for degreasing surfaces. Moderately they are recognized by terms such as solubilisation, saponification (alkaline hydrolysis of a fat or oil), soaking, emulsification, sequestration and deflocculation. In each of these processes the action requires surface wetting of the metal by the cleaning solution. Solubilisation, the dissolving of soils into solution, can occur when the soils have very similar polarity and chemical affinity for the cleaning media. Emulsification, the suspension of soils in solution, requires that the soils dispersible in the cleaning media. Saponification, turning the soils into soap, applies specifically to those soils that contain carboxylic acid and ester functionality that can react with alkaline cleaning media. Sequestration involves the deactivation of metallic ions in the soil to prevent them from interfering with the detergent action of the cleaner. Deflocculation is a process that breaks up large particles of aggregate soils into a finely divided material that is held in suspension in the solution to prevent redeposition on the part surface. The last two processes generally operate in conjunction with the first three processes mentioned. While alkaline cleaners are the most common, there are also acid cleaners and emulsion cleaners used for industrial applications. The cleaner selected must have the ability to remove a wide variety of soils, prevent redeposition, provide cleaning even when contaminated, provide foam control, be easily rinsed and be cost effective. Proper cleaning of some parts may require a combination of spray and immersion stages. The spray stage combines the chemical properties of the cleaner with the mechanical impingement of the solution applied under pressure. Immersion penetrates areas of the part that may be inaccessible to the spray. Spray or immersion processes can be used in manual batch operations or in automated systems with overhead conveyor. Batch systems will use a hand-held spray wand or small dip tanks. Conveyorized systems will use an in-line spray washer that has the proper number of stages. Batch systems are suitable for smaller volumes with less stringent quality standards. The list below shows some of the types of hand held systems and how they compare. Larger volumes or products with demanding quality standards will probably require a spray washer. Spray Wand Phosphatizing - Best suited for large bulky parts where dip tanks or conveyor systems would require more space and cost. Steam Cleaning - For small volume of heavily soiled parts. Melts grease. High Pressure Hot Water - Best for cleaning large bulky parts; should have 4-5 GPM, 1,000 PSI plus heat capacity at the nozzle of 160- 200 oF (71-93 oC). Cleaners may be classified according to their pH, a reference to the measurement of the relative alkalinity or acidity. pH is a measure of the ratio of hydrogen ions in solution to the number of hydroxyl ions in solution. If there are more hydrogen ions the solution will be acidic, if there are more hydroxyl ions the solution will be alkaline. On the pH scale, pure water is neutral and has a pH of 7. A pH of 0 to 7 is acidic and 7 to 14 is alkaline. Caustic soda has a pH of 13 or 14 while hydrochloric acid has a pH of less than 1. Cleaner pH varies with different products and substrate materials. Cleaner pH will typically range from 4.5 to 10.5.
• alkaline cleaners - mild, pH 9 - 10.5
- medium, pH 10.5 - 11.5
- high, pH >11.5
• neutral cleaners - pH 6.5 - 9
• acid cleaners - pH 1.0 - 5.5
Until the present time, it has been customary to clean oily, greasy, and/or waxy residues from metal articles by a vapor degreasing process. In a vapor degreasing process, the metal articles, at a temperature below the condensing temperature of a solvent for the oily, greasy and/or waxy residues, are suspended in vapors of refluxing solvent. The refluxing solvent condenses on the surface of the metal article, and the liquid solvent dissolves the oily, greasy, and/or waxy residues on the surface of the metal article. The condensing solvent with the high dissolving power for the contaminants to be removed from the surface of the metal article condenses on the surface of the article, dissolves the contaminants, and is returned to the source of the solvent vapor.
Solvents such as methyl ethyl ketone, methylene chloride, 1,1,1-trichloroethane, trichloroethylene, perchloroethylene, and the like are normally used in the vapor degreasing process. Since the oily, greasy, and/or waxy soils removed from the metal articles generally have a boiling point substantially higher than the boiling point of the refluxing solvent, the metal articles are contacted with a condensed solvent containing only relatively small quantities of the contaminants to be dissolved and removed from the metal articles.
Vapor degreasing is technically effective but economically and environmentally disadvantageous. The solvents are expensive, can be environmental pollutants, and require costly methods for reclamation and disposal. Special apparatus and processes are required to reclaim used solvent for reuse and to prevent solvent vapors from escaping from the vapor degreasing process. The solvent vapors are often hazardous to human health and some of them are suspected of promoting degradation of the earths ozone layer. In view of the drawbacks in the use of the vapor degreasing process, many attempts have been made to replace vapor degreasing with aqueous based cleaning compositions. However, to date the aqueous cleaning methods have not been entirely satisfactory, particularly in preparing metallic surfaces of relatively low density, such as those of aluminum and aluminum alloys, for use in the aerospace industry, where the requirements for cleaning are particularly stringent.