- Surface to be cleaned
- Dirt or soil to be removed
- Degree of cleanliness required
- Method of application of cleaning chemicals
- Disposal of the spent solution.
The dirt or soil to be removed affects to the selection of the cleaner composition. For many types of hydrophobic soils, a standard alkaline cleaner is perfectly satisfactory. If tarnish or oxides to be removed, a chelating cleaner works better.
Soil to be removed may be divided into two groups, organic and inorganic. Organic soils are mineral oil, animal and vegetable oil, and cleaning and pickling residue. These soils result from slushing, quenching and cutting oils, and buffing and drawing compounds used in fabrication or the metal object. They are never present as pure compounds but rather as complex mixtures.
Animal and vegetable oils can be saponified by reaction with the alkaline cleaner, mineral oils can’t work. This reaction is so slow, however, that removes very little oil. Emulsification, solubilisation, and preferential wetting are the primary mechanisms for the removal of both types of oil.
The ease of removal depends on the composition of the soil. Soils containing polar groups will be adsorbed on the metal surface, and if they contain high percentages of free fatty acids, metal soaps will form which are strongly attached to the metal surface. The bond is strengthened with time, particularly when the metal part is heated. These bonded soaps are very difficult to remove, their formation should be avoided if possible. The pickle and cleaner residue are either inhibitors, which are used to control the corrosive action of the acidic treating solutions, or metallic soaps that may form during the cleaning operation when an alkaline cleaner is used.
The inorganic soils, which include rust and tarnish, solid particle dirt, scale, smut, and cleaning residues, are usually removed by alkaline degreasing or an acid pickle. Rust and tarnish, being insoluble in water, are removed in acidic or alkaline chelating solution, with or without electric current. Water insoluble solid particles often adhere to oil present on the surface and are therefore removed simultaneously with the oil. Smuts which result from pickling are very troublesome and should be avoided. Inorganic cleaning residues may be films of oxides, phosphates, silicates, and the like remaining on the metal surface after alkaline cleaning and not removed by the rinses and mild acid dips that follow. Care in rinsing is the best way to eliminate trouble from this source.
Not all plating operations require the same degree of cleanliness of the basis metal. It is possible to plate from an alkaline cyanide plating solution articles that are dirty compared to those that may be processed in acid plating baths such as nickel plating. Therefore, a much shorter cleaning cycle may be employed for alkaline cyanide plating. It is poor practice, however, to contaminate a plating solution by using it as a combination cleaning and plating bath.
By definition, a chemically clean surface is one that has no film on it that might be detected by chemical or physical tests, obviously such a surface can be realized only with extreme difficult even in the laboratory. It is generally agreed, however, that surface substantially free of grease, oils, and gross oxide films are needed for the best electroplating. Therefore it is standard practice to clean as thoroughly as is economically feasible.
The method of application is very important owing to the effect of agitation on those cleaners that contain foaming surface active agents. For electro cleaners and spray cleaners, low surface active agent must be used.
The quality of water supply has a direct bearing on the type of cleaner that may be used. For example, where soft water is available cleaners containing soaps may employed, but with hard waters, the use of soaps should be avoided unless the water is properly treated, or the cleaner contains water softening, sequestering or chelating agents.
Disposal of waste solutions may be a problem and can influence the use of a particular compound. For example, a very satisfactory bath for the removal of oxide or tarnish films from copper and copper base alloys is a sodium cyanide solution, but disposal of the cyanide may be so troublesome as to make such a bath undesirable.