During post-metal treatment operations, metal surfaces must be thoroughly cleaned before a finish can be applied successfully. All foreign matter must be removed prior to painting or other surface preparation for acceptable adhesion to occur. In the aerospace and medical fields, this is especially important. Foreign matter includes:
Effective surface cleaning is achieved through a combination of detergency, chemical reaction, solvent dissolution and mechanical action. Principal methods employed in cleaning metal parts include spray, soak and electrolytic. ICL phosphates and phosphoric acids are used extensively in formulating both alkaline and acid cleaners. Alkaline cleaners are more effective in removing grease, oils, fatty soils and solid soils. Alkaline cleaners primarily contain water, builders and surfactants.
Acid cleaners are more effective when it comes to Metal Cleaning And Surface Treatment, smut, lime and milkstone. They are typically composed of mineral acids, organic acids or acid salts, in combination with a wetting agent and detergent. Inhibitors and cleaning solvents may also be added for specific applications. ICL phosphoric acid, monosodium phosphate (MSP) and sodium acid pyrophosphate (SAPP) are commonly used ingredients in acid cleaning formulations. Acid pickling refers to a relatively more severe treatment for the removal of scale from semi-finished mill products such as forgings or castings. Commonly used acids for pickling stainless steels include nitric, sulfuric, hydrochloric and phosphoric. Phosphoric acid/ethylene glycol monobutyl ether mixtures are widely used for cleaning grease, oil, light rust and scale from iron and steel.
Phosphoric acid or phosphate salts are typically used to enhance corrosion resistance and improve the adhesion properties of metal surfaces before additional coating operations occur. They provide a base for oils or other rust preventive treatments that lubricate and resist wear and provide a base for adhesives in plastic-metal laminations, resulting in a surface on steel that facilitates forming operations. The phosphating process is of major industrial importance for protecting iron and steel surfaces, and may also be applied as a protective treatment for zinc, aluminium, cadmium and magnesium-based metals. Phosphate coating solutions can be applied either by spray or immersion, depending on the application and coating weight desired.
The chemical polishing of aluminium, often called bright-dipping, is a popular means for obtaining a specular or bright finish on many types of aluminium fabrications and extrusions. Bright-dipping consists of immersing pre-cleaned aluminium parts into a phosphoric-nitric acid bath held at an elevated temperature. This process involves the selective attack on the aluminium surface protrusions, resulting in the levelling of the overall surface. A very smooth, specular finish can be obtained, which is then anodized, dyed and sealed. Examples of items with bright-dip finishes include lighting fixtures and reflectors, shower door frames, window frames, picture frames, lawn furniture and trim for automotive, luggage and appliance applications. Phosphoric acid is widely used in this application, together with fume suppressant and copper, which helps improve the specularity of the finish.