Metal Treatment

Metal Treatment is a key aspect of the  of the metal industry. Phosphates and phosphoric acid from ICL Phosphate Specialties play a critical role in this area.

The treatment of metal surfaces with phosphoric acid or phosphate salts to provide a coating of insoluble metal phosphate crystals is referred to as phosphating. These coatings affect the appearance, hardness and electrical properties of the metal surface. They also provide corrosion resistance and improve the adhesion of paint to the metal surface. Phosphate coatings are also used to provide a base for oils or other rust preventive treatments, to provide lubricity and resistance to wear, as a base for adhesives in plastic-metal laminations, and to generate a surface on steel that facilitates forming operations.

The phosphating process is of major industrial importance for the protection of iron and steel surfaces and may also be applied as a protective treatment for zinc, aluminum, cadmium and magnesium-based metals. Typical phosphating solutions contain phosphoric acid, a phosphate salt (MSP, DSP, or SAPP) and a variety of accelerators such as nitrates, nitrites, chlorates, peroxides, sulfites, molybdates, borates or citrates. Iron and zinc phosphate coatings are the most commonly used. Manganese phosphate, which is used as a base for oil or lubricants, represents the balance of the phosphating market. Phosphate coating solutions can be applied either by spray or immersion, depending on the application and coating weight desired.

The chemical polishing of aluminum, often called bright-dipping, is a popular means for obtaining a specular or bright finish on many types of aluminum fabrications and extrusions. Bright-dipping consists of immersing pre-cleaned aluminum parts into a phosphoric-nitric acid bath held at an elevated temperature. This process involves the selective attack on the aluminum surface protrusions, resulting in the leveling 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 a fume suppressant and copper, which helps improve the specularity of the finish.

Chemical polishing of metals is often done electrolytically. This process involves removal of the metal at the high spots of irregularities with little or no dissolution of metal in the low spots or “valleys.” The base metal must be reasonably smooth to produce a satisfactory finish. Stainless steel, steel, brass, aluminum, nickel, copper and zinc are among the metal surfaces that can be electropolished.

Highly concentrated solutions of phosphoric and/or sulfuric acids are frequently used for electropolishing. Chromic acid also finds some use in this application. More detailed information on these and other applications are available from our technical service group.

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Monosodium Phosphate

•Excellent carrier for liquid surfactants and phosphoric acid

Phosphoric Acid

Highly concentrated solutions of phosphoric and/or sulfuric acids are frequently used for electropolishing.

•Used in aluminum cleaner/brightener formulations
•Major phosphatizing ingredient

Sodium Acid Pyrophosphate (SAPP)

•Good builder for use in acid cleaners
•Can also sequester iron and copper

Tetrapotassium Pyrophosphate (TKPP)

Builders of choice where high alkalinity and prolonged high temperatures are required, such as soak tank cleaners

Tetrasodium Pyrophsphate (TSPP)

Used as builders in formulations containing caustic to aid in the rinsability of caustic