Acid Cleaning of Nonferrous Alloys

Aluminum Alloys. Acid cleaning of aluminum may be used alone or in conjunction with other acid, alkaline, or solvent cleaning systems. Vapor degreasing and alkaline cleaning may be required for removal of heavy oils and grease from workpieces before they are immersed in an acid bath. One of the main functions of an acid cleaner is the removal of surface oxides prior to resistance welding, painting, conversion coating, bright dipping, etching, or anodizing.

A mixture of chromic and sulfuric acids is commonly used to remove surface oxides, burntin oil,water stains or other films, such as the iridescent or colored films formed during heat treating. This acid mixture cleans and imparts a slightly etched appearance to the surface, preparing it for painting, caustic etching, conversion coating, or anodizing. Nonpolluting, proprietary products free of chromic acid are available for acid cleaning and deoxidizing.

When tungsten and molybdenum are slightly oxidized on the surface or after the heavily oxidized workpiece is cleaned with molten caustic, acid cleaning is used. The acid solution consists of 50 to 70 vol% concentrated nitric acid, 10 to 20% concentrated hydrofluoric acid, remainder water. The cleaning solution is best when maintained at temperatures of 50 to 65 °C (120 to 150 °F).

Tantalum and Niobium. After mechanical grinding, abrasive blasting, or alkaline cleaning, tantalum and niobium are cleaned further with an acid solution. This consists of 40 to 60 vol% concentrated nitric acid, 10 to 30% concentrated hydrofluoric acid, remainder water. This cleaning solution is best when maintained at temperatures of 50 to 65 °C (120 to 150 °F). After acid cleaning, the workpiece should be washed with water or rinsed thoroughly with a jet of water to remove any traces of acids.

Good ventilation and drainage systems should be installed in the acid cleaning or pickling room. A recycling system to remove the residues and to refresh the acid is preferred for both economical and ecological reasons.

Mechanical Cleaning Systems

Revised by Ted Kostilnik, Wheelabrator Corporation

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