Nonmechanical Cleaning

Nonmechanical cleaning methods applicable to cast irons include molten salt bath cleaning, pickling, and chemical cleaning with the latter method including acid cleaning, solvent cleaning, vapor degreasing, and emulsion cleaning. Articles on each of the aforementioned processes can be found in the Section on "Surface Cleaning" in this Volume.

Molten salt baths are excellent for cleaning complex interior passages in castings. In one electrolytic, molten salt cleaning process, the electrode potential is changed so that the salt bath is alternately oxidizing and reducing. Scale and graphite are easily removed with reducing and oxidizing baths, respectively. Molten salt baths are fast compared to other nonmechanical methods, but castings may crack if they are still hot when salt residues are rinsed off with water.

Pickling of cast iron is usually done prior to hot dip coating or electroplating. When pickling is used, the castings are cleaned in solutions containing sulfuric and hydrofluoric acids. The concentration of each acid depends on whether the primary purpose is to remove sand or scale. Increased hydrofluoric acid is needed to remove embedded sand from the casting surface, whereas sulfuric or hydrochloric acids are sufficient for simple scale removal. Table 4 gives the operating conditions for pickling iron castings. Before being pickled, castings must be free of oil, grease, and other contamination. After being removed from the pickling solution, castings are rinsed thoroughly in hot water. Residual heat permits self-drying, but drying may be accelerated by the use of fans. For complex shaped castings, baking ovens may be employed.

Table 4 Operating conditions and solution compositions for pickling cast irons

Operating

Sand

Scale

variable

removal

removal

Sulfuric acid, vol%

5

7

Hydrofluoric acid, vol%

5

3

Water, vol%

90

90

Temperature(a), °C (°F)

65-85 (150-185)

50 to over 85 (120 to over 185)

Average immersion time, h

4

4

(a) 50 °C (120 °F) is for slow pickling, 65 to 85 °C (150 to 185 °F) for average pickling speed, and over 85 °C (185 °F) is for fast pickling.

Acid cleaning is a process in which a solution of a mineral acid, organic acid, or acid salt, in combination with a wetting agent and detergent, is used to remove oxide, shop soil, oil, grease, and other contaminants from metal surfaces, with or without the application of heat. The distinction between acid cleaning and acid pickling is a matter of degree, and some overlapping in the use of these terms occurs. Acid pickling is a more severe treatment for the removal of scale from castings, whereas acid cleaning generally refers to the use of acid solutions for final or near-final preparation of metal surfaces before plating, painting, or storage.

Mixtures of 70 wt% phosphoric acid, 5 wt% ethylene glycol monobutyl ether, and 25 wt% water are used for removing grease, oil, drawing compounds, and light rust from iron and steel. This mixture is adaptable to immersion, spray, or wiping methods and leaves a light phosphate coating (100 to 320 mg/m2, or 10 to 30 mg/ft2) that provides a paint base or temporary resistance to rusting if the parts are to be stored.

Acid solutions of 40 to 60 vol% hydrochloric or 6 to 8 vol% sulfuric (often containing up to 1% inhibitor) are used at room temperature for removing soil and light rust. Stronger solutions of these acids are used in electrolytic baths for final cleaning of cast irons before electroplating. Various soils, including light rust, may also be removed by combining acid cleaning and barrel finishing.

Other Chemical Cleaning Methods. Organic solvents that were commonly used in the past include naphtha, benzene, methanol, toluene, and carbon tetrachloride. These have been largely replaced by chlorinated solvents, such as those used for vapor degreasing. Solvents effectively remove lubricants, cutting oils, and coolants, but are ineffective against such inorganic compounds such as oxides or salts. Emulsion cleaners are solvents combined with surfactants; they disperse contaminants and solids by emulsification. Emulsion cleaners are most effective against heavy oils, greases, slushes, and solids entrained in hydrocarbon films. They are relatively ineffective against adherent solids such as oxide scale.

After wet cleaning, short-term rust prevention is accomplished by the use of an alkaline rinse. This can be followed by the application of mineral oils, solvents combined with inhibitors and film formers, emulsions of petroleum-base coatings and water, and waxes. A summary of the temporary coatings used for corrosion protection is given in Table 5.

Table 5 Temporary coatings used for corrosion protection

Type

Coating structure

Thickness,

Removal method

Uses

fim (mils)

Oil

Non-drying mineral oil viscosity determines coating thickness

5-7 (G.2-G.3)

Seldom required; solvent rinsing, emulsion spray, or vapor degreasing

Finished automotive parts; galvanized irons

Solvent

Petroleum-based inhibitors and film formers dissolved in petroleum solvents

5-5G (G.2-2.G)

Seldom required; solvent rinsing or alkaline washing

External surfaces of machinery parts and tooling. Can be used outdoors

Emulsified

Petroleum-based coating modified to form a stable emulsion with water

5 (G.2)

Removal seldom necessary; solvent rinsing

Same as above, but suggested for indoor use

Wax

Wax layer applied heated or in a volatile solvent

38-75 (1.5-3.G)

Solvent rinsing or alkaline cleaning

Highly finished parts stored for prolonged periods

Source: Ref 7

Source: Ref 7

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