Cleaner Composition

A variety of mineral acids and solutions of acid salts can be used, either with or without surfactants (wetting agents), inhibitors, and solvents. The large number of compositions that are used may be classified as:

• Inorganic (mineral) acid solutions

• Acid-solvent mixtures

• Solutions of acid salts

Many acid cleaners are available as proprietary compounds, either as a liquid concentrate or a powder to be mixed with water. Compositions of several solutions used for cleaning ferrous metals are given in Table 1. Table 2 contains some possible operating conditions when cleaning ferrous metals.

Table 1 Typical composition of acid cleaners for cleaning ferrous metals

Composition of each constituent is given in percent by weight.

Table 1 Typical composition of acid cleaners for cleaning ferrous metals

Composition of each constituent is given in percent by weight.

Constituent

Immersion

Spray

Barrel

Wipe

Electrolytic

Phosphoric acid

70

70

15-25

Sodium acid pyrophosphate

16.5

16.5

16.5

Sodium bisulfate

80

80

80

Sulfuric acid

55-70

Nonionic wetting agent(a)

5

5

7-20

Anionic wetting agent

3

3

3

Other additives

(b)

(b)

(b)(c)

(b)(c)

(b)(c)

(b)(d)

(b)

Water

25(e)

25(e)

bal

bal

(a) Ethylene glycol monobutyl ether is used.

(b) Inhibitors up to 1 % concentration may be used to miminize attack on metal.

(c) An anti-foaming agent is usually required when the cleaner is used in a spray or barrel system.

(d) Small additions of sodium nitrate are often used as an accelerator in cleaning rolled steel; nickel nitrate is used in cleaning galvanized steel.

(e) Before dilution

Table 2 Operating conditions for acid cleaners for ferrous metals

Type of acid cleaner

Concentration

Temperature

g/L

oz/gal

°C

°F

Immersion

120 60-120

16 8-16

60

160 140

Spray

15-30

2-4

60 60

140 140

Barrel

15-60

2-8

Room

Room

Wipe

Room

Room

Sulfuric and especially hydrochloric acids are the most commonly used for cleaning operations. They are relatively economical to use and in some cases can be reclaimed by ion exchange or chilling methods whereby the dissolved iron is removed. Reclamation can have a significant positive impact on disposal and operating costs where large quantities of acid are consumed.

Typical operating concentrations are 20 to 60 vol% for hydrochloric acid and 4 to 12 vol% for sulfuric acid. Normally both are highly inhibited to minimize the attack of the base metal and process equipment.

Organic acids such as citric, tartaric, acetic, oxalic, and gluconic, and acid salts such as sodium phosphates, ammonium persulfate, sodium acid sulfate, and bifluoride salts, are used in various combinations. Solvents such as ethylene glycol monobutyl ether and other glycol ethers, wetting agents and detergents such as alkyl aryl, polyether alcohols, antifoam agents, and inhibitors may be included to enhance the removal of soil, oil, and grease.

Strength of the acid solutions varies from as weak as 5.5 pH for acid-salt mixtures to the equivalent of the strong acids used for pickling.

The phosphoric acid and ethylene glycol monobutyl ether mixtures (Table 1) are used for removing grease, oil, drawing compounds, and light rust from iron and steel. In various concentrations, these mixtures are adaptable to immersion, spray, or wiping methods and leave a light phosphate coating (110 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 sorted.

Chromic acid solutions are used occasionally to clean cast iron and stainless steel. A chromic acid formula used for cleaning stainless steel is 60 g/L (8 oz/gal) chromium trioxide, 60 g/L (8 oz/gal) sulfuric acid, and 60 g/L (8 oz/gal) hydro-fluoric acid in water, used at room temperature in an immersion system. Another solution used frequently for cleaning stainless steel is a solution of nitric acid (10 to 50 vol%) and hydrofluoric acid (1 to 3 vol%) in water. The steel is immersed in the solution at room temperature for 3 to 30 min.

Chromic acid solutions and mixtures containing chromic acid are often used as final rinses in acid cleaning-phosphating systems. The acid enhances the corrosion resistance of the coated surface. Paint applied following such a treatment gives greater protection against corrosion by salt and humid environments. Chromic acid is used in solutions of low pH when a strong oxidant is required. Nitric acid is also a strong oxidant, and a 10 to 20% nitric acid solution is used to brighten stainless steel.

For electrolytic cleaning applications, very high concentrations of sulfuric acid (Table 1) are recommended although hydrochloric acid may also be used. Phosphoric acid, however, is unsuitable due to its high gassing characteristic.

Various soils, including light rust, are removed by combining acid cleaning with mechanical action. Acid salts such as sodium acid pyrophosphate, sodium bisulfate, and mixtures of the two are sometimes used to clean ferrous metal parts in rotating barrels. (A formula is given in Table 1.) A solution with this formula may also be used for parts that are immersed or sprayed.

Additives such as oxalic acid occasionally are used with the acid salts when ferrous metal parts are being cleaned in rotating barrels. Oxalic acid attacks steel, but seldom to an objectionable degree. Thiourea is a good inhibitor, if inhibited oxalic acid solutions are required. The addition of fluoride salts to acid salts, such as 8 to 15 g/L (1 to 2 oz/gal) sodium fluoride or ammonium bifluoride, improves efficiency in the removal of silica sand from castings when parts are cleaned in a barrel or tank.

A formula used for wipe cleaning is also given in Table 1. Other cleaners used for wiping are 6 to 8 vol% sulfuric acid in water; 70% phosphoric acid, 5% wetting agent, and 25% water; and a paste made of 85 to 95% ammonium dihydrogen phosphate and the remainder wetting agent, used on a wet cloth or sponge.

Inhibitors are often included in cleaners used on ferrous metals to minimize attack on metal and lower acid consumption. Composition of inhibitors varies widely. Numerous byproducts, such as sludge acid from oil refineries, waste animal materials, waste sulfite cellulose liquor, offgrade wheat flour, and sulfonation products of such materials as wood tar, coal tar, and asphaltum, have been successfully used. These materials cost less than synthetic inhibitors but can vary widely in uniformity and effectiveness and may contain toxic or carcinogenic substances. For these reasons, synthetic inhibitors now dominate the market.

Synthetic inhibitors are usually complex organic compounds. One of the most common inhibitors for hydrochloric-acid-based cleaners was propargyl alcohol, which is poisonous and has been removed from most acid cleaners. Most often, a given compound or class of compounds will function most effectively with only one type of acid, so choosing the proper inhibitor should not be a haphazard process. Many proprietary compositions of these chemicals are available for use in various acid systems.

The amount of inhibitor used depends on the workpiece composition, acid cleaner formulation, temperature of operation, and nature of soil being removed. From 2 to 1% inhibitor before dilution with water is used. Higher percentages of inhibitor may be used for higher acid concentrations and operating temperatures. Once the optimum concentration is established for a particular operation, higher concentrations have no positive effect and result in increased cost.

Antifoaming agents may be required in acid spray cleaners to prevent excessive foaming. Sometimes foaming can be reduced by using naturally hard water or by adding small amounts of calcium chloride, up to -0 grains hardness. Addition of a plasticizer such as triethylhexylphosphate or one of the high-molecular-weight polyols (organic alcohols) reduces foaming. Because of variation in water and other conditions in a specific installation, several additives may need to be tried before foaming is brought under control. Silicones are usually effective as antifoaming agents, but they should not be used if parts are to be painted or plated, because of residual contamination. Paint or plating does not adhere to the silicone contaminated areas, resulting in a fisheye appearance at the contaminated spots.

Foaming agents may be desirable in certain immersion applications, to reduce acid fume evolution to the atmosphere and to provide an insulating blanket on the surface of the tank to decrease heat loss from evaporation. Proprietary inhibitors having controlled foaming properties are available.

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