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Sulfuric Acid Pickling. Sulfuric acid produces satisfactory results when used for batch pickling of carbon steel rod and wire (up to 0.60% C) and for continuous cleaning, if the iron concentration in the bath is less than 8 g/100mL. Table 1 lists the types of carbon and alloy steel products that are pickled in sulfuric acid and the ranges of acid concentrations and temperatures used.

Table 1 Solution concentrations and operating temperatures used for pickling carbon and alloy steel products

Product

Sulfuric acid concentration, wt%

Bath temperature

Minimum

Maximum

Minimum

Maximum

°C

°F

°C

°F

Bar, low-carbon

7

18

68

155

85

185

Bar, alloy

9

12

66

150

77

170

Billet, low-carbon

7

12

74

165

82

180

Billet, alloy

9

12

82

180

93

200

Pipe for galvanizing

7

15

71

160

88

190

Sheet for galvanizing

4

12

66

150

77

170

Sheet, tin plate (white pickle)

9

12

66

150

85

185

Strip, soft

6

12

77

170

88

190

Strip, alloy and high-carbon

7

12

66

150

77

170

Strip, continuous pickling

23

38

77

170

100

212

Tubing, low-carbon seamless

7

18

77

170

88

190

Tubing, high-carbon and alloy structural

9

18

71

160

93

200

Tubing (over 0.40% carbon)

9

18

60

140

71

160

Wire, soft

4

18

77

170

88

190

Wire, alloy and high-carbon

3

18

55

130

74

165

Fabricated parts (for tinning):

Initial pickle

5

10

66

150

88

190

Final dip

(a)

(a)

38

100

(a) Concentrated hydrochloric acid, 1.14 to 1.16 sp gr

(a) Concentrated hydrochloric acid, 1.14 to 1.16 sp gr

Commercial sulfuric acid is usually supplied at a concentration of 93 wt%, whereas hydrochloric acid is supplied at concentrations of 31 or 35 wt%. Prices for tank car lots of sulfuric and hydrochloric acids vary geographically within the United States. During 1993, costs were similar for 93 wt% sulfuric acid and 35 wt% hydrochloric acid. An advantage of using sulfuric acid is less fuming over pickling solutions. Disadvantages include darker surfaces and the production of smut, particularly on high-carbon steel, as well as a greater inhibiting effect on the sulfuric acid of iron salts in the bath.

Emissions from sulfuric acid pickling may include a spray (droplets of pickling solution resulting from acid attack on base metal that generates hydrogen gas); adequate ventilation must be provided to prevent localized corrosion of equipment and unsatisfactory working conditions.

Hydrochloric Acid Pickling. Hydrochloric acid is preferred for the batch pickling of hot-rolled or heat-treated highcarbon steel rod and wire. Continuous pickling operations also use hydrochloric acid to produce the very uniform surface characteristics required for both low- and high-carbon steel. The possibility of overpickling is minimized in these short-time operations. The acid also dissolves lead oxides that adhere to steel previously heat treated in molten lead baths.

Operating conditions for batch pickling in hydrochloric acid solutions typically involve acid concentrations of 8 to 12 g/100 mL, temperatures of 38 to 40 °C (10 to 105 °F), and immersion times of 5 to 15 min, with a maximum allowable iron concentration of 13 g/100 mL. Operating conditions for continuous pickling in hydrochloric acid solutions typically involve acid concentrations of 2 to 20 g/100 mL, temperatures of 66 to 93 °C (150 to 200 °F), and immersion times of 1 to 20 s.

Hydrochloric acid offers a number of advantages, when compared with sulfuric and other acids. It consistently produces a uniform light-gray surface on high-carbon steel. The possibility of overpickling is less than it is with other acids. Effective pickling can be obtained with iron concentrations as high as 13 g/100 mL. Rinsing is facilitated because of the high solubility of chlorides. The cost of heating the bath for batch-type operations is less than it is with sulfuric acid because of lower operating temperatures. The chief disadvantage of hydrochloric acid is the necessity for a good fume-control system.

Emissions from hydrochloric acid pickling include hydrogen chloride gas and must be adequately vented to prevent localized corrosion of equipment and unsatisfactory working conditions.

Other Acid Mixtures. Excessive contamination of the pickling bath by oiled steel results in nonuniform descaling and staining of the steel. To avoid this problem, oiled steel should be degreased before pickling. When pickling either oiled or degreased steel, the use of a wetting agent in the acid solution increases the effectiveness and efficiency of the bath, thereby reducing immersion time. Many commercial pickling inhibitors are formulated with a wetting agent.

Annealing smut and heavy-metal ions can be removed from the surface of steel to be cold drawn, porcelain enameled, or tin plated by adding sodium ferrocyanide to the acid pickling solution. A solution of sulfuric and hydrofluoric acids can be used to pickle castings that have burned sand embedded in the surface. The relative concentration of each acid is determined by whether the primary objective is sand removal or scale removal.

Determination of Acid and Iron Concentrations in Pickling Baths. Plants frequently report acid and iron-salt concentrations in weight/volume (w/v) units of grams per 100 milliliters (g/100 mL). Although these units are sometimes loosely referred to as "percent," concentrations in g/100 mL must be divided by the density of the solution in grams per milliliter (g/mL) to convert to true weight percent (weight/weight, or w/w, units). For this purpose, approximate equations for calculating densities have been developed from published data on sulfuric acid-ferrous sulfate solutions (Ref 4, 5, 6, 7) and on hydrochloric acid-ferrous chloride solutions (Ref 8, 9):

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