Effect of Process Variables on Scale Removal in Sulfuric Acid

The composition of scale on hot-rolled strip is primarily influenced by the cooling rate after coiling. When pickling with sulfuric acid, this is important because conditions that increase the amount of FeO in the scale (rapid cooling) render it more easily pickled (Ref 20, 21). With hydrochloric acid, the solubility of Fe3O4 is significantly greater than it is in sulfuric acid (Ref 22). Therefore, the relative amounts of FeO versus Fe3O4 in the scale layer are of less importance with hydrochloric acid. As the coiling temperature after hot rolling is increased, the scale thickness increases and pickling rates decrease (Ref 23).

The degree to which pickling rates are affected by concentrations of sulfuric acid and ferrous sulfate, as well as by temperature, is illustrated in Fig. 2 and 3. These bench-scale tests were made with specimens cut from the center and tail end of a hot-rolled coil (2.0 mm, or 0.080 in., thickness) of low-carbon drawing-quality steel. The respective scale thicknesses were 2.6 mg/cm2 (0.00475 mm, or 0.000187 in.) and 5.2 mg/cm2 (0.00953 mm, or 0.000375 in.). As might be expected, specimens with thicker scale required longer immersion times for scale removal than specimens with thinner scale under the same bath conditions. The time to remove scale decreased with increases in temperature from 80 to 100 °C (175 to 212 °F) and with increases in acid concentration from 5 to 25 g/100 mL. With sulfuric acid, increases in the concentration of ferrous sulfate exert an inhibiting action that increases the time for scale removal. The effect is greater when acid concentrations are 10 g/100 mL or lower. Pickling efficiency in a bath decreases with time, unless fresh acid additions are made, because the acid concentration drops while the ferrous sulfate concentration increases. Increased agitation in the bath increases the pickling rate. When specimens with thick scale are pickled in acid solutions of 10 g/100 mL or lower, increases in inhibitor concentration tend to slow down the pickling action.

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10 15 20 25 30 35 Concentration of sLilfuric acid, %

10 15 20 25 30 35 Concentration of sLilfuric acid, %

Temperature of acid solution, ISO 170 180 190 200 210

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Temperature of acid solution, ISO 170 180 190 200 210

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70 75 80 85 90 95 100 105 Temperature of acid solution, °C

70 75 80 85 90 95 100 105 Temperature of acid solution, °C

Fig. 2 Effect of acid concentration (a) and temperature of acid solution (b) on pickling time required to remove scale from sheet steel, 2 mm (0.080 in.) thick

Pickling Process Handbook
Fig. 3 Inhibiting action of ferrous sulfate on low-carbon drawing-quality sheet pickled for 2 min in sulfuric acid solutions of concentrations indicated. (a) Pickling time for complete scale removal. (b) Weight loss

In a separate bench-scale study (Ref 24), it was found that the influence of temper mill scale breaking (cracking the scale by imposing moderate room-temperature deformation to the workpiece) on the descaling time of hot-rolled strip in sulfuric acid solutions is pronounced. Descaling time is frequently half or less the amount required in a given solution without temper mill scale breaking, as illustrated in Fig. 4. The results of bench-scale experiments (unstirred solutions) with a commercial hot-rolled low-carbon steel with a scale weight of 3.4 mg/cm2 (0.0062 mm, or 0.00024 in.) are also shown in Fig. 4. For nontemper-rolled material, descaling times were decreased as the temperature increased from 82 to 105 °C (180 to 220 °F). The pickling times achieved by increasing the temperature from 93 to 105 °C (200 to 220 °F)

were about the same as those that resulted from maintaining the temperature at 93 °C (200 °F) and using temper mill scale breaking (3%) before pickling.

Pickling Process Handbook
Fig. 4 Effect of solution temperature on pickling time for hot-rolled low-carbon steel; comparison with temper mill scale breaking. All solutions contained 15 g FeSO4/100 mL. TR, temper rolled

Because the reductions in strip thickness introduced by temper rolling are relatively small, the effect of strip thickness profiles must be considered when used on pickling lines. Crown is an increase in thickness of the rolled center of strip as compared with the edges. For strip with some crown and feather edges, if the amount of reduction used for temper mill scale breaking is based on the center area with crown, then the thinner edge areas may not receive enough reduction to effectively crack the scale and enhance pickling. Commercial experience indicates that stretch leveling is at least as effective in cracking the scale as the use of a temper mill.

Decreases in pickling rates caused by increases in ferrous sulfate concentration were found to be less pronounced for more concentrated acid solutions. The time required for scale removal in tests at 93 °C (200 °F) was not affected by inhibitor usages up to 0.25 vol%, based on concentrated acid, but did increase when usages exceeded 0.50 vol% (0.25 or 0.50 gal inhibitor, respectively, per 100 gal concentrated acid).

The effect of strip speed, as well as the combined effects of acid and iron concentration, temperature, inhibitor usage, and degree of scale breaking on the pickling process was determined by using an apparatus constructed to simulate the motion of strip through a continuous-strip pickling line. Steel specimens were mounted on a cylindrical holder that could be rotated through a pickling solution. The solution was contained in a holder that had baffles to minimize bulk movement of the solution (Ref 2). Over the range of acid concentrations from 10 to 30 g/100 mL, descaling times were lowered by increases in strip speed from 0 to 30.5 m/min (0 to 100 ft/min), but the magnitude of the effect was not as great as that associated with hydrochloric acid solutions (which will be discussed below.) Only small decreases in descaling time were observed from 30.5 to 122 m/min (100 to 400 ft/min). Data obtained at a strip velocity of 122 m/min (400 ft/min), summarized in Table 2, should be pertinent to commercial continuous pickling in which line speeds can range from 1.5 to 6 m/s (300 to 1200 ft/min) or higher. Laboratory tests made with a well-stirred solution (mechanical stirring of 500 rev/min or greater) should give similar results to those in Table 2.

Table 2 Laboratory pickling tests using sulfuric acid solutions to remove scale from hot-rolled ingot cast steel

Temperature

Sulfuric acid concentration, g/100 mL

Ferrous sulfate concentration, g/100 mL

Time to remove scale, s

°C

°F

0%(a)

1.5%(a)

3%(a)

4.5%(a)

82

180

10

15

90

82

180

20

15

55

82

180

30

15

45

93

200

10

10

55

93

200

20

10

30

93

200

30

10

30

93

200

5

15

15

93

200

10

15

70

20

10

10

93

200

20

15

50

15

10

5

93

200

30

15

40

15

10

5

93

200

10

20

70

93

200

20

20

40

101

214(b)

10

15

40

103

217(b)

20

15

30

106

222(b)

30

15

20

(a) Degree of temper mill scale breaking in percent temper rolled.

(a) Degree of temper mill scale breaking in percent temper rolled.

(b) Solutions were at the boiling point during the test.

The time required to remove scale from hot-rolled strip in stirred sulfuric acid solutions is significantly decreased by temper mill scale breaking. For nontemper-rolled material, pickling at temperatures near the solution boiling point (as high as 105 °C, or 222 °F) resulted in scale removal times that were about half those found at 82 °C (180 °F). At 93 °C

(200 °F), a typical solution temperature on commercial continuous-strip lines that use sulfuric acid, the benefit to be derived by temper mill scale breaking is much greater than would be achieved if the steel were pickled at higher temperatures without temper rolling. Without temper mill scale breaking, the time required to remove the scale was lowered by increasing the acid concentration from 10 to 30 g/100 mL and decreasing the ferrous sulfate concentration from the 15 to 20 g/100 mL range to 10 g/100 mL. An effective commercial inhibitor, even when used at twice the recommended concentration (0.25 vol% based on the makeup H2SO4), did not affect descaling rates. However, an effective accelerator does increase scale removal by as much as 30%.

References cited in this section

2. R.M. Hudson and C.J. Warning, Pickling Hot-Rolled Steel Strip: Effect of Strip Velocity on Rate in H2SO4, Met. Fin., Vol 82 (No. 3), 1984, p 39-46

20. J.P. Morgan and D.J. Shellenberger, Hot Band Pickle-Patch: Its Cause and Elimination, J. Met., Vol 17 (No. 10), 1965, p 1121-1125

21. C.W. Tuck, The Effect of Scale Microstructure on the Pickling of Hot-Rolled Steel Strip, Anti-Corros. Methods Mater., Vol 16 (No. 11), 1969, p 22-27

22. B. Meuthen, J.H. Arnesen, and H.J. Engell, The System HCl-FeCh-^O and the Behavior of Hot-Rolled Steel Strip Pickled in These Solutions, Stahl undEisen, Vol 85, 1965, p 1722

23. L. Hachtel, R. Bode, and L. Meyer, Influence of the Coiling Temperature on the Pickling Behavior of Mild Steel Hot Strip, Stahl und Eisen, Vol 104 (No. 14), 1984, p 645-650

24. R.M. Hudson and C.J. Warning, Factors Influencing the Pickling Rate of Hot-Rolled Low-Carbon Steel in Sulfuric and Hydrochloric Acids, Met. Fin., Vol 78 (No. 6), 1980, p 21-28

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