Effect of Process Variables on Scale Removal in Hydrochloric Acid

The effect of hydrochloric acid and ferrous chloride concentrations, solution temperature, and scale breaking on pickling rates was studied in a series of laboratory tests with nonstirred solutions (Ref 24). It was found that the time required for scale removal decreases with increases in acid concentration and with increases in temperature. For an ingot-cast low-carbon steel with a scale thickness of 3.6 mg/cm2 (0.0066 mm, or 0.00026 in.), test data for nontemper-rolled specimens in solutions that contain from 1 to 14 g HCl/100 mL and up to about 30 g FeCl2/100 mL at temperatures of 66 to 93 °C (150 to 200 °F) can be summarized by an empirical equation:

where t is time in seconds for scale removal, CHCl is acid concentration in g/100 mL, and TF is the solution temperature in degrees Fahrenheit. For this steel, A = -2.22, B = -0.87, and D = 2824. From a limited number of tests made with specimens subjected to temper mill scale breaking, it was concluded that the times calculated by this equation are lower by approximately 10%. Inhibitor usages up to 0.50 vol% based on free acid did not affect time for descaling. The influence of iron buildup in hydrochloric acid solutions on pickling rate was not nearly as pronounced as the effect of iron buildup in sulfuric acid solutions.

In a subsequent study (Ref 3), the effect of strip speed on pickling time in hydrochloric acid was investigated. It was found that the time for scale removal decreased with an increase in strip velocity from 0 to ~1.3 m/s (0 to ~250 ft/min) (Fig. 5). As strip speeds were increased from 1.3 to 4 m/s (250 to 800 ft/min), there was no further decrease in descaling time. As expected, times were lowered by temperature increases from 66 to 93 °C (150 to 200 °F). The observed velocity effects for hydrochloric acid were greater than those observed for pickling in sulfuric acid, probably because of the depletion of acid that occurs near the steel surface during pickling in an unstirred bath and the higher acid concentrations usually used with sulfuric acid. Because descaling time in hydrochloric acid does not change for strip velocities above 1.3 m/s (250 ft/min), a number of tests carried out at 2 m/s (400 ft/min) are believed pertinent to continuous operations in which speeds can range from 1.5 to 6 m/s (300 to 1200 ft/min) or higher. These results were summarized by an empirical equation of the same form as that developed from still-bath data, except that A = -4.46, B = -0.56, and D = 3916. Similar equations have been obtained for other steels where coefficients A and B are negative and D is positive. For slow-pickling steels, A becomes less negative, whereas for fast-pickling steels, A becomes more negative. Equations of this type are useful for predicting the effect of changes in hydrochloric acid concentration and temperature on pickling time.

Strip velocity, Wmin 200 4-0 Q 600 800

Strip velocity, Wmin 200 4-0 Q 600 800

ra At

1

I

k

66 &C

k

V

___

80 aC

)

^ J 94 X

s.

Strip velocity, m/s

Strip velocity, m/s

Fig. 5 Effect of strip velocity on descaling time of hot-rolled low-carbon steel in 4 g hydrochloric acid/100 mL, 22.7 g FeCl2/100 mL

The effect of solution concentration and temperature on pickling has been studied in well-stirred hydrochloric acid solutions for aluminum-killed continuous-cast hot-rolled steels subjected to low coiling temperatures (LCT) of 566 to 593 °C (1050 to 1100 °F) and high coiling temperatures (HCT) (721 °C, or 1330 °F) after various degrees (up to 5%) of temper mill scale breaking (Ref 1). The average scale thickness on these steels was, respectively, 3.5 mg/cm2 (0.0063 mm, or 0.00025 in. ) and 5.0 mg/cm2 (0.0090 mm, or 0.00036 in.). Pickling tests were made on temper-rolled and nontemper-rolled material at temperatures of 66 and 88 °C (150 and 190 °F) in uninhibited solutions that contained from 2 to 16 g HCl/100 mL and with 16 to 18 g FeCl2/100 mL. The time required for complete removal of scale during pickling of nontemper-rolled LCT and HCT steels is shown in Fig. 6. The time required for complete removal of scale during pickling of these steels after 3% temper rolling is shown in Fig. 7. The data can be summarized using equations of the type mentioned above. Decreases in time for scale removal, going from 0 to 3% temper mill reduction, are much greater than the decrease in time associated with an increase in temper mill reduction from 3 to 5% (Table 3).

Table 3 Comparison of pickling times required for scale removal from hot-rolled AK-CC steels, as influenced by HCl concentration, solution temperature, degree of temper mill scale reaking and hot strip mill coiling temperature

Calculated from prediction equations developed for these steels. Solutions contained from 16 to 18 g FeCl2/100 mL.

Table 3 Comparison of pickling times required for scale removal from hot-rolled AK-CC steels, as influenced by HCl concentration, solution temperature, degree of temper mill scale reaking and hot strip mill coiling temperature

Calculated from prediction equations developed for these steels. Solutions contained from 16 to 18 g FeCl2/100 mL.

Temperature

Hydrochloric acid concentration, g/100 mL

Steel(a)

Time to remove scale, s

°C

°F

0%(b)

3%(b)

5%(b)

65

150

2

LCT

52

34

25

65

150

6

LCT

28

18

13

65

150

10

LCT

20

13

10

65

150

16

LCT

16

10

7

65

150

2

HCT

114

43

22

65

150

6

HCT

61

23

12

65

150

10

HCT

46

17

9

88

190

2

LCT

25

19

16

88

190

6

LCT

13

10

8

88

190

10

LCT

10

8

6

88

190

2

HCT

42

27

20

88

190

6

HCT

22

14

11

88

190

10

HCT

17

11

8

(a) Based on hot strip mill coiling temperature. LCT, low coiling temperature; HCT, high coiling temperature.

(a) Based on hot strip mill coiling temperature. LCT, low coiling temperature; HCT, high coiling temperature.

(b) Degree of temper mill scale breaking in percent temper rolled

Fig. 6 Influence of acid concentration and solution temperature on time required for scale removal from hot-rolled strip, where strip was not subjected to temper mill scale breaking
Fig. 7 Influence of acid concentration and solution temperature on time required for scale removal from hot-rolled strip, where strip was subjected to temper mill scale breaking (3% reduction)

For both LCT and HCT steels, the relative importance of degree of temper mill scale breaking is much greater for a solution temperature of 66 °C (150 °F) than it is for 88 °C (190 °F). Without scale breaking, the time required for scale removal under the same conditions of acid concentration and solution temperature is substantially longer for the HCT steel than for the LCT steel. With adequate scale breaking (3% or more reduction), and when using acid concentrations and temperatures that are favorable for pickling LCT material, commercially useful pickling rates can be achieved for HCT material.

References cited in this section

1. R.M. Hudson, Pickling of Hot-Rolled Strip: An Overview, Iron Steelmaker, Vol 18 (No. 9), 1991, p 31-39

3. R.M. Hudson and C.J. Warning, Effect of Strip Velocity on Pickling Rate of Hot-Rolled Steel in Hydrochloric Acid, J. Met., Vol 34 (No. 2), 1982, p 65-70

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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