Vol 112, 1987, p 71 12. J.K. Burdick, B.J. Meneghelli, and K.S. Fletcher III, On- Line Determination of Hydrogen Fluoride in a Mixed Acid Stainless Steel Pickling Bath, Iron Steel Eng., Vol 69, 1992, p 32-34 Inhibitors Inhibitors are added to acid pickling solutions in order to: • Minimize acid attack on the base metal with excessive loss of iron • Avoid pitting associated with overpickling, which contributes to poor surface quality • Reduce acid solution spray resulting from hydrogen that forms when acid attacks steel • Lower acid consumption • Minimize the risk of hydrogen embrittlement When used at appropriate concentrations, inhibitors should not appreciably affect the rate of scale or rust removal. A number of additives have been used in pickling solutions to inhibit acid attack on metals. Natural products, such as bran, gelatin, glue, byproducts from petroleum refining and coal coking, and wood tars were initially used. Modern inhibitors are largely formulations of wetting agents with mixtures of active synthetic materials, including nitrogen-base compounds (pyridine, quinidine, hexamethylene tetramine, and other amines or polyamines), aldehydes and thioaldehydes, acetylenic alcohols, and sulfur-containing compounds such as thiourea and thiourea derivatives (Ref 13). Frequently, two or more active ingredients provide a synergistic effect, whereby the mixture is more effective than the additive effect of the individual components. A good inhibitor should not exhibit "breakout," which is sludge that deposits on the work, a characteristic of many of the natural products formerly used. It should be stable at the temperature of the pickling bath and should not emit offensive odors. Modern inhibitors used with sulfuric acid often contain thiourea or a substituted thiourea with an amine. Most of the newer inhibitors developed for use with hydrochloric acid contain amines or heterocyclic nitrogen compounds as active ingredients. In sulfuric acid pickling, the ferrous sulfate buildup in a worked pickling bath also inhibits the activity of the acid and reduces the effectiveness of the solution for cleaning and brightening the steel. Most steels are reactive with acid and require inhibited solutions. Steels with high phosphorus contents (0.03% or above) are particularly prone to overpickling. Inhibited acid solutions are generally used in continuous strip lines and in oil well drilling operations to clean the internal surfaces of pipes. Although the immersion times during continuous strip pickling are substantially shorter than in batch operations, an excessive loss of base metal would occur during a line stop if inhibitors were not used. This would not only be objectionable because of the roughened overpickled surface, but also because of the effect on critical final-gage requirements of the product. In commercial practice, inhibitor concentrations are usually expressed in terms of percent by volume of the makeup acid used, because most inhibitors are liquids. For example, if an addition of 0.9 L (0.25 gal) of inhibitor is made with 380 L (100 gal) of concentrated acid, then the concentration of inhibitor is said to be 0.25 vol% of the acid. Additions are best made proportional to the acid additions to pickling tanks or to the acid volume in large storage tanks or truckload shipments of acid. A poor method of introducing inhibitor to pickling solutions is by adding inhibitor to the bath at certain time intervals that are not related to actual acid additions. Before inhibitor additions are made, the bath may be underinhibited, and just after additions are made, the bath might be overinhibited. It is generally agreed that the primary step in the action of inhibitors in acid solutions is adsorption onto the metal surface. The adsorbed inhibitor then acts to retard the cathodic and/or anodic electrochemical processes of the corrosion. When inhibitor concentrations are much below recommended levels, the adsorbed layer of inhibitor on the steel surface may be incomplete, which can result in preferential attack on unprotected areas. To help control inhibitor concentrations in pickling solutions, inhibitor manufacturers have proposed the inclusion in their formulations of various materials that can be used as tracers in determining inhibitor concentration. One scheme involved the use of fluorescent dyes and colorimetric analyses. However, because of color changes in the dye with time, this approach has not proven satisfactory. Another possibility is the inclusion of phosphates or phosphoric acid, which can be detected in either sulfuric or hydrochloric acid pickling solutions by colorimetric procedures. Inclusion of lithium salts with subsequent analyses by neutron activation or atomic absorption spectrometry also has been tried. For amine-based inhibitors, concentrations in pickling solutions can be estimated by determining the nitrogen concentration in the solution and comparing it with the known nitrogen concentration in the neat inhibitor. Analyses can be made by the Kjeldahl method, or if the inhibitor contains simple amines rather than heterocyclic amines, by using an ammonia selective ion electrode with suitable standards. Before a new inhibitor is introduced in a plant, some laboratory tests should be made to identify concentration dependence of inhibition under conditions appropriate for a particular operation. The degree of acid attack on the base metal can be measured by determining the weight loss or hydrogen evolution from a steel specimen of known size and weight that is immersed for a specified time in a solution containing known amounts of acid, iron salts, and inhibitor at a temperature typical of use. Results of inhibitor testing are often expressed as percent inhibition, defined as: 100 × [(weight loss, uninhibited) (weight loss, inhibited)] divided by (weight loss, uninhibited). This relationship is useful when a number of inhibitors are being compared under a selected set of conditions. A value of 90% inhibition, associated with proper usage of an effective inhibitor, implies that the corrosion rate is only 10% of the rate when no inhibitor is used. For plain carbon steels containing less than 0.40% C, and for batch pickling baths that contain 10 to 14 wt% sulfuric acid (1.82 sp gr) and operate at 71 °C (160 °F) or higher, strong inhibitors are used at concentrations of 0.25 to 0.50 vol% raw acid in the tank. When the concentration of ferrous sulfate reaches 30 wt%, the solution should be discarded, because this level of iron salt slows down the pickling process and may cause smut to form on the surface of the product. When iron levels approach this concentration in batch pickling with sulfuric acid, further additions of inhibitor may not be required. Plain carbon steels containing 0.40% C or more are pickled in similar baths with somewhat lower temperatures (60 to 66 °C, or 140 to 150 °F) and with ferrous sulfate concentrations of less than 20 wt%. With hydrochloric acid, strong inhibitors are used at concentrations of 0.125 to 0.25 vol% of raw acid. Because pickling rates in both sulfuric acid and hydrochloric acid tend to decrease when the pickling solution contains high levels of iron (higher levels are tolerable with HCl), especially when coupled with low acid concentration, commercial pickling bath additives, or accelerators, are sometimes used to enhance pickling rates. These proprietary materials are usually formulated with inhibitors to prevent excessive base-metal attack by the acid during scale dissolution. Uninhibited acid solutions are often used for pickling high-alloy steels, because more chemical action is required to remove the oxide. Alloy or plain carbon steels used in forming flat and shaped sections are sometimes etched by uninhibited acid solutions to produce a surface that retains the die lubricant during cold working. These solutions are used also for conditioning steel with slivers and sharp corners, as well as steel that is ground before coating for additional cold working. If an inhibitor is used when pickling alloy steels, concentrations that are somewhat less than those recommended for plain carbon steels are suggested. Reference cited in this section 13. R.M. Hudson and C.J. Warning, Effectiveness of Organic Compounds as Pickling Inhibitors in Hydrochloric and Sulfuric Acid, Met. Fin., Vol 64 (No. 10), 1966, p 58-61, 63 Precleaning Alkaline precleaning before acid pickling is beneficial for removing soils that do not readily react with acid, such as grease, oil, soaps, lubricants, and carrier coatings. A buildup of such materials in a pickling bath interferes with the pickling action, especially when the pickling time is short (20 s or less). A typical alkaline cleaning solution contains 20% sodium hydroxide, 30% organic chelating agents, 45% complex phosphates, and 5% surface-activating agents. The concentration of the cleaner in the precleaning solution is 30 to 45 g/L (4 to 6 oz/gal), and the operating temperature of the solution ranges from 82 °C (180 °F) to boiling. After immersion in the cleaning solution, the work is rinsed in water at room temperature. Chelating Agents. Lubricants used in cold-drawing operations contain compounds of calcium, zinc, magnesium, iron, or some other metal. Removal is facilitated when alkaline cleaners containing chelating agents are used. Chelates react with the metal ion to form a soluble metal chelate. Chelates include citric acid, ethylene diamine tetraacetic acid, gluconic acid, and nitrilotriacetic acid. Although all chelating agents react in a similar manner, certain chelates have a greater affinity for specific metal ions. The solution must contain an adequate amount of chelate and be of the proper pH (usually, basic) for effective cleaning. Chelates must be stable with respect to the particular environment (oxidizing or reducing, acid or alkaline). A procedure for determining the chelating power of the alkaline cleaning formula consists of mixing 10 mL of filtered cleaning solution with 10 mL of distilled water and 10 drops of saturated 5% ammonium oxalate solution. This mixture is then titrated carefully with a 2% solution of calcium chloride until one drop of the latter produces a faint, permanent turbidity. The number of milliliters of 2% calcium chloride required is equivalent to the chelating power. Equipment Storage Tanks for Acid. Hydrochloric acid can be stored in rubber-lined steel tanks or glass-fiber-reinforced polyester-resin tanks. Polyester-glass linings in steel tanks are not recommended, because permeation of acid through pinholes in the lining could result in attack on the steel. Hydrofluoric acid (70%) is usually stored in plastic-lined tanks, although low-carbon steel can be used at temperatures up to 38 °C (100 °F) if properly passivated. Concentrated nitric acid (94.2% or higher) is best stored in tanks made of 3003 aluminum. For less concentrated nitric acid, tanks should be of type 304L stainless steel (type 304 annealed stainless for nonwelded construction) or 15 to 16% high-chromium iron. Dilute sulfuric acid is more corrosive to iron and steel than more concentrated acid. A 93.2% sulfuric acid can be stored in iron tanks over a wide range of temperatures, but a 77.7% sulfuric acid must be stored below 38 °C (100 °F). A ≤ 74.4% sulfuric acid cannot be stored in unlined iron tanks or carried through unlined iron pipes. A glass or phenolic lining, among others, can be used. Batch Pickling Tanks and Auxiliary Equipment. Construction materials for pickling tanks include wood, concrete, brick, plastic, and steel. Acid-resistant linings provide protection for the outer shell of the tank and are commonly made from natural, pure gum, or synthetic rubber. Acid-resistant brick is used to line the sides and floor of the tank. Bricks are mortared with poured sulfur cement or an acid-resistant resinous cement. Drainage lines should be made of vitrified tile caulked with an acid-resistant cement. Figure 1 shows the materials used in constructing a 3.5 to 4.4 Mg (3.9 to 4.9 ton) capacity tank for the pickling of coiled steel. In a batch pickling operation, bar product to be pickled is placed in tiers on crates and racks made of an acid-resistant material, such as Monel metal. Coils of strip, rod, or wire are pickled by passing a C-hook or chain through the open center. The holders are raised and lowered by an overhead crane. Fig. 1 Materials used in construction of 3.5 to 4.4 Mg (3.9 to 4.9 ton) capacity tanks for pickling coils of steel After pickling, the product should be rinsed with high-pressure cold-water sprays to remove excess acid solution. It should then be placed in a hot-water rinse tank with sufficient overflow to ensure that the pH is not less than 5 or 6. Heating Methods and Temperature Control. In the past, the most widely used method of heating pickling solutions was the direct injection of live steam through steam jets. Although steam so introduced does provide some agitation of the solution, the steam condensate dilutes the pickling solution, requiring higher amounts of acid to be added to maintain concentration. Steam sparging also increases the volume of spent pickle liquor. Better heating methods include coil steam heat, heat interchangers, and, for small installations, electric immersion heaters. Heating equipment must be made of acid-resistant materials, such as carbon, lead alloy, stainless steel, or zirconium for use with sulfuric acid. Carbon and polytetrafluoroethylene-covered heat exchangers should be used with hydrochloric acid. Acid-resistant indicating and regulating temperature-control instruments are available for pickling solutions. Heaters can be centrally located or, in the case of continuous pickling, placed at strategic intervals along the tank. Continuous-Strip Pickling Lines. A few pickling lines make use of vertical towers in which one or two hydrochloric acid spray columns are used (Ref 14, 15). The acid spray columns are assembled and sealed in sections made of fiber- glass-reinforced polyester, with a tower height of 21.3 to 45.7 m (70 to 150 ft). The tank sections are made from rubber- lined steel. After use, acid flows into a sump and is returned to the circulating tank. The composition of the acid in the recirculation tank is typically maintained at 11 g/100 mL HCl and 13% FeCl 2 . It is passed through a carbon-block heat exchanger and delivered to the sprays at 77 °C (170 °F). Most lines of this type have acid-regenerating facilities. Entry and exit coil handling are similar to the more common horizontal lines. Continuous-strip pickling lines with horizontal pickling tanks are capable of handling coils that are welded head to tail. The entry section comprises a coil conveyer, one or two uncoilers, one or two processors, one or two shears, and a welder. Processors are integral with the uncoiling equipment and consist of a mandrel, hold-down roll, and a series of smaller- diameter rolls. As the strip is flexed through the processor, some cracking occurs in the scale layer, although not nearly as much as that imparted by a temper mill. Proper welding and weld trimming is essential to avoid strip breaks in the line. The section prior to the pickling tanks uses bridles for tensioning the strip; a strip accumulator, either in the form of wet looping pits or, for more modern lines, a coil-car accumulator; and, for many lines, a temper mill to crack the scale on the surface of the strip. A stretch leveler can replace the temper mill and not only effectively cracks the scale, but also contributes to superior strip shape. The pickling section usually contains three or more tanks. So-called "deep tanks" are typically 1.22 m (4 ft) in depth and up to 31.3 m (90 ft) in length. Acid tanks are steel shells with layers of rubber bonded to the steel. The rubber is protected from abrasion by a lining of silica-base acid-proof brick. Most lines have a cascade flow of pickling solutions countercurrent to the direction of strip movement. When fresh acid is added to the last tank, it will contain the highest concentration of acid. Acid concentrations will decrease from the last tank to the first tank, from which the spent pickle liquor is discharged. A rinse section follows the pickling section. An especially effective rinsing method used on many continuous lines is the cascade rinse system (Ref 16). Several rinse compartments are used, and fresh water is added to the last compartment. The solution in that compartment cascades over weirs into the preceding compartments. The excess overflows from the first compartment and is sent to the waste-water treatment plant (a portion can be used for makeup water in the pickle tanks). Each compartment contains less acid than the previous compartment. At the exit end of the line, there are usually an exit strip accumulator, steering rolls, a strip inspection station, dual side trimmers, an oiler, and two coilers. Pickling lines must have fume scrubbers to capture emissions/spray from the pickle tanks. In some modern lines, the pickling solution is contained in shallow tanks with liquid depths of approximately 0.41 m (16 in.) and lengths up to approximately 36 m (118 ft). Although they involve a cascade system, the solution in each tank is recirculated through a heat exchanger. During a line stop, the pickling solution can be rapidly drained from shallow tanks into individual storage tanks and then pumped back when the line starts up. Lines with deep tanks usually have strip lifters provided to remove the strip from the acid solution during an extended line stop. Tank covers may be made from fiberglass or polypropylene. Some lines have squeegee rolls, covered with acid-resistant rubber, located above and below the strip at each tank exit to minimize acid carryover from one tank to another. Maximum speeds in modern lines in the pickling section can be as high as 305 to 457 m/min (1000 to 1500 ft/min). Although sustained operation at such speeds is limited by other aspects of coil handling, the selection of pickling tank acid concentrations and temperatures must be such that complete scale removal is achieved during periods of high-speed operation. The combination of a pickling line and a cold reduction mill in tandem represents a new state of the art in continuous processing facilities (Ref 17). Another type of strip pickling line suitable for plants with moderate production requirements is the push-pull type, which has many of the features of the continuous-type lines, but no welder (Ref 18). Turbulent-flow, shallow-tank, continuous-strip lines that claim to provide more effective pickling action than conventional lines have been developed (Ref 19). References cited in this section 14. Spray Tower Pickles Steel, Iron Age, Vol 190, 4 Oct 1962, p 78-79 15. D.E. Poole, Hydrochloric Acid Pickling of Steel Strip, J. Met., Vol 17, 1965, p 223-224 16. J.B. Hodsden and W.L. Van Kley, Cascade Rinse System at Inland's 63-in. Con tinuous Strip Pickling Line, Iron Steel Eng., Vol 51 (No. 3), 1974, p 49-53 17. N.L. Samways, Modernization at Pittsburg to Make USS-POSCO More Competitive, Iron Steel Eng., Vol 64 (No. 6), 1987, p 21-29 18. G. Kuebler, Pushing for Pickling Productivity, 33 Metal Producing, Vol 30 (No. 9), 1992, p 28-31 19. F.G. Pempera and F.W. Delwig, Turbulent Shallow-Type Pickling Lines, Iron Steel Eng., Vol 65 (No. 3), 1988, p 33-36 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 Fe 3 O 4 is significantly greater than it is in sulfuric acid (Ref 22). Therefore, the relative amounts of FeO versus Fe 3 O 4 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/cm 2 (0.00475 mm, or 0.000187 in.) and 5.2 mg/cm 2 (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. 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 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/cm 2 (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. Fig. 4 Effect of solution temperature on pickling time for hot-rolled low-carbon steel; comparison with t emper mill scale breaking. All solutions contained 15 g FeSO 4 /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 Time to remove scale, s °C °F Sulfuric acid concentration, g/100 mL Ferrous sulfate concentration, g/100 mL 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. (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 H 2 SO 4 ), 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 H 2 SO 4 , 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-FeCl 2 -H 2 O and the Behavior of Hot- Rolled Steel Strip Pickled in These Solutions, Stahl und Eisen, 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 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/cm 2 (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 FeCl 2 /100 mL at temperatures of 66 to 93 °C (150 to 200 °F) can be summarized by an empirical equation: log t = A + B log C HCl + D (T F + 459) -1 (Eq 12) where t is time in seconds for scale removal, C HCl is acid concentration in g/100 mL, and T F 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. [...]... 1 5 -2 5 H2SO4(c) 7 1-8 2 16 0-1 80 3 0-6 0 Water rinse(d) Ambient Ambient Nitric-hydrofluoric acid dip 5-1 2 HNO3, 2- 4 HF 49 max 120 max 2- 2 0 Water rinse(d) Ambient Ambient Caustic permanganate dip(e) 1 8 -2 0 NaOH, 4-6 KMnO4(f) 7 1-9 3 16 0 -2 00 1 5-6 0 Water rinse(d) Ambient Ambient Sulfuric acid dip 1 5 -2 5 H2SO4(c) 7 1-8 2 16 0-1 80 2- 5 Water rinse(d) Ambient Ambient Nitric acid dip 1 0-3 0 HNO3 6 0-8 2 14 0-1 80 5-1 5... 400and 300-series hot bands are given in Table 7 Table 7 Pickling conditions for hot-rolled stainless steel strip following shot blasting Grade Concentration and acid type (g/100 mL) Temperature Time, s °C Austenitic 1 0-1 5 sulfuric plus 0-4 hydrofluoric 5 0-6 5 12 0-1 50 2 0-6 0 5-1 0 nitric plus 0. 2- 1 hydrofluoric Ferritic °F 5 0-6 5 12 0-1 50 2 0-6 0 1 0-1 5 sulfuric plus 0-4 hydrofluoric 5 0-7 0 12 0-1 60 2 0-6 0 5-1 5 nitric... Operating temperature °C Immersion time °F Sulfuric acid dip 1 5 -2 5 H2SO4(b) 7 1-8 2 16 0-1 80 5-3 0 min Water rinse(c) Ambient Ambient Caustic permanganate dip(d) 1 8 -2 0 NaOH, 4-6 KMnO4(e) 7 1-9 3 16 0 -2 00 20 min to 8 h(f) Water rinse(c) Ambient Ambient Sulfuric acid dip 1 5 -2 5 H2SO4(b) 7 1-8 2 16 0-1 80 2- 3 min Nitric acid dip 30 HNO3 Ambient Ambient 1 0-3 0 min (a) Acid solutions are not inhibited (b) Sodium chloride... raw-acid costs, yield loss, and waste-disposal costs Table 8 Pickling conditions for cold-rolled stainless steel strip after annealing Grade Concentration and acid type (g/100 mL) Temperature Time, s °C 5 0-6 5 12 0-1 50 1 5-6 0 5 0-6 5 12 0-1 50 1 5-6 0 1 0-1 5 nitric only Austenitic 5-1 5 sulfuric plus 0-4 hydrofluoric 5-1 0 nitric plus 0. 1-1 hydrofluoric Ferritic °F 5 0-6 5 12 0-1 50 1 5-6 0 1 0-1 5 sulfuric plus 0-4 hydrofluoric... 0-4 hydrofluoric 5 0-6 5 12 0-1 50 1 5-6 0 5-1 5 nitric plus 1-4 hydrofluoric 5 0-6 5 12 0-1 50 1 5-6 0 1 0-1 5 nitric only 5 0-6 5 12 0-1 50 1 5-6 0 Acid Pickling Schemes If scale has been preconditioned or if it is inherently easy to remove, then use of electrolytic sodium sulfate or electrolytically assisted sulfuric and nitric acids may be sufficient Use of these liquors should result in less base-metal yield loss than... 0%(b) °F 65 Hydrochloric acid concentration, g/100 mL 2 LCT 3%(b) 5%(b) 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... Hot-Rolled Strip: An Overview, Iron Steelmaker, Vol 18 (No 9), 1991, p 3 1-3 9 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) , 19 82, p 6 5-7 0 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 2 1 -2 8... from 600 to 750 °C (11 12 to 13 82 °F) Free HCl and most of the water is evaporated and leaves the reactor with the combustion gases Ferrous chloride reacts with the balance of water and oxygen to form hydrogen chloride and hematite: 2FeCl2 + 1 /2 O2 + 2H2O = Fe2O3 + 4HCl (Eq 15) Gases containing hydrogen chloride are washed with water to form an 18 to 20 wt% HCl solution By-product Fe2O3 can be used as a... p 3 0-4 2 26 R.M Hudson and H.M Alworth, Formability of Annealed Sheet and Black Plate as Affected by Scratches Before Cold Reduction, Iron Steelmaker, Vol 17 (No 2) , 1990, p 2 6-3 2 27 L.E Helwig, Rusting of Steel Surfaces Contaminated With Acid Pickle Salts, Met Fin., Vol 78 (No 7), 1980, p 4 1-4 6 Disposal of Spent Pickle Liquor Spent pickle liquor (SPL) from sulfuric acid pickling may contain from 2 to... 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 . Time, s 1 0-1 5 sulfuric plus 0-4 hydrofluoric 5 0-6 5 12 0-1 50 2 0-6 0 Ferritic 5-1 0 nitric plus 0. 2- 1 hydrofluoric 5 0-6 5 12 0-1 50 2 0-6 0 Austenitic 1 0-1 5 sulfuric plus 0-4 hydrofluoric. plus 0. 1-1 hydrofluoric 5 0-6 5 12 0-1 50 1 5-6 0 Ferritic 1 0-1 5 nitric only 5 0-6 5 12 0-1 50 1 5-6 0 1 0-1 5 sulfuric plus 0-4 hydrofluoric 5 0-6 5 12 0-1 50 1 5-6 0 Austenitic 5-1 5 nitric. Sulfuric acid dip 1 5 -2 5 H 2 SO 4 (c) 7 1-8 2 16 0-1 80 3 0-6 0 Water rinse (d) . . . Ambient Ambient . . . Nitric-hydrofluoric acid dip 5-1 2 HNO 3 , 2- 4 HF 49 max 120 max 2- 2 0 Water rinse (d)