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IC 
 
 8985 
 
 Bureau of Mines Information Circular/1984 
 
 Pickling of Stainless Steels— A Review 
 
 By Bernard S. Covino, Jr., John V. Scalera, 
 and Philip M. Fabis 
 
 UNITED STATES DEPARTMENT OF THE INTERIOR 
 
 ■" a' 11 * 
 
 
 '.afaw^iiwrewsawragpfflp 
 
Information Circular 8985 
 
 Pickling of Stainless Steels— A Review 
 
 By Bernard S. Covino, Jr., John V. Scalera, 
 and Philip M. Fabis 
 
 ' ■"■ •:• ■ 
 
 UNITED STATES DEPARTMENT OF THE INTERIOR 
 William P. Clark, Secretary 
 
 BUREAU OF MINES 
 Robert C. Norton, Director 
 
Library of Congress Cataloging in Publication Data: 
 
 V 
 
 <& 
 
 
 <* 
 
 Covino, B. S. (Bernard S.) 
 
 Pickling of stainless steels— a review. 
 
 (Information circular / United States Department of the Interior, Bu- 
 reau of Mines ; 8985) 
 
 Bibliography: p. 13-15. 
 
 Supt. of Docs, no.: I 28.27:8985. 
 
 1. Steel, Stainless— Pickling. I. Scalera, John V. II. Fabis, Philip 
 M. III. Title. IV. Series: Information circular (United States. Bu- 
 reau of Mines) ; 8985. 
 
 TN295.U4 [TS654] 622s [669'. 142] 84-600164 
 
k 
 
 4 
 
 £ CONTENTS 
 
 Page 
 
 i 
 
 V Abstract 1 
 
 j Introduction 2 
 
 / Effect of hot and cold working on pickling 3 
 
 Hot working 3 
 
 Cold working 4 
 
 Effect of annealing on pickling 4 
 
 •XEf f ect of conditioning on pickling 6 
 
 Degreas ing 6 
 
 Abrasive blasting 6 
 
 ^ Chemical conditioning 6 
 
 N. Reducing acids 7 
 
 Qv/N Oxidizing acids 7 
 
 Electrolytic acid conditioning 7 
 
 r Anodic conditioning « ... . rf 7 
 
 v~/ Cathodic conditioning '.'. .'..*.. 8 
 
 Alternating current conditioning ' 8 
 
 Electrolytic neutral conditioning 8 
 
 Salt bath conditioning 8 
 
 Reducing bath 9 
 
 Oxidizing bath 9 
 
 Electrolytic bath 9 
 
 Effect of pickling bath variables on pickling 10 
 
 Pickling operation 10 
 
 Bulk alloy dissolution 11 
 
 Solution composition 11 
 
 Acid concentration 11 
 
 Mixed acids 11 
 
 Temperature 12 
 
 Chromium-depleted zone dissolution 12 
 
 Research needs 12 
 
 References 13 
 
 ILLUSTRATION 
 
 /^l. Schematic of steps involved in processing stainless steels 2 
 
 \ 
 
 9\ 
 

 UNIT OF MEASURE ABBREVIATIONS USED IN 
 
 THIS REPORT 
 
 A/ft 2 
 
 ampere per square foot 
 
 ym/yr 
 
 micrometer per year 
 
 °C 
 
 degree Celsius 
 
 rpm 
 
 revolution per minute 
 
 min 
 
 minute 
 
 vol pet 
 
 volume percent 
 
 Um 
 
 micrometer 
 
 wt pet 
 
 weight percent 
 
PICKLING OF STAINLESS STEELS-A REVIEW 
 
 By Bernard S. Covino, Jr., John V. Scalera, and Philip M. Fabis 
 
 ABSTRACT 
 
 The Bureau of Mines is conducting a study of methods to improve the 
 efficiency of the process used to pickle stainless steels. A review of 
 the literature on pickling of stainless steels showed that the chemistry 
 of several process operations involved in the pickling of stainless 
 steels is not fully understood, and that further research could improve 
 the pickling efficiency. The benefits of this research would be a re- 
 duction in the annual loss of several thousands of tons of critical 
 minerals such as nickel and chromium, and a reduction in the amount of 
 solids and spent acid solution that are currently discarded. The con- 
 clusion from this review is that further research is needed in four 
 operations that either directly or indirectly influence the pickling 
 process: hot working, annealing, conditioning, and the actual operation 
 of pickling. Laboratory studies of the pickling operation are presently 
 in progress. 
 
 'Research chemist. 
 ^Materials engineer. 
 Avondale Research Center, Bureau of Mines, Avondale, MD. 
 
INTRODUCTION 
 
 The various operations involved in pro- 
 cessing stainless steels are given in 
 figure 1. After being worked by hot or 
 cold rolling, the steel is softened by 
 annealing. Oxide forms on the stainless 
 steels during this annealing process, 
 which, as shown in figure 1, occurs sev- 
 eral times. Conditioning is used to 
 facilitate the pickling process. Mixed- 
 acid pickling, or pickling by a solution 
 of two or more acids , is then used for 
 cleaning the oxide-covered stainless 
 steels. In addition to removing the an- 
 nealing scale, pickling also removes a 
 very thin (1- to 5-pm) region depleted in 
 chromium between the oxide and the bulk 
 stainless steel. Loss of chromium and 
 nickel from this region and the oxide is 
 inherent in this part of the pickling op- 
 eration. In practice, the stainless 
 steel may be left in the pickling solu- 
 tion longer than necessary, causing ex- 
 cessive dissolution of the bulk steel, 
 resulting in losses of several thousand 
 tons of chromium and nickel annually. 
 The combined dissolution products can 
 build up to a point where the action at 
 the pickling bath stops, resulting in a 
 sizable disposal problem when the bath is 
 replaced. The dissolved metals also in- 
 crease the use of acids in the pickling 
 bath by complexing or precipitating acid 
 salts. The need to study the pickling 
 process was formulated during discussions 
 between the Bureau of Mines and the Amer- 
 ican Iron and Steel Institute (AISI). 
 Both groups concluded that the problems 
 of loss of critical minerals, excess use 
 of acids , and disposal of spent solutions 
 could be lessened by a better understand- 
 ing of the entire pickling process. 
 
 The literature pertinent to the pick- 
 ling of austenitic stainless steels was 
 reviewed. Data bases such as Chemical 
 Abstracts, Metadex, Compendex, and NTIS 
 (National Technical Information Service) 
 were searched from 1900 to 1983 where 
 applicable. Although articles in all 
 languages were accepted in the search, 
 the review was done mainly on articles in 
 English. The general purpose of the re- 
 view was to assess the technology of 
 stainless steel pickling and to present a 
 
 critical examination of the mechanism of 
 mixed-acid pickling of stainless steels 
 in terms of all the important process 
 operations and operating parameters. 
 
 All such operations and parameters are 
 considered in light of how much knowledge 
 is available and what further knowledge 
 is necessary for a better understanding 
 and control of the pickling process. 
 This review begins by considering those 
 factors in the metal-forming operation 
 that can affect subsequent pickling be- 
 havior. The effect of annealing on pick- 
 ling is then addressed. Conditioning 
 treatments prior to pickling have a very 
 
 Ingot 
 
 
 Hot work 
 
 
 
 Anneal 
 
 
 
 
 
 
 
 
 
 
 
 ■ 
 
 j 
 
 
 
 
 Conditioning: 
 
 mechanical 
 
 or 
 
 chemical 
 
 
 
 1 
 
 j 
 
 
 
 Pickling 
 
 
 
 " 
 
 
 
 Cold work 
 
 
 
 
 
 
 ' 
 
 
 
 Anneal 
 
 
 
 1 
 
 
 
 
 Conditioning: 
 chemical 
 
 or 
 electrolytic 
 
 or 
 salt bath 
 
 
 
 
 • 
 
 
 
 Pickling 
 
 As necessary 
 
 
 
 
 
 ' 
 
 
 
 Finished 
 material 
 
 
 FIGURE 1. - Schematic of steps involved in process- 
 ing stainless steels. Indicated steps are repeated as 
 often as necessary to thin the material to the desired 
 thickness. 
 
significant effect on pickling and are 
 addressed next. Finally, the operation 
 of the pickling bath is considered. The 
 
 present understanding of the mechanism of 
 pickling of stainless steel is developed 
 in detail in this last section. 
 
 EFFECT OF HOT AND COLD WORKING ON PICKLING 
 
 
 In order to be inclusive, the effects 
 of hot and cold working on the pickling 
 of stainless steels are briefly consid- 
 ered. Working of the stainless steel 
 usually has no direct effect on the pick- 
 ling operation because an annealing op- 
 eration is interposed between the working 
 and the pickling steps. However, some 
 effects of hot and cold working can in- 
 fluence the annealing operation or remain 
 unchanged after the annealing operation. 
 Of the two types of working, hot working 
 presents the greatest potential problem 
 because of its ability to change the 
 chemistry and grain structure of the 
 stainless steel, resulting in subsequent 
 changes in the scale formed during 
 annealing. 
 
 HOT WORKING 
 
 Hot working, the process of mechanical 
 deformation of a material at temperatures 
 above its recrystallization temperature, 
 can significantly alter the grain size of 
 metals. The degree to which this altera- 
 tion occurs for alloys such as stainless 
 steels depends on the degree of deforma- 
 tion, the number and frequency of defor- 
 mation steps, and the initial and final 
 working temperatures. The degree of 
 deformation determines the stored energy 
 in a material that is the driving force 
 for recrystallization to occur. The fre- 
 quency of deformation steps and initial 
 and final temperatures determines the 
 rate of crystallization and the occur- 
 rence and rate of grain growth. Both a 
 lower deformation temperature and a 
 greater amount of deformation produce a 
 smaller ultimate grain size. It is 
 usually the temperature at which hot 
 working is completed, the finishing tem- 
 perature, that determines the average 
 grain size (1_). 3 
 
 ^Underlined numbers in parentheses re- 
 fer to items in the list of references at 
 the end of this report. 
 
 When hot working operations are fol- 
 lowed by an anneal, the major effect of 
 this altered grain size is on the result- 
 ing annealing scale. If the mill scale 
 conforms to the morphology of the metal 
 surface, then a fine-grained scale forms 
 on a fine-grained metal and a larger 
 grained scale forms on a large-grained 
 metal. Recent research (2) has shown 
 that the scale formed on smaller- 
 grain-size stainless steels contains more 
 chromium than that on larger-grain-size 
 steels. Oxide films containing more 
 chromium could significantly affect the 
 rate of pickling and possibly cause com- 
 positional changes, such as chromium de- 
 pletion, in the metal adjacent to the 
 oxide. 
 
 Severe problems can occur after hot 
 working if there is significant chemical 
 inhomogeneity or large quantities of in- 
 clusions in the initial ingot. Regions 
 of the ingot depleted of an alloy con- 
 stituent could be smeared in the rolling 
 direction, resulting in bands of differ- 
 ent composition (3). Another phenomenon, 
 similar in appearance to banding, is 
 fiber. This elongated structure consists 
 of nonmetallic inclusions which are elon- 
 gated as the steel is worked (4^) . While 
 the spontaneous recrystallization that 
 occurs during hot working is usually una- 
 ble to affect these localized composition 
 changes , annealing usually eliminates 
 them. If these variations in metal com- 
 position are still present after the an- 
 nealing step, however, preferential at- 
 tack of the banded regions or pitting 
 near the fibers could result in an ir- 
 regular surface morphology. Since stain- 
 less steel chemistry is closely con- 
 trolled, these structures are rarely seen 
 in commercial stainless steels; if pre- 
 sent, they would be expected to signifi- 
 cantly affect the pickling process. 
 
 Sensitization to intergranular corro- 
 sion may occur in austenitic stainless 
 
steels when they are subjected to a 
 working temperature range of 450° to 
 900° C or when the steel is slow-cooled 
 from 1,050° C (_5_ ) . A chromium-depletion 
 theory proposes that the precipitation of 
 M23C6 carbides along a grain boundary 
 results in a region depleted of chromium 
 adjacent to the carbides. In some cases 
 of very high purity (low-carbon) alloys, 
 although there is no detectable carbide 
 precipitation, a solute segregation 
 theory proposes that a chromium-depleted 
 region still exists (6) . The resistance 
 to the pickling solution is greatly re- 
 duced in these regions. Essentially, two 
 dissimilar metals are in contact and an 
 unfavorable anode-cathode area ratio is 
 present (7^, pp. 35-36). In this case, 
 the depleted zone sets up active-passive 
 cells with large-area alloy grains acting 
 as cathodes in contact with material in 
 the grain boundaries of limited area act- 
 ing as anodes. In addition, the grain 
 boundary carbides can be susceptible to 
 the mixed acid solution, and their dis- 
 solution would produce further surface 
 
 degradation. As with the banding and 
 fiber phenomena, however, any sensitiza- 
 tion should be removed in a proper an- 
 nealing step, making hot-work-induced 
 sensitization a minor problem area. 
 
 COLD WORKING 
 
 Cold working increases the stored 
 energy of a material and, when coupled 
 with an annealing process , can cause 
 grain size changes similar to those dis- 
 cussed for hot working. But, unless a 
 deformation-induced segregation or trans- 
 formation occurs that brings dissimilar 
 concentrations of atoms into contact (8) , 
 cold working has no other effect on the 
 pickling behavior of properly annealed 
 stainless steels. Simultaneous deforma- 
 tion and pickling never occur in the in- 
 dustrial processing scheme of stainless 
 steels; therefore, cold working is im- 
 portant only if new phases or the segre- 
 gation of alloyed components result from 
 the plastic deformation. 
 
 EFFECT OF ANNEALING ON PICKLING 
 
 Compared to hot and cold working, an- 
 nealing probably has a more significant 
 effect on the pickling rate of stainless 
 steels than any other process preceding 
 pickling. Annealing can be thought of as 
 a relatively uncontrolled oxidation re- 
 action; that is, the temperature can be 
 controlled fairly accurately, but the 
 atmosphere within the annealing furnace 
 is usually not controlled. Also, from 
 batch to batch of stainless steels, the 
 heat-up time, time at temperature, and 
 cool-down time are different for various 
 metallurgical reasons. This variability 
 in the annealing process can have a sig- 
 nificant effect on the mill scale formed 
 and thus on the ease of pickling of the 
 stainless steel. Numerous investigations 
 concerning the high-temperature oxidation 
 of stainless steels in various atmos- 
 pheres are available throughout the met- 
 allurgical literature. Although there is 
 considerable disagreement about the 
 structure, thickness, and growth mechan- 
 isms of the films, some agreement con- 
 cerning film characteristics exists. 
 
 Because of the short annealing times 
 involved in stainless steel processing, 
 the gaseous annealing environment reacts 
 preferentially with the more reactive 
 chromium component. This produces a re- 
 gion of the base metal near the oxide 
 that is significantly depleted in chromi- 
 um. Consequently, the thin oxide films 
 that form on these alloys during the ini- 
 tial oxidation stages are composed mainly 
 of Cr 2 3 with small amounts of Fe 2 3 or a 
 spinel phase FeFe ( 2 _ x) Cr x 4 where 0<x<2 
 (9-12) . These films are fairly adherent 
 and protective, and since their ionic 
 conductivity is low (13) , they prevent 
 diffusive penetration of other ions and 
 atoms through the scale. Although the 
 Cr 2 3 films are adequate barriers, the 
 iron oxide films are quite permeable, 
 especially to carbon (14) . Under condi- 
 tions of severe scaling, a stratified 
 structure occurs that contains Cr 2 3 , 
 FeFe (2 _ x )Cr x 4 , FeO, Fe 3 4 , and Fe 2 3 (8_, 
 15). In certain cases, as in extended 
 heating periods, the initially protective 
 scales then become nonprotective. 
 
The scaling behavior of stainless 
 steels under conditions similar to those 
 in annealing furnaces has not been ade- 
 quately studied. Brief exposures (1- to 
 5-min time at temperature) in complex 
 (0 2 , H 2 0, CO, C0 2 ) high-temperature (871° 
 to 1,038° C) environments should be stud- 
 ied. Complete characterization of the 
 scales resulting from these exposures 
 must be done in order to understand the 
 chemistry and structure of the complex 
 scales formed on stainless steels. It 
 would then be necessary to correlate the 
 ease of pickling of the steels with the 
 chemistry and structure of the annealing 
 scales, an area of research that could 
 have significant impact on the pickling 
 process as currently used. 
 
 Because of the elevated-temperature 
 anneal, three other metal-related phenom- 
 ena that could have an impact on subse- 
 quent pickling operations are embrittle- 
 ment, sensitization, and a transformation 
 to a dual-phase structure. 
 
 Embrittlement (_1_) may occur upon heat- 
 ing or slowly cooling certain ferritic 
 and high-nickel austenitic steels through 
 the 425° to 760° C temperature range. A 
 precipitation reaction occurs in this 
 temperature range producing a complex 
 structure FeCr phase, the sigma (a) 
 phase. Because this phase is rich in 
 chromium, the presence of a chromium con- 
 centration gradient between the a-phase 
 and adjoining phases may affect the pick- 
 ling behavior and corrosion resistance of 
 stainless alloys (8) . With proper tem- 
 perature control, however, this phase 
 will not form. 
 
 Elemental concentration gradients arise 
 in the alloy owing to diffusional pro- 
 cesses at elevated temperatures. For in- 
 stance, if an 18Cr-8Ni stainless steel 
 (i.e., 304) is heated to 1,150° C (a 
 common heat-treatment temperature) and 
 water-quenched, the single-phase austen- 
 itic alloy is formed. The quenched-in 
 austenitic structure is thermodynamically 
 unstable at room temperature, but the 
 transformation kinetics are extremely 
 slow. If the same alloy, assuming a 
 low carbon concentration, is heated to 
 1,200° C and allowed to soak at that 
 
 temperature for several hours , the FCC 
 austenite partially transforms to fer- 
 rite, a BCC structure. Therefore, a two- 
 phase (austenite-ferrite) microstructure 
 can be present. A problem arises in that 
 the austenite phase contains more nickel 
 and less chromium relative to the ferrite 
 phase (8) . Thus , an elemental concentra- 
 tion gradient is created between adjacent 
 grains. The susceptibility of the alloy 
 in this condition to localized corrosion 
 in a mixed-acid pickling bath may be sig- 
 nificantly enhanced. This enhancement is 
 attributable to a galvanic effect arising 
 between grains of different composition 
 in close proximity. 
 
 The final problem produced by elevated- 
 temperature exposure of stainless steels 
 is sensitization. Depending upon the 
 carbon content of the alloy, precautions 
 may have to be taken to avoid sensitizing 
 conditions during annealing. As men- 
 tioned previously, sensitization results 
 in intergranular precipitation of M 2 3C 6 
 carbides of high chromium content and 
 chromium depletion of regions adjacent to 
 these grain boundaries to below the 
 necessary 12 wt pet required for stable 
 passivity (_7, pp. 35-36). While this 
 would cause an increased localized at- 
 tack, sensitization is rarely a problem 
 in the production of austenitic stainless 
 steel. 
 
 In summary, hot working and, to a small 
 degree, cold working can have potentially 
 deleterious effects on the pickling of 
 stainless steels. Grain-size variations 
 can cause variations in scale chemistry, 
 and ingot inhomogeneity can cause banding 
 and fibering. Sensitization can occur in 
 both hot working and annealing, but em- 
 brittlement usually occurs only during 
 annealing. Typical annealing processes 
 are poorly controlled, resulting in vari- 
 ations in scale chemistry. All of these 
 problems do not necessarily occur during 
 present-day stainless steel processing, 
 but there is a very real possibility for 
 their occurrence. Therefore, in order to 
 fully understand the pickling process and 
 everything that affects it , the more im- 
 portant of these problems (grain-size 
 variations and annealing variations) 
 should be studied further. 
 
EFFECT OF CONDITIONING ON PICKLING 
 
 Conditioning of stainless steels is a 
 process used to prepare annealed stain- 
 less steel for the pickling process. Its 
 purpose is to alter the annealing scale 
 in order to reduce the time, temperature, 
 and acid concentration used in the pick- 
 ling process. The scale is either 
 cracked by thermal or mechanical means , 
 or constituents in the scale are chem- 
 ically or electrolytically altered to In- 
 crease their solubility in order to be 
 able to reduce either the amount of HNO3 
 and HF in the final pickling solution or 
 the times and temperatures in the pick- 
 ling process. 
 
 In contrast to the well-understood 
 mechanical conditioning process, the 
 chemical and electrolytic conditioning 
 techniques need to be studied further in 
 order to understand exactly what chemical 
 and structural changes result from the 
 respective technique. 
 
 DEGREASING 
 
 Although degreasing is usually done be- 
 fore annealing stainless steels, it also 
 may be necessary, prior to a conditioning 
 step, to remove surface contaminants such 
 as oil, grit, graphite, metal chips, or 
 other foreign matter that may have been 
 transferred to the steel surface. De- 
 pending on the type of foreign matter 
 suspected on the surface of the steel, 
 various techniques are employed. These 
 techniques include the use of vapor de- 
 greasing with organic solvents, water- 
 soluble emulsifiers, chelates, and water- 
 soluble alkaline cleaners (16) . Contam- 
 inants on the surface of the heat-treated 
 steel can often result in an oxidized 
 grime during scale conditioning, which 
 can severely inhibit the effectiveness of 
 the subsequent pickling processes. Sur- 
 face contamination also pollutes the 
 pickling solutions, which reduces their 
 effectiveness. 
 
 Once the surface has been effectively 
 cleaned, conditioning techniques help to 
 oxidize, crack, and loosen scale to en- 
 hance the effectiveness of final pickling 
 
 operations. Depending upon the nature of 
 the oxide film, various techniques are 
 employed. Both mechanical and chemical 
 techniques have been used successfully. 
 The basic mechanical technique for scale 
 conditioning is abrasive blasting. 
 
 ABRASIVE BLASTING 
 
 Abrasive blasting is one of the fastest 
 of all conditioning techniques and has 
 been successfully used in both batch and 
 continuous on-line processing of stain- 
 less steel strip (16-18) , although it is 
 used almost exclusively on the latter. 
 Abrasive blasting uses steel shot or sil- 
 ica in sizes ranging from 100 to 2,500 
 mesh depending on the material being con- 
 ditioned. The abrasive is either dropped 
 or directed by air pressure toward the 
 surface of the steel at angles and ve- 
 locities that allow the cracking, loosen- 
 ing, and partial removal of heavy oxide 
 scales without cold-working the steel's 
 surface. The technique has limited ap- 
 plication for batch-processing complex 
 shapes because some oxidized areas not 
 exposed to the shot blast are not totally 
 conditioned. The use of carbon steel 
 shot as an abrasive has been a topic of 
 controversy (19) . Opponents feel that 
 its use can embed iron particles in the 
 surface of the stainless steel, resulting 
 in future corrosion problems. This 
 would, of course, depend on what treat- 
 ments followed the carbon steel blasting. 
 The abrasive silica used in blasting must 
 be low in iron in order to avoid surface 
 contamination. As with most mechanical 
 descaling techniques, caution must be 
 practiced to avoid work-hardening the 
 steel surface. Heat-treated surfaces 
 with thin oxide films , such as cold- 
 rolled products, are usually not condi- 
 tioned using mechanical techniques. 
 
 CHEMICAL CONDITIONING 
 
 The choice of chemical conditioning 
 parameters varies significantly between 
 stainless steel processing plants. Some 
 of the parameters that must be taken into 
 consideration are the nature of the base 
 
metal and oxide being removed, strip or 
 batch processing, and chemical costs. 
 For example, when working with an inter- 
 mediate pickled 304 stainless steel, some 
 companies use a salt bath conditioning 
 process followed by an electrolytic 
 nitric acid bath before the final HN0 3 -HF 
 acid pickle; other processors go directly 
 from the salt bath into the final pick- 
 ling bath. The following paragraphs 
 describe some of the more common chemical 
 conditioning processes in use. 
 
 Reducing Acids 
 
 Reducing acid baths descale a metal by 
 reducing the oxides in the scale and also 
 liberate hydrogen at the oxide-metal in- 
 terface. The most common reducing acids 
 used in conditioning processes for an- 
 nealed stainless steels are H 2 S0 4 and 
 HC1. The specific acid solution used de- 
 pends on the nature of the base material 
 and oxide being treated; chemical costs, 
 as well as the time permitted for condi- 
 tioning, are usually determined by 
 whether the metal is prepared by either 
 continuous strip or batch processing. 
 Solutions of 10 to 15 vol pet H 2 S0 4 at 
 60° to 71° C are used for conditioning 
 heavy oxides (20) . H 2 S0 4 solutions are 
 relatively slow in comparison to solu- 
 tions combining H 2 S0 4 (6 to 10 vol pet) 
 with HC1 (6 to 10 vol pet) at 54° to 60° 
 C (_21, pp. 684-689). Although HC1 is not 
 as aggressive as H 2 S0 4 in attacking the 
 base metal at elevated temperatures (77° 
 to 93° C) , it is much more aggressive in 
 attacking iron oxides than is H 2 S0 4 . The 
 use of conditioning solutions containing 
 HC1 requires critical control because 
 ferric chlorides formed during the condi- 
 tioning process can result in pitting of 
 the base material ( 19 ) . Another condi- 
 tioning solution that can cause base met- 
 al pitting is H 2 S0 4 (8 to 11 wt pet) com- 
 bined with NaCl (5 to 6 wt pet) at 60° to 
 65° C U8). 
 
 Oxidizing Acids 
 
 In contrast to reducing acids , oxidiz- 
 ing acids descale by oxidizing the scale 
 to a higher oxidation state, thus in- 
 creasing the solubility of the scale. 
 
 HNO3 (8 to 10 vol pet at 38° to 54° C) is 
 the most commonly used oxidizing acid 
 (19) . Because of the greater cost of 
 HNO3 compared to H 2 S0 4 , however, its use 
 has been limited in the conditioning 
 processes. 
 
 ELECTROLYTIC ACID CONDITIONING 
 
 Experimental studies on electrolytic 
 pickling in acid solutions were described 
 by Tamba, Azzerri, Bombara, and others 
 (22-25). Industrial electrolytic condi- 
 tioning using an H 2 S0 4 bath can be traced 
 back to the 1920's and 1930' s where there 
 was a need for faster pickling techniques 
 to maximize output from on-line stainless 
 steel strip processing (26) . 
 
 The most common acids associated with 
 electrolytic conditioning are H 2 S0 4 and 
 HNO3. Electrolytic acid conditioning 
 techniques are used presently on both 
 martensitic and ferritic steels which, 
 during the annealing process , have devel- 
 oped tightly adhering thin oxide films 
 (21 , pp. 689-690). Austenitic chromium- 
 nickel steels can also be electrolytical- 
 ly conditioned, although this condition- 
 ing requires caution because of the 
 greater potential for surface pitting. 
 The probability of pitting is increased 
 for thicker oxide scales because breaks 
 in the oxide scale react more rapidly 
 than scale-covered base metal. This 
 highly localized activity can result in 
 pitting before the bulk of the scale is 
 removed . 
 
 Anodic Conditioning 
 
 There are three basic types of electro- 
 lytic conditioning: anodic, cathodic, 
 and alternating current (27) . Anodic 
 conditioning either electrically oxidizes 
 (dissolves) the surface or forces the 
 surface into a passive state by an ap- 
 plied anodic potential. When the surface 
 is being electrically oxidized, the base 
 metal is being attacked and the scale 
 dislodges. When the surface is passive, 
 then the base metal undergoes far less 
 attack than the oxide scale, releasing 
 oxygen gas at its surface. The oxygen 
 mechanically agitates the solution, 
 
uplifts the surface scale, and can oxi- 
 dize contaminants such as organic impuri- 
 ties. An example of an anodic electro- 
 lytic solution is 2 pet HN0 3 used at a 
 current density of 75 to 200 A/ft 2 (21, 
 pp. 689-690; 27_) . 
 
 Cathodic Conditioning 
 
 During cathodic electrolysis, the work- 
 piece is charged to act as a cathode. 
 The base metal is electrochemically pro- 
 tected while the oxide scale is being re- 
 duced. Cathodic electrolysis is faster 
 than anodic; hydrogen gas generated on 
 the metal surface helps to agitate the 
 solution and lift off the oxides. In 
 martensitic steels, however, this reac- 
 tion can result in hydrogen embrittlement 
 of the surface (27-28). 
 
 solution is 10 vol pet H 2 S0 4 used at 88° 
 C with a current density of 100 to 150 
 A/ft 2 (27). 
 
 Both ferritic and martensitic steels 
 undergo annealing processes which form 
 thin, tightly "skinned" oxides. Removal 
 of these oxides by mechanical means could 
 cold-work the surface. However, the use 
 of reducing acids, such as H2SO4, which 
 release hydrogen during the alternating 
 current descaling process when the work- 
 piece is the cathode, can be detrimental 
 to martensitic steels. As in cathodic 
 conditioning, these steels are subject to 
 hydrogen embrittlement. Alternative con- 
 ditioning techniques for martensitic 
 steels include those in which the libera- 
 tion of hydrogen on the steel surface is 
 eliminated. 
 
 Alternating Current Conditioning 
 
 Very low frequency (one cycle per sev- 
 eral minutes) alternating current elec- 
 trolysis is used in conjunction with 
 stainless strip processing where direct 
 electrical contact to the workpiece is 
 difficult. In fact, the current is usu- 
 ally reversed only from tank to tank in a 
 two-tank method, or, in a one-tank meth- 
 od, it is reversed once in the tank. The 
 basic reactions, however, are the same as 
 described in anodic and cathodic condi- 
 tioning. Operationally, the electrodes 
 are placed above and below the strip, 
 forming an anodic potential on one side 
 of the strip and a cathodic potential on 
 the other side (27) . During each alter- 
 nating cycle, the polarity is reversed on 
 the electrodes and, in turn, on the faces 
 of the stainless steel strip. A varia- 
 tion is the use of two tanks where the 
 strip in the first tank is made cathodic 
 with respect to the anodic electrode in 
 the tank (26) . Here the base metal is 
 electrochemically protected and hydrogen 
 gas is formed to lift off the oxides. 
 The strip then enters a second tank where 
 it becomes anodic with respect to the ca- 
 thodic electrode placed in the second 
 tank. Here the surface and scale are ox- 
 idized and release oxygen at the surface, 
 which uplifts the scale. An example of 
 an alternating current electrolysis 
 
 ELECTROLYTIC NEUTRAL CONDITIONING 
 
 Electrolytic neutral (the Ruthner 
 "Neolyte" process) conditioning is simi- 
 lar in mechanical design to electrolytic 
 acid conditioning in that alternating 
 cathodic and anodic electrodes are used 
 to polarize the workpiece and induce oxi- 
 dation and reduction of the surface scale 
 (29) . The electrolytic neutral pickling 
 involves a Na 2 S0 4 solution at tempera- 
 tures of 65° to 85 C, resulting in safer 
 operation conditions and reduced energy 
 costs (29) . The final stages in the pro- 
 cess result in a regeneration of the 
 Na 2 S0 4 . 
 
 SALT BATH CONDITIONING 
 
 The use of both aqueous and molten salt 
 baths as a pretreatment for acid pickling 
 has been successful in increasing the 
 ease of scale removal of all stainless 
 steel types, permitting the use of re- 
 duced acid concentration and decreasing 
 the time of pickling needed to remove the 
 scale Q6, JLjU 30-31). This reduction in 
 acid concentration and pickling time re- 
 duces the likelihood of hydrogen embrit- 
 tlement in susceptible alloys. Other 
 benefits of salt bath descaling are its 
 fast process time and uniform surface 
 finish (19). 
 
Salt bath treatments can be either re- 
 ducing, oxidizing, or electrolytic (16, 
 30 ) . Reducing and electrolytic baths 
 have proven to be more effective in at- 
 tacking heavy, tightly adhering oxide 
 scales. 
 
 Reducing Bath 
 
 Reducing salt baths consist of a molten 
 bath of NaOH (371° ±11° C) , containing 
 1.5 to 2.0 wt pet sodium hydride (16, 
 30 ) . The sodium hydride is formed in 
 generators along the side of the descal- 
 ing tank. Sodium and hydrogen gas react 
 to produce the hydride, which must be 
 constantly generated since the level of 
 sodium hydride in solution is depleted 
 during descaling. There are safety haz- 
 ards involved in the use of sodium and 
 hydrogen. Descaling takes place as the 
 oxide films are reduced to a lower oxida- 
 tion state: 
 
 M 2 3 + xNaH = xNaOH + M 2 3 . x . 
 
 After the workpiece is removed from the 
 salt bath, it is water-quenched. This 
 quenching thermally shocks the remaining 
 oxide layer, fracturing and loosening it 
 and making it more susceptible to the 
 acid pickling. 
 
 Oxidizing Bath 
 
 As with the previously described caus- 
 tic reducing salt bath, oxidizing salt 
 baths also use molten NaOH (60 to 90 wt 
 pet) at elevated temperatures (482° C) 
 (16, 30) . Other constituents include 
 sodium nitrate (7 to 32 wt pet) and so- 
 dium chloride (1.5 to 6 wt pet). Some of 
 the oxidizing reactions that take place 
 are (31)-- 
 
 2Fe0 + NaN0 3 = Fe 2 3 + NaN0 2 
 
 Cr 2 3 + 3NaN0 3 = Cr 2 6 + 3NaN0 2 
 
 2Ni0 + NaN0 3 = Ni 2 3 + NaN0 2 
 
 C + 2NaN0 3 = C0 2 + 2NaN0 2 
 
 As long as the bath is in contact with 
 air, the NaN0 3 can be regenerated by the 
 oxidation of the NaN0 2 formed. Trivalent 
 
 chromium is oxidized to the hexavalent 
 state, which readily dissolves into acid 
 pickling solutions. 
 
 Upon leaving the salt bath, the work- 
 piece is quenched with water. The ther- 
 mal shock involved in quenching the work- 
 piece disrupts the oxidized scale. After 
 the water quench, an acid pickling bath 
 is needed to dissolve any remaining 
 oxide. 
 
 An aqueous oxidizing salt bath consists 
 of NaOH (20 wt pet) and KMn0 4 (5 wt pet) 
 (32) . This strong oxidizing solution may 
 be used on all grades of stainless steel, 
 especially where a light, tightly adher- 
 ing scale has formed, such as is the case 
 in cold rolling or in some controlled 
 annealing atmospheres. The KMn0 4 -NaOH 
 solution may also be used on heavier ox- 
 ide scales. As with the other previously 
 mentioned molten salt baths , the treat- 
 ment is followed by acid pickling. 
 
 Electrolytic Bath 
 
 The use of an electrolytic technique in 
 combination with molten salt baths has 
 been applied successfully to continuous 
 stainless steel strip processing lines. 
 A typical bath consists of 75 pet NaOH, 
 10 pet NaCl-NaF, 14 pet Na 2 C0 3 , and 1 pet 
 other carbonates and is operated at 482° 
 C. The salt baths are neither oxidizing 
 nor reducing in nature until electrically 
 polarized. Polarization of the workpiece 
 is achieved by the use of a series of 
 cathodes followed by anodes in the neu- 
 tral salt bath (_16, 30). Opposite the 
 cathode, the strip acts as an anode with 
 oxidation taking place. As some of the 
 scale is dissolved into solution, it 
 reacts with the oxygen released during 
 the oxidation process to form insoluble 
 hydroxides. These hydroxides will not 
 interfere with the surface of the steel 
 strip as it becomes cathodic and under- 
 goes reduction. The strip becomes ca- 
 thodic as it passes below the anode. 
 As reduction takes place at the scale- 
 surface interface, hydrogen evolves, 
 lifting the scale off the surface. After 
 the sample passes through the salt bath, 
 it is water-rinsed and acid-pickled for 
 final scale removal and surface finish. 
 
10 
 
 EFFECT OF PICKLING BATH VARIABLES ON PICKLING 
 
 The pickling of stainless steels re- 
 quires three distinct processes. The 
 first process is the removal of the 
 thermally grown oxide scale for appear- 
 ance purposes and to facilitate further 
 cold working of the steel. The second 
 process maximizes the corrosion resist- 
 ance of the final steel product by com- 
 pletely dissolving the chromium-depleted 
 zone that is generally formed during 
 short high-temperature anneals in oxidiz- 
 ing environments. The third process dis- 
 solves the minimum amount of bulk steel 
 necessary to give the desired whitening 
 effect. These three processes occur, to 
 some degree, simultaneously during the 
 pickling operation and most probably are 
 interdependent. Therefore, to understand 
 the pickling operation, it is necessary 
 to understand the effect of pickling bath 
 variables, solution composition, and tem- 
 perature on the pickling operation as a 
 whole and on the dissolution of both the 
 chromium-depleted region and the bulk 
 steel. 
 
 PICKLING OPERATION 
 
 Before considering the effect of bath 
 variables on the pickling of stainless 
 steels, it is necessary to assess the 
 state of understanding of the mechanism 
 of pickling. It is generally agreed in 
 the literature (24) that the pickling of 
 stainless steels is accomplished by un- 
 dercutting the oxide and that the high 
 chemical reactivity of the chromium- 
 depleted zone essentially controls the 
 rate of the pickling operation. There 
 were, however, no in-depth studies of the 
 mechanism or even any proof found in the 
 body of literature reviewed here. While 
 it was difficult to determine how this 
 mechanism was discovered, it is clear 
 that little is known empirically about 
 the mixed acid pickling of stainless 
 steels. Other research indicates that 
 dissolution of the oxide is inconsequen- 
 tial in the actual removal of the oxide 
 (23) . What appears to be important is 
 that the oxide be sufficiently disrupted 
 
 to permit the penetration of the pickling 
 solution. Most changes in oxide porosity 
 occur during the conditioning process. 
 
 The ideal pickling solution is one that 
 will easily penetrate the thermally grown 
 oxide, rapidly dissolve the chromium- 
 depleted zone, and dissolve only a small 
 layer of the bulk steel. This solution 
 would optimize the rate of pickling while 
 minimizing the unnecessary and excessive 
 loss of bulk alloy. The most commonly 
 used pickling solution for austenitic 
 stainless steels is a mixed acid consist- 
 ing of HN0 3 and HF in various propor- 
 tions. By varying the ratio of HN0 3 to 
 HF, it is possible to pickle different 
 types of austenitic steels. This solu- 
 tion accomplishes all three processes 
 listed above and provides a very white 
 surface. There are, however, potential 
 problems of overpickling the steel, re- 
 sulting in excessive grain boundary at- 
 tack. This could affect the reflectivity 
 of the surface and decrease the corrosion 
 resistance. This mixed acid is also ex- 
 pensive and extremely dangerous. A pos- 
 sible alternative to HNO3-HF pickling was 
 reported in a series of research articles 
 (22-25) . This research resulted in a 
 technique (24) that used an applied po- 
 tential to dissolve the chromium-depleted 
 zone while minimizing the dissolution of 
 the base material in an H 2 S0 4 bath. The 
 researchers claim that a proper choice of 
 potential can optimize the pickling of 
 stainless steels while using the less 
 hazardous H 2 S0 4 solution. However, this 
 technique does not produce as reflective 
 a surface as produced in free (not poten- 
 tial controlled) pickling in HNO3-HF mix- 
 tures. The researchers also did not in- 
 vestigate the effects of iron, chromium, 
 and nickel buildup in the bath on the po- 
 tential controlled pickling or the quali- 
 ty of the pickled surface. As in HNO3-HF 
 solutions , these impurities could affect 
 the electrochemical reactions and the 
 rate of pickling, and could even lead 
 to pitting or increased intergranular 
 attack. 
 
11 
 
 Other pickling solutions have been ex- 
 amined in previous research efforts. In 
 one study (33) the solutions were select- 
 ed for their ability to dissolve the bulk 
 steel. This research reported on the re- 
 lationship of the composition of the 
 pickling bath to the dissolution rate of 
 304 stainless steel. Using various com- 
 binations of H 2 S0 4 , HC1, HN0 3 , and NaN0 3 , 
 it was shown that HC1 and HNO3 have about 
 an equal effect on the dissolution rate 
 of scale-free 304 SS, while H 2 S0 4 has a 
 much lower effect and NaN03 has a much 
 greater effect (NaN0 3 >HCl«HN0 3 >H 2 S0 4 ) . 
 The results of this study assumed that 
 each chemical in the pickling bath can 
 operate individually with no synergistic 
 effect. However, the same group of chem- 
 icals produced different results on an- 
 nealed and oxide-covered 304 stainless 
 steel. The NaN0 3 solution had the slow- 
 est pickling rate, while HC1 had the 
 fastest pickling rate. Both H 2 S0 4 and 
 HNO3 had intermediate rates. This sug- 
 gests that while dissolution may be im- 
 portant in the mechanism of pickling, 
 dissolution of the bulk alloy may not be 
 critical. When solutions containing HF 
 were tested, results showed that it had 
 as strong an effect on pickling rate as 
 HC1. HC1 is usually not used because the 
 FeCl 3 formed generally promotes pitting 
 of the stainless steel. No studies of 
 the effect of temperature on the pickling 
 of stainless steels were found in this 
 search of the literature. 
 
 BULK ALLOY DISSOLUTION 
 
 Solution Composition 
 
 The aforementioned report (33) suggest- 
 ed that dissolution of the bulk steel may 
 not be critical in determining the rate 
 of pickling of stainless steels. The 
 dissolution rate of the bulk steel does, 
 however, control the amount of bulk steel 
 lost to the pickling solution and to some 
 extent the amount of dissolved metal 
 species in solution. The factors that 
 could affect this dissolution rate are 
 temperature, acid concentration, dis- 
 solved metal concentration, and agita- 
 tion. Since many dissolution reactions 
 exhibit some dependence on convection and 
 
 diffusion, agitation of the solution 
 should be important. A study (34) was 
 done that showed that agitation, as simu- 
 lated by a rotating disk electrode, sig- 
 nificantly affects the dissolution of a 
 304-type stainless steel (Khl8N10T) in 
 HNO3 + NaCl solutions. Dissolution rates 
 for samples rotated at to 300 rpm were 
 not affected, whereas rates increased 
 steadily with increasing rotation speed 
 from 300 to 10,000 rpm. The investigator 
 assumed that the agitation affected main- 
 ly the cathodic reduction of nitric to 
 nitrous acid, but offers no evidence. 
 Another investigator has shown (35) , to 
 the contrary, that the anodic reaction of 
 nitric acid on platinum is diffusion de- 
 pendent while the cathodic reaction is 
 only reaction dependent. 
 
 Acid Concentration 
 
 The effect of HN0 3 concentration on the 
 dissolution of austenitic stainless 
 steels is minimal over a range of concen- 
 trations and temperatures that would be 
 used in pickling solutions. Isocorrosion 
 diagrams (7_, p. 243) for 18-8S4 steels 
 show that the corrosion rate ranges from 
 to 125 um/yr for HN0 3 concentrations up 
 to 50 wt pet from 30° C to the boiling 
 point. The dissolution behavior of sev- 
 eral of the more popular austenitic 
 stainless steels and of some iron- 
 chromium-nickel alloys has been thorough- 
 ly reviewed ( 36 ) elsewhere. The results 
 simply show that HNO3 solutions (<50 pet) 
 do not rapidly dissolve austenitic stain- 
 less steels. 
 
 Mixed Acids 
 
 To dissolve the austenitic stainless 
 steels , a mixed acid is usually consid- 
 ered. Mixtures of HNO3 and H 2 S0 4 in- 
 crease the dissolution rate of the steels 
 by about a factor of four over HNO3 alone 
 (7^, p. 243); however, this combination is 
 rarely used in pickling operations. The 
 most commonly used solutions for pick- 
 ling austenitic stainless steels contain 
 HNO3 and HF. The effect of this mixed 
 
 ^"S" means lower carbon content to pre- 
 vent carbide precipitation. 
 
12 
 
 acid on various austenitic steels has 
 been extensively studied and adequately 
 reviewed (37) . HF significantly in- 
 creases the rate of dissolution of aus- 
 tenitic steels in HN0 3 solutions. For 
 example, as little as 0.5 wt pet HF in 
 18 wt pet HN0 3 can increase the dissolu- 
 tion rate of 304 stainless steel by two 
 orders of magnitude at 60° C and by al- 
 most three orders of magnitude at 80° C 
 (37) . This corrosion rate is approxi- 
 mately 15,000 um/yr at 80° C. 
 
 Temperature 
 
 The above results suggest that tempera- 
 ture has a significant effect on the dis- 
 solution of austenitic stainless steels. 
 A study ( 37 ) conducted on 309SCb 5 stain- 
 less steel indicated that a temperature 
 change from 20° to 100° C increased the 
 dissolution rate in HNO3-HF solutions by 
 over two orders of magnitude. This re- 
 sponse to increasing temperature was 
 shown to be true regardless of solution 
 compositions from 4.5 wt pet HNO3 + 0.2 
 wt pet HF to 27 wt pet HNO3 + 2 wt pet 
 HF. These results imply that the activa- 
 tion energy for dissolution is not af- 
 fected by solution composition, although 
 the absolute magnitude of that dissolu- 
 tion rate was found to be affected by so- 
 lution composition. 
 
 CHROMIUM-DEPLETED ZONE DISSOLUTION 
 
 The dissolution behavior of the 
 chromium-depleted zone should be affected 
 by the same factors as those affecting 
 the bulk steel: temperature, acid con- 
 centration, and agitation. Dissolution 
 rates should be higher than those for the 
 bulk steel because of the reduced chromi- 
 um and nickel contents. Direct studies 
 of the dissolution behavior of this de- 
 pleted zone have not been conducted be- 
 cause of the extreme thinness of this 
 zone. Indirect studies, however, have 
 been done by fabricating alloys that sim- 
 ulate different regions of the depleted 
 zone. A study (38) of a series of iron- 
 chromium-nickel alloys (2 to 18 wt pet 
 Cr) showed that the dissolution rate in 
 sulfuric acid was high for very low con- 
 centrations of chromium. Surprisingly, 
 however, the dissolution rate passed 
 through a minimum at 12 wt pet Cr where 
 the dissolution rate was only 40 pet of 
 that for a 19 wt pet Cr alloy. Studies 
 of similar alloys in HNO3-HF would be 
 very important for developing an under- 
 standing of the pickling of stainless 
 steels. Equally important would be stud- 
 ies of the effect of acid concentration, 
 temperature, and agitation on the disso- 
 lution behavior of these chromium- 
 depleted alloys. 
 
 RESEARCH NEEDS 
 
 Based on this review of the literature 
 on the pickling of stainless steels, the 
 following areas have been identified as 
 needing further study: 
 
 3. The effect of the various condi- 
 tioning techniques on the structure and 
 composition of the scale related to its 
 effect on pickling rate. 
 
 1. The effect of hot working on the 
 grain size of metal and subsequent oxide 
 scale composition. 
 
 2. The effect of annealing parameters 
 such as furnace environment and time at 
 temperature on the annealing scale and 
 ultimately on the pickling rate. 
 
 S" means lower carbon to prevent car- 
 bide precipitation, and "Cb" means nio- 
 bium (columbium) added to prevent carbide 
 precipitation . 
 
 4. The dissolution behavior of bulk 
 steels and the chromium-depleted zone of 
 bulk steels, and the effects of the pick- 
 le bath variables, acid concentration, 
 temperature, and agitation, on the disso- 
 lution rate. 
 
 In conclusion, additional studies in 
 any one of the above four areas will con- 
 tribute to the understanding of the pick- 
 ling process. However, knowledge from 
 all four areas is necessary to develop 
 the relationships needed to quantify the 
 
13 
 
 pickling process. This quantification 
 should lead to an improvement in the ef- 
 ficiency of pickling, a reduced cost to 
 process stainless steels, and minimizing 
 the loss of critical metals such as nick- 
 el and chromium. Laboratory studies are 
 
 presently being done to understand the 
 effect of acid concentration, tempera- 
 ture, and dissolved metal concentration 
 on the pickling of 304 and 430 stainless 
 steels. 
 
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14 
 
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15 
 
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