W. A.
LUCE
J. H. PEACOCK
ANNUAL REVIEW
STAINLESS STEEL AND OTHER FERROUS ALLOYS T h e various tJYpes o f selective corrosion are receiving a greater amount of attention onsumption of nickel in the free world during 1963 was at a n all time high and indications are it will continue to increase in the future. However, production capacity in the free world still appreciably exceeds the 1963 consumption. The significance of the increased nickel consumption can at least be partially attributed to stainless steels. I t is estimated that over 30% of all the nickel consumed in 1963 was for stainless steels. This accounts for more nickel than any other single use. I n addition, nickel for stainless alloys showed a greater increase over 1962 than did use of nickel in any other area. Corrosion resistance of stainless steels continues to be the primary area of interest to the chemical industry. Particularly important are selective types of corrosion, such as pitting and stress corrosion cracking. Concentrated effort is being expended in a n effort to resolve these forms of corrosion. Quality, as related to corrosion resistance and mechanical properties, is receiving increased emphasis particularly with respect to castings. More severe service conditions, new and better production equipment, and rapidly advancing technology are primarily responsible for the trend toward improved quality.
C
Corrosion
Alloys with strong passivating tendencies such as stainless steels are prone to selective corrosion with an attendant rapid failure when the state of passivity fails in localized areas. Stress corrosion cracking is one of the most insidious problems of this type and there is continued emphasis on means for maintaining stronger passive films and on developing more inherently resistant alloys which do not depend on passivity for corrosion resistance. Studies of passivity-maintaining reactions offer a simplified means for controlling selective attack particularly in the presence of chlorides. Polarization curves (73A) for a number of common stainless alloys indicate when the material is relatively immune to attack. These can help in determining what service con-
ditions the particular alloy will resist. The anodic polarization characteristics of Type 316 stainless steel to strong sulfuric acid (75A) help explain passivation phenomena in the presence of various halide ions. Halides are known to increase corrosion problems in a practical sense and this work confirms their deleterious effect on polarization. The general corrosion picture is constantly being altered by developments within the stainless industry. New alloys are developed and modifications of existing compositions broaden their horizons. A cast, agehardening stainless steel, Alloy Casting Institute (ACI) designation C D - ~ M C Uoffers , a unique combination of high strength and excellent corrosion resistance to nitric and sulfuric acids ( 6 A ) . Iso-corrosion data depicting this resistance are shown in Figures l and 2. Much attention is also being given to the influence of ferrite on the properties of stainless steels. T h e cast 18 chrome-8 nickel alloys (ACI grades CF-8 and CF-8M) have a definite advantage in mechanical strength and corrosion resistance over their wrought counterparts (Types 304 and 316, respectively) in those areas where ferrite content is important. The manganese-containing 200-series stainless steels continue to be studied ( 7 7 A ) particularly with respect to the addition of such elements as molybdenum, which enhance their basic corrosion resistance to weakly oxidizing media. These alloys will find only limited acceptance within the chemical industry until their corrosion resistance is more firmly established. Selective Corrosion. Stress corrosion cracking is presently the most serious corrosion problem facing the chemical industry and although research work is continuing a t an accelerated rate, no simplified, practical answer appears imminent. The mechanism of stress corrosion cracking is better understood but its full implication is difficult to translate to nontechnical people who are ordinarily involved with the purchase of equipment. This is particularly the case with seemingly unimportant details in equipment fabrication which often determine the service life obtained. Intergranular corrosion problems had reached similar proportions a decade ago. While special alloys immune to sensitization were eventually developed, there seems to be no similar breakthrough in stainless technology to eliminate stress corrosion cracking. Hence, an economical solution to VOL. 5 6
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TABLE I. STRESS CORROSION SUBJECTS Subject
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industrial Applications
Reference
Review of reported failures Relation of hydrogen pick-up to stress corrosion cracking Effect of alloying elements Stress corrosion cracking in a purified 16 Cr-20 Ni alloy Stress cracking of Type 304 at 455'-615' F. Ordering, stacking faults, and stress corrosion cracking in austenitic alloys Influence of chloride content in potable water
Stress corrosion cracking in sea water Stress corrosion cracking in high temperature water
1
ZA 16A
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14A 7A
~
, i
I:
I
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5A
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SA 72A
this serious problem may be delayed several years. The basic concepts involved in understanding and controlling this phenomenon are important to the engineer first experiencing stress cracking ( 8 A ) . It does not confine itself to the liquid phase (7A) which broadens its scope. Table I indicates some of the specific subjects considered during the past year. Alloy development still offers the most promise as a means for eliminating stress corrosion cracking. It has been determined that cast alloys containing ferrite have less tendency to crack than equivalent wrought alloys and it may very well be possible to adjust the composition of stainless steels to render them crack-resistant As mentioned above, intergranular corrosion can be largely controlled on a practical basis by proper alloy specification but the subject still warrants attention since the basic mechanism of sensitization has never been universally accepted. Anomalies persist but it is the consensus ( 3 A ) that most failures can be prevented by approved fabrication techniques where both composition and prior thermal history are known.
W . A . Luce is Chief Metallurgist, J . H. Peacock i s Senior Materials Engineer with the Duriron Company, Inc., Dayton, Ohio. AUTHORS
Considerable information is available on the application of stainless steel to abnormally severe services. The trend toward higher temperatures and pressures in chemical processes continues and it is necessary to periodically reevaluate the application of stainless steels. The conventional 18-8 stainless steels were studied ( I B ) in various concentrations of the mineral acids and were compared to other materials inc uding noble metals. This study confirms previous findings that alloys resistant under less severe conditions must be retested at the higher temperatures to assure good serviceability. The increasing severity of chemical applications makes it advisable to utilize higher strength materials and design engineers (4B) are utilizing cast, austenitic stainless steels for high temperature petrochemical services because of the higher strengths compared to equivalent wrought materials. Higher quality standards are being met in castings and there is evidence that wider application can be made of castings in many areas previously assigned to wrought products. Nitric acid handling continues to be an important area of application for stainless steels but these alloys are often borderline for many of the more difficult services. The presence of chlorides is particularly troublesome but recently Type 329 stainless steel was found (6B) superior to other stainless alloys in nitric acid containing the chloride ion. Acetic acid can also be very corrosive to stainless steels particularly at hiyh temperatures. New information (3B)should help clarify the subject. Anodic protection is finding useful application in handling chemicals. An explosion in a phosphoric acid distribution system was attributed to the ignition of hydrogen which accumulated in the Type 304 storage tank as a result of corrosion. Data (5B) showed that anodic protecton could reduce the hazard to acceptable limits even at elevated temperarures. This illustrates how specialized techniques can often be successfully applied to difficult corrosion problems. Although corrosion failures will never be completely prevented, the correct analysis of the trouble is vital
400
Figure 7 . Iso-corrosion diagram f o r Alloy CD-4MCu in nitric acid 1 oc
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NITRIC ACID, 52
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A new stainless steel (20) consists of 14% Cr, 15.5y0 Co, 3.5% Ni, 2Y0 Mo, 0.5% Ti, and 0.03% C. This alloy combines exceptionally high strength with excellent notch toughness and ' corrosion iresistance. Adding cerium (0.015 to o.056y0) to a stainless steel was found (30) to reduce oxygen, sulfur, and nitrogen contents, reduce nonmetallic inclusions, enhance corrosion resistance in the active condition, provide increased resistance to stress and intergranular corrosion, and have no adverse effects on other properties. Sulfur is a common addition to stainless steels to improve machinability but the sulfur also tends to decrease ductility. Recent work (50) revealed that a distinct increase in machinability and mechanical properties can be obtained without sacrificing corrosion resistance when sulfur content is decreased and aluminum is added. Considerable development work continues on the precipitation-hardenable stainless steels, but with the exception of the CD-4MCu alloy cited in a previous section, most work centers around strength rather than strength and corrosion resistance. Another possible exception would be a new semiaustenitic precipitationhardenable alloy designated P H 14-8Mo. This alloy (60) is similar to PH 15-7Mo but it offers advantages in strength, toughness, weldability, high temperature stability, and stress corrosion cracking resistance. Numerous articles appeared in the literature in 1963 pertaining to the nickel-containing maraging steels. However, their primary application is in industries where exceptional mechanical properties are demanded. Very little has been published concerning the corrosion resistance of these alloys and the most probable reason is that these are not intended for applications where corrosion resistance is the primary criterion for selection. Vacuum technology offers considerable encouragement in producing more corrosion resistant stainless alloys. Vacuum melting, degassing, and pouring have produced noted improvements. Lower gas contents, fewer oxide inclusions, and cleaner alloys are significant factors resulting from vacuum practices. These factors can result in better mechanical properties and improved corrosion resistance. I t is foreseen that continued
since it will determine the next logical step in preventing a recurrence without resorting to uneconomical materials. An incorrect diagnosis may compound the problem. Even failure of welds may be due to more than one cause (2B). High Temperature
Although stainless steels have been widely used for elevated temperature applications there is renewed interest in the conventional low carbon grades. Many plants have difficult applications involving temperatures in excess of 1000" F., and long service life from this equipment is no less important than with aqueous solutions. Various high temperature properties are still under investigation and detailed information on the subject of creep was provided a t one meeting ( I C ) . High temperature mechanical properties are vital and must be viewed differently than with lower temperature conditions. Embrittling reactions may occur at operating temperature and these can seriously affect the serviceability of a part. The resistance of various alloys to high temperature environments varies considerably and it is important that the conditions be studied carefully before a particular material is selected. It was reported (2C) that the interaction of various atmospheres with stainless alloys can be predicted based on thermodynamic data provided either oxidation or carburization is the basic problem. Another study (3C) showed that the oxidation resistance of stainless steels can be affected by rare earth additives to the alloy and it is quite apparent that studies of this type help make high temperature alloys more predictible. Alloy D e v e l o p m e n t
Quality continues to be emphasized for stainless steels and related alloys. This emphasis emanates primarily from industries where improved mechanical properties are required. The chemical industry, however, will benefit because superior mechanical properties will be accompanied by improved corrosion resistance, particularly to selective types of corrosion.
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Figure 2. Iso-corrosion diagram for Alloy CD-4MCu in sulfuric acid
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advances in vacuum technology will offer many advantages to the chemical processing industry in the future. I n comparing wrought with cast stainless alloys the trend has been for wrought production, in general, to be somewhat more advanced than the cast counterpart. This is not totally unexpected when considering the vast difference in tonnage of wrought us. cast alloys and the accompanying difference in research and development effort. I n addition, procedures and methods adaptable to wrought production, permitting better quality, are often not applicable to production of castings. However, the demand for high quality stainless castings exists and through individual efforts and the efforts of organizations such as the Alloy Casting Institute, The American Society for Testing and Materials, and the American Foundrymen’s Society, marked improvement in stainless steel casting quality has been noted in the last few years. During 1963 in particular the number of articles appearing in technical journals pertaining to stainless steels for direct application in the chemical industry increased considerably. Many of these articles cited production, quality control, and technological advancements in production of stainless steel castings as having a direct bearing on extending the service life in corrosive environments. Manufacture and Fabrication. Fewer articles pertaining to conventional fabrication techniques for stainless steels appeared in the literature during 1963. Grinding and machining techniques actually are of little concern to the chemical industry because this phase of manufacture will seldom have any deleterious effect on the corrosion resistance of stainless alloys. The possible exception to this would be severe working of the alloys resulting in lowered general corrosion resistance or inducing stresses which aggravate stress corrosion. Utilization of plasmaarc for cutting and machining stainless steels is becoming more prominent. A description ( 4 0 )of plasma-arc cutting of stainless steels and other alloys cites new developments in this field. Data include recent improvements in cutting equipment and new applications. Other less conventional methods of metal removal incorporating ultrasonics and electron beam and laser techniques are also being investigated from various aspects. Still the most significant item to the chemical industry in the manufacturing and fabricating category is welding of stainless steels. This is the most important single item because catastrophic failures can result when stainless steel equipment is improperly welded. Concentration cell, intergranular, pitting, and stress corrosion can all be the direct or indirect result of poor welding procedure. I n weldments, porosity, inclusions, cracking, and lack of ductility are undesirable in any structure and this is particularly true with stainless steels being applied in corrosive media. Receiving detailed evaluations is the more recent development of friction, electron beam, and laser beam welding. Though these forms of welding are in their infancy and are restricted by geometry of the work54
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
piece and the equipment available, it is anticipated that significant developments, resulting in exceptionally high quality weldments, will be forthcoming. Miscellaneous Iron Base Alloys. The austenitic flake and spheroidal graphite nickel cast irons continue to find application in the chemical industry for a wide variety of mildly corrosive environments. In many instances they offer an economic advantage over cast irons and steel when the combination of initial cost and the all-important service life are considered. I n certain media the same is true of these alloys compared to more expensive corrosion resistant alloys. For instance, equipment constructed of austenitic cast iron is economical and suitable for certain areas in fertilizer and sulfuric acid plants where more corrosion resistant alloys are more common but not necessarily required. A rather recent development is the production of austenitic cast iron bar. Wick (70) describes the continuous casting procedure utilized to produce bar. Details are also provided on metallurgical characteristics, mechanical properties, applications, and advantages of the continuous cast bars us. sand-cast materials. A new high silicon cast iron ( 7 0 ) containing a nominal 14.50% of Si and 4.5% of Cr has broadened the range of applicability of the high silicon irons for corrosive environments. The new alloy, which can be produced only as castings, has as its main advantage exceptional corrosion resistance to severely oxidizing solutions. These include ferric chloride and solutions containing it, wet chlorine, chlorine dioxide, sodium hypochlorite, chlorinated organics, and chlorinated brines. REFERENCES Corrosion (1A) Barnartt, S . , Stickler, R., Van Rooyen, D., Corrosion Sci. 3, 9 (1963). (2A) Birchon, D., Eng. .Water. Design 6 , 724 (1963). (3A) Cihal, V., Bergnkudernie 15, 23 (1963). (4A) Douglass, D. L., Thomas: G., Roser, W. R., Corrosion 20, 15t (1964). (SA) English, J. L., Griess, J. C., Muter. Protection 2, 18 (1963). (GA) Flowers, J. W,, Beck, F. H., Fontana, M. G., Corrosion 19, 186t (1963). (7A) Hawkes, H . P., Beck, F . H., Fontana, M . G., Ibid.,p. 247t. (8A) Hoar, T. P., Ihid.,p. 331t. (9A) Kohl, H., Werkstofe Korrosion 14, 831 (1963). (10A) Logan, H. L., McBee, h f . J., Romanoff, M., M a t e r . R e f . Std. 3, 635 (1963). (11A) Nair, F., Semchyshen, M., Corrosion 19, 210r (1963). (12A) Neumann, P. D., Griess, J. C., Ibid., p. 345t. (13A) Prazak, M., Ibid.,p, 75t. (14A) Riedrich, G., Kohl, H., Bergund Huftenrnannische Monatshefte 108, 1 (1963). (15A) Riggs, 0. L., Corrosion 19, 180t (1963). (16A) Vaughan, D. A,, Phalen, D. I., Peterson, C. L., Boyd, W. K., Ibid.,p. 315t. Industrial Applications (1B) Bishop, C. R., Corrosion 19, 308t (1963). (2B) Campbell, H . C., Muter. Protection 2, 39 (1963). (3B) Eisenbrown, C. M., Barbis, P. R., Chem. Ene. 70, 148 (1963). (4B) Mason, J. F., Moran, J. J., Swales, G. L., “6th World Petroleum Congress Section VII.. Paper . 21.. 1963. (5B) Riggs, 0. L., Muter. Protection 2, 63 (1963). (GB) Tingley, I. I., Corrosion 19, 408t (19G3). High Temperature (1C) ‘,‘Joint International Conference on Creep,” 5 volumes published by The Institution of Mechanical Engineers, London, 1963. (2C) Littlewood, R., J . Iron and Steel Inst. 202, 143 (1964). (3C) Sehgal, S. D., Swamp, D., Trans. Indian Inst. 15, 177 (1962). Alloy Development a n d Miscellaneous (1D) Duriron Com any Bulletin A/2b, T h e Duriron Company, Inc., Dayton, Ohio (November, 19637. (2D) Hammond, C. hl., Cobalt 13, 8 (19G3). (3D) Kirschning, H. J., Hombeck, F., Schenck, H., Carius, C., Archiu Eisenhuttenwesen 34, 269 (1963). (4D) O’Brien, R. L., Wickham, R. J., Welding J . 42, 107 (1963). (5D) Oppenheim, R., Jansen, N., DEW Technische Berichfe 3, 58 (1963). (6D) Perry, D. C., Tanczyn, H., Clarke, W. C., M e l o l P70g. 83, 101 (1963). (7D) Wick, W. C., Modern Castings 44, 544 (1963).
