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.ill structural members above the spring line, trusses in the palm house, purlins, glaaing members, sills, belt courses, mullion coverings, radiator grilles, Rap pole brackets, m rind ow jambs, doors. and frames are of aluminum.
Aluminum in the Chernical Industry FRANCISC. FRARY,L41uniinum Company of America, New Kensington, Pa. The advantages of aluminum a s a material of construction for the chemical industry cmter around its high resistance to corrosion by m a n y substances, the colorless nontoxic character of its corrosion products and their inability to catalyze oxidation or other changes, and the combination of lightness, strength, and high thermal and electrical conductivity. T h e chief disadvantage of aluminum as a construction material is its sensitivmess to alkalies, halogens, and halogen acids. S o d i u m silicate and salts oj' chromic acid act as inhibitors of corrosion with certain reagents, while the presence of traces of heazy metals accelerates corrosion.
T
HE chemical industry is notoriously severe in its demands on materials of construction bwause of the corrosire character of many of its raw materials and products, and the injurious effects on the latter of impurities introduced bv corrosion. It has often been said that the greatest problem in connection with a new chemical process is usually the apparatus in which i t is to be carried out, and the most troublesome limitations in the design of this apparatus are often the properties of the materials which may he allowed to corne into contact with the chemicals IISPd.
T h e e$ects of temperature o n the strength of thc metal must sometimes be considered. I n designing chemical equipment, certain principles arp recognized which determine the choice of a metnl composition and method of fabrication best adapted to the conditions under which the equipment is to be used. The units m a y be either cast or wrought, and the usual fabricating and assembling methods m a y generally be employed. Consultation with experienced fabricators and tests of the material recommended, under the probable working conditions, are adtised in preparation for actual design and construction. cally and insoluble in some reagents-particularly most organic liquids, distilled water, and many organic acids. For certain purposes it is advantageous t o produce a thicker oxide coat by anodic treatment in suitable solutions (3) and improve the impermeabilitv of the oxide coating by "sealing" treatments, or impregnate the coating w i t h c o r r o s i o n inhibiting substances. I n other cases-for example, where brine is the corroding agentspecial alloys are found to be much more resistant1 than pure aluminum, and sometimes it is possihle t o add corrosion inhibitors, such as a dichromate,2 to the brine without interfering with the proces.. In moqt cases, however, t h p corrosion reiistance of a l u m i n u n i i n acid or neutral s o l u t i o n s i n c r e a s e s rapidly a s the purity of the metal increases. Broadly speaking, magnesium, m a n g a n e s e , c h r o m i u m ,
AI)VANT.\OEYOF ALUMISCMAS A STRUCTURAL 114TERIAL T h principal advantages of aluminurn center around its resistance t o corrosion b y c e r t a i n chemicals and the character of the c o r r o s i o n products formed. While the metal itself is very active chemically, it iq always (except ivhen amalRAYONBOBBINOF gamated) covered with a thin, adherent, naturally 9 9 . 6 PER CENT formed coating of it? oxide. This is inert chemiPUREALUMXNUM 1231
-
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1 llcoa 52s for wrought parts a n d llcoa 214 for castings are practically completely resistant t o common salt solutions a t ronm temperature 2 .\bout 1 per cent of the chlorine content of the Bodiurn chloride, magnesium ahloride, and/or calcium chloride
INDUSTRIAL AND ENGINEERING CHEMISTRY
1232
and possibly antimony are the only alloying elements commonly added to aluminum which do not reduce its corrosion resistance to most chemicals. One of the important advantages of aluminum lies in the fact that its salts are colorless, and s l i g h t corrosion, therefore, does not discolor or stain the materials with which it is in contact, or others with which such materials may later be used. For this reason, aluminum stills, tanks, tank cars, and drums are P E R C E N T A C E T I C A C I D BY W E I G H T standard equipment i n FIGURE 1. CORROSION OF ALUMI- the synthetic acetic acid NUM BY ACETIC ACID industry, and aluminum 1 , 1. Immersion a t 50' C. for 48 hours. equipment is becoming 3, 4. Immersion a t room temperature for 60 days. increasingly important 53SH = aluminum alloyed with 1.25 in the cellulose acetate per cent magnesium and 0.7 per cent silicon, full-hard temper. and the organic solvent PSO = commercially pure aluminum, annealed. industries. Figures 1,2, and 3 give the results of laboratory tests and show graphically the-very slow rate of attack of aluminum at room temperature by acetic acid at all concentrations except the most dilute. However, a t the other end of the curve, the rate of attack rises rapidly if even small amounts of acetic anhydride are present. Higher temperatures also increase the rate of attack by the dilute acid. Freedom from discoloration effects is also an important reason for the use of aluminum in the naval stores industry and in varnish making. The stearic acid industry is interested in the same property, and also in the fact that a cake of stearic acid solidified in a properly designed alumiiium pan does not adhere to it. The fact that the corrosion products, in the amounts formed, are nontoxic not only to man and animals (5)but also to yeasts and molds used in the fermentation industry, is important-for example, in the dairy cc..MIJor , , , industrv. the Droduction of citric a c i d f i o m sicrose and that of a I gluconic acid from glucose. Aluk i n u m seems to have no specific destructive effect on v i t a m i n s , even a t elevated temperatures (7), Moreover, as i t forms only one series of salts, aluminum causes no catalytic oxidation effects such k R C E N 7 A C E T I C A C I D BY WEIGHT. as may occur in air in the presFIGURE 2. CORROSION OF A~~~~~~~~ BY vERy ence of iron or copper salts. Such DILUTEA ~ T I ACID C AFcatalytic oxidation, in the presTER 60-D.4~ IMMERSION ence of traces of certain metals, AT TEMPERATURE has been shown to be the cause of the development of off flavors in dairy products (6) and may perhaps produce a similar effect in other foods. The fact that aluminum is the only common metal which, in its commercial form, does not appreciably catalyze the decomposition of hydrogen peroxide, accounts for its increasing use in the manufacture and shipment of this chemical. Its inertness toward sulfur, hydrogen sulfide, and organic sulfides is often useful-for example, in pipes for handling molten sulfur from the wells and cars for shipment of lump sulfur, and in the distillation and handling of so-called sour crude oils which are high in sulfur. Extensive tests have shown that aluminum is inert towards anhydrous sulfur dioxide and ammonia (liquid or gas), and I
Vol. 26, No. 12
the Underwriters' Laboratories have approved its use in refrigeration units employing these materials. Its good thermal conductivity is of importance in the fins, trays, etc., of refrigerators, and its high reflectivity and low emissivity have made aluminum foil important as a light-weight, noncombustible, and vermin-proof heat insulator for both refrigerated and steam-heated equipment. The high thermal conductivity is valuable in equipment when uniform heat distribution and avoidance of local overheating are important. Satisfactory f o r e i g n aluminum containers has 2 led to their approval in $ t h i s c o u n t r v . also. for 5 acid of 80 per cent conc e n t r a t i o n or better. g.mwI --t----!-_-L ,/T53--SHi ! The slow rate of attack 2 9M ""P E R C''' E N T ACETIC su ACID wA BY9oJ WEIGHT. wJ by this acid is FIGURE 3. CORROSION OF ALUMIshown in Figure 4. NUM BY STRONGACETIC ACID Small amounts of hyAFTER ~O-DAYIMMERSION AT d r o c h l o r i c or sulfuric ROOMTEMPERATURE acids greatly increase the attack, but larger amounts of sulfuric acid reduce it, as Figure 4 shows. Inertness to acid fumes, high moisture-proofing power, and long life in sunlight have made aluminum paint popular in many chemical plants for protection of both wood and metal, and to improve lighting conditions. Even dilute chlorine fumes, as in a cell room, seem not to injure i t seriously, but it will not stand a spray of caustic solutions. I n some cases, as in bobbins and spinners for rayon, the combination of light weight with adequate strength and resistance to shock (which causes breaking or chipping in stoneware, etc.) is of importance in addition to the chemical resistance of the metal. The fact that the metal can be cast or wrought in almost all forms and can be welded as well as bolted or screwed together, is an additional advantage in building equipment. A good example of the value of strength and stiffness, combined with chemical resistance, is found in the substitution of aluminum for block tin in the storage and handling of distilled water (2). Another interesting physical property was revealed in the mechanical tests of aluminum drums for shipping nitric acid, hydrogen peroxide, etc., where dropping full drums from a considerable height failed to burst them, even when they landed on a rail or I-beam. The low modulus of elasticity and the high ductility of the grade of aluminum employed enabled the drum to absorb the shock and stress of the impact by extensive general deformation rather than by local failure, such as would have occurred in a steel drum. The same property has appeared to advantage when aluminum tank cars or tank trucks have been involved in wrecks, avoiding spillage of their valuable and sometimes dangerous liquid contents. Standard designs for aluminum drums and tank cars have been worked out by the Manufacturing Chemists' Association and the Tank Car Committee of the American Railway Association, and approved by the Interstate Commerce Commission.
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DISADVANTAGES O F ALUMINUM Aluminum and its alloys are all rapidly attacked by fixed caustic alkalies (sodium, potassium, and calcium hydroxides, etc.) and by the halogens and halogen acids. This attack is to be ascribed to the solution or removal of the natural, protective oxide film and increases in vigor with increase in temperature and pH. Alkali carbonates likewise attack aluminum, but more slowly, and the same holds true for other soluble alkali metal salts which hydrolyze
December, 1934
INDUSTRIAL AND ENGINEERING CHEMISTRY
1233
of chromic acid (potassium or sodium dichromate, equal to phate). However, the action of these salts, mildly alka- 1 per cent of the chlorine content) and such additions to line soaps, etc., may be inhibited by the addition of a small refrigerating brines, to calcium chloride used for spraying amount of certain grades of sodium silicate. The “GC” grade coal carried in aluminum trucks, etc., have been successful. of the Philadelphia Quartz Company is of the type which No satisfactory inhibitors are known for the free halogen acids, seems to give the best results. Silicates do not, however, and their presence generally must be avoided. In the case satisfactorily inhibit caustic alkali solutions. Ammonia soh- of organic halogen compounds which hydrolyze slightly, howtions, if free from heavy metals and chlorides, form a thick ever (e. g., carbon tetrachloride), experience shows that impervious coating of aluminum oxide in a few days and anodic oxidation of the aluminum will give adequate protecthen have no further effect, so that aluminum has been SUC- tion, so long as copper, brass, or similar metals are not present. The i n j u r i o u s effects of cessfully used in condensers salts of heavy metals, such for ammonia-water mixtures. While aluminurn cannot FIGURE4. ACTIONOF NITRICACID- as copper, tin, Or lead, must be used with free halogens 25 SULFURIC ACID MIXTURESON COM- a l w a y s b e considered, and or halogen acids, or moist 2 MERCIALLY h E , ANNEALED ALUMINUMmaterials contaminated with them are often unsatisfactory 2 DURATION organic halides c a p a b l e of OF ACIDMIXTURE for contact with aluminum if hydrolysis in the presence of Cmvm EXPT. “01 H*S04 Y even a little water is present. water, the corrosive effect of Days 5% aqueous s o l u t i o n s of the % A 90 45-95 % The deposition of a minute B 30 60-80 f:5 speck of the heavy metal, chlorides of the alkalies and 2 C 30 60-80 D 30 60-80 5 by reaction of its salt with alkaline earths can be prelo E 30 60-80 aluminum, sets up a shortvented by the a d d i t i o n of COYENTRATION, PER CENT circuited b a t t e r y with the proper amount, of a salt
so as to produce an alkaline reaction (e. g., trisodium phos-
1234
INDUSTRIAL AND ENGINEERING
CHEMISTRY
Vol. 26, No. 12
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I N D U S T R I A L .4 K D E N G I N E E R I N G C H E M I S T R U
1235
December, 1934
I N D U S T R I h L AN D E N G I N E E R IN G C H E M IS T R Y
the aluminum as anode; this results in deposition of more heavy metal and hydrogen, and a rapid corrosion of the aluminum immediately adjacent to the heavy metal particle, with the formation of a sharp pit which may deepen rapidly a n d eventually cause perforation. It has been the author's experience that spectroscopic examination of the corrosion product in the bottom of most of the local pits which occur unexpectedly in aluminum apparatus shows appreciable amounts of one or all of these metals. Previous contact with copper, brass, or tin equipment caused the metals to be taken up by the solutions being treated, and the aluminum removed them and was corroded because of their presence. Certain organic compounds, if present, may be affected by the aluminum. Thus, certain fruit colors are reduced and decolorized by long standing in contact with aluminum, in the absence of air. Sometimes the color returns upon exposure to air. Although pure aqueous alcohol is not affected by aluminum, whisky that has been aged in wood cannot be stored for a long time in aluminum without danger of developing turbidity, because of precipitation of some of the dissolved organic matter. Red wines may be partly bleached, but, on the other hand, beer is not affected a t all so long as pure aluminum or the alloy 53s (containing magnesium and silicon as alloying additions) is used. Aluminum containing much iron, copper, or manganese may affect beer.
DESIGNO F ALUMIKUM EQUIPMENT Because of its low melting point (about 660" C. or 1220" F.) the strength of aluminum decreases as its temperature is raised ( 8 ) . This must always be allowed for in the design of chemical equipment. The lower modulus of elasticity, which causes the aluminum to yield elastically or bend more than steel a t a given unit stress, must also be considered in the design. Generally a slight increase in wall thickness is enough to compensate for this, as the stiffness of a metal sheet increases as the cube of its thickness. Tables and other information are available in the literature and in manufacturers' trade publications (literature citation 1, for example) showing compositions and properties of the pure metal and a great variety of alloys. Thme need not be repeated here. In attempting to use them, however, certain general principles must be observed. Aluminurn articles may be broadly divided into two groups-wrought and cast. I n general, an alloy suitable for wrought objects is not used for castings and vice versa. Pure aluminum is practically always used in a wrought form, as it is difficult to cast into any but the simplest shapes (ingots). Because of the crystallization shrinkage, it is difficult to produce any large castings of moderate to thin wall thickness which will be completely nonporous and impermeable to liquids and gases under pressure. For this reason, equipment for treating liquids or gases under pressure should generally be designed to be made of wrought metal wherever possible. Small simple castings such as pipe fittings, however, can be made tight with the proper technic and the use of certain alloys, and are regularly made and used for such work. Castings may also be used for apparatus where there is no pressure difference to be withstood, as for bubble caps in a n oil still, etc. The ease with which aluminum may be given special shapes by drawing, hammering, forging, or extrusion makes it practicable to produce almost any piece of equipment in the wrought metal. Most of such equipment for chemical industry is made of commercially pure aluminum (2s) or an alloy containing about 1.25 per cent manganese (3S), or one containing 1.25 per pent magnesium and 0.7 per cent silicon (53S), because these are most resistant, in general, to corrosion. For salt solutions, however, a new alloy (52s) containing a small amount of magnesium has shown a greater resist-
1237
ance than even pure aluminum, and with a casting alloy of a related composition (214) will probably give the best results obtainable with aluminum alloys. Pure aluminum is the weakest and 53s is the strongest of the materials just mentioned. The latter alloy is strong enough so that beer barrels made of it withstand a pressure of 500 pounds per square inch. Another factor to be considered in design is the effect of the conditions of use upon the initial strength of the material, as determined by its "temper." Like other metals, aluminum is hardened and strengthened by cold work (rolling, hammering, swaging, etc.) and may be obtained not only in the annealed (0),but also in the quarter-hard (l/qH), half-, threequarters, and full-hard (H) tempers. The hardening effect may be removed by annealing which takes place quite rapidly a t 300" C. (572 " F.) and more slowly a t lowertemperatures down to about 150' C. (302" F.). Unless the amount of cold working has been abnormally large, no annealing takes place a t 100" C. (212"F.),even after several years. If the equipment is to be used a t somewhat higher temperatures, a higher eventual strength may be obtained by starting out with half-hard rather than full-hard material, as the greater internal stress in the more highly worked full-hard material lowers the annealing or softening temperature (4) so that after some time at, for example, 200" C. (392" F.), the halfhard sheet remains stronger than the full-hard material. The alloy 53s may be considerably strengthened by heat treatment (quenching from about 500" to 510' C. or 932' to 950' F.) and is then further hardened by heating ("aging") a t temperatures of 100" to 150" C. (212" to 302' F.), but may be softened again a t somewhat higher temperatures. As to low temperatures, aluminum increases in both strength and ductility as the temperature drops, and does not show any tendency to develop brittleness a t low temperatures, as far as is known. There are no allotropic or other changes which are brought about in aluminum by low temperature. Because of the many variable factors influencing the life of aluminum and its behavior in any chemical process, consultation with experienced manufacturers of aluminum equipment is desirable before the design is determined. The adaptability of the metal or alloy for the proposed use should be determined in advance, if possible, by submitting a suitable piece of the metal to the exact environment in which it is proposed to employ it. On the basis of such information, a reliable decision may be made as to the use of the metal and the design of suitable equipment. &KNOTVLEDGMEKT
The data on which the curves are based, and much other specific information in this paper, are the result of work carried out in the Aluminum Research Laboratories under the direction of H. V. Churchill and J. R. Akers, to whom the author is also indebted for many valuable suggestions.
LITERATCRE CITED (1) dluminum Company of America, "hlcoa Aluminum and Its .Illoys," 1932. (2) Churchill, H . V., IXD. EKG.CHEM.,Anal. Ed., 5, 264 (1933). (3) Edwards, J. D., Ibid.,News Ed., 11, 328 (1933); Bengston and Pettit, Am. Machinist, 77, 76-9 (1933). (4) Howell, F. A, and Paul, D. h.,"Metals and Allloys,'' Mellon Institute, 1934. (5) Mellon Inst. Ind. Research, Bibliographic Series Bull. 3 (1933). (6) Rogers, L. A , Associates of, "Fundamentals of Dairy Science," A. C. S. Monograph No. 41, Chemical Catalog Co , New Pork, 1928. (7) Schwartze, Nurphy, and Hann, J . Sutrition, 2, 325-52 (1930); Schwartze, Murphy, and Cox, I b i d . , 4, 211-25 (1931). (8) Templin, R. L., Braglio, C., and Marsh, K., Trans. Am. Soc. Mech. Eng., 50, 25 (1928) RECBIVED September 19, 1934 Presented before the meeting of the American Institute of Chemical Engineers, Pittsburgh, Pa , November 1 4 to 17, 1934
