I
F. S. BADGER Haynes Stellite Co., Kokomo, Ind.
High Performance Materials
Alloys and Reactive Metals There are no more expensive and limiting problems facing the chemical industry than those of corrosion. Decisions should be based on savings that can be made by proper choice of materials-not on what has been used
AMONG
the early alloys to be introduced to the chemical industry were the high-molybdenum, nickel-base Hastelloy alloys. These were developed for their resistance to the mineral acids. Because of their strength, even a t elevated temperatures, they resisted fabrication into the diverse wrought forms required by the chemical industry. And more recently, a cobalt-base chromium-tungsten-nickel alloy, Haynes alloy No. 25, developed for sheet applications a t 1700' to 1800' F., was found to be outstanding for a number of special corrosion applications. In particular, it proved to be the best commercial alloy for handling red fuming nitric acid, but was limited by weld attack in the vapor phase. Experimental work developed that chromium was not helpful in this nitric acid application and that nickel could be favorably substituted for cobalt. The optimum composition was 55 nickel, 45 tungsten. This alloy, while still in the development state, is currently being tested in the field for specific corrosion applications. I t is expected to be commercially available as sheet, bar, and wire in the near future. A sheet of this alloy was included in rapid-heating rate tests devised to duplicate, in the laboratory, conditions 'present in the skin of a missile. Surprisingly, it lasted two and a half times as long as the next best superalloy under these conditions, when heated to a maximum temperature of 2300' F. Many of the premium corrosion alloys were originally used in the wrought form primarily for patching failed equipment of cheaper materials, but as the chemical industry gained in ex-
1608
perience and confidence in these materials, more and more complete largesized equipment has been fabricated from them. This is currently true of tantalum, where one large usage is in the patching of glass-lined equipment. Fabrication of medium-sized to large equipment is also in the offing. In the cobalt- and nickel-base hightemperature alloys, the stepped-up analyses and the special treatments developing the optimum high-temperature properties may be detrimental to the best corrosion resistance. Thus, there are three general methods of altering an alloy to achieve improved hightemperature properties. The first is
Good Resistance Chromic acid lactic acid Hypochlorites Moist chlorine Molten sulfur Chlorinated hydrocarbons Chlorine dioxide Aqua regia Food products Poor Resistance Hydrofluoric acid Phosphoric ucid Fluorides Oxalic acid Bromine
Titanium may be metallurgical bargain
INDUSTRIAL AND ENGINEERING CHEMISTRY
solid solution hardening, in which the maximum amount of the beneficial elements is put into solid solution, which is generally the most desirable form for corrosion resistance. The second method consists of the formation of carbides and compounds out of solution in relatively coarse form, either directly from the melt or by precipitation through heat treatment. This robs the matrix of some of the elements promoting corrosion resistance. If precipitated constituents are concentrated at the grain boundaries, this condition may lead to localized impoverishment of the desirable elements in the grain boundary areas? which tends to promote corrosion penetration along these grain boundaries. Thirdly, the lattice may be strained by the formation, through heat treatment, of fine intermetallic compounds which promote high-temperature creep strength by retarding the movement of dislocations. Similarly, straining of the lattice by cold work may be used. In general, corrosionwise, lattice strain is considered undesirable. Thus, for optimum corrosion properties a highly alloyed single-phase solid solution theoretically has the best corrosion resistance. Of particular importance in the fabrication of chemical equipment are the formability and weldability characteristics of alloys, which make practical the construction of complicated equipment. Metallurgical stability in the weld zone is highly desirable but not always present in these high alloys. Such instability may call for full annealing of equipment after fabrication or of modification of the alloy analysis or special stabilizing heat treatments prior to welding.
The cobalt- and nickel-base alloys appear to be near their limit of hightemperature application at the present time when highly stressed at 1800’ to 1900’ F. ; however, various applications in the production of power call for temperatures of 2000’ F. and much higher. For these, the refractory metals such as molybdenum, chromium, tungsten, tantalum, and niobium (columbium) are particularly promising. They are all reactive in nature, as, at the high temperatures where they are melted and worked, they combine avidly with the gases of the atmosphere and others contacted during processing, such as nitrogen, oxygen, and hydrogen. Consequently, special methods of melting, hot-working, and welding have to be used, and the fabrication of these metals is a difficult and expensive operation. However, because of their importance to the military, the development of these metals is receiving extensive government support. As they become commercially available, they will be of interest to the chemical industry because of outstanding corrosion-resistant properties in many service media. Titanium is attractive to the military because of its lightness combined with high strength at the lower end of the high-temperature range. This metal is a good example of what “forced-draft” development by government support will do in reducing costs. I t is also a lesson that even this type of development cannot be expected to move a metal out of its essential economic field to compete with lower cost materials-in this case, magnesium, aluminum, and stainless steels; however, the development of light-weight alloys of titanium for usage a t temperatures over 1000’ F. should bring this metal a promising future in the aircraft industrv. Titanium is currently available to the chemical industry as pure metal with optimum fabricating i n d corrosion properties. It should be seriously considered by the chemical industry as a metallurgical bargain under the present conditions of reduced usage by the military. The general corrosion fields in which it is useful are shown in the box on page 1608. Mobybdenum is the refractory metal which, to date, has received the most government support in its development. While it has a very severe high:temperature drawback in the re1a;ively &low temperature of vaporization of its oxide, this should not greatly limit its chemical applications. The production of zirconium has been government supported because of its importance in the atomic energy program. Zirconium has outcorrosion properties. sed for many years beca its excellent corrosion resistance, has recently received greater impetus in its production because of unique properties important for capacitors’ in the
Figure 1.
Tantalum sheet is the largest ever produced-28
X 64 X 0.030 inch’
Uormol 3olllnp Point
>0.2001PY
Figure 2. Stainless steels, Hastelloy F, and titanium-hold up at all concentrations of the boiling acid at atmospheric pressure
Legend
Belling Data Not Valid At The80 Acid Strengths
Normal CO.0101PY
Bo1i I n (I
Paint 0.010-0.050IPY
Ha8t.B Ho8t.C
>0.2001PY
Em b r l ttled
achbric asid don test
Figure 3. Tungsten and molybdenum remain unaffected at all concentrations VOL.
bo, NO.
11
NOVBMBER 1958
1609
Normal
0.2001PY
NoDoto
Phosphoric acid corrosion test
Phosphoric Acid X 6
4b
-2'0
6'0
Figure 4. Tantalum, molybdenum, zirconium, tungsten, and Hastelloy alloy B are useful under both conditions at most concentrations
Legend Normal
0.2001PY Embrlttled
181
Sulfuric acid corrosion test Sulfuric Acid%
0
20
Figure 5. Tantalum in the best-although embrittlement above 75% acid
new electronic industry. Originally, tantalum was produced by powder metallurgy methods, but arc melting of tantalum is now permitting production of considerably larger forms in the wrought conditions with superior welding properties. Fabricating costs probably will be cut in the immediate future. Figure 1 shows what is believed to be the largest single sheet of tantalum produced to date: 28 inches wide, 64 inches long, and 0.030 inch thick. Because of widespread corrosion experience with this metal, it should move forward rapidly as large fabricated equipment becomes feasible. Columbium is now being actively supported and developed as a base for high-temperature alloys and has atomic energy applications as a pure metal. I t also has unique corrosion-resistant capabilities. Currently, the development of tungsten is being carried out at an
16 1 o INDUSTRIAL
40
60
80
it i s subject to hydrogen
increasing rate. Reason: It promises to be one of our most important missile and space materials because it has the highest melting point of all metals. While its high density is a drawback and it is particularly difficult to fabricate, it is also a most promising entrant into the corrosion race. To demonstrate the capabilities of these metals, corrosion tests were made in comparison with alloys now used in the widely used mineral acids of various concentrations at their atmospheric boiling points and at a temperature of 190' C. and the corresponding pressures developed by testing under totally enclosed conditions. Figure 2 shows the results in bar form of these tests for nitric acid in terms of inches per year (IPY). The atmospheric boiling point of 70% nitric acid is 120' C. Under these oxidizing conditions molybdenum is rapidly attacked. However,
AND ENGINEERING CHEMISTRY
stainless steels, Hastelloy F, and titanium are excellent at all concentrations of the boiling acid at atmospheric pressure. Here Hastelloy C is useful only up to 20% acid concentration. At 190' C., zirconium, tantalum, and tungsten are practically unaffected, while titanium has a useful limit of 25% and the stainless steels and Hastelloy alloy F can be used at only concentrations below 5%. In Figure 3 are shown results of similar tests with hydrochloric acid. The constant boiling concentration of this acid is 20.2% at 110' C. In the boiling acid at atmospheric pressure, titanium and Hastelloy alloy C are useful only at very low concentrations. At the higher temperatures and pressures Hastelloy alloy B is useful to 2.5% acid, columbium to lo%, zirconium and tantalum to 30%. Tantalum suffers from hydrogen embrittlement above 30% rather than from corrosive attack, but tungsten and molybdenum remain practically unaffected at all concentrations. The data for phosphoric acid, which when concentrated boils at 160' C., are shown in Figure 4. The tests at 190' C. are not much more severe than the atmospheric tests. Here there are a few gaps in our results where no data are currently available. Tantalum, molybdemum, zirconium, tungsten and Hastelloy alloy B are useful under both conditions at most concentrations. In Figure 5 , sulfuric acid tests are shown which may not have as much importance as the tests with other acids, because the concentrated sulfuric acid under atmospheric pressure boils at 330' C., considerably higher than the tests at 190' C. under enclosed conditions. Tantalum is the best material, although it is subject to hydrogen embrittlement above 75% acid. Tungsten and zirconium are similar, although zirconium's limit of usefulness is about 80% as against 90% for tungsten. Columbium is roughly equivalent to Hastelloy alloy B in atmospheric tests, both being inferior to molybdenum. The chief drawback to satisfactory service of these refractory metals at temperatures over 2000' F. is their very poor resistance to oxidation. The comparative oxidation rates of four of these materials are shown in Figure 6 in flowing air at 2000' F. Columbium and tantalum are fairly close in this respect and greatly superior to molybdenum, with tungsten in an intermediate position somewhat worse than columbium and tantalum. Despite the wide difference between the best and worst materials on this chart, even the best are several orders of magnitude poorer in oxidation resistance than is required for successful high-temperature applications unless their surface is protected. The severity of this problem is emphasized in Figure 7 and the approach to
HIOH PE R,F0 R MA NC E MATER IA LS IO0
.50
805
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45
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X
.30
t b 4.86 o/o Cr
Mg./Sq Cm 2-
.95 20 .I5
.io
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400 800 TIWE-MiNlWES
800
two
Figure 7. Oxidation rates in air at 800' C. are compared for columbium and a columbium-chromium alloy with other metals and alloys
Figure 6. The oxidation rates of columbium and tantalum are fairly close and greatly superior to molybdenum. Oxidation test was in flowing air at 2000' F. (1093' c.)
a solution being used is indicated. This chart is a log-log representation of oxidation rates us. time at 800' C. of columbium compared with some standard metals. One of the best oxidationresistant materials is Nichrome. This material has such a low oxidation rate that it hardly shows on this chart. Pure nickel is an order of magnitude worse in its oxidation rate than Nichrome. Iron is an order of nitude worse in its oxidation, columbium is almost a third orde magnitude worse than iron. Thus,
up to its melting point. However, alloying of just under 5% of chrom with columbium improves its oxida
elements promoting increased resistance are currently being e
importance to the chemical industry of this development work is that some of the alloys produced in the program are likely to have superior properties to those of the pure metals themselves. These may be lower corrosion rates, lower initial casts, or better fabricating qualities. I n effect, a whole new family tif
%
corrosion-resistant alloys may be in the making for the benefit of the chemical industry. These will be the by-products of the attempts to achieve the ultimate in high-temperature refractory metals. iate potential for these alloys in rrosion field could not support cost of the work involved; th ey become available, these alloys may radically change our chemical
als with the hope of the best properties of produced by powder ques, and by the same produced various comas the carbides and nitrides ny materials, being
commercial success as thermocouple protection tubes and also withstand immersion in molten metals. Modified with molybdenum or tungsten and used in of rocket nozzles, they have sh tanding resistance to the bl solid propellants. Silicon
nitride is attractive because it is not wetted by some molten metals. Molybdenum disilicide has excellent hightemperature properties and, hence, is useful in the form of heating elements. As the chemical industry goes to more severe operating conditions, these materials may find many specialized applications despite their lack of ductility and impact strength. For example, glasslined equipment has been satisfactory under severe conditions, though many consider it both fragile and expensive. For these materials to be widely used industrially and to be made commercially available, they must in the long run be economically attractive. Commercialization of exotic materials and production in quantity inevitably result in lowered costs. But at the same time, do not let the current prices of the reactive metals scare you away from considering the use of these materials. There are no more expensive and limiting problems facing the chemical industry than those of corrosiop. Your idea as to the value of the solution of these problems should not be based on the costs of previously used materials, but rather on the savings to be made by successfully combating the corrosion problems. The minimization of production equipment down-time will warrant the high capital investment involved in choosing these more expensive materials. The use of these materials to serve a t higher temperatures and pressures in production will lead to increased earnings. Marketing changes go hand in hand with progress, and new processes may prove to be entirely dependent upon these exotic materials as they become commercially available. RECEIVED for review April 17, 1958 ACCEPTED August 15, 1958 Division of Chemical Marketing and Economicb, 133rd Meeting, ACS, San Francisco, Calif., April 1958. VOL. 50, NO. 11
NOVEMBER 1958
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