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1 I/ECIMaterials of Construction Review
Titanium by H. B. Bomberger and H. A. Clampett, Jr., Crucible Steel Co. of America, Midland, Pa.
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Declining prices and an abundance of excellent performance data are stimulating greater usage of titanium in the process industries. Application is no longer restricted to severely corrosive conditions
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Platinized titanium, having electrode characteristics similar to platinum, looms as an inexpensive and highly attractive electrode material -especially for the chlorine industry
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titanium industry in the United States continues to grow at a healthy rate; last year’s shipments of mill products totaled about 3200 tons, a 20% increase over 1958. Although uncertainties in military requirements and improvements in competitive materials make forecasting uncertain, titanium’s unusual mechanical and corrosion properties, abundant ore supplies, and increasingly attractive prices should result in vigorous future growth for this young industry. With steadily declining prices and an abundance of technical knowledge, new applications are developing in both military and civilian fields. At present, most of the U. S. titanium is employed in aircraft engines and frames where optimum performance is required. A recent announcement stated that a new recoilless weapon is expected to require an additional 1000 tons of titanium alloy in the next five years. The chemical industry employs a small portion of the current production under some of its most severe conditions. A large increase from critical to more general applications is expected with further downward trends in price. Commercial interest is more apparent in Japan where 250 tons were employed by the chemical industry in 1959more than the total employed by the American industry for all years through 1959.
Chemical Properties Corrosion properties of titanium have been well established by a number of investigators in both laboratory and field tests. Most of these data have been discussed in previous reviews and are tabulated in a recent publication (3A).
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The metal appears to be completely unaffected by all natural environments, including sea water. It has outstanding resistance to chloride and hypochlorite salt solutions, nitric acid, hydroxides, wet chlorine, aqua regia, and many other chemicals. Oxidizing agents and certain metal ions, even in small amounts, serve as powerful inhibitors in otherwise corrosive solutions. The reducing-type acid solutions tend to be corrosive. Hydrofluoric acid and certain concentrations of hydrochloric, sulfuric, phosphoric, oxalic, trichloracetic, and unaerated formic acids corrode the unalloyed metal. Dry halogen gases and liquids. ionizable fluoride compounds, and hot? highly concentrated solutions of aluminum, calcium. and zinc chlorides also attack titanium. An extensive study on titanium and other materials was made, by Gegner and Wilson (&), in a wide variety of chemical plant exposures including alkali, chloride, hypochlorite, nitric acid, and sulfuric acid solutions, gaseous chlorine, and solutions containing free chlorine. These data agreed well with earlier laboratory results but also showed that chlorine gas is a very effective inhibitor for titanium in solutions containing u p to 547, sulfuric acid at ambient temperatures. A number of metals were tested as container materials for uranium extraction, with hot hydrochloric-nitric acid solutions, from spent elements jacketed in zirconium and stainless steel. Titanium and tantalum appear suitable for these processes ( 7 A ) . A recent study was made on the corrosion resistance of scvcral alloys. In general, all commercial titanium alloys have a high order of corrosion resistance, but most alloys appear to have slightly
AND ENGINEERING CHEMISTRY
less corrosion resistance than unalloyed titanium in corrosive and sulfuric and hydrochloric acid solutions ( 2 4 ) . It was found ( 5 A , 8*4,70A) thar small am.ounts of certain noble metals, when alloyed with titanium, markedly improve its corrosion resistance in reducingtype acids and that these addirions do not impair the metal’s resistance to oxidizing media. -4s little as 0.1% of platinum, palladium, rhodium, or ruthenium markedly improved titanium’s resistance to hydrochloric and sulfuric acids. Similar additions of the other noble metals were somewhat less effective. In view of the improvements reported, an alloy containing 0.1 to 0.2y0 palladium appears quite practical. The passivity imparted to titanium in reducing-type sdutions by noble metals occurs \\-hen a small amount of corrosion leaves the noble metal? in elemental form, a t the corroding surrace. These low-hydrogen overvoltage areas permit sufficient galvanic current io flow to stabilize (anodize) the exposed titanium surface ( 70A4). Detailed potentiostatic studies were made in hydrochloric and sulfuric acids to explain this behavior (5-4, 6A, 9 A ) . These studies also showed that an applied anodic current confers exceptional corrosion resistance to titanium in hydrochloric and sulfuric acids. Protection was achieved by coupling titanium to relatively cathodic materials, such as 18-8 stainless, Hastelloy F? carbon? and platinum. Anodic current from an external source is also very effective. As little as 0.08 pa. per sq. cm. passivates titanium in 677, sulfuric acid, giving a surface film resistance of 17,500,000 ohms per sq. cm. (77A). A detailed study was made of the corrosion and electrode characteristics of
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Nonmilitary applications for titanium are increasing considerably as the price continues to fall
titanium when coupled to a large number of other metals, and when these metals were added to the test solutions ( 7 A ) a plot of the corrosion rates in 2 M hydrochloric acid us. the couple potential yielded a polarization curve similar to the anodic polarization curve for titanium with an impressed voltage. Similar but less extensive work was done in sulfuric acid. The platinum metals, where employed as ions or couples, inhibited the corrosion of titanium most effectively. Coupling with antimony resulted in the highest corrosicn rates in hydrochloric acid ; however, ionic additions of antimony markedly reduced corrosion. Potential measurements were made to explain these differences and to show the influence of a number of other cation additions.
The largest single application for titanium in the process industries was described by Simons (77B). A nickelcobalt leaching plant a t Moa Bay, Cuba, contains about 37,000 pounds of titanium heat exchangers, valves, pipes, and reactor parts. The purification plant, located at Port Nickel, La., utilizes an additional 8000 pounds of titanium equipment. The leaching process involves treating a thick ore slurry with 98% sulfuric acid between 400" and 500" F. above 500 p.s.i. The acid is consumed rapidly to a residual under 370, yielding a highly erosive-corrosive liquor containing 36% fine iron ore in suspension. Under these conditions, titanium is superior to other materials as the cobalt, nickel, or other metallic ions inhibit corrosion. Tita-
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erosion-corrosion resistance was also reported for titanium impellers tested in thorium oxide slurries a t 1500 p.s.i. and 280" C. (6B). Prescott (9B) cited other practical applications, including a let-down valve taking a 2700 p.s.i. pressure drop in a corrosive liquid. The titanium valve compared strikingly with other materials in its ability to withstand liquids under high velocity and turbulence. High velocity heat exchangers were suggested which would have higher heat transfer rates but reduced surface-area requirements. The successful use of the 6A1-4V alloy for high-pressure (5000 p.s.i.) helium storage bottles a t temperatures to -320" F. was discussed by Hurlich (3B). A number of interesting applications in Columbia-Southern Chemical's plant were recently described. These included large valves for chlorinated brine, 55% calcium chloride at 220' F., 1770 hypochlorous acid, and 45y0 chlorine-saturated sulfuric acid ; tubing for a chlorine-hydrochloric acid atmosphere ; a cooling coil for a mixture of calcium hypochlorite, free hypochlorous acid, and free lime; a liner for 20% brine saturated with chlorine and for wet chlorine a t 190" to 205" F. I n most cases the service has now exceeded two years and, except for a small pit found in a hairpin heating coil in 62% calcium chloride a t 310" F., there was no visible corrosion. Titanium jigs and fixtures are being used economically in a number of surface treating processes. including: anodizing in sulfuric and chromic acids;
Applications
As noted in previous reviews, a full line of titanium mill products and alloys with a wide range of mechanical properties are now available for the consumer. Most fabrications have been reduced to routine operations, and the metal has had extensive testing in military and commercial applications. Detailed information can be obtained from the literature cited in earlier reviews and from the producers. The important advantages titanium offers over other materials of construction for jet engine compressors were discussed by Wile (73B). Surprisingly, titanium alloys are among the least expensive materials employed, and they are less costly and less difficult to machine than the super-alloys. Important savings in weight, high order of performance, and outstanding service reliability were also cited. Over 1,000,000 flight hours have accumulated without a single service failure.
This 300-gallon titanium sauce tank was constructed b y B. H. Hubbert Co. for a leading food producer
VOL. 52, NO. 10
OCTOBER 1960
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a Materials of Construction Review inch in diameter) a t current densities of 68 amperes per square foot a t high tide to 46 amperes per square foot a t low tide. Preiser and Tytell (7B, 8B) reported on the economic advantages and excellent performance of platinumclad titanium and tantalum anodes for a variety of cathodic protection systems. Platinum-clad anodes are considered for many marine, chemical, petroleum, and water-handling applications. Presumably titanium cladded with a platinum alloy containing lOy0iridium would give even better service because of its very low attrition rate ( I B ) . Shreir (70B) noted that tantalum is preferable as a base material because it has a higher breakdown voltage (about 160 volts) than titanium (about 12 to 14 volts). Liferafure Cited Titanium cloth i s woven by Cambridge Wire Cloth Co.
pickling in nitric, nitric-sulfuric, and certain solutions of hydrochloric, phosphoric, and sulfuric acids; nickel and chromium plating; electropolishing in phosphoric-sulfuric acid ; hot dip galvanizing; and alkaline cleaning and etching ( 4 B ) . Major savings in maintenance-as much as five times the initial cost in a year-were realized by replacing conventional racks with titanium racks in anodizing and brightening operations (5B). Typical applications for titanium castings were described and their costs and performance compared with other materials by Aschoff (2%). Titanium pumps have been operated for 10 to 20 times longer than previously used materials in hot solutions containing chlorine, chloride salts, sodium hypochlorite, and nitric acid. The food processing industries have found titanium (unlike other structural metals) to be completely unaffected by detergents and the most corrosive food products. Some of the more serious corrosion problems have been solvedeliminating maintenance and, more important, food contamination-by the use of titanium equipment. Interestingly, foods also have less tendency to stick to titanium than to most other metals. This little-known property is important to food processors (3A, 72B). Previous reviews and a current summary (3A) have noted a number ofimportant applications for titanium. Marine, petroleum, food processing, pulp, and transportation industries are expected to employ ever-increasing amounts with continued price improvements. Recent developments indicate
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that titanium should become very useful in certain electrochemical applications-particularly in the chlorine-caustic industry. Although titanium is virtually immune to corrosion in brine and wet chlorine, it is not suitable as an anode material because the surface oxide resists current flow ( 3 A ) . However, as shown earlier by Cotton, when titanium is given a very thin film of platinum its anode characteristics appear to be equivalent to pure platinum, but a t considerably less cost. This is true even if the titanium is incompletely plated. Many laboratory tests and a few plant trials are under wa)--with diaphragm and mercury cells-to determine the feasibility of platinized titanium anodes. The insoluble, low-chlorine overvoltage platinized electrodes should offer advantages in less maintenance, freedom from carbon and oxygen contamination, lower power costs, increased production, and more optimum cell designs. A number of tests on simulated diaphragm cells indicate platinum attrition rates of the order of 0.4 to 0.6 gram per ton of chlorine. Similar anodes are being considered for chlorite, perchlorate, electrodialysis, electroplating, chlorinating, organic oxidizing, and other cells which require, or would benefit from, the use of an insoluble, low-overvoltage anode. Considerable interest is also developing in the use of platinized-titanium anodes in a variety of cathodic protection systems. For example, 15,COO square feet of steel piling in polluted estuarine water have now been protected for more than six months by four platinized titanium rods (2 feet long by ’/8
AND ENGINEERING CHEMISTRY
Chemical Properties (IA) Buck, R., Sloope, B. W., Leidheiser, H., Jr., Corrosion 15, 566t-570t [November 1959). (2A) Chem. Eng. Progr. 55, 94, 96, 99 (July 1959). (3.4) Crucible Steel Co. of America, Pittsburgh, Pa., “Making the Most of Titanium in the Chemical Process Industries” (1959). ( 4 A ) Gegner, P. J., Wilson, W. L., Corroszon 15,341t-505t (July 1959). (5A) Hoar. T. P., Platznum Metals Rev. 4, 59-64 (April 1960). (6A) Otsuka, R., J . Electrochem. Sac. Japan (overseas ed.) 27, E41-3 (Spring 1959). (7A) Peterson, C. L.. Miller, P. D., others, INDENG.CHEM51, 32-7 (1959). (8A) Stern, M., Bishop. C. R., Trans. Am. Sod. Metals 52, 239-56 (1960). (9A) Stern, M., Wissenberg, H., J . Electrochem. Sac. 106,755-9 (1959). (10A) Ibid., pp. 759-64. (11A) Sudbury, J. D., Riggs, 0. L., Jr., Shock, D. A., Corroszon 16, 91-102 (February 1960). Applications
ilB) Barnard. K. N.. Christie. G. L. ‘ Gage, D. G., Corrosion 15, 41-6 (November 1959). (2B) Crucible Titanium R e v . 7, 4-5 (November 1959). (3B) Hurlich, A., J . Metals 12, 136-8 (1960). (4B) Imperial Chemical Industries, Ltd., Birmingham, England, “Titanium for Chemical Plant No. 8. Titanium for Jigs” (January 1960). (5B) Jones, J. C., Prod. Finishing (London) 5,31 (December 1959). (6B) Moyers, J. C., Oak Ridge National Lab. Rept. CF-59-7-35,lO (July 1959). (7B) Preiser, H. S., Platinum Metals Rev. 3, 38-43 (April 1959). (8B) Preiser, H. S.,Tytell, B. H., Corrosion 15,56-60 (November 1959). (9B) Prescott, G. R . , J . Metals 12, 143-6 (1960). (10B) Shreir, L. L., Platinum Metals Rev. 4,15-7 (January 1960). (11B) Simons, C. S., Corrosion 15, 95-8 (April 1959). (12B) Steel 146,88 (February 1960). (13B) Wile, G. J., J . Metals 12, 132-3 (1960).
