October 1948
Corrosion resistance as well as other properties of titanium metal make it an attractive material.
T
is indeed the modern wonder metal, if the tremendous increase in interest and activity in connection with this metal during the past two years can be used as a yardstick. When research on titanium-chromium alloys was begun a t The Ohio State University approximately two years ago, only four companies or institutions were engaged in the study of titanium and its alloys, including the United States Bureau of Mines, which has done much of the pioneering development on titanium. Today there are approximately sixty different concerns working on titanium and its alloys. One of the major attributes of titanium is its good resistance to corrosion. Practically no actual data have been reported in the literature on the corrosion resistance of titanium and AND ENGItitanium-rich alloys. This issue of INDUSTRIAL KEERING CHEMISTRY contains the Second Annual Review of Chemical Engineering Materials of Construction. However, as none of the articles covers titanium, the writer felt it desirable t o devote this column to a discussion of titanium metal, which may be of considerable interest from the corrosion standpoint in the near future. ITANIUM
11% chromium was prepared by arcmelting in an inert atmosphere. This alloy showed a tensile strength of 150,000 pounds per square inch in the stress-relieved condition. The specific gravity of this alloy is 4.8 and its strength-weight ratio is 31,200. As far as we know,this strength-weight ratio is better than any other metal or alloy, including the high strength aluminum alloys. The lightness, strength, and corrosion resistance of titanium and titanium-base alloys suggest many uses for these materials, including aircraft construction.
Mechanical and phgsical properties Titanium is a relatively light material, possessing a density or specific gravity of 4.5 as compared to 7.9 for iron or steel and 2.7 for aluminum. Some of its properties are shown in Table I. The strength and hardness of the metal are dependent upon the nature and extent of the impurities present. Metal of exceptionally high purity would show lower values than those listed in Table I. During our research program a t Ohio State for the Air RIat6riel Command, an alloy containing 89% titanium and ~
~~
~~
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~~~
TABLE I.
PROPERTIES O F TITANIUM METAL Annealed Cold Worked 79,000 121,000 Tensile strength lb./sq. inch 65,000 113,000 Yield strength, Ib./sq. inch 25 7.5 Elongation. Yo Modulus of elasticity X 1 0 8 16.8 15.4 Brinell hardness 185 255 4.5 4.5 Specific gravity 1820ilOO (3300f180" F.) Melting point, C. Like steel Color
W e t corrosion resistance The few publications and press releases on titanium practically all state that this metal possesses corrosion resistance comparable to that of "stainless steel." As a simple, broad, general statement, that is satisfactory, but the situation depends upon the purity of the titanium, the nature of the corrosive environment, and the type of stainless steel, of which there are many. Gee, Long, and Waggaman of the Bureau of Rlines reported in Materials and Methods (January 1948) that titanium metal is inert to concentrated nitric acid; practically unattacked by 5% solutions of hydrochloric acid, ammonium hydroxide, sodium hydroxide, and acetic acid; soluble in higher concentrations of hydrochloric acid; readily attacked by sulfuric acid; and shows no visible signs of corrosion after 30 days' exposure to salt spray. Other information in the literature indicates that titanium is attacked by hydrofluoric acid, more so if nitric acid is added to the hydrofluoric; not attacked by cold or boiling water; and reacts with molten potassium hydroxide. A few tests were made in the Corrosion Research Laboratory a t Ohio State University (Table 11). The corrosion resistance of titanium to sulfuric acid is rather poor and actually shows greater corrosion than ordinary carbon steel in TABLE 111. CORROSION TESTSON 89% Ti-ll% Cr ALLOY Corrosion Rate, Mds per Year
TABLE 11. CORROSION TEsw AT ROOMTEMPERATURE ON
Corrosion Medium 3% HZ804 65% HzS04 93% HZs04 0.5% HC1 0 . 7 5 % HC1 1.0% HC1 3% NaCl 10% HNOa
10% HAC
TITANIVM ?VIETAL Corrosion Rate, Mils per Year First Second period period Cumulative, 48 h o d s 120 hodrs 168 hours 20 0 6 540 160 265 150 275 240 0 0.3 0.2 0 1 0.6 0 1.6 1 85 0 25 0 0 0 0
0
0
Condition Corrosion Medium 65% HNOs, 50' C. Remarks Numerous pits Numerous pits Numerous pits No visible pitting No visible pitting No visible pitting Few amsll pits No measurable weight loss No measurable weight loss
65% "Os,
boiling
3% NaC1, boiling
10% HzSO4, 50' C.
of
Alloy0 A.F. W.Q. b A.F. W.Q. A.F. W.Q.
A.F. W.Q.
First period, 48 hours
1 0.8 5 5 0 0 95 205 3700
A. F. W.Q. 3000 3% HzSO4, boiling A.F. 1400 W.Q. 1200 a A.F., as forged. W.Q., water quenched. b Water quenohed from 2200' F.
10% HzSOr, boiling
99 A
Second Third Curnulaperiod, period, tive, 48 48 144 hours hours hours 0.5 0.7 0.4 0.6 4 i 4 6 4 5
.
0 0 235 270
. ..
... ... ...
0 0
.. .
0 0 1 fi.5
235 3700 3000
1400 1200
L O O K TO THE LEADER FOR THE 65 and 9370 (66' B6,) sulfuric acid. The corrosion in dilute hydrochloric inckases rapidly with concentration, indicating destructive attark by higher strength acid, Titanium shows excellent resistance to nitric and acetl"cacids and the behavior is similar to 18-8s stainless steel, Additional tests were not made because of the very limited supply of metal. Corrosion tests were also made on the 89yo titanium, 1 17' chromium alIoy mentioned above (Table 111). The corrosion rehistance in the "standard" nitric acid (65% boiling) test is essentially the same a8 that of quench-annealed 18-8s stainless steel. High temperature q u e n c h ~ a n n e a ~of~ nthe ~ 8c1-1I alloy showed no appreciable effect on corrosion resistance im these tests. dBFg
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Fine titanium powder oxidizes rapidly and i s pyrophoric. Solid pieces can be ignited in a stream of oxygen but stop burning when the oxygen is removed. Titanium reacts with steam at 800" C. and with ammonia, chlorine, hydrogen chloride, fluorine, hydrogen, iodine, and nitrogen a t high temperatures. At high temperatures titanium is very reactive and in fact is used as a getter in vacuum tubes. Oxidation tests in air at 1600" F, mere made on three titanium-base alloys containing 4, ll, and 16% chromium. A double layer scale formed on these alloys. The inner layer, piobably chromic oxide, is dense and adherent, and the outer layer, probably titanium oxide and nitride, flakes off on cooling. The alloy containing 16% chromium formed a more dense outer scaPe and showed the least tendency to flake off, In addition, the oxidation rate for this alloy was much lower than for the other two. Pa88ieatiQn Titanium tends t o passivate in a manner somewhat similar for 3% sulfuric acid to 18-86 as shown by the data in Table 1% and 3% sodium chloride, After a high initial rate the specimens showed no attack during the second period. The 89-4 1 alloy does not exhibit passive tendencies in 10% sulfuric acid a t 50" C, as shown by Table 111. W A. Hroll reported that concentrated nitric acid passivates titanium and improves its resistance to sulfuric and hydrochloric acids, Research on the passivation of titanium is under way in the Corrosion Research Laboratory,
Prodrcctiom Yaturd resource^ of titanium ore8 are very plentiful and the problem of supply is relatively ~ n h p o r t a n t . The production of titanium metal is difficult because of its extreme activity, Most of it made to date involves powder metallurgy methods, including compacting and sintering, Equipment for melting titanium must be devised M o r e the cost of the metal can be substantially reduced. The basic problem concerns refractories to contain the molten metal and this problem remains substantially unsolved, Good, but not very reproducible, results were obtained in our laboratory using vacuum melting and specially sintered beryllia crucibles in that ductile high titanium-chromium lrlgots were obtained. One reason for the need of successfu%melting concwns the scrap value of the rnet:il---for example, platinum alloy spinnerets are widely used in the rayon industry, but the use of tantalum spinnerets i s limited. Platinum has a high scrap value, whereas the value of ~antrtlunascrap is prac&ica.liynil.
