Abstract:
An excellently corrosion-resistant titanium-base alloy comprises, all by weight, either from 0.005% to less than 0.2% ruthenium or from 0.005% to 2.0% palladium or both, at least one of from 0.01% to 2.0% nickel, from 0.005% to 0.5% tungsten, and from 0.01% to 1.0% molybdenum, and the remainder titanium and unavoidable impurities.

Description:
BACKGROUND OF THE INVENTION 
     This invention rleates to an excellently corrosion-resistant titanium-base alloy. 
     Titanium has come into extensive use as an industrial material, replacing conventional corrosion-resistant materials by dint of its greater corrosion resistance. It is particularly resistant to corrosive attacks of oxidizing environments such as of nitric acid, chromic acid, chloric acid, chlorine dioxide, and chlorate. Also, it is inert to sea water and other chloride-containing corrosive environments. In a non-oxidizing acid such as hydrochloric or sulfuric acid, however, titanium fails to prove as anticorrosive as in above said environments. Efforts to overcome this disadvantage have led to the introduction of its alloys, typically Ti-Pd, Ti-Ni, and Ti-Ni-Mo alloys, in some sectors of industry. The Ti-Pd alloy is high-priced because it uses expensive palladium, whereas the Ti-Ni and Ti-Ni-Mo alloys have a common drawback of poor workability. These drawbacks have hampered widespread use of the titanium alloys. 
     Thus, much remains to be settled before successful employment of titanium in severely corrosive environments despite the excellent corrosion resistance inherent to the metal element. Titanium alloys developed to attain partial improvements in this respect have not proved satisfactory either, with many shortcomings yet to be corrected. 
     SUMMARY OF THE INVENTION 
     The present invention has now been perfected with the foregoing in view. It is directed to a titanium-base alloy which exhibits a profound anticorrosive effect in rigorously corrosive environments not only of oxidizing acids such as nitric acid but also, and in particular, of non-oxidizing acids. The alloy is, moreover, resistant outstandingly to the crevice corrosion that frequently occurs in solutions wherein chlorine ions are present. 
     The alloy is a titanium-base alloy of a composition containing one or two of 
     from 0.005% to less than 0.2% by weight ruthenium and 
     from 0.005% to 2.0% by weight palladium 
     and one or more of 
     from 0.01% to 2.0% by weight nickel, 
     from 0.01% to 1.0% by weight molybdenum, and 
     from 0.005% to 0.5% by weight tungsten. 
    
    
     DETAILED DESCRIPTION 
     In the composition according to the present invention, the ruthenium content has the lower limit fixed at 0.005 wt% because a smaller ruthenium proportion brings a too slight improvement in corrosion resistance for practical purposes. More then 0.005 wt%, preferably more than 0.01 wt%, is required. The upper limit of less than 0.2 wt% is set because a larger addition is uneconomical in that the anticorrosive effect is saturated and the ruthenium cost increases non-negligibly. 
     The minimum amount of palladium is specified to be 0.005 wt% because a less amount of the element is of little practical significance in improving the corrosion resistance. An amount of at least 0.005 wt%, preferably at least 0.01 wt%, is needed. The maximum palladium amount is specified to be 2.0 wt%. Saturation of the anticorrosive effect and the high palladium cost make a larger addition economically unjustified. 
     Nickel should be used in an amount of at least 0.01 wt%. When added in a smaller amount, it will not improve the corrosion resistance to a practically beneficial degree. Preferably, at least 0.1 wt% nickel is added. On the other hand, the nickel amount should not exceed 2.0 wt%. A greater nickel proportion adds little to the anticorrosive effect but renders the resulting alloy difficult to work and fabricate. A nickel amount of 1.0 wt% or less is preferred. 
     The lower limit of the molybdenum content is 0.01 wt%. The addition below this limit is impractical, with a negligible improvement in corrosion resistance. The upper limit of 1.0 wt% is placed because more molybdenum no longer produces an appreciable improvement but rather reduces the workability of the alloy, making it difficult to fabricate. 
     For tungsten the lower limit of 0.005 wt% is fixed since the addition below this limit is little contributory to the corrosion resistance and is impractical. A preferred amount is 0.01 wt% or more. The upper limit of 0.5 wt% is set on the grounds that a larger percentage of tungsten creates little more favorable effect but decreases the workability and presents difficulty of fabrication. 
     Next, the effectiveness of the titanium alloy according to the present invention will be explained below in comparison with conventional corrosion-resistant titanium alloys. 
     The corrosive environments used for tests were, for general corrosion tests, 
     1. 1% H 2  SO 4 , boiling, and 
     2. 5% HCl, boiling, and for crevice corrosion tests, 
     3. 10% NaCl, pH=6.1, boiling. 
     Table 1 summarizes the results of the tests carried out using 1% H 2  SO 4 . 
     Among the materials tested, pure titanium and conventional corrosion-resistant titanium alloys are designated by Nos. 1 to 7. Ternary alloys prepared in accordance with the invention are represented by Nos. 8 through 51 and quaternary and further multicomponent alloys of the invention by Nos. 52 through 62. 
     Test material Nos. 8 to 13 are (Ti-Ru-Ni) alloys embodying the invention in which the Ni proportion was varied. A Ni content as small as 0.01 wt% (No. 8) proved effective, and the corrosion rate was sharply lowered with 0.1 wt% or more. The favorable effect of Ni addition is readily distinguishable by comparison with No. 3. 
     
                       TABLE 1______________________________________Results of general corrosion tests(1% H.sub.2 SO.sub.4, boiling)                       Corrosion rateNo.   Composition (wt %)    (mm/y)______________________________________ 1    Pure titanium         10.4 2    Ti--0.15Pd            0.278 3    Ti--0.04Ru            0.280 4    Ti--0.6Ni             6.55 5    Ti--0.8Ni--0.3Mo      1.69 6    Ti--0.02W             9.74 7    Ti--0.1Mo             9.42 8    Ti--0.03Ru--0.01Ni    0.271 9    Ti--0.03Ru--0.06Ni    0.15610    Ti--0.03Ru--0.12Ni    0.07811    Ti--0.03Ru--0.6Ni     0.06012    Ti--0.03Ru--1.0Ni     0.05913    Ti--0.03Ru--2.0Ni     0.05414    Ti--0.01Ru--0.6Ni     0.08515    Ti--0.04Ru--0.6Ni     0.07616    Ti--0.07Ru--0.6Ni     0.07517    Ti--0.11Ru--0.6Ni     0.06918    Ti--0.20Ru--0.6Ni     0.05819    Ti--0.04Ru--0.01W     0.24120    Ti--0.04Ru--0.05W     0.14421    Ti--0.04Ru--0.1W      0.10822    Ti--0.04Ru--0.5W      0.08923    Ti--0.01Ru--0.02W     0.27124    Ti--0.1Ru--0.02W      0.07325    Ti--0.2Ru--0.02W      0.06626    Ti--0.04Ru--0.01Mo    0.23127    Ti--0.04Ru--0.3Mo     0.17728    Ti--0.04Ru--1.0Mo     0.19229    Ti--0.01Ru--0.1Mo     0.27530    Ti--0.1Ru--0.1Mo      0.17731    Ti--0.2Ru--0.1Mo      0.10032    Ti--0.05Pd--0.01Ni    0.26633    Ti--0.05Pd-- 0.1Ni    0.09334    Ti--0.05Pd--1.0Ni     0.07135    Ti--0.05Pd--2.0Ni     0.06936    Ti--0.01Pd--0.6Ni     0.27537    Ti--0.1Pd--0.6Ni      0.06238    Ti--1.1Pd--0.6Ni      0.03339    Ti--2.0Pd--0.6Ni      0.02940    Ti--0.07Pd--0.005W    0.25341    Ti--0.07Pd--0.09W     0.19442    Ti--0.07Pd--0.5W      0.18843    Ti--0.01Pd--0.05W     0.27144    Ti--0.15Pd--0.05W     0.14345    Ti--2.0Pd--0.05W      0.03346    Ti--0.05Pd--0.01Mo    0.19947    Ti--0.05Pd--0.3Mo     0.18848    Ti--0.05Pd--1.0Mo     0.17649    Ti--0.01Pd--0.1Mo     0.27250    Ti--0.15Pd--0.1Mo     0.23151    Ti--2.0Pd--0.1Mo      0.08452    Ti--0.05Ru--0.5Ni--0.02W                       0.04953    Ti--0.05Ru--0.5Ni--0.1Mo                       0.04554    Ti--0.04Ru--0.02W--0.1Mo                       0.11355    Ti--0.05Pd--0.5Ni--0.02W                       0.07756    Ti--0.05Pd--0.5Ni--0.1Mo                       0.07357    Ti--0.04Pd--0.02W--0.1Mo                       0.09458    Ti--0.05Pd--0.05Ru--0.5Ni                       0.04359    Ti--0.05Pd--0.05Ru--0.5Mo                       0.10160    Ti--0.05Pd--0.05Ru--0.5W                       0.10861    Ti--0.05Ru--0.02W--0.1Mo--0.5Ni                       0.07362    Ti--0.05Pd--0.02W--0.1Mo--0.5Ni                       0.084______________________________________ 
    
     It should be clear from these why the lower limit was fixed at 0.01 wt%. The upper limit of 2.0 wt% is placed because a larger addition of Ni does not produce a correspondingly favorable effect but affects the workability of the alloy seriously. 
     Nos. 14 to 18 are (Ti-Ru-Ni) alloys embodying the invention with varied Ru proportions. A Ru content of only 0.01 wt% (No. 14) exhibited its beneficial effect. The effectiveness of Ru addition is obvious in contrast with No. 4. Thus, it will be appreciated that the lower limit is 0.005 wt%. The upper limit of 0.2 wt% for Ru addition is required since a higher percentage addition is little contributive to rise the anticorrosive effect for the added amount of unduly raises the Ru cost. 
     Nos. 19 to 22 represent (Ti-Ru-W) alloys according to the invention with varied W contents. The corrosion rate was noticeably retarded by the addition of 0.005 wt% (No. 19), demonstrating the advantage derived from the W addition over No. 3. Hence, the lower limit of 0.005 wt% for W addition. The upper limit of 0.5 wt% is chosen because more W seriously affects the workability of the alloy. 
     In Nos. 23 to 25, (Ti-Ru-W) alloys of the invention, the Ru content was varied. With 0.01 wt% Ru (No. 23) the favorable effect is evident, as contrasted with No. 6. Thus, the lower limit is 0.005 wt%. The upper limit of 0.2 wt% is necessary because more Ru does not give a marked effect but raise the Ru cost to excess. 
     Nos. 26 to 28 are (Ti-Ru-Mo) alloys embodying the invention with varied Mo contents. The corrosion rate began to slow down with 0.01 wt% Mo (No. 26), indicating the merit of Mo addition in contrast with No. 3. For this reason the lower limit of 0.01 wt% is put to Mo addition. The upper limit of 1.0 wt% is placed to avoid a larger Mo percentage which will reduce the workability of the resulting alloy. 
     In (Ti-Ru-Mo) alloys of the invention, only the Ru content was varied in Nos. 29 to 31. Ru addition evidently took its effect with only 0.01 wt% (No. 29), and its favorable effect makes a sharp contrast to No. 7. In view of this, the lower limit of Ru addition is set at 0.005 wt%. The upper limit is 0.2 wt% because a larger Ru content does not add an accordingly desirable effect but merely boosts the Ru cost. 
     Nos. 32 through 51 represent Ti-Pd alloys with the addition of Ni, Mo, or W in accordance with the invention. The data suggest practically the same tendency as observed with the Ru-containing alloys already described. In brief, the addition of Ni, Mo, or W remarkably improves the corrosion resistance of the Ti-Pd alloys. 
     Nos. 52 through 62 represent the alloys of four or more components embodying the invention. It must be understood that all are superior to conventional corrosion-resistant titanium alloys. 
     Table 2 shows the results of tests conducted using 5% HCl, boiling. 
     
                       TABLE 2______________________________________Results of general corrosion tests(5% HCl, boiling)                       Corrosion rateNo.   Composition (wt %)    (mm/y)______________________________________ 1    Pure titanium         29.7 2    Ti--0.11Pd            6.20 3    Ti--0.02Ru            9.51 4    Ti--0.6Ni             83.3 5    Ti--0.8Ni--0.3Mo      71.7 6    Ti--0.02W             33.1 7    Ti--0.1Mo             44.6 8    Ti--0.03Ru--0.01Ni    5.39 9    Ti--0.03Ru--0.06Ni    2.2010    Ti--0.03Ru--0.12Ni    0.68511    Ti--0.03Ru--0.6Ni     0.57912    Ti--0.03Ru--1.0Ni     0.50413    Ti--0.03Ru--2.0Ni     0.49814    Ti--0.01Ru--0.6Ni     0.47915    Ti--0.04Ru--0.6Ni     0.39016    Ti--0.07Ru--0.6Ni     0.33117    Ti--0.11Ru--0.6Ni     0.36018    Ti--0.20Ru--0.6Ni     0.29919    Ti--0.04Ru--0.01W     0.35220    Ti--0.04Ru--0.05W     0.29121    Ti--0.04Ru--0.1W      0.20322    Ti--0.04Ru--0.5W      0.19423    Ti--0.01Ru--0.02W     5.8824    Ti--0.1Ru--0.02W      0.93325    Ti--0.2Ru--0.02W      0.42826    Ti--0.04Ru--0.01Mo    1.9827    Ti--0.04Ru--0.3Mo     1.0328    Ti--0.04Ru--1.0Mo     1.4129    Ti--0.01Ru--0.1Mo     6.0730    Ti--0.1Ru--0.1Mo      1.3231    Ti--0.2Ru--0.1Mo      0.7532    Ti--0.05Pd--0.01Ni    5.0133    Ti--0.05Pd--0.13Ni    0.54334    Ti--0.05Pd-- 1.0Ni    0.49535    Ti--0.05Pd--2.0Ni     0.42636    Ti--0.01Pd--0.6Ni     3.4737    Ti--0.1Pd--0.6Ni      0.37838    Ti--1.1Pd--0.6Ni      0.14139    Ti--2.0Pd--0.6Ni      0.09340    Ti--0.07Pd--0.005W    2.8841    Ti--0.07Pd--0.09W     1.3142    Ti--0.07Pd--0.5W      1.0743    Ti--0.01Pd--0.05W     6.3444    Ti--0.15Pd--0.05W     0.88345    Ti--2.0Pd--0.05W      0.69146    Ti--0.05Pd--0.01Mo    7.0347    Ti--0.05Pd--0.3Mo     5.3248    Ti--0.05Pd--1.0Mo     4.3749    Ti--0.01Pd--0.1Mo     6.4350    Ti--0.15Pd--0.1Mo     1.0351    Ti--2.0Pd--0.1Mo      0.74552    Ti--0.05Ru--0.5Ni--0.02W                       1.9453    Ti--0.05Ru--0.5Ni--0.1Mo                       1.8854    Ti--0.04Ru--0.02W--0.1Mo                       1.9155    Ti--0.05Pd--0.5Ni--0.02W                       2.0056    Ti--0.05Pd--0.5Ni--0.1Mo                       2.0357    Ti--0.04Pd--0.02W--0.1Mo                       2.2158    Ti--0.05Pd--0.05Ru--0.5Ni                       0.35559    Ti--0.05Pd--0.05Ru--0.5Mo                       0.70360    Ti--0.05Pd--0.05Ru--0.5W                       0.81761    Ti--0.05Ru--0.02W--0.1Mo--0.5Ni                       0.22162    Ti--0.05Pd--0.02W--0.1Mo--0.5Ni                       0.296______________________________________ 
    
     The corrosive environment was more rigorous than with 1% H 2  SO 4  and the corrosion rates were generally higher. However, the alloys embodying the invention all remained superior to the ordinary corrosion-resistant titanium alloys. 
     Crevice corrosion tests were conducted and the results as in Table 3 were obtained. 
     As the corrosive conditions, an aqueous solution of 10% sodium chloride was used, with pH=6.1 in a boiling state. 
     Crevice corrosion occurred in pure titanium and a Ti-0.15Pd alloy before the lapse of one full day. A Ti-0.8Ni-0.3Mo alloy corroded in two days. The alloys embodying the invention, by contrast, were all more resistant to crevice corrosion. It will be seen from the table that the alloys according to the invention are superior in resistance to crevice corrosion as well as to general corrosion. 
     Aside from the resistance to the afore-described corrosive attacks, the alloys according to the invention have excellent resistance to hydrogen absorption. Table 4 gives the results of tests on this subject. 
     The data were obtained from tests performed using platinum as the counter electrode and a bath voltage of 6 V and then allowing the test material to absorb hydrogen from hydrogen bubbles formed and directed to the alloy surface. The table clearly indicates that the alloys of the invention absorbed less hydrogen than pure titanium does. 
     
                       TABLE 3______________________________________Results of crevice corrosion tests(NaCl = 10%, pH = 6.1, boiling)No.  Composition (wt %)   1     2   3   4   (day)______________________________________Comparative alloy 1   Pure titanium        X     X   X   X 2   Ti--0.15Pd           X     X   X   X 3   Ti--0.05Ru           Δ                           X   X   X 4   Ti--0.8Ni--0.3Mo     O     Δ                               X   X 5   Ti--0.02W            X     X   X   X 6   Ti--0.1Mo            X     X   X   X 7   Ti--0.6Ni            O     X   X   X 8   Ti--0.05Ru--0.5Ni    O     O   O   O 9   Ti--0.05Ru--0.05W    O     O   Δ                                   X10   Ti--0.05Ru--0.1Mo    O     O   X   X11   Ti--0.05Pd--0.5Ni    O     O   O   O12   Ti--0.05Pd--0.05W    O     O   Δ                                   X13   Ti--0.05Pd--0.1Mo    O     O   Δ                                   X14   Ti--0.05Ru--0.5Ni--0.02W                     O     O   O   O15   Ti--0.05Ru--0.5Ni--0.1Mo                     O     O   O   O16   Ti--0.05Ru--0.02W--0.1Mo                     O     O   O   Δ17   Ti--0.05Pd--0.5Ni--0.02W                     O     O   O   O18   Ti--0.05Pd-- 0.5Ni--0.1Mo                     O     O   O   O19   Ti--0.05Pd--0.02W--0.1Mo                     O     O   O   X20   Ti--0.05Ru--0.02W--0.1Mo--0.5Ni                     O     O   O   O21   Ti--0.05Pd--0.02W--0.1Mo--0.5Ni                     O     O   O   O______________________________________ O: No change Δ: Color change X: Crevice corrosion 
    
     
                       TABLE 4______________________________________Results of hydrogen absorption tests             Item                     H.sub.2 conc. increasedCondition Test material   by H.sub.2 abspn. (wt %)______________________________________6 v × 3 hours     Pure titanium   0.0040(25° C.)     Ti--0.05Ru--0.5Ni                     0.0001     Ti--0.05Ru--0.01W                     0.0007     Ti--0.05Ru--0.05Mo                     0.0013     Ti--0.05Pd--0.5Ni                     0.0001     Ti--0.05Pd--0.01W                     0.0009     Ti--0.05Pd--0.05Mo                     0.00066 v × 24 hours     Pure titanium   0.0059(15° C.)     Ti--0.05Ru--0.5Ni                     0.0004     Ti--0.05Ru--0.01W                     0.0013     Ti--0.05Ru--0.05Mo                     0.0030     Ti--0.05Pd--0.5Ni                     0.0005     Ti--0.05Pd--0.01W                     0.0017     Ti--0.05Pd--0.05Mo                     0.0036______________________________________ 
    
     As has been described hereinbefore, the alloy according to this invention is strongly resistant to such highly corrosive non-oxidizing acids as sulfuric acid. It also possesses excellent resistance to crevice corrosion and hydrogen absorption. The proportions of the alloying elements added are small enough for the alloy to be worked almost as easily as pure titanium and made at low cost. It will be understood from these that the alloy of the invention is a novel titanium alloy that eliminates the disadvantages of the existing corrosion-resistant titanium alloys and exhibits greater corrosion resistance.