US3457103A

US3457103A - Process for protecting titanium and titanium alloys against corrosion by oxidizing acid media

Info

Publication number: US3457103A
Application number: US628268A
Authority: US
Inventors: Hans Keller; Karl Risch; Winfried Althen
Original assignee: Hoechst AG
Current assignee: Hoechst AG
Priority date: 1962-12-07
Filing date: 1967-04-04
Publication date: 1969-07-22
Anticipated expiration: 1986-07-22
Also published as: BE641012A; DE1300416B; GB1052064A; NL299448A

Description

United States Patent 3,457,103 PROCESS FOR PROTECTING TITANIUM AND TI- TANIUM ALLOYS AGAINST CORROSION BY OXIDIZING ACID MEDIA Hans Keller, Frankfurt am Main, Karl Risch, Hoiheim,

Taunus, and Winfried Altheu, Kelkheim, Taunus, Germany, assignors to Farbwerke Hoechst Aktiengesellschaft vormals Meister Lucius & Bruning, Frankfurt am Main, Germany, a corporation of Germany No Drawing. Continuation-impart of application Ser. N 0. 319,567, Oct. 28, 1963. This application Apr. 4, 1967, Ser. No. 628,268 Claims priority, application Germany, Dec. 7, 1962, F 38,483 Int. Cl. C23c 11/08; B4411 1/36 U.S. Cl. 117135.1 3 Claims ABSTRACT OF THE DISCLOSURE Oxidizing acid media, especially nitric acid, aqua regia and chromosulfuric acid, do not, or only slightly attack titanium and titanium alloys even at temperatures exceeding 100 C., when acting in the presence of silicon or siliceous substances. The anticorrosive effect is observed in practically all siliceous substances, even in those known to be inert towards oxidizing acid media. The corrosion attack in the vaporous phase is particularly avoided by vaporizable or sprayable siliceous compounds. Especially suitable are elementary silicon, iron silicide, precipitated silicic acid, quartzand glass powder, kaolin, asbestos, and among the volatile or sprayable siliceous compounds the silicon halides, the halogeno-silanes and, in particular, silicone oils.

The present application is a continuation in part of the copending application Ser. No. 319,567, now abandoned.

The present invention provides a process for protecting titanium and titanium alloys against the corrosion caused by oxidizing acid media at temperatures exceeding 100 C. By oxidizing acid media there are, in particular, understood nitric acid and mixtures of nitric acid and hydrochloric acid, so-called aqua regia, moreover the mixtures of concentrated sulfuric acid and chromic acid, known as chromosulfuric acid.

Oxidizing acid media of this type vigorously attack titanium and titanium alloys which may especially contain aluminum, vanadium, molybdenum and palladium, at elevated temperatures, in particular at temperatures exceeding 100 C.

It has now been found that the corrosion of titanium and titanium alloys caused by oxidizing acid media can be noticeably reduced or completely avoided even at temperatures exceeding 100 C. by allowing the oxidizing acid media to act upon the titainum or the titanium alloy in the presence of silicon or a siliceous compound.

This observation is surprising since most of the compounds of silicon are practically inert towards oxidizing acid media and since siliceous compounds which are not inert under such conditions are converted into practically inert compounds by hydrolysis.

It now appeared that protection is obtained when the oxidizing acid medium contains, for example, quartz sand or glass powder. The protection is even more effective in the presence of siliceous compounds which are less inert. Compounds of this type are, in particular, precipitated silicic acid, iron silicide, alkali metal silicates. kaolin and asbestos powder. It is a matter of course that elementary silicon is also effective.

With the use of solid and non-volatile siliceous substances it is, naturally, only possible to protect the part of the metal surface which comes into contact with the "ice liquid phase of the oxidizing acid medium. The parts of the apparatus which come into contact with the vapor of the oxidizing acid medium or in which the vaporous phase is condensed, can, however, be protected by volatile or sprayable siliceous substances. Members of this group of substances are, for example the silicon halides such as SiCl the halogeno-silanes such as SiHCl the siloxanes such as (SiH O and, in particular, the silicone oils.

Silicone oils are particularly suitable for carrying out the process of the invention since they are easy to handl-e and to dose and since their handling involves no hazards.

In apparatuses which are in contact partly with the liquid phase and partly with the vaporous phase of an oxidizing acid medium, it may be advantageous to use various siliceous substances for the protection against corrosion. It is, for example, possible in the case of distilling columns, to introduce kaolin into the sump and to spray silicone oil into the vaporous phase.

The amount of siliceous substance is not critical. The spaces to be protected should, however, always contain such an amount of siliceous substance that, per square meter of the surface to be protected, 10 milligrams of silicon are present, when siliceous compounds are used, it should be an amount equivalent to that of the silicon. When compounds are used which are volatile with the vapor of the oxidizing acid medium, the silicon level may be maintained by a continuous or portionwise feed. In many cases, non volatile siliceous substances can be added to the oxidizing acid medium in an excess sufiicient to do without a feeding over prolonged intervals.

In the following example and tables there are described a number of corrosion tests for illustrating further particulars of the process of the invention and of the protection of titanium and the alloys thereof against oxidizing acid media. However, the examples are not intended to limit the invention thereto.

Example For the corrosion tests tubes of pure titanium were used which served as testing vessels and as corrosion samples simultaneously. They had an inside diameter of 20 mm., an inner height of mm. and an inner surface of 97 cm. One end of the tubes was closed by welding with a pure titanium plate. On the other end the tube was closed so at to be gastight by means of a metal cover and a sealing plate of polytetrafiuoroethylene with the use of a clamping device. The titanium tubes were filled to about /5 of their height with the corrosive medium, placed in thick-walled steel containers and heated at the required temperature in a furnace with circulating air. The temperature was maintained constant by means of a regulator. In the following Tables 1, 2, and 3 are indicated the test results obtained, i.e. the anticorrosive effect on pure titanium against nitric acid and mixtues of nitric acid and hydrochloric acid.

In Table 1 are recited the corrosion values obtained in the treatment of pure titanium with nitric acid of varying concentrations and at different temperatures.

In Tests 1 to 7 severe care was taken that the results were not influenced by siliceous substances. That is the reason why no glass vessels were used. As a measurement for the corrosion there is indicated in the last column of Table 1 the linear corrosion velocity in millimeters per annum. In Tests Nos. 1, 2 and 7 the nitric acid formed on the pure titanium a firmly adhering layer of corrosion products which could not be removed quantitatively without carrying off unattacked titanium. Therefore, reliable results cannot be given.

Table 2 contains the experimental data with respect 3 to the stability of pure titanium towards HNO with different additions under varying conditions.

In column 7 of Table 2, V stands for vaporous phase and L for liquid phase. The plus means that no corrosion could be observed. The minus indicates that the nitric acid condensed in the vaporous phase did attack the metal. The corrosion velocity in each case was about 2 millimeter per annum, corresponding to the value indicated in Table 1 for an about 30% nitric acid at 100 C. to indicates that the protection was not uniform, some spots of the metal surface having been attacked.

Table 3 indicates the linear corrosion velocity of titanium under the action of mixtures of nitric acid and hydrochloric acid of varying concentrations and at different temperatures and illustrates the anticorrosive effect obtained by carrying out the process according to the invention.

the same concentration and temperature of the acid, a corrosion did not take place when a very small amount of silicone oil was added to the nitric acid. Neither on the parts in contact with the vaporous phase nor on the parts in contact with the liquid phase of the nitric acid a corrosion could be observed. With the additions used in Tests Nos. 816 only the parts in contact with the liquid phase of the acid were protected against corrosion.

The weaker anticorrosive effect obtained in Tests Nos. 11 and 14 is mainly due to the fact that glass powder and quartz powder are attacked to a very small extent only by nitric acid so that the protection develops only slowly.

The tests have further revealed that in the liquid phase the protective layer on the metal was only durable when in the acid a small silicon content was continuously maintained. The volatile siliceous substances SiCL; and silicone oil reacted under the conditions of Tests 17 to 35 with Concentration Tempera- Duration Linear corrosion the acid, whereby a nonvolatile siliFeol-ls Teaction Product Test of a 1n We e of test, velocltyy 20 was formed in the liquid. This reaction product guaranteed number percent C. hours min/year a protection of the titanium in the liquid phase against 1 3 48 xg g gg g corrosion for a longer period of time whereas the initial 2 200 43 Highe 'than intest protective effect on the titanium in the vaporous phase 3 30 200 144 2 ceased earlier. 41:11:11: 00 200 48 3I5I 25 In Tests 18-35 about 20 milligrams of phenylmethylg 22 2 22g g-gsilicone oil (density 1.02 g./cc., heat stability in air at 71111111 75 164 11s Iii glierthani te t 250 C. 1000 hours, viscosity 200 centistokes at 25 C.) was added to about cc. of acid medium. After the The values could not be exactly defined. respective duration of the tests the titanium exposed to TABLE2 Concen- Stability of titatration of Tenipera- Duration Amount of nium in IINOQ in ture of of test, addition percent HNO; hours Addition in g. V L

30 200 144 Si (powder) 0. 2 30 200 144 Iron silieide 0.2 30 200 144 3101161120.. 0.1 30 200 144 Quartz powder. 0. 2 30 200 48 Na silicate 0.00 30 200 (la-silicate. 0. 02 30 200 0.2 30 200 0.1 30 200 0.15 30 200 0.2 30 200 0. 02 3 200 0.02 10 200 0. 02 30 245 0.02 30 205 0. 02 00 200 0. 02 00 205 0. 02 05 140 0. 02 75 164 0. 02

1 The volume of acid filled in was about 30 cc. 1 V=vaporous phase; L=liquid phase.

3 A phenyl-methyl silicone oil was used; density 1.02 g /co., viscosity 200 centistokes at 25 0; thermal stability in air at 250 0. 1,000 hours.

TABLE 3 Percentage in acid Linear corrosion v1 Protecting efiect of 0.02 g. silicone oil added to the 1 V=vapor0us phase; L=liquid phase.

+ means a thin layer wasiormed 011 the titanium; a loss in weight was not observed. to means the protective layer was not uniform, some spots of the metal surface were corroded.

the attack of the vaporous phase of the acid was not yet corroded. Further tests showed that a permanent protection of the titanium in the vaporous phase could be achieved when a small amount of silicone oil was added in convenient intervals to the acid. A sample of titanium metal covered with a thin film of silicone oil and placed into the titanium tube likewise showed an effective protection against corrosion. The weight of the silicone film was 5 milligrams. After a time of action of 15 hours of a 60% HNO at 200 C. the titanium tube and the samples of titanium in the liquid as well as in the vaporous phase did not shOW any corrosion.

In all cases where an anticorrosive effect was obtained this effect was extended to the gap due to construction on the bottom plate attached by welding and on the sealing plate of the used test tube.

For the tests with titanium alloys corresponding metal samples were placed into the specified tubes of pure titanium. Tests were carried out with the following alloys: titanium with 6% of aluminum and 4% of vanadium, titanium with 5% of aluminum and 2% of molybdenum, and titanium with 0.2% of palladium.

The results are summarized in Table 4. It is obvious that the anticorrosive eifect obtained with pure titanium is also reached with titanium alloys.

What is claimed is:

1. A process for protecting titanium and titanium alloys, having as an alloying component aluminum, vanadium, molybdenum or palladium with the base metal being titanium, against corrosion by a strong oxidizing acid medium at temperatures exceeding 100 C., which comprises allowing the oxidizing acid medium to act upon titanium or titanium alloys in the presence of silicon and a siliceous substance selected from the group consisting of iron silicide, Si .xH O, quartz, sodium silicate, calcium silicate, glass, kaolin, asbestos, SiCl halogenosilanes, and silcone oils.

2. A process as claimed in claim 1, wherein the oxidizing acid medium is allowed to act upon titanium or titanium alloys in the presence of SiCl 3. A process as claimed in claim 1, wherein kaolin, asbestos, quartz sand, glass powder, precipitated silicic acid, sodium silicate or calcium silicate is used as the siliceous substance.

TABLE 4 Protecting efiect of 0.02 g. Percentage of v (mmJyear) silicone oil added to the in acid mixture Duration when the acid acid mixture 1 of test at Titanium is free from Test number HCl HNOa 200 C. alloy additions V L 0 30 48 P 2. 4 0 30 48 V 3.5 0 30 24 M 1.1 0 60 22 P 0 60 22 V 0 60 22 M 10 24 P 10 22 P 3. 1 10 20 22 V 0.5 10 20 22 M 1. 3 10 22 P 10 45 22 V 10 45 22 M 1 Composition of Ti alloys: P=Ti with 0.2% Pd; V=Ti with 6% Al. 4% V; M=Ti with 5% Al, 2% Mo.

2 See footnote 1 of Table 3. 3 Not determined.

References Cited UNITED STATES PATENTS 1,890,595 12/ 1932 Valenta.

2,661,286 12/1953 Swazy et al 175.5 2,678,875 5/1954 Spooner 252387 X 2,711,364 6/1955 Beach 1343 X 2,711,974 6/1955 Happe 117-127 2,739,047 3/ 1956 Sanz.

2,961,110 11/1960 Cooke et a1. 117135.1 X 2,981,610 4/1961 Snyder et a1.

3,010,854 11/1961 Satterfield 252-793 X 3,231,374 1/1966 Sciambi.

OTHER REFERENCES Bishop, Corrosion, vol. 19, 1963, pp. 308t, 313t;

TA462 C58, Sci. Lib.

Gordon et al., Surface Coatings and Finishes, 1954, p. 221, Chemical Pub. Co. Inc. New York, N.Y., TP936 RALPH S. KENDALL, Primary Examiner U.S. Cl. X.R.