US2697051A

US2697051A - Treatment of titanium metal surfaces

Info

Publication number: US2697051A
Application number: US310132A
Authority: US
Inventors: Mellgren Svante; Oliver W Moles
Original assignee: Nat Lead Co
Current assignee: NL Industries Inc
Priority date: 1952-09-17
Filing date: 1952-09-17
Publication date: 1954-12-14
Anticipated expiration: 1971-12-14
Also published as: NL180884B; NL88126C; DE966015C; FR1083807A; BE522839A

Description

United States Patent TREATNEENT F TITANIUM METAL SURFACES Svante Mellgren, Laurence Harbor, and Oliver W. Moles,

No Drawing. Application September 17, 1952, Serial No. 310,132

7 Claims. (Cl. 134-30) This invention relates to a method for pickling refractory metals. More specifically, it relates to a method for removing scale such as oxide coatings and the like from a titanium metal surrace.

'l'itanium metal particularly at elevated temperatures has a strong amnity for oxygen, nitrogen, hydrogen and the like, and contact between the metal and these gases, boil]. in the initial production steps and also in subsequent fabrication steps, orten results in the formation of surface scale or coatings of the oxide, nitride, etc. when this type scale or coatin is formed on such titanium metal products, the propertiesare often modlned to such a degree that the roducts are no longer suitable for the purpose for which they had been intended. rurtnermore, such surrace scale adheres very tenaclously to the titanium metal and employment of pickling processes such as, for example, the acidic washes customarily used tor the removal of scale rrolll iron and steel have proved ineffective.

An OOJCCE or this invention, therefore, is to provide a satlslactory process tor removing such surrace scale or coating lronl titanium metal in order to make the metal suitable tor commercial utilization. Another ob ect is to provide a quick and emclent method for reclaiming titanium rrom titanium metal products which are so contamlnated. l hese and other oo ects will become obvious from the following more complete description of the instant invention.

.Dl'OklQly speaking the instant invention contemplates a process tor removing scale from a titanium metal surrace which comprises treating said surface with a molten alkali metal polysulnde until said scale 18 loosened trom said surface and removing said polysulfide and said loosened scale rrom said surtace.

Alkali metal polysulfides which are suitable for employment m the instant process may be produced in a number or ways. rot example, an alkali sulfide such as sodium sulfide tNaaS) may oe combined with additional free sull'ur to yield compositions corresponding to the general formula NazSa wnere x is greater than unity. A particularly effective composition comprises either sodium or potassium polysulfides in which from about 0.1 to parts of free sultur are present in addition to that normally combined as NazS or ms. Mixturesof sodium and potassium polysulfides may also be used.

The polysultide composition must be in the molten state. lhe temperatures at which such compositions melt vary considerably and is in part dependent upon the amount of free sulfur present. Satisfactory scale removal has been accomplished at temperatures or from about 250 C. to 500 C., naturally depending upon such factors as bath composition, apparatus size, design, etc.

When the titanium metal surface is treated with the molten polysulfide, the surface scale is loosened very rapidly, and generally, the metal product need be exposed to the action or' the polysulfide for only a relatively short period of time. According to the instant process the treatment time should not exceed about ten minutes. Obviously shorter periods or contact may also be employed dependent upon the amount of scale and type of surrace to be treated. However, it may be desirable on occasion to extend the treatment time beyond the ten minute period indicated above. For instance, the size of the article or the method of application of the polysulfide may be such that a longer time period would be more satisfactory. Accordingly, it has been found possible to buffer the action of the molten polysulfide by the addition of halide compounds such as, for example, the alkali and alkaline earth ice metal halides including magnesium halides. These compounds n general are miscible with the polysulfides employcd in the instant process. Obviously, the amount of halide compound which is used may vary and will depend on the degree of bufiering action which is desired and upon the temperature to be employed. For practical utilization of such bufl'ered polysulfide melts it has been found convenient to limit the amount of halide to that amount which will permit formation of a molten polysulfide-halide mixture at temperatures of from 250 Cv to about 500 C. Furthermore, the addition of the buffer compound generally should be limited so as to permit the treatment time to be extended up to about one hour since longer periods are not particularly desirable in commercial operations.

In a further embodiment of the instant invention it may be desirable to rinse the metal surface from which the scale has been loosened with water to remove any remaining traces of loosened scale and polysulfide which may be present. The surface of the metal then possesses a fresh, clean metallic luster.

The actual method of treating the titanium metal surface with the molten polysulfide may, of cource, vary; however, according to one preferred means, a molten bath of polysulfide is provided in which the titanium metal article which contains the surface scale may be appropriately immersed for a time sufiicient to loosen said sca e.

in order to describe the instant invention more clearly, the following examples are presented:

Example I A piece of titanium bar stock, which had been hot rolled in an oxygen atmosphere and had been contami-- Example II A titanium metal ingot which was covered with a surface scale of titanium oxide and titanium nitride was immersed in a molten'bath comprising a mixture of sodium polysulfide and potassium polysultide, the composition of which corresponded closely to the formula NamrsKzS. The ingot was permitted to remain in contact with the molten polysulfide mixture for a period of five minutes at a temperature of about 500 C. The ingot was then withdrawn from the polysulfide melt and subsequently cooled. Again, the surface scale had been completely reinoved, and the ingot possessed a bright and shiny metallic uster.

By the examples presented and by the preceding discussion, it has been demonstrated that according to the process of the instant invention, it is possible to effectively and swirtly remove surface scale such as oxide coatings from titanium metal by treating the metal surface with alkali metal polysulfide. This quick and efficient operation permits the reclamation of large quantities of titanium metal which would otherwise not be readily available for commercial utilization.

While this invention has been described and illustrated by the examples shown, it is not intended to be strictly limited thereto, and other variations and modifications may be employed within the scope of the following claims.

We claim:

1. A process for removing adherent scale from a titanium metal surface which comprises treating said surface with a molten alkali metal polysulfide until said scale is loosened from said surface and removing said polysulfide and said loosened scale from said surface.

2. A process for removing adherent scale from a titanium metal surface which comprises treating said surface with a molten alkali metal polysulfide for a period of not more than about ten minutes to loosen said scale from said surface and thereafter removing said polysulfide .and

said loosened scale from said surface.

3. A process for removing adherent scale from the surface of a titanium metal product which comprises immersing said product in a molten alkali polysulfide for a period of not more than about ten minutes until said scale is loosened from said surface, substantially immediately thereafter Withdrawing said product from said molten polysulfide, and rinsing With Water to remove anyremaining traces of loosened scale and polysulfide.

4. A process according to claim 1 wherein said polysulfide comprises sodium polysulfide.

5. A process according to claim .1 .Wherein'sa'idpo'lysulfide comprises potassium polysulfide.

6. A process for removing adherent scale from a titanium metal surface which comprises treating said surface with a molten alkali metal polysulfide and a compound selected from the group consistingof alkali metal halides and alkaline earth metal halides including magnesium halide .until said scale .isloosened from said surface and removing said polysulfide, said compound and said loosened scale from said suit-face.

7. A process for removing adherent scale from a titanium metal surface which comprises treating said surface with a molten alkali metal polysulfide and a compound selectedrfrorn the group consisting of alkali metal halides and alkaline earth metal halides including magnesium halide for a period of not more than about one hour to loosen said scale from said surface andthereafter removing said polysulfide, said compound and said loosened scale from said surface.

References Cited :in the file of this patent The Canadian Mining and Metallurgical Bulletin for January 1951, Montreal, pages 2'528 (photostat in Div. 3) Ti Digest.

Claims

1. A PROCESS FOR REMOVING ADHERENT SCALE FROM A TITANIUM METAL SURFACE WHICH COMPRISES TREATING SAID SURFACE WITH A MOLTEN ALKALI METAL POLYSULFIDE UNTIL SAID SCALE IS LOSSEDEN FROM SAID SURFACE AND REMOVING SAID POLYSULFIDE AND SAID LOSSEDEN SCALE FROM SAID SURFACE.