US3038824A

US3038824A - Passivation of reactive metal surfaces

Info

Publication number: US3038824A
Application number: US735603A
Authority: US
Inventors: Virgil F Novy; Craig G Kirkpatrick
Original assignee: NUCLEAR CORP OF AMERICA Inc
Current assignee: NUCLEAR Corp OF AMERICA Inc
Priority date: 1958-05-15
Filing date: 1958-05-15
Publication date: 1962-06-12
Anticipated expiration: 1979-06-12
Also published as: GB898389A

Description

June 12, 1962 v. F. NOVY ErAL 3,038824 PASSIVATION OF' REACTIVE METAL SURFACES Filed May l5, 1958 3 Sheets-Sheet 1 ...nv i

FIG.5

V/RCr/L. E NOVY CRn/G G fr/RKPAT/e/CK INVENTORS ATTORNEY `Tune 12, 1962 v, F. NOVY EIAL 3,038,824

PASSIVATION 0F REACTIVE METAL SURFACES Filed May l5, 1958 I5 Sheets-Sheet 2 FIGJO VlRG/L E NOVY CRA/G G. /f/HKPATRlcK INVENTORS MMM L. Shu@ ATTORNEY June 12, 1962 v. F. NOvY ErAL 3,038,824

PAssIvATIoN 0F REACTIVE METAL suRFACEs Filed May l5, 1958 3 Sheets-Sheet 3 FIG. I3

CRA/6 G. l K/RKPA TORS ATTORNEY rates fihce 3,038,824 PASSii/ATIN @E REAQTVE METAL SURFACES Virgil F. Novy, Altadena, and Craig G. Kirkpatrick,

Granada l-lliils, Caiif., assignors to Nuclear Corporation of America, line., New York, NPY., a corporation of Michigan Filed May l5, i958, Ser. No. 735,603 Claims. (Cl. 14S-6.14)

This invention relates to a chemical method for indefinitely passivating the surfaces of air reactive metals and, more particularly, this invention relates to a method of indefinitely passivating the surfaces of rare earth metals such as cerium, lanthnnum, praseodymium and neodymium metals, and the like.

IDuring and after the final polishing of rare earth metal samples such as cerium, `for example, an oxide coating forms. Heretofore, before the true surface structure of the cerium metal could be revealed, it was necessary to remove this coating. There were two methods available to accomplish this removal, i.e mechanical and chemical; the mechanical method is undesirable however, due to the necessity of using a coarse abrasive to remove all traces of oxide from the low arcas and from the grain boundaries. The use of such mechanical methods of removal leaves the sample surface with traces of smeared metal and fine scratches. The mechanical method might possibly be utilized in an inert atmosphere or a vacuum chamber, but the disadvantages of operating under such conditions are obvious.

In accordance with the present invention, it has been found that a chemical treatment of the polished metal surface, i.e. treatment of the polished sur-face lwith an acid solution is efhcacious to passivate the surface of active metals, such as cerium, to such an extent that corrosion in air is retarded for a period of time in excess of 150 hours. Since the usual handling procedures for freshly prepared surfaces of air reactive metals is to dip them in heavy oil and then cover the metal surfaces with oilsoaked rags, it is apparent that the present invention affords many advantages over the heretofore known methods of handling such air reactive metals. This bulky and crude procedure formerly employed is unnecessary after the metal surface has been treated in accordance with this invention and the treated surfaces may be handled indetinitely in air with only reasonable care being required to prevent damage to the treated surface.

In the process of the invention, a freshly ground or polished surface of a reactive metal is first cleaned with the aid of a solvent, such as ethyl alcohol, to remove any traces of the original solvent used in the original grinding or polishing step. The polished surface is then dipped or rubbed or swabbed for a period of about to 3() seconds in an acid solution, such as a nital solution, nital being a mixture of 5% by weight of nitric acid and 95% by weight of ethyl alcohol. The nital changes the dark colored film, which forms almost immediately on the surfaces of unpassivated metals, to a different, and usually lighter, color. This color or deposit is then removed from Ithe surface by treating the surface with a suitable solvent such as an alcohol, for example ethyl alcohol. This process is repeated, starting with the nital `dip or rub or swab until there is no color change detectable upon treatment with the nital. After a last `alcohol rinse, the surface is dried and the true metallic lustre is revealed. Metallic surfaces so treated are indefinitely passivated and will not tarnish in the atmosphere at ambient conditions.

Among the solvents which may be used to clean the reactive metal surface to remove the original grinding solvent are alcohols such as ethyl, methyl, isopropyl, and butyl, and other compounds such as ethers, and ketones, such as acetone. Among the oxidizing agents which may be employed are mixtures of acids such as nitric, chromic,

erchloric, and pieric, with solvents such as ethyl alcohol, glycerine, glycol, and petrolatum. Other oxidizing agents such as ferrie chloride, may also be used. The acid concentration in the acid solution may be in lthe range of about 1 percent to 25 percent, preferably 5 percent to l0 percent.

The treatment time with the acid solution may be in the range of about 3 seconds to 60 seconds, preferably about l0 to 20 seconds. The temperature of the acid solution which is used lfor treatment of the polished reactive metal surface may be in the range of about 60 to F.

Generally speaking, it is necessary to `dip or rub or swab the polished reactive metal surface in the acid solution about l to 6 times, providing an intermediate rinse with a solvent such as alcohol, before no color change will be detected upon a succeeding treatment with the acid solution.

The invention will be further illustrated by reference to the following specific example:

Example I A `series of reactive rare earth metal samples having freshly ground or polished sur-faces were first finely polished with the aid of ethyl alcohol to remove any traces of the original solvent. Each surface was then dipped for a period of 20 to 30 seconds in a solution of nital (5 percent nitric acid, percent ethyl alcohol) and the dark colored oxide film, which forms almost immediately on the surfaces of unpassivated metals when exposed to air, was changed to a different and `lighter color. This color was removed from the surfaces by swabbing with ethyl alcohol and the nital `dip and alcohol treatment were repeated 3 times, i.e. until no color change was detectable upon treatment with the nital. After rinsing each of the surfaces with ethyl alcohol, the surfaces were dried and the true metallic lustre was revealed.

in the accompanying drawings, a series of photomic-rographs are shown, depicting both unpassivated and passivated reactive metal surfaces, the passivation treatments being conducted as above described.

iFIGURE l is a photomicrograph of a cerium metal sample showing a polished surface which had been exposed to air at room temperature (75 to 85 F.) for a period of hours. The print shows the corrosion film formed on lthe surface of the metal. Further exposure to air resulted in progressive corrosion.

'FIGURE 2 is a photomicrograph of a sample of the same cerium metal shown in FIGURE 1 passivated in accordance with this invention. In the passivating treatment, the corrosion film which had formed during the short interval of ltime between polishing and passivation (l to 5 minutes) was removed and the surface of the metal was stabilized as shown.

FIGURE 3 shows the same surface of the cerium metal sample shown in FIGURE 2 after the metal surface had been exposed to air at room temperature (75 to 85 F.) for a period of 45 days. Microscopic examination of the sample, after exposure to air at room temperature for a period of 60 days, revealed the same structure with the absence of a corrosion film, indicating that the `sample would remain in this stable condition indefinitely.

FIGURE 4 is a photomicrograph of another sample of cerium metal showing a polished surface thereof. The print lshows the corrosion film which formed on the metal surface after exposure to air for a period of only l to 3 minutes.

IFIGURE shows the same metal surface shown in FIGURE 4 after passivation in accordance with this invention. This passivated surface was studied microscopically after exposure to air at room temperature for a period of hours and no change in the structure could be ascertained. The absence of any additional corrosion indicated that the metal surface would remain stable indefinitely.

FIGURE 6 shows a typical corrosion film `formed on the surface of lanthanum metal.

FIGURE 7 shows a typical corrosion lm formed on the surface of praseodymium metal, and FIGURE 8 shows a typical corrosion Ifilm formed on neodymium metal.

Typical passivated surfaces of the metals shown in FIGURES 6, 7 and 8 are shown in yFIGURES 9, 10 and 11, which are passivated surfaces of lanthanum, praseodymium and neodymium respectively.

LFIGURE 12 shows the amount of intergranular corrosion which occurred during a 20 hour exposure to air at room temperature of the passivated surfaces of the lanthanum metal. An unpassivated surface of lanthanum metal exposed to air at room temperature for the same length of time under the same conditions was completely corroded and was impossible to photograph due to the corrosion powder or residue which completely covered the surface.

FIGURE 13 shows the pit-type of corrosion which occurred during a 20 hour exposure to air at room temperature on the passivated surface of neodymium metal. An unpassivated surface of the neodymium metal exposed to air at room temperature for the same length of time and under the same conditions was also impossible to photograph as the corrosion film completely obscured the grain boundaries shown in FIGURE 8.

Both samples of passivated cerium metal, s hown in FIGURE 5, and passivated praseodymium metal, shown in FIGURE 10, were unaffected after exposure to air at room temperature for 20 hours, under identical conditions.

It will be obvious to those skilled in the art that many modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.

What is claimed is:

1. A method of passivating the polished surface of a rare earth metal which comprises treating the polished surface with a solvent capable of removing therefrom all traces of material present on said surface after the polishing operation to remove said material from the surface, apply-ing an oxidizing agent to the thus treated surface,

and removing the deposit `formed by the oxidizing agent, by treating said surface with a solvent for said deposit, and drying the surface.

2. A method according to claim 1 in which the solvent is an organic solvent.

3. A rare earth metal having a lustrous polished surface passivated according to the method of claim 1 to render the metal of the surface more resistant to corrosion in air than natural polished metal, whereby said polished surface retains its natural lustre after polishing.

4. A method according to claim 2 in which the solvent is an alcohol.

5. A method according to claim 1 in which the oxidizing agent is nitric acid.

6. A method according to claim 1 in which the oxidizing agent is nitric acid in admixture with an alcohol in the proportions of about 1% to 25% of the acid and the balance the alcohol.

7. A method according to claim 6 in which the alcohol is ethyl alcohol.

8. A method of passivating the polished surface of a rare earth metal lwhich comprises treating the polished surface with an alcohol to remove from said surface all traces of material present thereon after the polishing operation, applying to the thus treated surface an oxidizing agent comprising a mixture of nitric acid and alcohol, again treating the surface with an alcohol to remove therefrom the deposit formed by the oxidizing agent, and drying the surface.

9. A method according to claim 8 in which the alcohol is ethyl alcohol.

10. A rare earth metal having a lustrous polished surface which is passivated according to the method of claim 8 to render the metal of the surface resistant to corrosion in air, whereby to retain its metallic lustre after polishing.

References Cited in the tile of this patent UNITED STATES PATENTS 2,408,155 Thornbury Sept. 24, 1946 2,523,892 Warf Sept. 26, 1950 2,777,785 Schuster et al. Ian. 15, 1957 FOREIGN PATENTS 103,593 Australia Mar. 29, 1938 OTHER REFERENCES Metals Handbook, 1948 edition, page 394. Levy: The Rare Earths, p. 126, par. 1, 2nd ed., Longmans, Greene & Co., 1924.

Claims

1. A METHOD OF PASSIVATING THE POLISHED SURFACE OF A RARE EARTH METAL WHICH COMPRISES TREATING THE POLISHED SURFACE WITH A SOLVENT CAPABLE OF REMOVING THEREFROM ALL TRACES OF MATERIAL PRESENT ON SAID SURFACE AFTER THE POLISHING OPERATION TO REMOVE SAID MATERIAL FROM THE SURFACE, APPLYING AN OXIDIZING AGENT TO THE THUS TREATED SURFACE, AND REMOVING THE DEPOSIT FORMED BY THE OXIDISING AGENT, BY TREATING SAID SURFACE WITH A SOLVENT FOR SAID DEPOSIT, AND DRYING THE SURFACE.