US3011923A

US3011923A - Surface treatment of molybdenum metal

Info

Publication number: US3011923A
Application number: US10002A
Authority: US
Inventors: Charles O Coffer
Original assignee: Individual
Current assignee: Individual
Priority date: 1960-02-19
Filing date: 1960-02-19
Publication date: 1961-12-05
Anticipated expiration: 1978-12-05

Description

United States Patent 3,011,923 SURFACE TREATMENT OF MOLYBDENUM METAL Charles 0. Cofier, Livermore, Califl, assignor to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Filed Feb. 19, 1960, Ser. No. 10,002 4 Claims. (Cl. 134-3) This invention deals with a process of removing surface scale from molybdenum, and in particular of removing oxidic scale.

Molybdenum metal has a great many uses in the industry because of its high melting point and its great strength at temperatures up to about 870 C. It has been used for chemical apparatus which have to be operated at such high temperatures, and it now also finds extensive use as structural material of nuclear reactors, and in particular as the cladding material for fuel elements of reactors using fast neutrons. The latter use is possible because of the comparatively low neutron-capture cross section for fastand intermediate-energy neutrons of molybdenum and also because of the inertness of molybdenum to alkali metals which are used as coolant in such reactors and there come in contact with the cladding of the fuel elements.

For the use of molybdenum in neutronic reactors just described it is particularly important that the surface of the molybdenum be free from oxide and any other compounds that might have formed on the surface; this is necessary because the oadde of the surface scale reacts with sodium and forms sodium oxide, 2. highly undesirable reaction.

Molybdenum has been descaled heretofore by immersion in a molten mixture of sodium hydroxide and sodium nitrite in a proportion of about 70 and 30% by weight, respectively, at a temperature of between 260 and 370 C. This descaling bath requires a chemical aftertreatment for cleaning the surface from the adhering descaling material; this has been accomplished by immersion in various acids.

The above-described method has a number of drawbacks. The use of molten salts at the temperature necessary and described above is rather hazardous. Furthermore, the molybdenum is attacked by the sodium hydroxide-sodium nitrite bath, and a metal loss of up to three mils for an immersion of five seconds has been observed. Furthermore, some of the after-treatments were found not to be too satisfactory, because they also often attacked the molybdenum; this, for instance, was found to be the case for an after-treatment solution of a 25% nitric acid containing 2% by volume of hydrogen fluoride. Sandblasting for the after-treatment was found not to be feasible for parts having a complex surface structure.

It is an object of this invention to provide a process for the surface treatment of molybdenum articles having a scale on their surface which requires a comparatively low temperature.

it is another object of this invention to provide 'a process for the surface treatment of molybdenum articles having a scale on their surface which does not useany molten salts and thus does not involve the same hazards.

3,011,923 Patented Dec. 5, 1961 ICC It is finally also an object of this invention to provide a process for the Surface treatment of molybdenum. articles having a scale on their surface which uses bath compositions that do not attack the molybdenum metal.

These objects are accomplished by immersing the molybdenum article to be treated in a mixture of sodium hydroxide and potassium permanganate solutions, rinsing the article with water, and immersing the article in an aqueous acid bath.

It was found that the oxidic scale on the molybdenum articles is converted, by the sodium hydroxide-permanganate solution, to substances that can be easily removed by a pressure-spray rinse; that is in contradistinction to the oxidic scale originally formed on the molybdenum which adheres firmly to the surface and cannot be removed by merely rinsing; After the water rinse a black smut still remains on the surface of the molybdenum article. The smut, however, can be readily removed by immersion in a second bath containing one or several acids that do not attack the molybdenum. For instance,

a mixture of sulfuric, hydrochloric and chromic acids has given excellent results, and the preferred proportion of this acid mixture is 15% by weight of sulfuric acid, 15% by weight of hydrochloric acid, by weight of water and from 60 to 100 grains of chromic acid per liter of the mixture. The temperature of the first, the descaling, bath ranges preferably between 65 and C., While for the second smut-removing bath room temperature, about 25 C., is satisfactory.

Instead of mixing sodium hydroxide and potassium permanganate for the preparation of the descaling bath, sodium hydroxide may also be fused with manganese dioxide and the melt thus obtained can then be dissolved in water. A solution thus prepared is just as satisfactory for the purpose of this invention as is a sodium hydroxidepermanganate mixture.

In the following an example is given to illustrate the process of this invention.

Example A piece of molybdenum metal which had a heavy coat of scale on its surface was immersed in a solution containing 10% by weight of sodium hydroxide, 5% by weight of potassium permanganate and 85% of water; the temperature of the bath was about 75 C. Immersion was carried out for seven minutes after which the molybdenum article was withdrawn and thoroughly rinsed with tap water. Thereafter the molybdenum article Was immersed for eight minutes in a solution containing 15 by weight of sulfuric acid, 15 by weight of hydrochloric acid, 70% by Weight of water and eight grams of chromic acid per milliliters. of acid solution. The solution had a temperature of 27 C. After withdrawal of the molybdenum article from this acid bath its surface was found to be completely free from scale, smut and had a chemically clean metallic appearance.

It will be understood that this invention is not to be limited to the details given herein but that it may be modified within the scope of the appended claims.

What is claimed is:

l. A process of removing scale from molybdenum articles comprising immersing said articles in a descaling bath containing a mixture of sodium hydroxide and potas- 3 sium permanganate and being at a temperature of between 65 and 85 C.; rinsing the articles with water; and immersing said articles in an acid bath consisting of sulfuric, hydrochloric and chromic acids.

2. The process of claim 1 wherein the descaling bath 5 contains by weight of sodium hydroxide, 5% by weight of potassium permanganate and 85% by weight of water.

3. The process of claim 2 wherein the acid bath contains about by weight of sulfuric acid, about 15 by weight of hydrochloric acid, of water and from 60 to 100 grams of chromic acid per liter of the mixture.

4. The process of claim 3 wherein the acid bath contains grams of chromic acid per liter of the mixture.

References Cited in the file of this patent UNITED STATES PATENTS Re. 22,887 Spence June 3, 1947 2,041,331 Gann et al May 19, 1936 2,883,311 Halpert Apr. 21, 1959 FOREIGN PATENTS 713,219 France Aug. 10, 1931

Claims

1. A PROCESS OR REMOVING SCALE FROM MOLYBDENUM ARTICLES COMPRISING IMMERSING SAID ARTICLES IN A DESCALING BATH CONTAINING A MIXTURE OF SODIUM HYDROXIDE AND POTASSIUM PERMANGANATE AND BEING AT A TEMPERATURE OF BETWEEN 65 AND 85*C., RINSING THE ARTICLES WITH WATER, AND IMMERSING SAID ARTICLES IN AN ACID BATH CONSISTING OF SULFURIC, HYDROCHLORIC AND CHROMIC ACIDS.