US3287181A

US3287181A - Treatment of intergranular sulfur corrosion in metals

Info

Publication number: US3287181A
Application number: US322256A
Authority: US
Inventors: Steverding Bernard
Original assignee: Individual
Current assignee: Individual
Priority date: 1963-11-07
Filing date: 1963-11-07
Publication date: 1966-11-22
Anticipated expiration: 1983-11-22

Description

United States Patent 3,287,181 TREATMENT OF INTERGRANULAR SULFUR CORROSION IN METALS Bernard Steverding, Huntsville, 'Ala., assignor to the United States of America as represented by the Secretary of the Army ,7 No Drawing. Filed Nov. 7, 1963, Ser. No. 322,256 12 Claims. (Cl. 148-203) The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon.

The present invention relates to a treatment for'prevention of intergranular corrosion in metals as well as rejuvenation of metal which has been effected by intergranular sulfur corrosion.

In the past much diificulty has been encountered in maintaining the desired characteristics of metal when the metal has been exposed to sulfur containing vapors at elevated temperatures.

This phenomenon is especially troublesome in missile thrust chambers where the sulfur content of the fuels used is about 0.03 to about 0.05 percent by weight. To further reduce the sulfur content of these-fuels would be impractical because of the excessive cost of .this type of purification. To provide a nozzle cooling means in liquid propelled missile systems, the missile fuel is cycled through a multitude of small tubes contained in the walls of the thrust chamber. Nickel and nickel-base alloys are commonly used for the construction of thrust chambers and under normal conditions these alloys are quite resistant to granular corrosion of any type.

Under certain uncontrollable conditions small superheated areas may occur within the thrust chamber cooling tubes in the vicinity of'the nozzle throat. The temperature at these so-called hot spots is often high enough to cause local evaporation of the fuel and the sulfur contained in the fuel vapor then becomes extremely active as an initiator of intergranular corrosion.

Other metals, of course, may be used in such a system and would be effected in a similar manner to a greater or lesser extent. 7

The thrust chamber is often used repeatedly in ground tests and therefore must be very durable to achieve satisfactory testing of missiles. Because corrosive attack within the tubes is very difiicult to detect, it will be seen that the probability of failure because of intergranular corrosion caused by sulfur becomes even greater when the nozzle must be reused. The problem, therefore, becomes not only one of prevention of intergranular corrosion but also one of rejuvenation of previously fired thrust chambers. This rejuvenation should be possible for the most severe sulfur attack, as long as mechanical fracture does not occur.

Accordingly, an object of my invention is to provide a treatment which will restore the original characteristics of metals which have been effected by intergranular sulfur corrosion.

Another object of my invention is to provide a treatment of intergranular sulfur corroded metals which will be simple and economical while consuming a relatively small amount of time.

Still another object of my invention is to provide a treatment which will prevent sulfur corrosion in metals.

Much experimentation has been performed in an effort to alleviate the deterioration of metals which have been exposed to sulfur under corrosive conditions. Probably the worst defect of metals caused by sulfur corrosion is the sulfur build-up between the grains. This phenomenon is referred to as intergranular corrosion and often penetrates the metals to extensive depths. This infiltration 3,287,181 Patented Nov. 22, 1966 by sulfur between the grains of metals weakens the binding connection between the grains thus causing extreme brittleness, as well as loss of the metals most desirable characteristics of strength and toughness. In order to remove the sulfur from the grain boundaries the inventive treatment contemplates heating the metal in a vacuum to the point at which the sulfur coagulates into spheroidal particles which tend to relocate away from the grain boundaries. The original grain binding characteristics of the metal are thus restored. The temperature of the treatment should be kept below the recrystallization tern perature of the metal so that no grain growth need occur.

It has been found that once treated in this manner a metal becomes a great deal less susceptive to further intergranular sulfur corrosion. Thus, for maximum protection a metal would be subjected to conditions causing intergranular corrosion, and then treated so as to eliminate the corrosion. By this method the metals resistance to further corrosion would be greatly increased.

Experimentation has shown that similar results occur and the decrease in pressure need not be so great when the metal is heated in an atmosphere of hydrogen. However, to avoid oxidation, hydrogen of great purity must be used if the treated metal is an alloy containing chromium.

It has been found in the case of nickel which is commonly used in thrust chambers, that satisfactory results are obtained by vacuum firing the metal at temperatures between about 500 C. and about 1100 C. at an absolute pressure of between about 10- mm. Hg and about 10* mm. Hg for a period of between about 1 hour and about 48 hours, while excellent results have been achieved 10- mm. Hg and 10* mm. Hg for a period of between 10 and 18 hours.

During this study one nickel sample was severely corroded by exposure to H S vapor at 600 C. for onehalf hour. The rejuvenation treatment was performed at a pressure of 10 mm. Hg and a temperature of 700 C. for 14 hours. The results of this treatment were excellent. The corroded sample had been completely embrittled because of infiltration by the sulfur into the grain boundaries. After the restoration treatment the sulfur had coagulated into small, well distributed particles well away from the grain boundaries. Further tests revealed that the sample had regained its original characteristics of strength, ductility and toughness, while its resistance to further intergranular corrosion hadbeen greatly increased.

Further study has indicated that for other metals such as zirconium alloys, steel, super alloys and transition metals the temperature range must be varied as well as the pressure and time. The time, for example, may range up to about 48 hours, the temperature variation may be from about 200 C. to about 1500 C. and the pressure may range from about l0 mm. Hg to about 10" mm. Hg. For most metals of the type used in thrust chambers results were good when using a temperature of between 600 C. and 1200 C., a pressure of between 10- mm, Hg and l0 mm. Hg for a period of between.

3 The treatment would thus be useful in laboratories not equipped to reduce pressures to the limits necessary when utilizing a normal atmosphere.

Thus, it will be seen that a metal which has been previously afiected by integranular sulfur corrosion canbe rejuvenated by vacuum firing. Seconadly and even more important is the fact thatthe meal is made more resistive to such corrosion than it was in its original state. As a final step toward even further resistance to intergranular corrosion the metal may be cold rolled. This treatment will cause the grain boundaries to run more closely parallel to the surface of the metal thereby decreasing the depth of the lateral grain boundary which in turn reduces the depth to which the corrosion may advance.

It is thus within the concept of my invention that in addition to the treatment of thrust chambers presently in use the treatment can be in the fabrication of new thrust chambers. In the treatment of the new materials of fabrication the materials would be subjected to an intentional exposure to sulfur gases before being vacuum fired.

While the foregoing is a description of the preferred embodiment, the following claims are intended to include those modifications and variations that are within the spirit and scope of my invention.

I claim:

1. A'method of treating intergranular sulfur corrosion in nickel and nickel base alloys which comprises heating the metal in an atmosphere of hydrogen to a temperature of between about 500 C. and about 1100 C. at. a pressure between about .5 and about .1 atmosphere for a period of time of between about 1 hour and about 48 hours.

2. A method of treating intergranular sulfur corrosion in metals which comprises taking said metal after it has been exposed to a sulfur vapor environment suflicient to cause intergranular sulfurf corrosion in said metal andheating said metal to a temperature of between about 200 C. and about 1500 C. at a pressure between about 10- mm. Hg and about 10" mm. Hg for, a period of not more than about 48 hours.

3. A method of treating intergranular sulfur corrosion in metals which comprises taking saidmetals after they have been exposed to a sulfur environment sufiicient to cause intergranular sulfur corrosion in said metals and heating said metals in an atmosphere of hydrogen to a temperature of between about 200 C. and about 1500 C. at a pressure that is maintained between about .5 and about .1 atmosphere for a period of not more than about 48 hours.

4. The method of treating intergranular sulfur corrosion in metals which comprises taking said metalsafter they have been heated to an elevated temperature and exposed to sulfur vapor and heating said metals to a temperature between about 600 C. and about 1200 C. at a pressure of between about 10* mm. Hg and about 10- mm. Hg.

and exposed to sulfur vapor and heating said metals in 5 an atmosphere of hydrogen to a temperature between about 600 C. and. about 1200 C. and at a pressure that is maintained between about .5 and about .1 atmosphere.

6. A method of making-metals resistant to sulfur corrosion comprising; exposing said metals to sulfur vapor, heating said metals to a temperature of between about 650 C. and about 800 C. ate pressure .of between about mm. Hg and about 10- mm. Hg for a period of time of between about 10 hours and about 18 hours.

7. A method of making metals resistant to sulfur corrosion comprising; exposing said metals to sulfur vapor, heating said'me-tals in an atmosphere of hydrogen to .a temperature of between about 650 C. and about 800 C. at a pressure of between about .5 and .1 atmosphere for a period of time of between about 10 hours and about:

18 hours. r

8. A method as set forth in claim 7 wherein said metal is cold rolled as a final step.

9. A method of making metals resistant to sulfur corrosion comprising; exposing said metals to sulfur vapor, heating said metals to a temperature of between about 500 C. and about1l00 C. at a pressure of from about 10- mm. Hg to about 10 mm. Hg for a period of time of between about 1 hour and about 48 hours.

10. A method as set forth in claim 9 wherein metals are nickel and nickel base alloys.

11. A method of making metals resistant to sulfur corrosion comprising; exposing metals to sulfur vapor, heat-.

ing said metals to a temperature of between about 200 maintained between about .5 and about .1 atmosphere for a period of not more than about 48 hours.

12. A method as set forth in claim 11 whereinsaid.

metals are nickel and nickel base alloys.

References Cited by the Examiner UNITED STATES PATENTS 1,201,633 10/1916 Ruder 148-16 1,842,200 1/1932 R-amage 148-20.3

2,314,816 3/1943 Brown et a1 148-20.3

2,662,839 12/ 1953 Becker et a1 148-20.3

OTHER REFERENCES Metals Handbook, 1948 edition, The American Society for Metals, Cleveland, Ohio, relied on page 1042.

HYLAND BIZOT, Primary Examiner. 7 DAVID .L. RECK, C. N. LOVELL, Assistant Examiners.

said

C. and about 1500" C. in an atmosphere of hydrogen

Claims

9. A METHOD OF MAKING METALS RESISTANT TO SULFUR CORROSION COMPRISING; EXPOSING SAID METALS TO SULFUR VAPOR, HEATING SAID METALS TO A TEMPERATURE OF BETWEEN ABOUT 500*C. AND ABOUT 1100*C. AT A PRESSURE OF FROM ABOUT 10-**3 MM. HG TO ABOUT 10-**6 MM. HG FOR A PERIOD OF TIME OF BETWEEN ABOUT 1 HOUR AND ABOUT 48 HOURS.