Embrittlement of Silver by Oxygen and Hydrogen

Transcription

1 AMERICAN INSTITUTE OF MINING AND METALLURGICAL ENGINEERS Te'chnical Publication No (CLASS E, INSTITUTE OF METALS DIVISION. NO. 396) DISCUSSION OF THIS PAPER IS INVITED. Discussion in writing (a copies) may be sent to the Secretary. American Institute of Mining and Metallurgical Eneneers. 29 West 39th Street. New York. N. Y. Unless special arrangement is made, discussion of this paper will close June I Any discuss~on offered thereafter should preferably be in the form of a new paper. Embrittlement of Silver by Oxygen and Hydrogen BY D. L. MARTIN* AND E. R. PARKER,* MEMBER A.I.M.E. DURING the heat-treatment of silver specimens for tensile tests it was observed that the bars blistered and became brittle when heated in a hydrogen atmosphere. (New York Meeting, February 1943) The microstructure of a cross section of the tube is shown in Fig. 2. The embrittlement is extremely severe at the edge; the blisters result from the severe deforma- FIG. I.-BLISTERS ON SILVER TUBE EOR~IED BY ANNEALING IN HYDROGEN AT 850 C. TOR ONE HOUR. X 3. PIG. 2.-MICROSTRUCTURE OF SECTION THROUGH BLISTERS SHOWN IN FIG. I. X 100. Etched with potassium bichromate followed by ammonium hydroxide-hydrogen peroxide. To check this unexpected result, a wide tion of the surface material by large undervariety of commercial silvers were heated lying gas pockets. The effects resulting in hydrogen at 850 C. Only a few of the from hydrogen embrittlement of silver materials developed blisters, the most have been observed by numerous investisevere of which is the tube pictured in gators over a long time'f2,3 although it was Fig. I. only recently that a clear explanation was ~ffered.~ However, the damage resulting Manuscript received at the oflice of the Institute Dec. I, from hydrogen embrittlement has not *Research Laboratory, GeneralElectric Co.. been generally recognized; and it is of Schenectady. N. Y. 1 References are at the end of the paper. advantage to know more about this phe- - Copyright. 1943, by the American Institute of Mining and Metallurgical Engineers. Inc. METALS TICTINOI.OGY, April Printed in U. S. A.

2 2 EMBRITTLEMENT OF SILVER BY OXYGEN AND HYDROGEN nomenon because silver is being used in increasing amounts as a substitute metal. HISTORY Embrittlement of oxygen-bearing copper by annealing in a hydrogen atmosphere on the occlusion of hydrogen by metals, found that silver acquired a beautiful frosted appearance on the surface when heated in hydrogen and by repeated heating it became brittle. Beilby and Henderson2 studied the action of ammonia on metals at high temperatures. For silver, they found that at 8o0 C. the wire became coated with rounded blisters or bubbles, and the elasticity of the metal was greatly reduced. They attributed the change to the rapid formation and decomposition of an unstable nitride. Bone and Wheeler3 studied the catalytic action of silver on the hydrogen-oxygen reaction. They found that by heating silver in air just short of the melting point and then in a hydrogenoxygen mixture at 4o0 C. "a change took place both in the outward appearance and mechanical properties." Fig. 3 illustrates very clearly the changes in the appearance of the silver gauze after this treatment. The behavior of the silver was attributed to the breaking up of a hydride. Dornblatt4 briefly mentions that the formation of blisters is due to the water vapor formed within the metal by the interaction of hydrogen and oxygen. FIG. 3.-EFFECT OF HYDROGEN-OXYGEN MIX- TURE ON SILVER WIRE GAUZE. AS SHOWN BY AND \\'HEELER.$ BONE a, original appearance; b, gauze after airtreatment near the melting point followed by 400 C. exposure to a hydrogen-oxygen mixture. has been thoroughly inve~tigated.~-~ This effect is attributed to the formation of water vapor within the metal, by interaction of oxygen and hydrogen, with sufficient pressure to cause local rupture along grain boundaries and thus weaken the metal. A review of the literature has revealed several examples of embrittlement of silver when heated in a hydrogen atmosphere. Graham,' in his experiments in r866 I. The silver specimens used in the series of experiments to be described were cut from a rod of fine silver* 44 in. in diameter, which experiments proved was not embrittled when heated only in hydrogen. However, samples of this material were made brittle by first heating in air at 850 C. and then in hydrogen at 850 C. The changes in microstructure and tensile properties are of interest. The loss of ductility and strength through hydrogen embrittlement is indeed considerable, as shown in Table I, in which the tensile ' Fine silver contains approximately 0.05 per cent impurities. Spectrographic analysis of the silver used showed traces of Cu and Fe; Zn, Ni. Sn, Pb, A1 were nil.

3 D. L. NARTIN AND E. R. PARKER 3 properties of sound and embrittled silver and copper are compared. embrittlement."* Such embrittlement was produced by first saturating the silver with TABLE I.-Telzsile Treatment Silver I. Heated in Nz at 850 C. for I hr Heated in air at 85o0C. for I hr Heated in alr at 850 C., for I hr. then in H? at 8soqC. for I hr.... Tough-pitch Copper I. Heated in Nz at 2. Heated in Hz at 8io0C. for I hr..i 24,100 Propevlies of Silver arcd Copper ~trengt'h ' 850 C. for I hr Reduction of Area. Per Cent y: --- ~ Elongation, Per Cent in I In nil nil The loss of strength and plasticity is clearly understood after comparison of the microstructures of the material before and after the hydrogen treatment, as illustrated in Fig. 4. Silver forms an oxide, Ag20, which decomposes at about 350 C. in air. At higher temperatures oxygen exists in solution in the metal. For most treatments the specimens were maintained at temperature and the atmosphere changed from air to hydrogen. A few samples, however, were water-quenched after saturation with oxygen (by heating in air at 850 C.) and then reheated in hydrogen; no difference was noticed in the results. When silver containing dissolved oxygen is heated in hydrogen a decline in strength and ductility results. This is in accord with the results of Rhines and Anderson7 on copper; they found that dissolved oxygen in copper containing no Cu20 caused a mild susceptibility to hydrogen embrittlement. 2. The reverse of hydrogen embrittlement was also found to occur; i.e. silver containing hydrogen was embrittled by heating in air. For convenience in discussion, we have termed this effect "oxygen. a FIG. 4.-EFFECT OF HYDROGEN ON SILVER CON- TAINING OXYGEN. X 100 Etched with potassium bichromate follo~\ed by ammonium hydroxide-hydrogen peroxide. a, heated in alr at 850 C. for one hour; b, heated in air at 850 C. for one hour and then in hydrogen at 850 C. for one hour hydrogen at a high temperature, followed by heating in air. The resulting structure * Strictly speaking:, the terms "hydrogen embrittlement" and oxygen embrittlement " are both misnomers, since both gases are equally necessary lo:, the formation of water vapor. The term water-vapor embrittlement" would be a more general and accurate description of the effect.

4 4 EMBRITTLEMENT OF SILVER BY OXYGEN AND HYDROGEN is shown in Fig. 5. No embrittlement was found in other samples heated only in hydrogen. To check these results, a piece of silver was degassed by heating for PIG. 5.-EFFECT OF OXYGEN ON SILVER COX- TAININC HYDROGEN. X joo. Etched with potassium bichromate followed by ammonium hydroxide-hydrogen peroxide. n, heated in hydrogen at 8j0 C. for one hour; 6, heated in hydrogen at 850 C. for one hour then in air at 850 C. for one hour. 5.i hr. in vacuum at 850 C. The piece was cut into two samples; one was heated only in hydrogen for I hr. at 850 C., the other was heated first for I hr. in hydrogen at 850 C., then the atmosphere was changed to air, without change of temperature, and held for another hour. No embrittlement was found in the sample heated only in hydrogen, but marked embrittlement resulted in the sample heated first in hydrogen then in air. However, the oxygen embrittlement was not as severe as the hydrogen embrittlement (see Fig. 4). Oxygen embrittlement could occur only if the oxygen diffused into the silver before the hydrogen could escape. This is in accordance with the known rapid absorption of oxygen by silver. 3. The grain size at the surface of thc silver specimen heated first in air (Fig. 4) is much smaller than that of the samplc heated first in hydrogen (or in vacuum), Fig. 5. Rhines and Grobell observed a similar fine-grained zone in silver and silver alloys after an oxidizing anneal. Small amounts of soluble impurities frequently have profound effects on the annealing characteristics of the base meta1,l2!l3 accordingly it was considered possible that oxygen in solution might, in this case, be responsible for the finegrained surface zone. To check this, a sample was water-quenched from 850 C. after saturation with oxygen; cold-worked and then reheated for I hr. in air at 850 C. If soluble oxygen inhibited grain growth the silver sample should be fine-grained throughout. However, this was not truethe fine-grained structure was confined to the surface; hence, some other factor was responsible for the retarded grain growth. To see whether the metal in the center of the rod behaved in the same way as the metal near the surface, a rod j.is in. in diameter was machined to X6-in. diameter and then heated in air for I hr. at 8j0 C. In this sample coarse grains extended to the surface, indicating that the fine-grained structure found near the surface of the x6-in. bar heated in air was characteristic only of the metal near the outside of the rod. Hence it was concluded that the original bar of silver was

5 D. L. MARTIN AND E. R. PARKER 5 not homogeneous. Something inherent in the surface material, apparently introduced in the manufacturing process, caused fine grains when the metal was heated in air, but not when heated in hydrogen or vacuum. 4. To determine the minimum temperature at which silver, containing oxygen, would be embrittled by hydrogen, a series of samples was first heated in air at 850 C. for 7 hr. and then quenched; individual specimens were heated for I hr. in hydrogen at various temperatures ranging from 450" to 850 C. The embrittlement at lower temperatures was less severe and did not occur below 5o0 C. Samples of the original material developed fine grains near the surface when heated in air, but coarse grains throughout when heated in hydrogen or vacuum. Gold and platinum dissolve little or no oxygen aiid therefore are not embrittled when heated in a hydrogen atmosphere after an air treatment. It appears that embrittlement resulting from the union of dissolved or combined oxygen with hydrogen is common to copper and silver, and probably to any metal that contains reducible oxides or that will dissolve appreciable oxygen in the solid state. If water vapor can be formed within any metal, embrittlement is possible. To be susceptible to hydrogen embrittlement, a metal must contain considerable oxygen, either combined or dissolved, which is free to react with hydrogen; thus platinum and gold, which do not form stable oxides and dissolve little or. no o~ygen,*~~ should not become embrittled in hydrogen. Samples of these metals were heated in air at a high,temperature and then in hydrogen; the gold and were free from grain-boundary fissures. Solid silver will dissolve sufficient oxygen at high temperatures to become embrittled if subsequently heated in a hydrogen atmosphere (above 5o0 C.): In the use of silver, embrittlement can be avoided by never exposing to an atmosphere containing.hydrogen material that has previously been heated in air. Oxygen embrittlement of silver, the reverse process of hydrogen embrittlement, is also possible-the metal is first heated in hydrogen and then in air. The authors wish to express their appreciation to Mr. Roy Adams for conducting the heat-treatments, and to Mrs. C. B. Brodie and Miss F. E. Wiley for the photomicrographs. I. Graham: Phil. Trans. (1866) 156, Beilby and Henderson: Trans. Chem. Soc. (1901) 79, Bone and Wheeler: Phil. Trans. (1906) 205-A. I. 4. Dornblatt: Silver in Industry, chap. 5, 180. New York. Reinhold Publishing Corporation. 5. N. B. Pilling: Trans. A.I.M.E. (1919) 60, L. L. Wyman: Trans. A.I.M.E. (1933) 104s 141; (1934) 111, 305; (1940) 137, F. N. Rhines and. W. A. Anderson: Trans. A.I.M.E. (1941) 143, C. E. Ransley: Jnl. Inst. Metals (1930) '9. F. M. G. Johnson and P. Larose: Jnl. Amer. Chem. Soc. (1927). 49, J. H. Simons: Jnl. Phys. Chem. (1932) 36, F. N. Rhines and A. H. Grohe: Trans. A.1.M.R. (1942) 147, F. Hudson. T. M. Herbert. F. C. Ball and E. H. Bucknall: Jnl. Inst. Metals (1929) 42s R. M. Brick. D. L. Martin and R. P. Angier: Amer. Soc. Metals Preprint No. 37 (1942).