Determination of Hydrogen in Magnesium, lithium, and Magnesium-Lithium Alloys 31. W. MALLETT, A. F. GERDS, AND C. B. GRIFFITH
Battelle Memorial Institute, Columbus, Ohio This investigation was begun in connection with a study of porosity in magnesium-lithium alloys. Hydrogen was suspected of being the major cause of porosity. Determination of hydrogen in the light metals has generally led to erratic results; hence a workable method was sought. Two satisfactory methods of determining hydrogen in magnesium-lithium alloys were developed: the tinfusion method and the method of extracting hydrogen at 630" to 800' C. from the specimen sealed in a steel capsule. The tin-fusion method is more rapid and is suitable for determining the hydrogen content of pure magnesium. It might be used to determine hydrogen in pure lithium if precautions are taken to protect the cut surfaces of the lithium from the atmosphere while i t is being weighed and loaded into the furnace. The apparent hydrogen content of magnesiumlithium alloys i s increased on exposure to the atmosphere. With a method of analysis available, one can isolate the sources of hydrogen entering the melt. This makes possible control of the hydrogen content of castings.
Extraction from Specimen Contained in a Steel Capsule. The samples are sealed in welded-steel capsules, made by oxyacetylene welding 1/8-inch steel disks into one end of steel tubes 1.5 inches long, 0.75 inch in outside diameter, with 0.038-inch wall. The capsules and steel plugs are sand blasted, degreased with ether, and degassed in a vacuum furnace at 750' C. for 4 to 8 hours. The entire process of sample preparation (cleaning by abrading, washing in petroleum ether, and placing the specimens in the capsules) is carried out within a dry bos containingan argon atmosphere. After weighing, the capsules are loaded and a steel plug is forced into the end. The loaded capsules are reweighed and then welded in a closed chamber u hich has been first evacuated and then filled to atmospheric pressure with argon, The welding is done with an argon-shielded tungsten arc. After welding, the capsules are stored in a desiccator until ready to be analyzed. For the extraction process, a loaded capsule is placed in a quartz furnace tube (see Figure l ) , which is then evacuated and heated to 800" C. Liquid nitrogen is used on the cold trap to freeze out any water vapor desorbed from the walls of the apparatus. About 11 hours are required to complete the extraction from a sample of magnesium-l6% lithium alloy. The hydrogen content of the evolved gas is determined by means of an Orsat apparatus.
T
HE determination of hydrogen in magnesium, aluminum, and similar metals has generally given erratic results when the hydrogen was extracted by diffusion from the specimen uhen heated in vacuum. Some error is caused by the reaction of the hot metal with traces of residual water vapor in the system to produce the metal oxide and free hydrogen. $nother complicating factor is the volatility of these metals when heated. Combustion methods are more complicated and subject to similar interfwences. The present investigation was an attempt t o surmount these difficulties and devise a satisfactory method for determining hydrogen in magnesium, lithium, and magnesium-lithium allo!.. .
.
Figure 1. General Gas-Extraction Train
Results obtained by this method of analysis are summarized in Table I. The hydrogen value obtained for lithium by this method is gleater than that obtained by the tin-fusion method. This difference is believed to be caused by lithium's diffusing through the capsule walls and reacting with water adsorbed on the walls of the apparatus. This results in an interference similar to that occurring in the warm extraction method. Although this method of extraction is time-consuming, the analytical values obtained for magnesium-l6% lithium alloy agree with those obtained by the tin-fusion method described below.
ANALYTICAL METHODS
Three nlethods of extracting the hydrogen u-ere tried: (1) n-arm e,traction at 5000 c.,(2) extraction at 6500 to 8000 c. from the specimen contained in a sealed *tee1 capsule, and (3) tin fusion at i50°
,
F*rn.,c~ section
c.
Tin-Fusion Method. The sample is dissolved in a molten-tin Warm Extraction at 500' C. The magnesium-l~thium alloy bath a t 480" C. The bath remains fluid after addition of the sample is placed in an Alundum boat and inserted in a quartz specimen, and the hydrogen is iapidly evolved with no visible furnace tube, which is then evacuated and heated to 500" C. Liquid nitrogen is used on the cold trap (see Figure 1) to collect any water vapor desorbed from the walls of the apparatus. Gas was evolved a t a rather high rate (0.04 ml. Table I. Hjdrogen in Lithium and Magnesium-Lithium Alloys per minute) for nearly 3 hours. The run was dis(By iacuum extraction from samples contained in sealed steel capsules) continued after 3 hours because of the excessive Sam le Hydrogena, vaporization of magnesium and lithium. RydroRun TVeigKt, Weight gen in the extracted gas was determined in an No. Grams % Sample Remarks Orsat apparatus. The'magnesium and lithium vapors reacted with the water adsorbed on the walls of the apparatus, giving a spurious source of hydrogen. Therefore, this method of extraction is not applicable to magnesiuni-lithium alloys, nor is it likely to be applicable t o either magnesium or lithium metal.
16
0.78
0.19
Lithium17629
19
2 25
0 0056
Mg-16% Li alloy 2663
38
5 24 0 0054 Mg-16% Li alloy 2663 Preciaion of analssis is & l o % of reported values.
116
After 36 hours a t 750' C , rate of gas e.volution was 0 006 ml. per min. Lithium dtffused through steel capsule Extracted 7 hours a t 150' C. and 6 hours a t 800' C. Extracted 1 1 hours a t 800' C .
V O L U M E 25, NO. 1, J A N U A R Y 1 9 5 3
117
Table 11. Hydrogen in Magnesium, Lithium, and MagnesiumLithium Alloys (BYtin-fusion method) Run No.
7 47 26 8
Sample Weight, Grams 3.98 2.90 2.36 2.66
Hydrogena, Weight
%
Sample Commercial magnesium rod Commercial magnesium rod Mg-16% Lialloy2663 Mg-16% Li alloy 2663 spiked with hydrogen
0.0012 0.0008 0.0052 0.025
Remarks
...... ......
Hydrogen equal to 0.0203% added to alloy 2663 a t 500' C. before analysis 9 1.53 0.095 Lithium 17629 Sample exDosed to air about 10- minutes after cleaning surface Lithium 17629 Sample loaded in tin-foil cap14 0.645 0.068 sule inside argon dry box Sample loaded in tin-foil capLithium 17639 15 0 954 0.037 sule inside argon d r y box 1.1-5.370 Lialloy 2694 Edge of casting 3 30 0.0039 40 Edge of casting 2.84 k&3% Li alloy 2694 41 0.0042 Core of casting 3.22 RIg-5.3% Li alloy 2694 50 0.0050 Sample exposed t o air about 0.190 Lithium 69315 45 0.060 10 minutes after cleanine surface 46 0.403 0.051 Lithium 69313 Sample loaded into tin-foil capsule inside argon dry box Precision of runs 7 and 47 is i0.0003C;o hydrogen. Precieion of all other runs is + l o % of reported value. Q
The results obtained by the tin-fusion nicthod of analysis are summarized in Table 11. The hydrogen values obtained by this method for magnesium and magnesium-lithium alloys are reproducible. The accuracy of the results mas checked by analyzing a sample of magnesium16% lithium alloy with which a measured amount of hydrogen had reacted (see runs 26 and 8). This sample gave a hydrogen value equal to the original content of the alloy, plus the amount of hydrogen added. The values obtained for the hydrogen content of lithium were very erratic. This appears to be principally caused by the effects of exposure of the metal to the atmosphere. Considerable difficulty was encountered in preparing a sample of lithium entirely free from air-corrosion products even when an argon dry box was used. It may, however, be possible by exercising extreme precautions to eliminate the formation of air-corrosion products. DISCUSSION
indications of evaporation of the specimen constituents. The disadvantage of the excessive volatilization found in the warmextraction method and the slo-ivness of extraction from a specimen contained in a steel capsule are eliminated by using the tinfusion method. This method is similar to a method of hydrogen extraction which has been used successfully for hydrogen in steel ( 1 1.
In the tin-fusion method, a borosilicate glass furnace tube is used (see Figure 2). A borosilicate glass test tube containing 100 grams of commercial tin is placed in the vertical section of the furnace tube. The sample, a 30-gram degassed steel weight, 15 grams of C.P. tin, and a steel pusher rod are stored in this order in the horizontal arm of the extraction tube. The system is evacuated and liquid nitrogen placed on the cold trap. After the tin bath has been outgassed a t 450" C., the sample and steel weight are pushed into the bath (at 450' C.) with the steel pusher rod manipulated by means of a magnet. Xearly 95% of the total gas is evolved in the first 15 minutes. dfter the evolution rate drops to 0.01 ml. per minute, the C.P. tin is dropped into the bath to ensure complete solution of the sample and to stir the bath. ilbout 1 hour is required to complete the extraction of hydrogen from the sample. There is no visible indication of evaporation of niagnesium or lithium from the bath. When pure lithium is analyzed, the temperature of the bath is lowered to 300" C. before the sample is dropped, in order to prevent a violent reaction, which ma) be caused either by the rapid evolution of hydrogen, or by an exothermic reaction between the tin of the bath and the lithium metal. Immediately after the specimen is dropped, the temperature is rapidly raised to 450" C. t o complete the extraction.
AW
I
'
d
ij
-Chroml S1eel copscrewsampit
-
llrt-
vwnd furnoce
Pyrex test tub+ Pyrex furnocc t
Figure 2.
Details of Furnace Section L'sed for Tin-Fusion Method
Precision of Hydrogen Determinations. I t is general experience that multiplicate determinations of gas content of metals agree to about 10 relative %. This includes errors introduced by sampling, difference in surface preparation, gettering of extracted gas by vaporized metals, and spattering of the molten sample. The relatively high analytical values are most affected by these losses. At low gas contents, the uncertainties imposed by limitation in measuring gas volumes determine the precision of the results. The buret of the Orsat apparatus used in the present study has 0.10-ml. graduations and readings can be estimated to 0.05 ml. Thus the best precision which can be claimed for a 1-gram sample is 10.0009 weight yohydrogen.
Table 111.
Spectrographic Determination of Lithium in End Plug of Steel Capsule Position of Sample Lithium, Weight %
C u t 1, outermost cut 0.048 cut 2 0.084 cut 3 0.100 Cut 4,innermost cuta 0.205 a A "winch thickness of original Q/sr-inch-thiok end plug remained after deepest cut had been taken. ~
~
~-
Reaction of Iron with Magnesium-Lithium Alloy. In all the steel capsules used for extracting hydrogen from either lithium or niagnesium-l6'% lithium alloy, the odor usually associated with acetylene was noted when the samples were removed from the capsules. A magnesium-l6% lithium alloy, subjected to the hydrogen extraction treatment a t 800' C. for 11 hours, Jvas removed from the capsule and analyzed for residual hydrogen by the tin-fusion method. The sample failed to dissolve in the tin bath a t 450' C. Carbon and iron were determined in a part of the unreacted sample and a sample of the original ingot. While there was no significant change in the carbon content, the analysis showed that appreciable amounts (0.06 weight %) of iron dissolved in the alloy while it was held a t 800' C. in the capsule. At the same time the lithium may have reacted xith the inclusions in the steel capsule. Diffusion of Lithium in Iron. Hydrogen from a sample of lithium was extracted through the malls of a steel capsule by heating for 36 hours a t '700" to 800" C. in a vacuum. The capsule was cut open and the lithium a-as found to have adhered to the walls of the capsule. The lithium n-as broken away and prepared for tin-fusion analysis. A facing cut was taken on the lathe from the outside surface of one of the end plugs of the capsule.
ANALYTICAL CHEMISTRY
118
Chips were then taken a t four depths from the outer surface of the plug. The results of spectrographic analysis of these chips, shown in Table 111, indicate that the lithium had begun t o penetrate through the steel end plug of the capsule. After the chips had been removed for analysis, the remainder of the end plug was examined metsllographically. The microstructure of the end plug is shown before and after exposure to lithium in Figures 3 and 4, reppectively. Before exposure, the microstructure consists of the typical cold-rolled structure of steel (Figure 3). Two types of microconstituents e m be clearly noted in the bright a-ferrite matrix: (1) black stringerlike patches of pearlite and (2) gray inclusions, which are probably sulfide.
Lc 'i'
Figure 4.
Steel E n d Plug Reaction w i t h L i t h i u m (1OOX)
Mafcir. Large grains of =-ferrite Dark nresa. Lithium reaction p-duct removedduringnam~llepolishin~. 4% pieral etch
Figure 3. Cold-Rolled Structure of Steel Capsule E n d Plug, as Rolled (lOOX) Matrix. o-Ferrite Black areas. Pearlite Gray ~Uinsern. Sulfide8 4% pisral etch
Bright field
Both aonstituents are oriented in the direction of rolling. Figure 4 shows the microstructure after exposure to lithium. The dark voids are the lithium reaction products, while the light matrix, with the eqniaxtxed grains, is the a-ferrite which has recrystallized and now has a grain size of ahont No. 5 (SAE classification). The voids also have a stringerlike appearance similar to the original inclusions. It, therefore, appears that the lithium reacted with the sulfide inclusions and with the cementite of the pearlite. Because of their stringerlike and somewhat continuous form, almost complete permeation of the end plugs may occur along the path of these inclusions. Lithium also diffused readily through the walls of the Armco iron capsule, in this case attacking the stringerlike oxide inclusions. Certain types of stainless steel may he more satisfactory capsule materials. EFFECT OF EXPOSURE TO AIR ON MAGNESIUM-LITHIUM ALLOYS AND LlTHlUiM
An experiment was performed to test the efficiency of removing hydrogen from lithium or magnesium-lithium alloys through the walls of a steel capsule in argon instead of vacuum. The capsule, containing maguesium-l60/, lithium dloy, was first heated a t 700' t o 800° C. for 21 hours in argon and then 4 hours additionally in vacuum a t 8M)" C . This latter extraction showed the residual hydrogen content to he O;oOl~o. The sample was then removed from the capsule in a dry box contzining argon, wrapped in tin
foil, weighed, then analyzed by the tin-fusion method. A relatively high value of the hydrogen oontent, 0.0039%, was found. The increase in the hydrogen content was ascribed, in part, to the hydrolysis of resetiou products by the moisture in the atmosphere on the removal of the alloy from the oapsules. It wm aleo noted that lithium tarnished rapidly in air. Because of this, a study was made of the effect of exposure to air on the apparent hydrogen contents of lithium and magnesium-lithium alloys. Samples of magnesium-16% lithium alloy, each having approximately the mme ratio of surfsce area. to weight, were exposed t o the atmosphere a t room temperature for from 0.5 to 72 hours. No increase in the hydrogen content was detected after air exposure of 2 hours or less. However, after 20 hours' exposure, the rspparent hydrogen content of the specimens had increased by 50%, and after 72 hours by 200%. Lithium metal corrodes rapidly in air. The'hydrpgen content of specimens (having surface areas ranging from 10 to 17 sq. cm. per gram) including the corrosion layer, had increased 40% after 2 hours' exposure, and about 500% aftter 21 hours. As the amount of air-corrosion products increased, 1,he hydrogen also increased in both the magnesium-16% lithium alloy and lithium. The hydrogen values were determined by tin-fusion analysis a t 450' C. The hydrogen ma>, be from either the corrosion products on the
Table IV. Tin-Fusion Analysis of Magnesium-Lithium Alloy Exposed to Atmosphere sample R.un ho.
Weight. Grams
26e
2.36 2.34 2.98 2.47 2.58 2.21 2.82
2o 32 51 23 24
Hydrogen". Weight
7%
0.0052 0.0055 o.oo51
0.0077 0.0080 0.015 0.0CFO
Expwure Conditioish Rsiative Room Time. humidity. temp.. hours % 0 F. 0.1 .. 48 77 46 78 24 82 20 63 49 78 72 57 85 24 Stored in desiccator over anhydrous CaSO. (Drieritel
F5
..
*Precision of ?nalysis !i &IO% of reported values. b Control speoime?. Time qf exposure 1s that normally given incidental t o loading apeaimen in extraotlqn apparatus. r Hwnidity and temperature I" eaoh case initid condition of exposure.
119
V O L U M E 2 5 , N O . 1, J A N U A R Y 1 9 5 3 COYCLUSION
Table V.
Run No. 35c 33 34 37
Tin-Fusion Analysis of Lithium Exposed to Atmosphere Sample Weight. Gram 0,167 0.204 0.261 0,273
Hydrogen", Weight 70
0.10 0.09 0.14 0 57
Exposure Conditionsh Relative Room Time, humidity, temp., hours 70 F.
..
0.1 0 5 2 21
24 38 29
82 79 81
Precision of analysis is + l o % of reported valiiPs. Humidity and temperature in each case initial condition of exposure. Control specimen, only exposure incidental t o loading specimen in extraction apparatus. a h C
surface or absorbed hydrogen within the sample. The above data are the result of single determinations, and there was some inhomogeneity within the sample. Howevei , the trend toward increased hydrogen content of magnesium-l6% lithium alloys and lithium after exposure to air appears real. The complete analytical results of the samples exposed to air are shonn in Tables IV and V. No attempt was made to correlate the relative humidity with the increase in the hydrogen content of the specimens.
The tin-fusion method has been successfully used for the determination of hydrogen in magnesium and magnesium-lithium alloys. This method probably would be applicable to lithium also, if the sample could be adequately protected from reaction with atmospheric moisture during preparation for analysis. The apparent hydrogen content of magnesium-lithium alloys, and especially lithium, increases when exposed to air. Satisfactory results also were obtained by extracting hydrogen from magnesium-16% lithium alloy contained in welded-steel caapsules. Permeation of the capsule walls by lithium from the alloy did not affect the results. However, with unalloyed lithium, interference from this source was sufficient to invalidate the results. S o magnesium samples were analyzed by this method. The tin-fusion method is preferred over the sealed steel capsule method because it is simpler and more rapid. LITERATURE CITED
(1) Carney, D. J.. Chipman, John, and Grant, S . J., .J. Metals, 188, 397-403 (1950). RECEIVED for review July 3, 1952. Accepted October 1 , 1952. Work done under Contract W-7405-eng-92 for the U. S. Atomic Energy Commission.
Determination of Small Amounts of Zinc in Aluminum Alloys S
Rapid Electrograv imetric Method Utilizing Isotope Dilution Technique KURT THECRER AND THORIAS R. SWEET The Ohio State L'niversity, Columbus, Ohio This work was undertaken with the intention of overcoming a number of the difficulties ordinarily encountered in the electrogravimetric determination of zinc. It was found that this could be accomplished by utilization of the isotope dilution technique and the radioactive tracer zinc-65. In this way, the loss of a portion of the zinc by coprecipitation, mechanical loss of solution, etc., during the removal of interfering substances, or by the incomplete deposition of the metal on the cathode was without effect. A rapid method for determining small amounts of zinc in aluminum alloys w-as developed. The relative error in the method will generally limit its application to samples containing no more than a few per cent of zinc.
ANY methods have been reported for the electrogiavimetric determination of zinc. Some of these involve the deposition of zinc from a sodium hydioxide solution ( 4 ) , ammonium chloride or sulfate solution ( 7 , b'), cyanide solution ( I ) , acetate solution ( 6 ) ,and citric acid solution (9). Procedures for the electrolytic determination of zinc in aluminum alloys have been suggested ( 2 , 3,6, 8 ) . In the proposed method the zinc need not be quantitatively deposited and only a Teighable portion of the pure metal must be obtained on the cathode. Therefore the conditions do not have to be so carefully controlled as in the ordinary electrogravimetric method for zinc and the duration of electrolysis may be decreased. The zinc solution mal- be purified, prior to deposition more quickly and conveniently than is possible in the conventional type of electrogravimetric analysis. For example, if a reagent is added to remove interfering ions by precipitation, the solution may then be quickly centrifuged and most of the clear supernatant liquid decanted. The time that would otherwise be consumed in slow filtration and washing is eliminated.
Zinc45 is an especially desirable active isotope to use for an analysis of this type, because it possesses a long half life of 250 days and an easilv measurable gamma radiation. I t can be obtained from Oak Ridge in a high state of purity and at B sufficiently high specific activity. The initial cost is low and i t has a long shelf life METHOL)
A standard curve is prepared by adding a known volume of an
active zinc solution to solutions containing various known weights of pure zinc. =ifter a sufficient quantity of sodium hydroxide is added to each solution, it is diluted and electrolyzed in the cell shonn in Figure 1. The zinc is deposited on a weighed copper disk cathode that fits in th,e base and serves as the bottom of the cell hfter the electrolysis, the disk containing the deposited zinc is weighed and its activity is measured. Bv dividing the activity (in counts per minute) by the weight (in milligrams) of the zinc deposited, the specific activity is calculated for each deposit. If S . A . is the above described experimentally determined specific activity of a zinc deposit, W is the weight in grams of
