VOl. 2, so. 3
AiliALYTICAL EDITION
266
X-Ray Examination of Commercial Galvanized Iron by a Modified Reflection Method' Charles W. Stillwell a n d George L. Clark DEPARTMEKT OF CHEMISTRY. UNIVERSITY
A modified technic for obtaining diffraction patterns by reflection h a s been described, this method being particularly suitable for metals a n d alloys. An analysis of galvanized iron wire by this method has brought out t h e following points: (1) There is no solid solution of zinc i n iron adjacent to t h e steel base; (2) t h e compound next t o t h e steel base, formerly designated a s FeZna, has been identified as Fe,Znlo; (3) t h e outer layers of the coating contain FeZn,, a solid solution of zinc i n FeZn;, a solid solution of iron i n zinc, a n d a phase with a body-centered cubic lattice t h e composition of which is unknown. A similar analysis of galvanized Armco sheet steel brought o u t t h e following points: (1) There is no solid solution of zinc i n iron adjacent to t h e steel base; (2) there is no evidence of FeaZnlo; (3) t h e intermediate
OF I L L I N O I S , r R B A K A . ILL
layers are composed essentially of a solid solution of zinc i n FeZni, a n d t h e unidentified body-centered cubic phase, t h e latter being more concentrated near t h e surface of the coating; (4) t h e surface of t h e coating is pure zinc a n d t h e crystals exhibit preferred orientation. The occurrence of t h e body-centered cubic lattice, a form which has not been found i n t h e iron-zinc system studied i n equilibrium, suggests t h a t there may be i m portant aspects of metallic systems not i n equilibrium which have been missed i n t h e equilibrium studies. I t has been demonstrated t h a t a complete description of zinc a n d other metallic coatings can be obtained only by cooperative microscopic a n d x-ray analysis, t h e former t o give a picture of t h e distribution of t h e several phases a n d t h e latter t o determine t h e s t a t e of combination of t h e metals i n these phases.
...... HE purpose of this investigation has been twofold:
T
first, to identify the various iron-zinc phases present in galranized iron-to which end two conimercial galvanized irons were examined, a wire prepared by the hot-dip process and a hot-dipped Armco sheet; second, to deinonstrate the precision and adaptability to problems of this nature of the so-called reflection method of obtaining x-ray diffraction patterns. Several very satisfactory microscopic studies of etched zinc coatings have been made, but it is admitted bhat the inicroscope falls short of a complete solution of the problein. Bablik ( 1 ) finds the ordinary galvanizing made u p of three layers: (1) a very thin layer, very rich in iron, next to the iron base; ( 2 ) a thicker layer of a mixture of two components designated FeZiia2and FeZn,; (3) a thicker, outer layer of pure zinc. E. in. f . measurements on the several layers seem to confirm this ( 2 ) . Rawdon (9) finds four layers: (1) a narrow layer of FeZn, next to the iron base; (2) a layer of FeZna and FeZn,; (3) a thick layer of FeZn, crystals in a softer matrix; (4) pure zinc. These four stages were obtained by holding a wire in contact with molten zinc a t a t'emperature just above the melting point of zinc for 4 hours, so that the system more nearly approached equilibrium than it does in actual galvanizing practice. Finkeldey (5) has identified four layers: (1) relatirely pure zinc; (2) a wide band of two phases, a hard zinc-rich compound dispersed in relatively pure zinc; ( 3 ) a narrow layer consisting entirely of a zincrich compound; (4)next to the steel, a very thin layer of a compound less rich in zinc than the preceding layer. Finkeldey points out that metallographic technic falls short in two respects: First, the compounds cannot be positively identified as FeZiia and FeZn,, although they may be so designated with fair certainty with the help of the phase-rule diagram 1 Received March 5 , 1930. Presented under the title "X-Ray Analysis of Binary Systems under Equilibrium and Son-Equilibrium Conditions. Iron-Zinc Alloys and Commercial Galvanized Iron,'' before t h e Division of Physical and Inorganic Chemistry a t t h e 79th Meeting of t h e American Chemical Society, Atlanta, Ga.. April 7 t o 11. 1930. 2 A recent crystal analysis of the compound designated FeZnj shows t h a t i t cannot have this formula and is in reality FeaZnio. T h e formula FeZna is used in quoting work already published t o avoid confusion.
for the iron-zinc system. Second, the very thin layer neyt to the steel base is hard to find with the microscope unless it has been brought out by annealing the sample for soine time. It is difficult to prove that it is present in samples which have remained in the bath only the short time generally permitted in practice. There also seems to be some doubt as to whether or not there is a layer of solid solution of zinc in iron next to the steel base. An attempt to clear up these uncertainties by means of an x-ray investigation seems very desirable. Reflection Method Diffraction patterns may be obtained by reflection with the help of a n oscillating spectrograph. The specimen is mounted a t a definite angle in relation to the beam, the adjustments being more or less troublesome. This method has generally been used only qualitatii ely and is not well adapted to the determination of interplanar spacings, since it is difficult to measure the distance from specimen to film; and the undiffracted beam is not registered on the film except when the beam hits the specimen at a grazing angle. A reflection method of great precision was first suggested by Davey ( 4 ) five years ago, and his method has been modified to fit the particular needs of this work. Davey pointed out the convenience of using specimens of metals in the form of fine wires rather than preparing a finely divided powder of the metal. X wire--or plane surface-properly annealed will present niinute crystals completely chaotic in arrangement and, if this wire is placed in the usual specimen position on a quadrant cassette, x-rays will be reflected from the surface of the metal and give results entirely comparable with those obtained froin a powder, provided certain sources of small errors are taken into consideration. Since the sheets used by the writers were large, it was convenient to bend them over a mandrel of small radius and to replace the quadrant cassette by a flat film held in the vertical cassette used for taking pinhole pictures on the General Electric diffraction apparatus. The wire sample, placed with its length parallel to the slit, is moved upward from below until it all but obliterates the beam, the adjustment being made by sighting through a
July 15, 1930
I S D U S T R I A L r l S D E S G I S E E R I S G CHEMISTRY
fluoroscope. Since the surface of the specimen is curved, there is provided automatically a limited but sufficient area to which the x-rays are incident a t an angle such that the resulting diffraction pattern will fall on the film. B y raising the film holder in the cassette so that the undiffracted beam is a t the bottom, the whole height of the film is utilized. The bottom of the film is protected by a sheet of lead in order to eliminate the general scattering which fogs the film if the undiffracted beam is allowed to strike it throughout the exposure. Since the method is highly sensitive, it is essential to use a zirconia filter, placed over the slit nearest the target, to eliminate all but the k'u radiation. There are two principal factors affecting the accuracy of the method as a means of determining interplanar spacingsnamely, the nieasurenient of the distance from the undiffracted beam to the several diffraction lines and the measurement of the distance from specimen to film. The distances on t'he film may be nieasured with great precision if the image of the undiffracted beam is sharp, since the zero point is the edge of the beam furthest from the diffraction pattern, this edge being defined by the top of tlie specimen. The diffraction lines, however, are not always sharp. The main source of error is in measuring the distance froin the film t'o the particular narrow area on the curved surface oi the specimen from which the beam is reflected, and included in this is an error introduced by the fact that on a rounded surface the source of the pattern will be slightly different for every line (,:I. This error is minimized with a cylindrical specimen and may be determined within 0.2 mm., particularly if the radius of curvature of the specinien is small. Under the less favorable conditions n-here the curved surface is not syinnietrical, such as one formed b y bending a sheet. tlie error need not exceed 0.5 mm. as a maximum. Table I shows the actual error in the determination of the interplanar spacings for iron, assuming a difference of 0.5 mni. in the iiieasureiiieiit of approximately 7.0 cni. The rnaximuiii error in the lattice coiiataiit (a01 is seen to be 0.45 per cent. Tlie niaxiniuiii error in the determination of any one interplanar spacing as compared with the value calculated from tlie be$ available precision data for a-iron (Table 11) is 0.014 A. for the 222 planes. Spacings as small as this are not significant in the anaiyses presented in this paper, however, and we may take 0.007 A. for t'he 310 planes as tlie rnaxiniuiii 5ignificant error, a discrepancy of about 0.7 per cent. K h e n the samples have been prepared by bending sheets of varying thickness, a further clieck on the nieasurenieiit of the distance may be secured by taking two pictures a t different specimen-to-film distances, calculating the interplanar spacings for each and averaging the results. I n Table I1 data are presented for the determination of the lattice constant of alpha-iron froin three different patterns obtained with three slightly different specimen-to-film distances. The data are compared with the interplanar distances calculated from the accepted lattice constant for alpha-iron ( 7 ) . The actual error is well within that already claimed for the method. It is evident that the influence of certain minor causes of error (lack of perpeiidicularity of film to x-ray beam. lack of flatness of film, etc.) must lie within the limits of error demonstrated in Table 11,and i t is well to check the apparatus in this way before attempting quantitative measurements. K i t h these procedures it is always possible to determine the interplanar spacings with sufficient accuracy to identify the several crystal phases which may he present, but the degree of precision is not sufficient to demonstrate conclusively a very slight shift in the lines due to the existence of a solid solution. A coiiclusive method of checking this is available when coatings and surface filiiis are under examination. I n the present work the zinc was dissolved completely from the sample lying half way across the slit and left on the other
267
half, thus producing two patterns side by side on the same film and affording a direct comparison. Having eliminated the possibility of a solid solution, it is then possible to use the lines of this known crystal form as a calibration to determine the exact position of lines of other crystal f o r m occurring on the same film. This, of course, will eliminate all the errors so far enumerated. Fortunately this procedure could be applied to some extent t o the problem in question, since the pattern of alpha-iron is superimposed on several of the other patterns. Table I-Error i n Determination of Interplanar Spacings for AlphaIron Introduced by Error i n Measuring Specimen-to-Film Distance FROM INTERPLANAR SPACISG CALCD. DISTAXCE O N FILM FILM-TO-SPECIMEN DISTANCE hkl 7.20 cm. 7.22 cm
.
Cm.
.4
2 65 3.95 5 13 7.60 8 9.5
2 Old 1 42" 1,160 0 894
A. 2 023
110 100 '111
1.429 1 167 0 897 0 821
0 818
310
'122
Table 11-Interplanar Spacings for Alpha-Iron Determined a t Different Film-to-Specimen Distances, Compared with Values Calculated f r o m Best Published Lattice Constant a2 - 2.855 A. d h k l C.4LCD. dhi.1 FOR sPECILlES-TO-FILMDISTANCE iiki 7.03 c~n. 7 . 2 3 cm. 7.30 cm. d. ;l. A '4
(10
=
2.551
2 840
2 840
Any reflection method is very useful becauic of its reiisitivity and also for obtaining diffraction patterns of $ d a c e coatings and of substances which are highly opaque t3 x-rays. The method described has the follonkg additional advantages o w r tlie method iiqing the oscillating spectrograph: (1) There is no special equipment needed. ( 2 ) The sample may be adjusted more simply and more rapidly. (3) Measurements accurate to within 0.5 per cent are possible. The undiffracted beam is registered on the film and the distance from specimen to film may be accurately determined.
The greatest precision is not clainied for the technic which has been described: hut i t is sufficiently accurate to distinguish between several cryst,al forms whose lattices have already been established. It has been succcssfully applied to this prohleni and may be used t,o advantage in nuiiierous problems of this sort in which the size and shape of the specimens would either prevent the use of the quadrant cassette or introduce inaccuracies of similar magnitude. The only limitations of the method are that the specimen must be curved and rigid, and it' is suitable for metal speciinens of any sort. X-Ray Analysis of Galvanized Iron Wire
Tlie zinc coating n-as applied to this wire by a special hotdip process, differing from the ordinary process in that the coating is annealed for a brief period after dipping. The wire base is a low-carbon steel. The coating has a thickness of 0.715 ounce per square foot. It was found t,hat the coating could be completely dissolved ijy immersing for about 1 minute in 6-1' hydrochloric acid. Iccordingly, a series of samples was prepared by renioi-iiig successive layers of the coating as follows: SaMPLB
0 1 2 3
ETCrrIsG
TIME
Jlinules 0 15 2.5
35
SAMPLE
ETCHISC TIME
4
Minutes 40
6
50
45
ANALYTICAL EDITION
268
Zinc
Figure 1-Diffraction
VOl. 2, No. 3
1 2 4 6 Iron Patterns of Galvanized Iron Wire Taken a t Varying Distances from t h e Surface of t h e Coating, Compared with t h e Pattkrns for Pure Zinc and Pure Iron
Diffraction patt'erns of these samples were obtained using the General Electric diffraction apparatus with inolybdenum target tube and the reflection method already described. The patterns, together with those for pure zinc and for the steel base, are reproduced in Figure 1. It may be seen a t a glance that there are two patterns distinctly different from each other and froni zinc and iron, most clearly developed in patt'eriis 1 arid 4. The intermediate patterns merely iridicate transition stages. Photomicrographs have shown that the several layers in the coating are very uneven, so that any one reflection would be likely to show the distinctive patterns for two or more phases superimposed. The time of exposure for pure zinc and pure iron was 24 hours, while the time for all the others was 60 to 72 hours, indicating that these patterms result from a mixture of several crystal forins no one of which predominates greatly. The lines of alpha-iron are visible in the patterns of samples 4,5 , and 6. I n order to determine TThether or not there exists a solid solution of zinc in iron, one-half of a specimen was etched down to the pure iron, while the other half was ebched for 40 seconds, thus affording a direct comparison bet,xeen pure iron and sample 4. The resulting pattern is similar to that reproduced in Figure 2, shoning that there is no solid solution. The data for t,he zinc-iron compound nearest the iron base indicated on pattern 4 in Figure 1 are recorded in Table 111. The alpha-iron lines superiniposed on the pattern and labeled in Table 111 serve as a calibration for the other patt'ern. The lattice constant for these lines, which are also recorded in the third coluiiiii of Table 11,is 2.849 * 0.009 A,, as compared with the generally accepted yalue 2.855 A. The error is therefore 0.3 per cent or less, while the niaxiniurii error for any single interplanar spacing is that for the 211 planes, which is 0.005 A. It' is obrious, therefore, t'hat the only significant source of error in t'his case is that in measuring the distance from the zero point t'o the several lines, many of which are weak and indistinct,. I n the last three columns the interplanar spacings of Fe3Zn10, as debermined b y Osawa and Ogawa (S),are recorded for comparison. There are several discrepancies which should be pointed out: (1) ,The reflection from the 321 planes is present in the writers pattern but absent from that of Osawa and Ogawa. T h e latter deduced the space group for FesZnlo, however, merely
by comparison with the pattern obtained for gamma-brass ( I O ) , which pattern was subsequently analyzed completely (,y) and assigned to the space group T:, I t is a modified body-centered cubic lattice with 52 atoms t o the unit cell. The 321 reflection was present on LVestgren and phragmen's brass pattern. (2) There is not a satisfactory reflection from the 332 planes on the present writers' pattern, (3) The writers observe a reflection from the 541 planes while Osawa and Ogawa identified one from the 622 planes, while both these are absent from gamma-brass. (4) There are two discrepancies of minor importance in the higher orders.
The direct comparison of the two pat'terns may perhaps be more clearly made by means of a chart, and such a chart is reproduced in Figure 3. The dissgreement between esperimental values and published data for t'he line? 22, 24, and 26 is not significant, since these lines are scarcely visible on the film aiid are therefore subject t o coiiiparatively large errors of measurement. Table 111-Patterns
4 a n d 5, Figure 1, Galvanized Wire (Distance specimen t o film, 7.23 cm.) FILMDISTASCE d h k l h'+k*+l? a. dhbl D A T A FOR FeZnio Cm. Intensity Intensity i z ? + k ' + l ? 2.07 2.22 2.47 2.53 2.63 2.72 2.95 3.13 3 47 3,i5 3.95 4.13 4.38 4.42 4 59 4 82 5.i 3 5.33 5.53
W VI%-
m vs W
v 1Y \-\I'
v 1v
vw W
m
2.550 2.380 2.198 2 110 2 019 1 971 1.836 1,742 1.593 1.490 1.426
12 8 53 14 8 90 (No 1 strongest) 18 8 95 (Iron, 110) 22 9 23 24 8 98 26 8 89 (Iron, 200 K g J 36 8 94 (Iron, 200) 8 90 42
VW
1,376
v \v
46 S 95 1.320 1.302 48 9 02 30 8 93 1.264 1,217 54 8 95 1 160 (Iron, 211) 62 8 90 1.130 1,100 66 8 96
VW
m
rn m vw
2.5i5
ms
2.108
YS
18
1.904 1.821 1.733
ms m mw
22 24 26
1.487 1.450
ms mw
36 38
1.343 1.317 1,290 1.264 1.213
m IY mm mw
44 46 48 50 54
mw vs
1.131 mw ms 1,100 W W 1.083 ms 72 8 95 1,033 w 1 055 5.88 ms 1,040 74 8.98 5.95 W 1.048 ms 1,026 Five of t h e lines belong t o other crystal forms, as indicated.
12
62
66 68
72 74 76
The present problem, however, is not to establish with the greatest precision the composition and the lattice structure of the several phases in the system. Such a study could not be made on surface coatings, since there is no way of knowing
I.VDUXTRIAL A N D ENGINEERING CHEMISTRY
July 15, 1930
the coinposition of the phase corresponding to any particular lattice. It is merely necessary to identify the patterns obtained as either body-centered FeaZnlo or close-packed hexagonal FeZn, or pure zinc, or solid solutions of these. Of course the lattices are markedly different, and according to the best analysis of the system available these are the only t x o compounds which occur in the entire range. I n spite of the irregularities in the analysis, therefore, the evidence is sufficient to show that the layer in the zinc coating immediately adjacent to the steel base is FeaZnlo. The analysis of pattern 1 is somewhat more difficult and nct quite so satisfactory. The complete data are shown in Table IT'. The large nuinber of lines indicates a mixture of tn-o or more phases and in the last coluinn each line is identified with one of several phases. T h a t phase designated A is analyzed in Table V, phase B in Table VI, and phase C in Table T'II. Table IV-Pattern
1, Figure 1, Galvanized Wire
(Distance, specimen t o film 7.23 c m ) FILM DISTAKCE dhkl LINE BELOSGSTO P H A S E Intensity
Cm. 1 2 2 2 2 2 2 3 3 3 3 3 3 4 4 4 4
4 4 4
4 5 5 6 5 5 6
7 8
92 13 18
2.730 2 474
vw
w
48 54
m m
60 85 05 28 3.5 51 58 67 00 10 20 31 59 67 75 84 00 25 49 56 95 38 60 92
-4
A, B A FeZn- C (2,135)n FejZn;a-strongest line B
2,424 2.148 2.105 2 056 1 892 1.782 1.670 1.640 1,581 1,550 1.518 1.412 1.384 1.357 1 330 1.265 1,250 1 230 1.217 1.186 1.143 1,108 1.108 1.046 0 998 0,895 0.835
m vs 1-w
vw
m m
V W
m m vw \-1v
.4
N o t accounted for A, B B . FeZni (1.626)
269
C, Pattern 1, Galvanized Wire INTENSITY h? k? 12 h kl a0 2.148 vs 2 110 3 04 1.550 m 4 200 3 10 1.250 S 6 211 3 06 1.108 vs 8 220 3 13 0.998 S 10 310 3 15 0.895 m 12 222 3 n9 0.835 W 14 32 1 3 12 Body-centered cubic lattice, ua = 3 . 1 0 Table VII-Phase
dhkl
+ +
A.
Th: phase A proves to be close-packed hexagonal with a. = 3.17 A. and an axial ratio of 1.58. The c o y o u n d FeZn7 is also close-packed hexagonal with = 2.778 A. and an axial ratio of 1.607 (8). The phase 9, then, is isomorphous with FeZn, and is in fact a solid solution of zinc in FeZn7. corresponding to the €-phase in the Cu-Zn Since the atomic radius of iron is 1.26 and that of zinc 1.37 A. we would evpect the lattice to expand, as it does. when zinc is dissolved in FeZn7. This solid solution is not the predominating phase in sample 1 because all the lines are relatively weak. The phase C is shown to beobody-centered cubic with the side of the unit cube Q = 3.10 A The preaence of this lattice cannot be explained, but is considered more a t length below. The phase B is close-packed hexagonal with a. = 2.602 A. and an axial ratio of 1.883. I t i s therefore isoniorphous with pure zinc, in d i i c h a = 2.665 A. and the avial ratio is 1.865 (6). We would expect the lattice to be somewhat smaller when some of the zinc is replaced by iron. The significant criterion is the axial ratio. which will indicate whether we are
.1
A , FeZni .A .A, B B , FeZni
A, C B
A B , FeZn;
A B,C K 012
(1 393) (1 274)
(1.181)
1
C
C C
T h e figures in parentheses are t h e interplanar distanceslfor FeZn; published by Osawa and Ogawa (8).
Table V-Phase
2 730 2.474 2.424 1.892 1.670
=
+ hk) +
2.75 A. dooi = 5.00 k. c / a = 1.58
B, Pattern 1, Galvanized Wire
INTENSITY
-----SIX2
Exp.
w
2 056 m 1 670 m 1 640 m 1 330 vw 1 265 m 1 230 m m 1 108 k2 sin2 8 = 24-O l h 2 sin BNO = 0 . 1063 sin Bo01 = 0 0715 aa = 2 . 6 0 2 k.
200 214 364 450 492 542 614 656 692 706 800 856 964 1148
hkl 100 002 101 102 on3 110 111 103 200 112 201 004 202 104 210
dioa =
3.17 A.
Table VI-Phase
2 474
204 212 351 449 500 542 627 652 680 707 802 845 955 1146 50 1 2
w m vw
+
dhkl
SIN^ 8--Exp. Calcd. 167 164
vw
m 1.581 vw 1.518 m 1,412 vw 1.384 VW 1.357 vw 1.330 vw 1.250 S 1.217 W 1.143 vw 1,046 v 1v sin2 0 = 164 jh2 k2 sin BIOS = 0.128 sin Boa1 = 0.0706 ua
A, Pattern 1, Galvanized Wire
INTENSITY
dhkl
+ + hk) +
204 296 447 465
R---.
Calcd. 204 296 449
459 704 783 786 827 816 1020 1031 51 1 % dim = 2 255 A. doai = 4 94 A. c/a = 1.885
707
hkl 002 101 102 003 103 111 004 201
Figure 2-Diffraction Pattern from a Single Specimen of Galvanized Armco Sheet Iron the Right-Hand Side of Which Has Been Et'ched Down t o the Iron Base While Only Part of the Coating Has Been Rembved f r o m t h e Other Side. T h e Continuity of t h e Iron Lines Shows t h e Absence of Solid Solution i n t h e Left-Hand Pattern
dealing with zinc, a solid solution of iron in the zinc lattice, FeZn7, or a solid solution of zinc in the FeZn7 lattice. The first two will have an axial ratio in the neighborhood of 1.86, while the last two mill have an axial ratio nearer 1.60. I n pattern 1, therefore, we have some evidence of FeZn7 and perhaps a trace of FesZn,o. These two phases are not too clearly indicated. We have conclusive evidence of a solid solution of zinc in FeZn,, a solid solution of iron in zinc, and a predominating body-centered cubic phase the composition of which is unknown. The concentration of the body-centered cubic lattice is greater in pattern 1 than in layers nearer the steel base.
A,VALYTICAL EDITIOS
270
22
Z26 L
+_Izo
J
26
I
42 46
48
62
62 66
Figure 3-Graphical Comparison of Data in Tab,e 111, the Interplanar Spacings of Pattern 4, Figure 1, and t h e Interplanar Spacings of FesZnlo T h e scale is in Angstrom units. T h e figures a t t h e sides are t h e values for ha kz 1 2 . T h e length of t h e lines indicates relative intensities.
+ +
It is interesting to compare these results with those obtained microscopically. Finkeldey noted that the layer which has definitely been identified as FeaZnlo is usually hard to find with a m:croscope unless the sample has been annealed for some time. while the x-rays detected it in diffraction patterns 4, 5 , and 6. The microscope indicates that usually the layers are unevenly distributed. This justifies the existence of characterdie lines for at least two crystal lattices on each of the patterns reproduced; in fact, patterns 1 and 2 include all the substances found microscopically in the outer layers, so that the x-ray method is not so satisfactory in showing the relative position of the layers. It does. however, show the several states of combination of the metals in these layers, a task which the microscope cannot perform. T o relate the several metallic compounds and solid solutions obtained to the layers observed microscopically, it might be desirable to have photomicrographs corresponding to each of the diffraction patterns. The patterns show that the layer next to the steel base is FeaZii,o.and also that there is no pure zinc 011 the outer surface. hi^ is ill agreement with the microscopic evidence f o r thin c o a t i n g s .
Analysis of Armco Galvanized Sheet
These samples were taken from Armco low-carbon steel sheet which had been coated by the hot-dip process with Prime Western zinc showing the following impurities : lead 1.0; iron, 0.05; tin, 0.04; copper, 0.02; and cadmium, 0.30 per cent. The microscopic examination of this product agrees essentially with those described above. There are four visible layers: a narrow layer next the iron, another narrow layer, and a third, much wider layer penetrating up into the pure zinc a t the outer surface. The layers are unevenly deposited. The third, wide layer appears very hard and brittle and is apparently largely responsible for cracking in the coating when such cracking occurs. Five samples were prepared by bending the sheet almost double, giving a curved surface as nearly regular as possible, and then etching in 6 N hydrochloric acid for the times indicated below: SAMPLE 1 2 3 4
5
SECOXDS
n
10 20 30
40
The diffraction patterns, together with those for pure zinc a n d alpha-iron, are reproduced in Figure 4. It may be seen
T'ol. 2 , No. 3
that nuxber 4 shoivs ouly the alpha-iron pattern. The iron pattern carries over into patterns 3 and 2 , but the lines groxv somewhat weaker as would be expected. Pattern 1 is essentially pure zinc, although the varied i n h s i t y (not yitlth) of the lines makes it rather difficult to recognize the zinc pattern. This variation in intensity is evidence of preferred orientation of the zinc crystals, the visible evidence being the spangles of zinc on the surface of the coating. It is interesting to note that there is no evidence of preferred orientation in any phases other than the pure zinc. I n Table T'III the data for pattern 1 are recorded and compared with the interplanar spacings for pure zinc, as determined by Hull (6). 1, Figure
Table VIII-Pattern
4, Arrnco Sheet
(Distance, specimen to film 7.11 cm.) FII,MDISTAXCE dhkl HULL'SDAT.AF O R ZINC Intensity dhkl Intensity
Cm. 2 2 2 3 3
4 4 4 5
5 5 5 6
7 8 8 9
10 26 53 18 70 11 24 68 03 13 33 92 92 45 03 23 00
2 2 2 1 1 1 1 1
w m vs W
vw m 5-S W
S
1
w
1
S
1
w
1 0 0
m 1%.
n n
m S
0
m
Table IX-Patterns
;:
4
85
m
93 20 29 50 59
is
on
5 18 5 30 5 60 5 82 5 95 6.15 6.68 6.90 7.10 7 23 7.38
7 87 8 18
vw
m
s \Y
vs
m m vW VW
vw s vu' vw Vu. w s R
w
1.427 { 2 n 1 404 1 332 1.307 1.262 1 243 1.163 1.133 1.116 1 072 1.049 1.030 1 007 0 955 0 935 0 920 0 911 0 899 0 866
0.847
5.473 315
-2.080 A . 684
( K O radiation)
1.339 1 333 1.235 1.173 1 132 1 121 1 044 0 947 0 908 0.857
I and 3, Figure
P.ATTERN 2 Distance film to specimen, 7.05 cm. Film distance d h i l Phase Intensity Cm. 2 07 m 2 480 2.1 2 12 2 428 2.4 5 2 26 iv 2 282 2 0 2.148 2C 2 42 2 50 2.084 Zinc a 58 2.025 Iron 1 894 1 . 8 6 4 1 2.1 ( 2 95 1.7961 1,7271 1 3 08 3 18 1.683 Zinc 3 24 1 . 6 5 2 2.1 3 33 1.615 2.1, 2 D 1.537 2C, 2.1 3 53
3 3 4 4 4
468 305 079 686 488 338 331 233 166 149 119 040 940 899 860 847 810
20 Zinc 2 0 2C 2.4 Iron 2 0 , 2.1
4
4, Arrnco Sheet
P A T T E R3N Distance film t o specimen, 7.30 cm. Film distance d h k i Phase Intensity Cm. 2 19
(2 37')
2.430 v v w 2 262 7%-
2 50
m
2.148
2 67
s
2.028
Iron
3.28 3.35
v U'
1 687 1.657
Zinc 3d
3 3 3 4 4 4
4
4 4 5
60 64 92
no
10 35 45 66 75 18
YW
vs
1 556 1. :40 1.448 1.425 1.402 1.332 1.310 1 260 1 243 1.162
W
w W
s vIY 5-1v
\.n-
m vu.
3c
3; I Iron 3A, 3 0 Zinc 30
3c
3.A Iron
2C
5.53
m
1.107
3c
2d 2 0 Iron
6.05
VIY
1.043
3.1
6 40 6 90 7 10
m vw vw
1 002 0 995 0 939
3C, iron
7 8 8 9
s
vw
0.895 0.863 0 848 0 811
Iron 3D 3C Iron
2.4
20 2.1 2.4 Iron 2D
2C
68 20 48
14
vw m
30
I n order to deterLninemh3ther or not there exists a solid solution of zinc in iron, a diffraction pattern was obtained of a sample from half of which all the coating had been dissolved and the other half of which had been etched for 20 seconds. This gives the line for pure iron adjacent to those for sample 3 thus eliminating the possibility of a small difference in the specimen to film distance causing a slight shift in the lines. The result indicates that there is no solid solution of zinc in iron. The pattern in reproduced in Figure 2 . The data for patterns 2 and 3 are recorded in Table IX, and the several phases to which the lines belong are indicated. Phase 2A is analyzed in Table X. It is a close-packed hex-
I S D U S T R I A L A N D ENGISEERING CHEMISTRY
July 15, 1930
Zinc F i g u r e 4-Diffraction
1 1 2 4 Iron P a t t e r n s of Galvanized A r m c o S h e e t I r o n , T a k e n a t Varying D i s t a n c e s f r o m t h e S u r f a c e of t h e C o a t i n g , C o m p a r e d w i t h P a t t e r n s f o r P u r e Zinc a n d P u r e I r o n
agonal lattice with Q = 3.21 8. and an axial ratio of 1.54, and is a solid solution of zinc in FeZn,. Phase 2C is analyzed in Table XI and corresponds to the body-centered cubic phase, C, found in the galvanized wire. Phase 2 0 is probably the pattern formed b y the diffraction of the MoKg radiation from the planes of alpha-iron, and oiie MoKp line of 2C. The evidence is a bit confusing, since this D phase is more prominent in pattern 2 than in pattern 3, and the iron lines for K a radiation are more prominent in 3. The agreement is too close to be overlooked, however. There is also a trace of pure zinc recorded on pattern 2. T a b l e X-Phase
2A, P a t t e r n 2, A r m c o S h e e t
---Sir\.? e---. Exp. Calcd. 203 204 "1" 912 360 365 456 459 480 483 534 530 612 620 807 816 9iO 977 1088 1103 1127 1140 1433 1437 147s 1460 1508 1501
ISTESSITY
dhll
2 480 2 428 1 864 1 652 1 615 1 537 1 41i 1 243 1 135 1 072 1 049 0 935
m
m v I;-
0 920 0 911
sin? e = sin @loo = sin Bcoj = aa =
161 ( h ? 0 . 127 0.0715
271
+ k ? + h k ) + 51 Pdioa
= 2.782
hkl 002 101 102 003 110 111 103 004 104 203 210 105 204 30 1
.k.
doni = 4.94.4. c,in = 1 . 2 4
3.213.k.
T a b l e XI-Phase 2C, P a t t e r n 2, A r m c o S h e e t ISTESSITY k2 k? 1 .1 2 k kl
+
1 ,
310 Body-centered cubic lattice, aa =
3.07 3.09 3.15 (3. 18) 3.11 3.16
3.09 A.
It should be noted t h a t several of these diffraction patterns exhibit an unusual variation in the breadth of the lines. One would expect the lines to be very sharply defined, and in many cases they are, but there are numerous very broad lines and some darkened portions resembling bands (two such are indicated in pattern 2 , Table IX, in brackets). Since the rays penetrate to a finite distance, the broadened lines could be caused by diffraction by a solid solution of
varying composition. The broadening could also be due to distortion or strain of the lattices, a condition which doubtless exists in surface coatings. The x-ray daba agree as well as would he expected with the iiiicroscopic data up t'o a certain point. Thus. there is no solid solution of zinc in iron and there is an outer layer of pure zinc. Of the three intermediate layers found microscopically, two are accounted for on the x-ray patterns. One of the thin layers is identified as a solid solution of zinc in FeZn,; the thicker layer is probably composed of the body-centered cubic phase, C. There is no pattern of a third phase to account for the other thin layer nearer t'he iron. One would suspect it to be FesZnlo, but the characteristic lilies of this pattern are absent from the films. The possibility of the bodycentered cubic lattice being that of a solid solution of zinc in iron has not been overlooked, but it seeiiis s-ery improbable because it occurs nearer the outer surface in the zinc-rich region. It is true that in the sheet coating all the phases are niixed and there are no marked differences on layer patterns 2 and 3, but in the wire coating there is a definite layer of FeaZnlocoiiipletely separating the iron base from the bodycentered cubic phase. The results of this st'udy conflict with or supplement the published data for t'he iron-zinc system (8)in two respects. -1solid solution of iron in zinc has been found in t'he coat'ing of the wire, whereas none was reported by Osawa and Ogawa. This, of course, is of small consequence, since they doubtless would have found one had they analyzed a sample containing more zinc. The existence of the body-cenbered cubic lattice is significant, however. It is not likely that the impurities in the zinc are sufficient to be responsible for this phase, which means that it must be a compound of iron and zinc; and because it' occurs t'omard the outer layer it is probably richer in zinc than is FeZn7. Of course. in the x-ray analysis of the A-ole-Since the completion of this paper, fairly good evidence has been found t o indicate t h a t t h e body-centered cubic lattice is t h a t of the compound FeZnio [cf. Schueler, Trnnr. I m . Electrochem. S O L . ,47, 201 (1925)l. Osawa and Ogawa could not have found this form in their x-ray study, since their sample richest in zinc had t h e approximate composition of FeZni. T h e existence of body-centered cubic FeZnio can, of course, be checked.
iron-zinc system the samples were always brought to equilibrium before being analyzed, while in the case of a galvanized coating no such condition is realized. The cubic lattice rnay
ANALYTICAL EDITION
272
be either a metastable phase or a phase which is stable in such a narrow range of composition that it is missed in the usual method of analysis when the system is allowed to come to equilibrium. The results at least indicate that it is not safe t o draw too close an analogy between the equilibrium diagram and a metallic system not in equilibrium, a condition so often found in practice; and they also suggest that there may be many interesting aspects of alloys not in equilibrium which have been missed in working out the niany metallic systems under equilibrium conditions.
1'01. 2,
so. 3
Literature Cited (1) Bablik, .S/nhZ Eisen, 44, 223 11924).
(2) Bablik, M e t a l I n d . ( L o n d o n ) , 26, 481 (1925). ( 3 ) Bradley and Thewlis, Proc. R o y . SOL. ( L o n d o n ) , Al22, 678 (1926). (4) Davey, Gett. EZec. Rev., 28, 586 (1923). ( 5 ) Finkeldey, Proc. Am. SOL.T e s t i n g Materials, 26, Pt. 2, 304 (1926). (6) Hull, Phys. Reo., 17, 571 (1921). (7) International Critical Tables, Val. I, p. 340, McGraw-Hill, 1926. (8) Osawa and Ogawa, Z . Kris:., 68, 177 (1928). (9) Rawdon, Proc. A m . SOL.Tesling Materials, 18, Pt. I , App. I11 (1918). (10) Westgren and Phragrnen, Phil. Mag.. SO, 311 (1925).
Determination of Perchlorate' H. H. Willard and J. J. Thompson UNIVFRSITY OF MICHIGAX, ANN ARBOR,~ I I C H .
T
HE usual methods of determining perchlorate involve reduction to chloride either by fusion or by means of
Experimental
It was found that if perchlorate is heated Fvith starch and titanous sulfate. The latter may be used as a standard reducing agent or simply as a means of forming chloride. sulfuric acid under proper conditions, it is completely reduced, A review of various methods is given by Lamb and Mar- mainly t'o chlorine, Tyhich is absorbed and reduced to chloride den ( 1 ) . Senften ( 2 ) has described a method differing en- in alkaline arsenite solution and determined in the usual tirely in principle, in which the perchlorate is mixed with po- way. The potassium perchlorate, made from very pure materials, tassium dichromate and concentrated sulfuric acid aiid gently heated to boiling. The gas is absorbed in a saturated solution was further purified by recrystallization, aiid dried at 300" C. of sulfur dioxide and for 15 minutes ( 3 ) . TKOdeterminations mere made with the reduced to c h l o r i d e . apparatus already described (5), using the following proAlthough no explana- cedure: tion is given. the reacThe weighed sample of approximately 1 gram, 0.8 gram of tion seeins to be as fol- chlorine-free starch, and a few glass beads are placed in the flask. l o m : Upon h e a t i n g The apparatus is connected, having 1 gram of arsenious oxide the solution of dichro- in 100 cc. of a 10 per cent chlorine-free sodium hydroxide solution iiiate and sulfuric acid in the absorption flask. the c h r o m i u i i i is reSole--All joints are well lubricated with a viscous paste made by mixing d u c e d ; the hot per- phosphorus pentoxide with warm, sirupy phosphoric acid ( 4 ) . R u b b e r chloric acid o x i d i z e s bands are used on the glass hooks. i t a n d is r e d u c e d t o Through the dropping funnel 35 cc. of concentrated sulfuric acid chloride. Soiiie qerious are added and the solution is brought t o the boiling point in 20 disadvantages are the minutes. It is boiled 5 minutes, then 25 cc. of fuming sulfuric acid containing 20 per cent SO3 are added and the boiling is formation of insoluble continued for 10 minutes. An excess of a saturated solution of anhydrous chromic wl- permanganate is slowly added t o the boiling solution to espel fate, causing bumping, any ch!orine remaining in the flask as hydrochloric acid. T h e arid the loss of per- water is drained from the condenser and the solution boiled 3 minutes. The solution in the absorption flask is transferred chloric acid due to yola- to a beaker and acidified with dilute nitric acid, using methyl tilization before reac- orange as indicator. Excess of silver nitrate is added to precipition with c h r o m i u m tate the chloride, then 2 ce. of concentrated acid. The precipit a k e s p l a c e . T h i s tate is filtered on a Gooeh crucible, washed with water containing a few drops of nitric acid, and then with acetone, dried at 135' C., method Fvas carefully and weighed. tested. but the results were low. Antimonous The results are shown in Table I. Figure 1-Apparatus for Determining and arsenious o x i d e s It is desirable to work with a smaller apparatus; therefore Small Quantities of Perchlorate were then tried as re- one was designed as illustrated in Figure 1. This apparatus ducing agents in concentrated sulfuric acid, but somewhat was satisfactory for samples \-arying in weight from 0.03 to low results were obtained. 0.4 gram. K i t h samples of approximately 0.03 gram the I n recent work on the determination of halogens in organic amounts of reagents used were: 0.03 gram starch, 0.05 coinpounds (.?), potassium persulfate was eriiployed to facili- grain arsenious oxide, 15 cc. of a 10 per cent solution of sotate the decomposition of carbonaceous material. Sonie diuin hydroxide, 6 cc. of 95 per cent sulfuric acid, and 3 cc. of samples of persulfate were found to be contaminated with funling sulfuric containing 20 per cent sulfur trioxide. K i t h perchlorate to so great a n extent that a blank had to be run a 0.4-gram sample the amounts Tvere: starch, 0.25 graIn; to determine the chlorine present in this form. This sug- arsenious oxide 0.25 gram; 95 per cent sulfuric acid, 20 CC.; gested the application of this method to the determination of fuming sulfuric acid, 5 cc. The time required for a complete per chlorate. analysis was 45 minutes. The results of these analyses are shown in Table I. 1 Received March 22, 1930.
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