THE ELECTRON MICROSCOPE Calibration and Use at Low Magnifications CHARLES J. BURTON, R. BOWLING BARNES, AND T. G . ROCHOW Stamford Research Laboratories, American Cyanamid Company, Stamford, Conn.
ITH the advent of the electron microscope about ten years ago, the scientific world became aware of the fact that undreamed of magnifications of small objects were possible. I n almost every field of endeavor a wide variety of objects, some familiar and some not, were photographed a t high magnification. Many papers and popular articles have appeared describing this “supermicroscope” and stressing the fact that “magnifications as high as 100,000 diameters are possible”. Statements pointing out the gigantic proportions which familiar objects assume a t this magnification are popular, One reads that ‘(an average human hair would become as large as a giant redwood tree” or that “an individual bacterium would have a diameter equal to that of a baseball”. It is true that electron micrographs of good quality may be obtained at extremely high magnifications. It is also true that such high magnifications have a popular appeal,
and that they have certain specialized uses for the scientist, such as the testing of electronic lenses a t high magnifications and the convenience of measuring distances in the range 0.1 to 0.004 p (1000 to 40 A,). Nevertheless the most generally useful degrees of magnification are merely those required to make the electron-microscopical images perceptible by the human eye. To the microscopist, however, the important features of the electron microscope are the high resolving power (down to 0.004 p ) together with remarkably great depth of focus (>10 p at greatest resolution). The numerical aperture of the electron microscope is very low (ca. 0.002) ; therefore, ultimate resolution does not decrease appreciably with increase in focal length of the objective (decrease in magnification). By decreasing the magnifications to the order of 200 to 3000 diameters, the fields of view are much more closely comparable to those of the optical microscope, and the
FIGURE 1. COMPARISON OF HIGH-POWERED PHOTOMICROGRAPH AND LOW-POWERED ELECTRON MICROGRAPHS OF CALCIUM CARBONATE (ARAGONITE), SHOWING THE HIGHRESOLVING POWER AND DEPTHOF Focus AVAIL^ ABLE IN THE ELECTRON MICROSCOPE (a) 240 X ; (b) I180 X ; (e) 2OOOX; ( d ) 3600 X ; (e) 7000 X : f l optical hotomicrograph (not same field) at 1130 X: ( 8 ) portion of (e) opticshy enlsrgec?to 17,000 X .
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-
1
ll0V
4
, I
OBJECTIVE
LENS 4
Y
!
(tocusing controt)
..
I
/
I
I
,,
I I
&PROJECTION
II
I,
I‘
!/
’
‘,
>$
I (magnification I
LENS control)
FIGURE 2. SCHEMATIC OUTLINE OF ELECTRICAL EQUIPMENT FOR THE
ELECTRON MICROSCOPE
The cathode is actually heated by a rectifier-filter system instead of the battery indicated above.
increased ability to correlate the images by means of the two microscopes has been so helpful that i t was considered desirable to describe the method used.
Range As delivered, the R. C. A. electron microscope can be used directly a t any magnification between approximately 1100 and 15,000 diameters; the desired magnification is obtained by adjusting the current in the magnetic projection lens t o the correct value as noted on a predetermined calibration curve. I n practice it is found most convenient to use the microscope at magnifications between 4000 and 10,000 diameters. This choice results from the fact that a t these intermediate magnification values a happy combination of the brightness of the image on the fluorescent plate, the size of the photographic field, and the time of exposure is obtained. The latter goes up roughly as the square of the magnification, and it is inconvenient if this time is either too short or too long. Above about 12,000 diameters, the fluorescent image is relatively weak for satisfactory focusing and the field is small. At 15,000 diameters this image is hard t o see clearly, and the field which can be photographed a t one time on a 2 X 3 inch picture is only 3.4 X 5.1 p . At 1100 diameters, on the other hand, the image is very bright; and while the field takes in a circle a t the object screen roughly 16 p in diameter. this field on the photograph occupies a circle only 18 mm. in diameter. Fortunately the resolving power of the objective is so great that with negatives taken a t any of these magnifications, subsequent optical enlargement is warranted. Just as in optical microscopy, the proper interpretation of the results obtained is the most important phase of the work. I n practice, however, extreme difficulties are often encountered in attempting to interpret intelligently electron micrographs a t the high final magnifications generally used. The interpretation of these electron micrographs is complicated by se.i.era1 factors. First, the field is so small that often only part of the object is seen at any one time. Smaller objects which can be seen in their entirety are often photographed in the complete absence of their neighbors, which makes ready comparison difficult. Second, while a similarity exists between optical and electron micrographs of the same material, the difference in the two types of emanation used for image formation should always be kept in mind, when
Vol. 34, No. 12
interpreting the respective micrographs. Third, the electron image is devoid of all helpful differences of visible color. Fourth, the analogs of the optical phenomena of polarization and double refraction so useful in the interpretation of optical images have not been observed in electron microscopy. I n fact, electron images of familiar objects frequently present appearances so novel and unexpected that it is difficult or even impossible to obtain much real information from their examination. The jump from the unaided eye a t 1 diameter to the electron microscope a t 4000 diameters or greater is usually too large; accordingly, in such cases it has been found most helpful to make preliminary examinations with the optical microscope for purposes of orientation and identification. Nevertheless, a gap still exists between the useful ranges of the visual microscope and the present electron microscope. In order to use the electron microscope to fullest advantage and the wealth of information which can be furnished by optical microscopists, it is desirable to lower the magnification of the electron microscope and thereby increase its field of view. “Useful” magnification is defined as the magnification which, with a given objective, is necessary to produce a distance between two points of at least 0.2 mm., the approximate limit of resolution of the unaided eye. The optical microscope may be used with visible light a t useful magnifications up to about 1400 diameters. But a t these magnifications the depth of focus is so small (down to ca. 0.06 p ) that an entire field of small objects can be in sharp focus only if all of them have a thickness equal to or less than the depth of focus. The use of ultraviolet light makes possible the taking of photomicrographs a t somewhat higher useful magnifications (ca. 2000 diameters) but increases the difficulties attendant upon small depth of focus. The so-called slit ultramicroscope does make it possible to detect the presence of objects which cannot be resolved by the ordinary optical microscope, but not to determine the size, shape, or internal structure of these objects.
-
OBJECT
IMAGE
FIGURE 3. COMPARISOX OF MAGNETIC AND GLASSLESSES
Table I shows that, for purposes of bridging the gap referred to above, the upper useful limit of the optical microscope is about 1400 diameters, leaving the range from 1400 to about 4000 diameters still practically inaccessible. A practical method has been worked out whereby the useful range of the electron microscope may readily be extended on the lower side, so that magnifications as low as those easily obtainable in the optical microscope may be used. With this method neither the useful resolving power nor depth of focus of the electron microscope are lost, and the field is materially increased; thus direct comparisons between optical and electron images a t the same magnifications are possible. In this way the interpretation of electron micrographs at higher magnifications has been greatly simplified.
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Figure 1 demonstrates the fact that resolving power and depth of focus of the electron microscope have not been sacrificed a t the lower magnifications.
Lowering the Magnification The electron microscope as originally sold can be used from 1100 up to about 15,000 diameters (using 60-kv. electrons). Because of the small size of the image, however, the range .from approximately 1100 to 4000 diameters is of limited usefulness. Since all distances between the object and the fluorescent screen of the microscope are k e d , these changes in magnification must be obtained by varying the current in the projection lens (Figure 2), which in turn varies the focal length of this lens by the desired amount. I n order to obtain still lower magnifications without making any major changes in the microscope, such as redesigning the projection lens, it is clear that some modification of the size of the intermediate image produced by the objective lens must be made. As in the case of geometrical optics, so in electron optics we have the relation 1
1-1
:+;-$ where p
distance from object to lens distance from lens to image focal length of lens Also, as in light optics, the magnification of the object by the magnetic lens is equal to the ratio q / p . Figure 3 shows more clearly the analogy between geometric and electron optics. The objective lens in the electron microscope has a pole piece designed to concentrate the magnetic flux and thereby make the focal length as short as practicable. The slight changes in this focal length which are necessary in order to bring a particular object into sharp focus at the intermediate image plane are made by varying the current in this lens. Thus with the image distance q a fixed quantity, it is clear that if provisions can be made for varying the values o f f and p through a sufficient range, almost any desired magnification of this image can be obtained. One obvious way to increase f by a k e d amount is to remove bodily the objective pole piece. i f p is increased proportionately by reducing the length of the brass object holder (Figure 4) from 17 to 10 mm., the range of magnifications from 250 to 2900 diameters can be produced readily by the projection lens. Since, however, the removal of the pole piece is somewhat time consuming, it seemed desirable to work out a more practical method for producing low magnifications. Actually, the focal length of the objective lens can be increased sufficiently to compensate for such a change in p merely by varying the current through the lens. For ex= q = f =
TABLE I. COMPARISON OF MAGNIFIERS Useful Focal Magni- Depth of Limit.oj Numerical Length fication Foous5, Resolutionb, Instrument Aperture Mm. 'Diemete& p P 1 100-200 ii 0.1 * 20 40 g % d lens 10 Binocular microsoope 0.1 32 25 80 2.5 200 0.25 16 4 Research microscope 1 8 0.50 Research microscope 0.9 400 0.5 4 Research microscope 700 0.85 0.3 0.2 Research microscope 4 0.95 800 0.08 '0.26 Oil immersion objective (4200B.) 1.38 3 1,400 0.06 0.15 Ultraviolet objeotive (2700 A.). 1.38 3 2,000 0.04 0.10 Electron microscope lo# 0.004 0 Depth of focus calculated for circle of confusion of 50 P . b Figures given for oblique illumination.
.....
...
FIGURE4. SPECIMENHOLDERS OF VARIOUSLENGTHS OF MAGNIFICATION RANGE WHICHALLOWCHANGE
ample, if the pole piece is left in place, the 10-mm. object holder used, and the current in the objective lens adjusted properly, the variation of the magnification which can be produced by changing the current in the projection lens is from about 490 to 6500 diameters. Specimen holders of other lengths give corresponding ranges'. By changing the brass cap which carries the specimen mount from a holder of one length to another, the same objrect may be studied at any magnification between 500 and 22,000 diameters. The following table shows the results of these changes:
Length of Holder Pole piece out 10 mm. Pole piece in 10 mm. Pole piece in 17 mm. Pole piece in 19 mm.
Magnification Range Produced by Projection Lens 250-2900 490-6500 1100-14,400 2000-21,900
Max. Field Diameter a t Lowest Magnification. P 72 37 16 9
The electron micrographs shown in Figures 5, 6, and 7, when judged from the points of view of resolving power, depth of focus, field size, ease of orientation, and identification of any one small object, illustrate the advantages of being able to use the electron microscope at low as well as high magnifications.
Methods of Calibration Depending on Visual Microscope The problem of calibrating the magnification of the electron microscope accurately has proved to be a rather serious stumbling block in applying the instrument to quantitative studies of small objects such as pigments. This is a field of immgnse importance, and the high resolving power and great depth of focus of the electron microscope make this new tool ideal for the purpose, provided accurate linear measurements can be made. Therefore, a considerable amount of time has been devoted in this laboratory to a critical study of the various methods of calibration. The method most frequently used for calibration of the electron microscope is to make a direct comparison of photomicrographs of some well-defined specimen with electron micrographs of the same specimen. Thus, an easily recognizable portion of the test specimen is observed in the electron microscope and photographed over the entire range of available magnification by varying the projection lens current. The sample is then removed and the same portion of matter is observed in a visual microscope. Since the calibration of the visual microscope can be made by comparison of a standard stage micrometer with its image on the photographic ground glass, the dimensions of the selected test 1 Since the completion of this work, R. C. A. has suggested a similar method for obtaining low magnifications in their "Eleotron Microscope Note 2" furnished to all users of the R. C. A. electron microscope.
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MICROGRAPHS OF BLUE DYE PIGFIGURE 5. ELECTRON MENT, SHOWING THE LINEARITY OF ELECTRONIC MAGNIFICATION AND POSSIBILITY OF IDENTIFYINQ AND STUDYING A
SINGLESMALLPARTICLE AT VARIOUS MAGNIFICATIONS (a) 140 X (complete 200-mesh opening). ( b ) 250 X . (c) 1080 x * ( d ) optical enlargement of (a) t o 3000 X I d o w i n g the ekcellent resolhion of the low-magnification electron micrograph.
object are readily measured. It is easy then to determine the magnification range of the electron microscope by carefully measuring the dimensions of the test object as recorded on successive photographic plates, corresponding respectively to various values of the current in the projection lens. This procedure of calibration is open to criticism from several points of view. In particular, the absolute accuracy of the visual microscope in measuring the size of particles decreases rapidly as the particles approach the limit of resolution of the microscope. It is obvious that any error in measuring the selected test specimen by means of the visual microscope will impose the same error upon the ultimate calibration of the electron microscope. The precision of the visual microscope improves as larger particles are measured. With the electron microscope as it is delivered, the maximum field diameter is approximately 16 p. Moreover, an object larger than about 2.5 p (the maximum field diameter a t 22,000 diameters) necessitates a two-step calibration. That is, the electron microscope must first be calibrated internally by photographing some well-defined particle which is visible over the entire magnification range of the instrument. Then by photographing another particle which approaches 16 p in diameter, the actual magnification of one of the projection lens “stops” can be determined by comparison of this electron micrograph with corresponding optical measurements. Thus, the ordinate of the internal calibration will be set. By using the methods which have been outlined for obtaining low magnifications, it is possible to increase considerably the precision of this method of electron microscope calibration. With a 10-mm. specimen holder and with the objective pole piece removed, it is possible t o view a maximum field of
Vol. 34, No. 12
FIGURE 6. FURTHERELECTRONIC hf.4GNIFICATION PIGMENT PARTICLE SHOWN IN FIGURE 5
OF
(a)2500 X : ( b ) 4000 X ’ ( c ) 5400 X :. ( d ) optical enlargement of (c) t o give t o h magnification of 19,000 x
approximately 72 p. This means that some large objectfor example, a single opening of the 200-mesh specimen support-can be viewed in its entirety as shown in Figure 5. A visual microscopic measurement of one of these 60-70 p openings is much more accurate than a measurement of a 2.5 p particle, If it can be proved that the electron microscope is a linear magnifier-that is, if the relative magnifications of any two particles of different sizes, regardless of the absolute magnification, is constant-then it should be possible to obtain a fairly good calibration of the instrument by comparing magnified images of some small, well-defined particle supported in the opening of the mesh with the measured diameter of the mesh itself. After making many measurements on a number of specimens of widely differing types, the linearity of electronic magnification has been thoroughly established. Thus, this technique of “calibration a t low magnifications” is a definite improvement over the methods previously described. However, even this last procedure of calibrating has a definite limitation which is imposed by the relatively small depth of focus of the visual microscope. An electronic image of one of the wire mesh openings shows a sharp, well-defined silhouette of the hole. An optical image of the mesh, on the other hand, appears fuzzy, and the image of the opening can be varied in size by focusing the microscope up or down. Thus, an uncertainty is introduced into the final calibration.
Nonoptical Means of Calibration Because of the severe limitations inherent in any method for calibrating the electron microscope which is dependent on the visual microscope, other methods have been sought. The first technique suggested was that of determining a critical dimension by weight. For example, an extremely
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This work suggested the possibility of using replicas of diffraction gratings as a means of calibrating the electron microscope, since the actual grating spacing of the replica could be determined rather accurately by observing the spectrum formed in a spectrometer. The same replica could then be observed in the electron microscope at various magnifications. Several methods have been suggested in the literature for preparing replicas suitable for eyamination in the electron microscope. Zworykin and Ramberg advocate the “double replica” process in which a thin layer of silver is deposited on the surface being studied. This is then stripped off, and a dilute solution of collodion is flowed over the replica surface and allowed to dry. The silver can be selectively dissolved in nitric acid, leaving a collodion replica which is sufficiently thin to be studied successfully in the electron microscope. This method, although apparently satisfactory, 500 X 1000 x is rather tedious and involved. FIGURE7. PHOTOMICROGRAPHS OF THE FIELDSHOWN IN FIGURES 5 A much more rapid method consists in mechaniAND 6 cally stripping a thin film from the surface of The optical objective had a focal len th of 4 mm a numerical aperture of 0 96, and the specimen. This leads to a negative rather a depth of focus of about 0.08 p . T i e sample w& mounted pn a nitrocelluiose,film stretched across an opening in the metal screen used for takin the electron microthan a positive replica of the surface. The chief graphs. The photomicro raphs show that the depth offocus ,of the qbjective IS !esa objection to this method is the possibility of than the depth of field. getter optical results are obtained using a microscope slide but the present photomicrographs serve as a direct com arison with the eleotro; distorting the film during stripping. By careful micrographs shown in Figures 5 an8 6. manipulation of the film, however, any distortion can be minimized, and experience has shown that results can be duplicated. Choice of the replica material is an extremely important fine quartz fiber (e. g., . .t
I-5 v) z?
U0 cF
%
Y
DISTANCE ON NEGATIVE (miIli met ers) FIGURE 9, DENSITOMETER TRACEACROSS NEG.4TIVE E R E 8B
OF
FIG-
The spacing between the minima of the curve is 4.2 mm. Since the grating space was found t o be 0.83 p, the magnification of the electron micrograph was 6100 diameters. The positive print shown in Figure SB has been optically enlarged to 12,400 diameters.
Formvar solution (0.5 per cent in ethylene dichloride) is flowed directly on the grating surface and allowed to dry. By carefully immersing the grating surface under a wellswept water surface, the Formvar replica is floated off. A 200-mesh nickel screen support is dropped on the film and is picked up, following the usual techniques which hare been frequently outlined. l17hen the grating replica is observed in the electron microscope, a well-defined image of the surface is seen, as shown in Figure 8. Five different strippings from the diffraction grating showed not more than one per cent variation in the grating spacing as measured on the electron micrographs ; this indicates the possibility of duplicating the calibration whenever it appears desirable. The possibility of shrinkage of the Formvar replicas was carefully considered. To check this, several additional replicas were prepared by the method outlined above. They were picked up from the water surface by a somewhat different procedure. An iron washer having a hole of roughly one-half inch diameter was thinly coated on one face with rubber cement. After the cement had become tacky, the
sin 0
sin 45" 7'
0.7056 =
'
A similar set of measurements was also made on the original reflection grating. I n this case the line at 5896 A. was observed and the average diffraction angle found to be 44" 20'. A calculation analogous to that outlined above gives for the original grating spacing d = 0.844 ,u These results indicate that there is approximately 1.5 per cent shrinkage of the Formvar replicas. However, this shrinkage appears constant since, as noted previously, not more than one per cent difference has been observed in the spacing of lines as measured on the electron micrographs.
(millimeters)
MAXIMUMELECTRONIC AS A FUNCTION OF THE MAQNIFICATION SPECIMEN HOLDER LENGTH
FIGURE11.
--
..
PROJECTION L E N S CURRENT (mliliomperes)
FIGURE 10. CALIBRATION CURVESFOR ELECTRON MICROSCOPE
ObTTiously, once the spacing between lines of the replica grating has been determined and once the fact has been established that the shrinkage of the films after being formed is negligible, a magnification calibration over the entire range of the electron microscope can be readily obtained merely by making a set of electron micrographs while varying the projection lens current. One difficulty in obtaining calibrations in this way is that a t extremely high magnifications the lines of the grating become more diffuse, and it becomes somewhat difficult to obtain an accurate measure of the separation of lines in the magnified image. There are at least two possible methods of eliminating this difficulty. First, and perhaps simplest, is the procedure of initially making an accurate internal calibration of the electron microscope. To accomplish this, i t is necessary only to select some well-defined test specimen which can be observed over the entire range of available magnification. By making a series of electron micrographs, these images can be measured rather accurately and a precise
December, 1942
INDUSTRIAL AND ENGINEERING CHEMISTRY
internal calibrat i o n obtained. By observing the grating replica, it is possible to determine accurately the actual magnification a t any arbitrary value of projection lens current. Thus the ordinates of the internal calibration curve can be set on an absolute scale of linear dimensions. A second method is also capable of yielding satisfactory results. B y using a microdensitometer, the contours of the lines molded in the replica can be determined by measuring the FIGURE12. ELECTRON MICROGRAPH OF ACTIVATEDCHARCOAL ( X 6300), recorded density ILLUSTRATING USE OF DIFFRACTION in the electron GRATING REPLICAS AS INTERNAL micrographnegaSTANDARDS OF MAGNIFICATION tives. Thus, Figure 9 shows densitometer curves made on the negative shown in Figure 8B. The separation between lines on the magnified image can be accurately determined by measuring the distances between the minimum points of the curve. Figure 10 shows a set of calibration curves obtained in this way for the various specimen holders while using 60-kv. electrons. It is obvious that the maximum attainable magnification without changing lenses is 21,900 diameters. This could probably be increased somewhat by using a specimen holder slightly longer than 19 mm. However, such a procedure would entail making certain major changes in the microscope itself. These changes have been unnecessary in the work undertaken in this laboratory. Several additional precautions are necessary in any study where an exact measure of the electronic magnification is necessary. Figure 11 shows the variation in the maximum attainable magnification as a function of the holder length, the focal length of the projection lens being kept constant. Since this curve is so steep in the vicinity of 17 mm. (the length of holder furnished with the instrument), i t is important that the object holder assembly be seated properly in the microscope and that the brass cap be mounted firmly in place. A slight change in the object distance p , no matter how it is produced, can lead to a considerable change in the final magnification. It is also important that the objective current and the accelerating potential of the electrons be noted during the calibration procedure. For exact work it is necessary that these values be reproduced in subsequent pictures. Since the magnification of the objective lens at constant electron voltage is approximately proportional to the square of the objective current, it is possible to make corrections to the value of the magnification if the objective current varies from the value at which the instrument was calibrated.
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It will also be found that the calibration will change slightly with time as a result of other causes. These consist mainly of changes in the projection coil current and changes in the electron accelerating voltage as the standard batteries in these two systems become older. Therefore, it is advisable to recalibrate the instrument before any work is done which requires extreme accuracy. A diffraction grating replica prepared as described above can be mounted permanently in a specimen holder. Thus, a t frequent intervals this holder can be inserted in the electron microscope and a new calibration readily prepared. The shrinkage of these films over long periods of time is being investigated in an effort to determine the feasibility of using a good replica as a permanent standard. Another method which is possible whenever high accuracy is essential is to use the grating replica itself as a support for
FIGURID13. ELECTRON MICROSCOPE, SHOWINGTHE TELESCOPE USED IN FOCUSING THE FINALIMAGE AND THE PERISCOPE IMAGE WKICHVIEWSTHE INTERMEDIATE
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INDUSTRIAL AND ENGINEERING CHEMISTRY
the material being studied. In this way the replica will serve as an internal standard of calibration. Figure 12 illustrates the possibilities of this method of determining precise linear dimensions. This procedure of using an internal standard will entail some slight loss of resolution because of the introduction of chromatic aberrations by the replica film, which must be somewhat thicker than the usual thin films. However, for objects which are far above the limit of resolution of the electron microscope, the chromatic aberration can be neglected.
Vol. 34, No. 12
When low magnifications are used, it has sometimes been found difficult to focus the images on the fluorescent screen. A low-power telescope (2.5 diameters) focused on the viewing screen has proved helpful in arriving at the optimum focus. As Figure 13 shows, the telescope has been inserted through one of the observation ports of the electron niicroscope. The optical system of the telescope is such that the fluorescent screen is viewed almost normally. The periscope arranged to view the intermediate image has also been of help in orienting specimens.
Corrosion of Metals and Alloys by Flue Gases J
LOUIS SHNIDMAN AND
J
JESSE S. YEAW Rochester Gas and Electric Corporation, Rochester, N. Y.
F
LUE products, resulting from the combustion of most industrial fuels, exhibit a surprising activity with respect to the corrosion of materials with which they come into contact from the time they leave the burner or fuel bed until they are finally discharged into the outer atmosphere. The presence of the oxides of sulfur has generally been justly blamed for much of the difficulty (1-11). The suggestion has been made that the oxides of nitrogen, which always occur in small quantities in the products of combustion, catalyze, accelerate, or otherwise promote the activity of these vapors (11). A field study (2) conducted recently, in which a total of 783 domestic units in ten cities was inspected, indicated that: 1. The condensation of vapors, which are formed during the combustion of fuel gases containing sulfur, results in the corrosion of combustion chambers and flue pipes. 2. Other constituents of the products of combustion do not appear to be significant from a practical standpoint with respect to corrosion. 3. Sulfur must be reduced to less than 2 grains per 100 cubic feet (4.6 grams per 100 cubic meters) to eliminate corrosion problems in the field (Figure 1).
Section of Coke Conueyor
It has been demonstrated that the elimination of sulfur from fuel gases does not necessarily eliminate corrosion ( I I ) , but the field survey (9)did reveal that corrosion by sulfurfree gases would not create serious field problems. The complete removal of sulfur from industrial fuels is not practicable a t present. The substitution of corrosion-resistant materials for those which are susceptible is one possible method for overcoming this corrosion problem, and the testing of certain metals and alloys in flue gas atmospheres is the subject of the following discussion.
