INDUSTRIAL AND ENGINEERING CHEMISTRY
2892
viscosities to intrinsic viscosities; experimental errors encountered during the milling and in the analytical procedures; and variations in the types of polymer. When these factors are taken into consideration, the agreement in the calculated values shown in Tables XII, XIII, and XIV for the relative number of polymer chains attached to each unit of black is believed t o constitute reasonable substantiation of the assumption. Thus the polymer apparently attaches to the black at one, or a t most a very few, of the functional groups of the polymer chain and the number of chains is relatively independent of the loading of black above 25 parte per 100 parts of rubberprovided the polymer does not contain gel of a type that Fill not enter into the black complex and the molecular weight of the bound polymer is between 200,000 and 1,000,000 LITERATURE CITED
(1) Baker, IT. O., and Walker, R.
W.,private communication to
Office of Rubber Reserve.
(2) Dannenberg, E. M., and Collyer,
Vol. 43, No. 12
H.J., IND.ENG.CHEM.,41,
1607 (1949). (3) Drogin, I., and Bishop, H. R., “Today’s Furnace Blacks,’’ Charleston,W. Va., United Carbon Co., Inc., 1948. (4) Drogin, I., Bishop, H. R., and Wiseman, P., India RubbeT W o r l d , 120,693-7 (1949); 121, 67-66 (1949). ( 5 ) French, D. M., and Ewart, R. H., Anat. Chem., 19, 165-7 (1947). (6) Johnson, B. L., Iun. ENG.CHEX., 40,351 (1948). (7) Kennedy, T. J., and Higuchi, T., private communication b (8) (9) (10) (11)
Office of Rubber Reserve. Kolthoff, I. M., Gutmacher, R. G., and Kahn, A,, Ibid. Kolthoff, I. M., and Kahn, A., I b i d . Mullen, J. W., and Baker, W. O., Ibid. Sperberg, L. R., Svetlik, J. F., and Bliss, L. A , IND. ENQ.
CHEM., 41, 1641 (1949). (12) Sweitzer, C. W.,Goodrich, W.C., and Burgess, K. A,, Rubber Age, 65, 651-62 (1949).
RECEIVED March 7, 1961. This work was sponsored by the Office of Rubber Reserve, Reconstruction Finance Corp., in connection with the government synthetic rubber program.
anical Breakdown of SoapR. J. MOORE AND A. M. CRAVATH Shell Development Co., Emeryville, Cui$. o n e of the important practical characteristics of a soapbase lubricating grease is its progressive softening upon prolonged mechanical agitation. This investigation was undertaken in order to find out what causes this softening and what determines its rate. It was found, from electron micrographs of sodium-, calcium-, lithium-, and barium-base greases, that breakage of the soap fibers occurred during work softening. The ratio of length to diameter of the fibers was the factor of predominant influence on consistency, and its change when the fibers were broken appeared to explain the observed change in consistency. Usually, the consistency (reciprocal of the Shell microcone penetration) was found to be a linear function of the logarithm of the time of working. A theoretical explanation of this relation, which seems to apply to many other cases of the mechanical degradation of small particles, shows that the law is approximately followed when the breakage of particles is due not to gradual fatigue or wear, but to the fluctuating stresses to which the particles are subjected and which occasionally reach unusually high values exceeding the initial breaking strengths of the particles.
T
HE changes in consistency of lubricating greases which occur when they are subjected to shearing forces have been the subject of considerable speculation. Chemical and/or physical changes in the system soap-oil which comprises the usual greases have been alluded to in order to explain observed changes in consistency, but on the whole the system and its mechanical degradation have been poorly understood. Solvation of soap by oil, for example, has been a common expression in this field, as has hydration or dehydration as the occasion demanded; the use of the term dehydration stems no doubt from the chemistry of lime-base greases which show a marked change on going from the hydrate to the anhydrous form. More recent work has established that most soap-base greases are true gels, or two-phase systems comprising cryetalline soap
and more or less soap-free oil (7, 9). The electron niicroscope studies reported by Farrington and Birdsall ( 1 ) defined the form taken by the soap-namely, a mass of fibers of about 0.1- to 1.0-micron width and of varying length up to 0.1 mm. These microscopic soap fibers should not be confused with the visible structures in the so-called fiber greases, such as some of the sodabase greases. -4lubricating grease can be defined as an aggregation of soap crystals in which oil is held by capillary forces, and the specific grease properties-consistency and non-Newtonian floware a consequence of the distribution of the soap in fine filaments or fibers. The present paper describes the mechanical breakdown of grease in terms of the disintegration of the fibers constituting the thickening agent as observed n-ith the electron microscope. EXPERIMENTAL
In evaluating the various devices used to effect mechanical breakdown of greases, it became apparent that while the rate of breakdown might differ markedly, the mechanism was the same. Accordingly, most of the work described here was carried out with the Shell roll tester, although similar results were obtained with other types of apparatus. The shear conditions of the roll tester are not completely known, but the severity correlates well with field experience with truck wheel bearings. By using this apparatus, a convenient amount of grease-about 75 grams-can be thoroughly and reproducibly sheared to cause breakdown in a reasonable time. At convenient intervals the grease samples were removed from the roller and penetration values were obtained with the Shell microcone ( 2 ) . This is a 74” aluminum cone 21 mm. high, with a weight such that the total weight of cone and plunger of the ASTM penetrometer is 58.3 grams. This cone is preferred because of its accuracy in the consistency range of interest-Le., between manufactured consistency and failure-and also because as a simple cone, in contrast to the complex ASTM cone, it gives rise to penetration values, whose reciprocals are roughly a linear function of yield value (6). I n making a consistency determination, the grease is immediately removed from the rolling apparatus and quickly brought to 25” ct 0.2” C. by placing the grease on a steel plate in a refrigerator. The grease is then loaded into the test cup and promptly tested. In this way variable results due
December 1951
INDUSTRIAL AND ENGINEERING CHEMISTRY
to hardening or thixotropy are minimized. Direct com arison OF micropenetrations with the usual ASTM penetration dPepends on the type of grease being tested, but in general a micropenetration of 230 dm. corresponds to about 390 to 400 ASTM. In the present work the limit of useful consistency from the standpoint of grease lubrication has been arbitrarily taken as a micropenetration of 230 dm.
z
'-1
I
c
ROLLING TIME, HOURS
Figure 1. Mechanical Stability of Commercial Greases
I
5
10
ROLLING T I M E , HOURS
2893
Selected examples of the electron micrographs obtained from each mechanical stability test are shown in Figures 3 to 7. Specimens for the electron micrographs shown here were prepared from the barium grease by simply smearing with a glass fiber over the Formvar film on the specimen screen. The other samples were diluted with a little xylene or cymene before smearing and were washed with heptane after the diluent evaporated. When excessive mechanical work was avoided, specimens prepared in other ways showed the same fibers, but often had inferior resolution and contrast and also had more tendency for the fibers to be concentrated in separate clumps instead of uniformly distributed. Study of the individual micrographs shows some striking changes in the length of the soap fibers on rolling. In the hydrous soda-base grease shown in Figure 3, a many-fold change in average length takes place as the ASTM penetration increases from about 240 to >400. The set of micrographs in Figure 4 representing the changes on working a lime-base grease shows only a small average shortening of the fibers after 336 hours in the roll tester, and significantly, only a relatively small change in consistency of the grease was observed during this interval. The barium grease prior to working differed somewhat in that the unworked grease fibers shown in Figure 5 are very fine, less than 0.01 pin diameter, and are gathered in skeins several microns long. This grease also contains many dense lumps, up to 1p in diameter, which are probably barium carbonate and other salts that appear by virtue of the severe oxidizing conditions used in the grease preparation. After 98 hours working in the ASTM motor-driven worker, which is equivalent to about 24 hours in the roll tester, most of the fibers are in skein fragments as shown in Figure 5. The most obvious change in fiber structure observed coincident with the decrease in consistency is the progressively lower length to width ratios of the soap fibers. By analogy with the extensive work on the relationship of length to diameter ratio of high poly-
A
Y
Figure 2. Mechanical Stability of Anhydrous Soda-Base Multipurpose Grease Numbered arrows designate figures for aorresponding electron micrographs
In expressing the penetration results, it was found convenient to define consistency as the reciprocal of the micropenetration, which would then be directly proportional to yield value as demonstrated earlier by Schroeter (6). These data are shown plotted in Figures 1 and 2 as reciprocal penetration vs. log time. It has been pointed out by McLennan and Smith (3) that greases actually have no true yield value and in the broad sense this is true; from a practical standpoint, however, the yield value of a grease is that stress below which no movement is observed in the time of interest in a specific lubrication problem. Five types of greases were examined and the data are summarized in Table I.
TABLE I. PROPERTIES OF GREASESEXAMINED Initial ASTM Pene- Soap, Soap Type trstion % ' Oil Component Sodium hydrous 240 80/210° 8 U 9 California 25 V I calciud 310 a 300/100° S.U.S., California 25 V I Barium 245 a Sodium, multipurpose type 300 8 500/100° S.U.S.,Cal/forn/&55 V I Lithium, multipurpose type 280 6 500/100° S.U.S.,California 55 V I a Commercial, not determined.
C
B
Figure 3. Electron Micrographs (X4000) of RollTested Hydrous Soda-Base Grease A . Unworked B. Roll tested 1 hour C. Roll tested 4 hours
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
2894
A Figure 4.
Vol. 43, No. 12
B
Electron Ilicrographs (X8000) of RollTested Lime-Rase Grease A . Unworked B . Roll tested 336 hours
mers and the viscosity of theii solutions ( 6 ) , it may be inferred that the consistency of a soap grease is also related to the anisometry of the fibers. Confirmatory evidence for this hypothesis is seen in the breakdown of the multipulpose soda-base grease, data and micrographs for which are shown in Figures 2 and 6. In this case, initial fiber breakage and loll-ered consistency are followed by fiber shredding which gives rise to a higher length to width ratio and an increase in coiisistency. Ultimately, even these finer fibers were broken and consistency again became less. Finally, this degraded grease can be reheated to dissolve the soap and cooled to permit the fibers to reform, after which the entire cycle can be repeated. If loss in consistency is caused by breakage of the soap fibers, an obvious goal in grease compounding would be the formation of fibers whose tensile strengths would exceed the stresses encountered in a given service. This was at least partially realized in an experimental lithium multipuipose-type grease-lithium 12-hydroxystearate soap-in vi hich the consistency reached a micropenetration plateau of 195 din. after about 100 hours. Subsequent roll testing for 1000 hours did not decrease the consistency further. The resistance to breakdoan of the fibers is evident in Figure 7 , taken of this grease after 336 hours of rolling. The fresh untvorked grease is also shown in Figure 7 . The relative stability and dimensions of the soap fibers can also be correlated with the amount of soap required to compound a grease. A much higher concentration of the relatively fragile hydrous soda soap fibers is required to achieve even moderate mechanical stability compared with soda-base fibers found in the multipurpose-type grease. The weight ratio of soaps in two examples is in excesE of 4 to 1 to achieve the same mechanical stability as measured by the roll test. Lime-base grease, on the other hand, has relatively stable fibers below the dehydration temperature, but their length to width ratio is low, so that a high concentration is needed to achieve a useful consistency. THEORY OF THE MECHANISM AND RATE OF BREAKDOWIV
In describing the change in consistency with time of shearing, a linear relationship was observed in many instances between reciprocal micropenetration-Le., yield value-and log time. [Although it has been reported that the penetration itself versus log time gives rise t o a straight line (S),this is not true over a significant range of penetrations. ] This linear relationship held not only for shearing in the roll tester but with the ASTM worker, as well as with stirred kettles. The only difference was in the slope of the resulting breakdown curve. Examination of empirical data from other fields, such as the beating of wood pulp in paper mills (8) and the ball milling of minerals, also showed linear relations between some variable related to the particle size or shape and logarithm of time. An example closely related to grease breakdown is that of the decrease in the viscosity of a polymer
B
Figure 5 .
Electron Micrographs (X6500) of RariumBase Grease A . 7hw-orked E . 98 hours in ASTM worker
solution with shearing. The viscosity is known to be related to the average length of the polymer molecules present (6) and a qualitative discussion of changes taking place has been recently presented ( 4 ) . Thus the mechanical breakdown of dispersions of small particles often seems to follow a log-time law. It is of interest to find the physical basis of this law as a step toward understanding the mechanism in those systems where it applies. Fundamentally the problem is that of the relation between the breaking stresses of the particles, the frequencies with which these stresses are produced, and the effect of breakage on the viscosity or other observed property of the system. If viscous drag is responsible for the stress, a longer fiber, such as a polymer molecule or soap crystal, is subjectd to a greater cumulative pull than a shorter one in a region of the same fluid stress. In a ball mill, a larger particle has greater chance than a smaller one of being crushed between colliding balls. Since, however, in the complicated systems of practical interest there is a distribution of stress from frequently occurring low values to much higher values that occur rarely, even a small particle has a finite, though small, chance of being broken. The rate of breakdown will decrease continually as the size of the fibers present decreases. -4variable such as viscosity or penetration will not fall off exactly as a linear function of logarithm of time and the exact relation would be too complicated to calculate. However, the significance of the validity of the empirical relation as a good approximation can be euplained. If y is the measured variable dependent on particle size, for instance, consistency, the empirical relation is y = a - b In t with a and b constants. This is equivalent to
Hence another way of describing the empirical relation is to say
INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1951
2895
D
Figure 6. Stages in Fiber Degradation of Experimental Multipurpose Sodium-Base Grease Unworked, X8000 B . After 22 hours rolling, X8000 C. After 314 hours rollinp.. X 16.000 D . After 480 hours rolling, X 16,000
A.
that the rate of decrease of the consistency or other measured variable which depends on particle size falls off as an exponential function of the variable itself. By making some approximations this relation can be transformed so that its significance is more easily understood. The ranges over which y varied in the sets of data found to obey the law were usually not over 2 to 1. It is assumed that, over this small range, the rate at which y decreases is simply proportional to the rate of particle breakage, and also that y = g hZ where g and h are constants and is an average particle length. Hence
+ Figure 7 . Fiber Degradation of Experimental Lithium Multipurpose Grease (X7000) A . Unworked B. Rolled 336 hours
or (3) where k and c are constants. The empirical relation implies that the rate of particle breakage falls off about exponentially with average particle length. Though this derivation was restricted to a small range-about 2 to l - o f y and E, it is not restricted in the range of dyldt, which varied 100 times in some of the empirical tests. The relation is not obtained merely by restricting the range of the variables so much that any two-constant equation could be fitted. The rate of breakage often varies exponentially with particle size because it is often the frequency of a relatively rare event that happens only when a number of randomly varying factors combine to cause a stress exceeding the breaking strength. In the grease and polymer solution examples, these factors include fiber dimension, orientation, and position, and fluid strain rate. In ball milling, the radial and tangential velocities of the colliding balls, and the particle dimensions, position, and orientation are involved. It is the rule rather than the exception for a quantity determined by a number of such statistically fluctuating variables to exceed a given limit with a frequency that varies approximately exponentially with the value of the limit in the region of
the tail of the distribution function where a large change in frequency resultb from a small change in the limit. Examples are the rate a t which molecules are given activating energies in a rate process; the frequency of large errors in any normal Gaussian error distribution; and the number of unusually high speed molecules in a gas. Since the fluid stress required to break a particle increases gradually as the particle size decreases, the size is a measure of that stress. The relation will be approximately linear over a Timited range. Therefore whenever the breaking frequency varies about exponentially with the necessary fluid stress, and this is usually the case, then the rate of breakage will also vary approximately exponentially with particle size and the empirical relation to logarithm of time will hold approximately. Alternative choices for the variable y-e.g., y = consistency or alternatively its reciprocal, penetration-will give different shapes to the curves of y us. In t. In some cases the different errors so compensate that y us. In t gives a fairly straight line over a relatively large range. In any case, however, the above discussion shows why the plot of y us. t is very sharply curved and why replacing t by In t removes most of the curvature. Conversely, the validity of the In t law for a particular system is an indication that the breakage of the particles is caused by sudden, relatively un-
2896
INDUSTRIAL AND ENGINEERING CHEMISTRY
usual accidents, whose frequency obeys an exponential probability law, rather than by the cumulative effects of many ordinary encounters acting through wear or fatigue. In the remainder of this paper the analysis presented above is carried out in more detail. That the y = a - b In 1 relationship follows when certain simplifying assumptions are carried to extremes will be proved and a more complete explanation will be given of why and when the In t relationship is followed by real systems. The extreme simplifying assumptions are that the particles are strips all having the same width, thickness, and strength, and thus differing only in length, L; that a particle breaks when its stress reaches a fixed value; that the changes of y and of aver-
Vol. 43, No. 12
haved two-constant expression. The use of q = iejL, where i and j are constants, is convenient. As the degradation process continues, the size distribution curve, rn(L),shifts toward smaller values. To help visualize this process, an imaginary possible succession of m curves has been drawn in Figure 8. An initial curve, labeled In t = - m, was assumed. The chance of breakage in time, dt, for a fiber of length L was assumed to be Pdt = eLdt,and the breaks were assumed to be near the middles of the fibers. From these assumptions, the m t o In t = change occurring in the time interval from In t = - 18 was calculated. Similarly the remaining curves were calculated, each from the preceding. The shaded area shows a change during a doubling of the time. From one solid curve to the next, the time increases e2, which is 7.4 times. Since P varies rapidly with L, only the longer particles of those present at a given time are being broken at an appreciable rate. The much lower rate of breakage of smaller particles becomes important only later on when there is no longer an appreciable number of more rapidly breaking larger particles, and the time interval required for an appreciable change of the system has become much greater. Hence, an approximation to m in the integrand need be valid only over the righthand part of the distribution curve. As such an approximation m ( L ) = m,(L u)ekuwas chosen by sliding an earlier curve mo(L) to the left a distance u and magnifying its ordinates by a rather slowly varying factor ekuin which k is a constant. The adequacy of this approximation for the simplified grease is easily shown. The fibers will usually break near their middles. Therefore m ( L ) receives most of its addition from breakage of fibers of about 2L, at the time when the latter are breaking most rapidly. P ( L ) is so much smaller than P ( 2 L )that most of the breakage a t 2L and addition to m ( L ) , is completed before much breakage at L. During the time that most of the breakage is occurring,
-
+
L
Figure 8.
Successive Length Distribution Curves
age L are small enough to give whatever accuracy is required in various approximations which become exact for infinitesimal ranges; and that P , the probability per unit time that a given particle be subjected to a stress, s, or greater, varies as P = c1e-c2s, and so rapidly that even a small change in s alters the probability manyfold. However, it appears that all that is really necessary for validity of the In t law is that when the particles are classified according to their breaking probabilities per unit time, PI then q ( P ) ,the change in y caused by breaking a gram of particles of given P and representative distribution of length and other characteristics, be related to P by P = ae-bg with large enough b. No high accuracy of the approximations is required, for the present theory is not capable of predicting more than that most of the very sharp curvature is eliminated when y is plotted against In t instead of against t. It has been assumed that P = cle-C@o for a fiber of length LO. For another fiber of greater length, L, the probability of getting into the same environment or fluid stress is the same, but the resulting fiber stress is greater by a smoothly varying factorf(L)that is, s = sof(L). Hence P = cre-Cz8/f(L). If we approximate l/f(L) by cQ- c4L,and let 86 be the breaking stress, thentheprobability of breaking per unit time becomes P = cle-Cz*a(ca-c4L) = aleb+. This relation actually applies much more generally than the present derivation. If mdL is the weight fraction of the particles having lengths between L and L dL,and p(L) is the change in the observed property, y, resulting from the breaking of unit mass of particles of length L, then
+
(4) The function p(L) may change somewhat with time as the size distribution, m,changes, but over the limited range involved this change may be neglected. The variation of q with L will be regular and small compared to the rapid variation of P with L and p may be approximated closely enough by any reasonably well-be-
m ( L ) = nzBe-Pt where me is independent of t and is approximately the envelope of the m curves. Introduction of the previous expression for P and rewriting gives
m ( L ) = n ~exu , i--ale%)
If u = l/h In(t/t,), the preceding equation becomes
If m,(L) K e - k L , m,(L)/m,(L f u ) = e - k L k(L + = e k u exactly. Most of the change in rn is given by the mo(L u)teim. Therefore the slowly varying term me(L)/mB(L u ) does not require high precision. Moreover me actually is concave upward and comparatively slowly varying, so that me a e - k L is a good approximation over a considerable range. Thus the whole approximation m(L) = m,(L u ) e k u i s shown to be quite accurate when the assumptions here made for the simplified grease apply. However, validity of this approximation is not restricted to cases where all particles have the same width and thickness. It suffices that the righthand part of the distribution curve be carved off rather steeply with a shape largely determined by the rapid exponential variation of breakage rate with particle size. Then the shape will not change rapidly as long as the same exponential describes the variation of breakage rate. The change will be largely a displacement to the left; what change in shape does occur will be small enough to be approximated by the magnification term eku. Introduction of the approximations for p, P, and m gives +
+
+
+
(7)
December 1951 =
=
INDUSTRIAL AND ENGINEERING CHEMISTRY
Lrn + + Lrn + q(L
e@ - j - b3u
u)e-juP(L
q(L
u)e-blum,(L
u)P(L
+ u)ekudL
+ u)m,(L + u)d(L+ u )
The integral is practically equal to the constant
JI;"
for P practically vanishes for the small values of the argument between o and u. Hence
where a2 and bs are constants. Now y depends on the distribution curve m(L) in such a manner that for a series of successive equally spaced curves, approximately equally spaced values of y will result if the range covered is not too great. Moreover, the way in which y depends on the distribution will not involve any sudden changes which would severely restrict this range. Since equally spaced rn curves mean equally spaced values of 21 and also equally spaced corresponding values of u, u is a linear function of y and hence
-9 = aze-b2cs(y dt
- vo)
= ase-bsv
which the particles of a given L were identical, and then show that the contribution of each species to the change of y was linear in In t, and hence that the sum total change of 21 was linear in In t. It is simpler, and appears sufficient for the present, to replace the actual size descriptive variable L in the preceding argument by
L' = qPmodL,
(8)
which has been shown to be equivalent to the empirical law y = a - b In t. Although it was assumed to simplify the preceding discussion that all particles of the same size L are identical, it is not necessary to do so. One way of generalizing the theory might be to divide the particles into separate species within any one of
2897
1
In P/al so that particles of bi
a given L' will all have the
same P,but will have the size L' only on the average, individuals varying in L. If it is assumed that this spread of L about the average L' does not prevent y from depending in a smooth and regular way upon the distribution curves, now m(L') instead of m(L), then the preceding argument still holds for the real case where the particles are not completely described by a single size variable. ACKNOWLEDGMENT
The authors gratefully acknowledge data obtained from G. F. Green, Fiberboard Products, Inc., Port Angeles, Wash., on experimental ball mill testa of selected wood pulp types. LITERATURE CITED
(1) Farrington and Birdsall, Oil Gas J., 45, No. 46, 268 (1947). (2) McFarlane, Inst. Spokesman, 6, No. 12 (1943). (3) McLennan and Smith, ASTM Bull., No. 152, 71 (1948). (4) Morris and Schnurman, Nature, 160, 674 (1947). (5) Sohroeter, Congr. mondial pbtrole, 11,434 (1933). ( 6 ) Simha, J.Applied Phys., 13, 147 (1942). (7) Stross and Abrams, J . Am. Chem. SOC.,73, 2825 (1951). (8) Tech. Assoc. Pulp Paper Ind.,Standard Testing Methods, Designation T-224 sm-45. (9) Vold, Hattiangdi, and Vold, IND. ENC.CHEM.,41, 2539 (1949). RECEIVEDSeptember 19, 1950. Presented before the Division of Colloid CHEMICAL SOCIETY, ChiChemistry at the 118th Meeting of the AMERICAN cago, Ill.
Corrosion of Steel in Molten
Sulfur ANDREW DRAVNIEKS Engineering Research Department, Standard Oil Co. (Indiana), Chicago, Ill. Corrosion of steel vessels by hot sulfur-bearing media causes constant difficulties in many industrial processes. Little is known about the basic corrosion reactions involved. The simplest ease of this type is studied in the present work. Between 300" and 450" C. the corrosion of steel in sulfur is limited by two processes, one chemical and another mechanical. A thin film consisting principally of ferrous sulfide grows by diffusion of reactants, presumably iron ions, through the film. The rate of corrosion is inversely proportional to the thickness of the film. With increasing thickness, mechanical strains produce film rupture followed by temporary local acceleration of attack. Kinetic constants of the reaction are given, and the film structure is described. The paper clarifies the fundamental mechanisms in sulfide formation on steel in molten sulfur. This instance of corrosion falls into the type of film-growth reactions complicated by a cracking-heating process.
S
ULFUR compounds attack most of the metals and alloys
used in chemical engineering. With the greater use of oils with high sulfur content in the petroleum industry, sulfur corrosion increases considerably in importance. The corrosion products usually are sulfides, and even in the corrosion of alloy steels in sulfur dioxide-containing flue gases, the sulfides often are reaction intermediates. However, little is known as to the mechanism and the rate law for the corrosion of metals with sulfide formation, even in liquid sulfur. The behavior of
metals in this medium must be understood before studies in even more complicated media are undertaken. LITERATURE
Hackerman and Shock (6,11) foundonly a slight discoloration of steel held in contact with dry sulfur for 3 hours at 130' C. However, theyreported enormous corrosion ratesof several inches per year if water was present simultaneously and the steel coupons were in contact with both sulfur and water. West (14) reports a corrosion rate of steel in sulfur of 0.025 mm. or 0.001 inch per year at the melting point of sulfur, 115' C., and 12.5 mm. or 0.5 inch per year at the boiling point, 445' C. Gel'd and Esin (5) suggest that the sulfide scale grows by iron diffusion outwards, in an analogy to Pfeil's (8) picture of oxide scale growth on iron. The change of the reaction rate with time and temperature has not been studied systematically in the iron-sulfur system.
