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By E. C. Smith

THE CHAIRMAN.-Mr. Earle Smith- has kindly offered to make some remarks in connection with segregation in the product, Mr. Smith: E. C. SMITH,* Cleveland, Ohio-I will start this off by a story of the code days of the American Iron and Steel Institute. In those days, Republic did not have a big strip mill, and the question came up of the check analysis of rimmed steel. I suggested that they let me be the chairman of the Committee on Accumulating the Information on the Variation of the Composition of the Ingots of Rimmed Steel. Somebody wanted to know why I wanted to be the chairman. I said, "We don't make any of the big ingots of rimmed steel, and no matter who makes them they are so bad that you don't dare publish the information. So if I collect it and hide it you can't charge me with having done anything that really disrupted the commercial situation. So, everybody furnished me with a rather complete story as to the horrible variations in big rimmed ingots. I took it out to my house and put it away, and it is still there. Now, segregation in the big ingot is no longer an "-academic-matter-as far as II am concerned. ¬I am in a situation like that of the German who was told in 1936 about the coming war. He said: "Until it is your throat that is going to be cut, you don't pay any attention to what is happening to the other man." That is the way with the problem that we have in front of us. Recently I have been told that the comptroller was questioning the payment for certain articles of stamped steel that had been made from rimmed steel, because upon check analysis of the rimmed steel it had been determined that these stamped articles were not within the compositions that were included in the specification, and therefore he as the comptroller had no alternative. It was not meeting the specification; therefore, no payment. So I would say to all of you: You can very seriously consider this matter of segregation now and on into the future on the basis of the fact that it is actually going to cost your companies, which are making these products, money unless there is some new idea about the infallibility of quantitative chemists who analyze some of these products or lawyers who read specifications. I don't know which man is going to be the major factor in the future. If the National Bureau of Standards could in some way show the specification writers the variation between good chemists analyzing for what they expect to be umpire results, the writers would certainly realize that some of the specifications to which we have calmly signed our names as being O.K. are well, as one man said to me, they might be filled by the devil, and they, certainly could be filled by the Lord, but they could not be filled by mankind. I would like to inject two or three things into a segregation study which I want you to think about, because I would be a little surprised if there is very much discussion of them. In the first place, we generally make steel in the United States in the basic open hearth, and I do not think there has been nearly enough consideration given to segregation within the bath before it moves into the ladle. I am sure that any of the people who handled the 9400 series of the NE series when it contained the 0.60 per cent silicon have more respect than they had previously for the segregation that is within the ladle; that is, before the metal reaches the ingot mold, the frozen ingots and the segregation in them. Another point to which I think we ought to give serious consideration is the segregation of iron oxide in the ingot-iron type of material, where the distinction between that and the steel segregation is that it is actually segregating iron oxide. Normally we are segregating some form of carbon or sulphur, or something like. that, with the stamping steel industry now coming down into the ranges of chemistry, where the segregation of iron oxide will be just

Jan 1, 1944

The pressure of the gas in equilibrium with sphalerite has been determined in the temperature range of 680' to 825°C, using the Knudsen orifice method. A comparison of these experimental pressures with those calculated from thermal data and from other equilibrium measurements shows that the vapor above sphalerite is predominantly dissociated ZnS. Equations have been given for correctly calculating dissociation pressures using the Knudsen orifice method. It has been shown that the experimentally determined pressure is the same, whether the zinc sulphide is sphalerite or not, or a mixture of wurtzite and sphalerite. CONFLICTING points of view appear in the literature on the constitution of the vapor in equilibrium with solid zinc sulphide in the vicinity of 800°C. By comparing the dissociation pressure calculated from thermodynamic data and the vapor-pressure determination of ZnS by Veselovski,1 Lumsden2 has concluded that the vapor consists largely of dissociated ZnS. Sen Gupta,' however, concludes from his spectroscopic determinations that the vapor is largely ZnS molecules. In view of the fact that the thermodynamically calculated' dissociation pressure is higher than that experimentally measured by Veselovski, it seemed in order to repeat Veselovski's measurements. Experimental Procedure The method used for the determination of the pressures in this papel- is the Knudsen effusion cell. The apparatus and procedure were described in a previous paper- from this laboratory on the determination of the vapor pressure of silver. The only difference is that the Knudsen cell in this work is made from platinum and there is no external cover around the cell. The cell is an ordinary platinum crucible of 2.2 cm top diameter with a capsule cover. It was thought that platinum might stand up at these temperatures to the solid and gaseous ZnS, since it was found that the weight of the platinum cell itself did not change appreciably on heating ZnS in it at the working temperatures. To insure that reaction of the zinc sulphide with the cell was not giving' a false value, a stabilized zirconia cell was employed for check runs. Fig. 1 shows the comparison, which is satisfactory. Veselovski previously had measured the vapor pressure of ZnS using a silica Knudsen effusion cell. On repeating his experiment in this laboratory, it was found that ZnS at-tacked the silica cell, giving it a marked frosty appearance. This led to the belief that Veselovski's result:; may be in error. Also, he was operating at pressures above the range ordinarily considered safe for the Knudsen method. The effusion rate was measured by weighing the cell before and after each run. The weight loss during heating to temperature and cooling down was measured and subtracted from the weight loss during the actual run. The zinc sulphide used in this investigation was from two sources: Fisher cp grade, and a sample of pure sphalerite supplied by Mr. E. A. Anderson of the New Jersey Zinc Co. Before and after the series of runs with Fisher ZnS, X-ray analysis showed that both wurtzite and sphalerite were present. However, the ratio of sphalerite to wurtzite increased. All measurements were made below the transition temperature which has been reported" to be 1020°C. The data obtained in this investigation are tabulated in Table I. The pressure was calculated by the usual Knudsen formula" on the assumption that ZnS molecules were effusing. From these data, using pure sphalerite in the platinum Knudsen cell, the vapor pressure of ZnS, in mm of Hg, as a function of temperature is given by the solid line in Fig. 1. The best straight line, as determined by the method of least squares, is given by 14405 logpzns =-14405/T +11.032. A comparison of these results with Veselovski's shows that his results are about 50 pct lower. Discussion The vapor in equilibrium with solid zinc sulphide in the temperature range of this study will consist of Zn, S2, and ZnS mol, since other species of zinc and sulphur' are relatively unstable. The question to be settled is whether or not ZnS is largely dissociated. The derivation8 which follows gives the method of calculating the pressure of zinc and sulphur over solid ZnS, assuming complete dissociation, from Knudsen cell data. The free energy of the reaction 2 ZnS(solid) ? 2 Zn(gas) + S2(gas) is given by ?F?° = -RT In K = —RT In p12p2 where p1 is the zinc pressure and p is the sulphur pressure. If dissociation occurs in a closed system,

Jan 1, 1955

By W. F. Kieschnick, Eugene Harrison, W. J. McGuire

W. J. McGuire, et al, are to be commended for their undertaking of a mathematical solution of a very difficult problem. Unfortunately, however, a mathematical approach requires the application of several assumptions. These assumptions appear to be unrealistic and lead to answers which do not describe what actually happens when hydraulically fracturing oil and gas wells. Considering laboratory confirmation of breakdown phenomena, the authors appear to have tested their theories only on cement specimens and on samples of Austin limestone, much too small to provide any fracture system. This work resulted in the formation of vertical fractures. If the authors had tried similar experiments on thick walled cylinders made from almost any sandstone cores, they would have found that, using crude oil as the breakdown fluid, horizontal fractures would almost always occur, and at pressures much lower than any calculated. They would also find that by confining the fluid to within the bore (using oil base mud for example) on similar samples, the pressures required to burst the cylinders would be considerably higher and most of the fractures would be vertical. This breakdown pressure behavior has been duplicated in wells in Texas, Oklahoma, Kansas and in Wyoming. Considering field data the phenomena of different breakdown pressures for different breakdown techniques can be further illustrated. Most production and service personnel will agree that a breakdown can be more easily obtained if injection into a formation can be established prior to the occurrence of the breakdown. This is true whether the formation being treated is completed as open hole or as a perforated interval. This is clearly illustrated by a Lakota well in Wyoming, completed open hole at a total depth of 7,358 ft. An attempt to vertically fracture this well failed when a bottom hole pressure of 10,326 psi was insufficient to break down the formation. A non-penetrating type fluid (oil base mud) was in the well at the time the breakdown was tried." The oil base mud was then cleaned out of the well and replaced by a 30" API gravity crude oil. With this oil in the hole the formation breakdown was easily accomplished at a bottom hole pressure of 3,607 psi. This large difference in fracture pressures would be impossible according to the theories presented by McGuire, et al. The authors have used as an example the breakdown pressures experienced when acidizing Permian Basin wells. During acid treatments of limestone and dolomite the "breakdown" (drop-off in pressure) seldom occurs until some injection of acid has been accomplished. In these cases the breakdown is most likely to result from the chemical reaction of acid and rock in already existing vugs and fractures rather than from making a new fracture by hydraulic pressure. If this is true, then results in the Permian basin should not be used to validate the authors' calculations. *** AUTHORS' REPLY to ROSCOE C. CLARK and HENRY F. COFFER The purpose of our laboratory experiments in which thick-walled rock cylinders were hydraulically fractured was to determine the validity of the "thick pipe" formula for brittle materials, and not to predict nor demonstrate directly the orientation of field fractures. Our conclusions concerning field results resulted from calculations involving the "thick pipe" relationship as well as considerations of overburden stresses, rock strengths, and the geometry and dimensions of the field system. Clark suggests that had the models been more porous or contained weak bedding planes, horizontal fracturing would have occurred. This is undoubtedly true provided external stresses similar to those in the earth's crust are nor imposed. However, if we were going to design experiments to represent directly the field case we would impose the proper stresses on the models. It is generally recognized that the vertical compressive stress in the earth's crust arising from the weight of the overburden is approximately 1 psi/ft of depth. Then, as an example, even though a horizontal bedding plane has zero strength, the formation cannot be separated to form a horizontal fracture unless the hydraulic pressure exceeds the stress due to overburden. And in those cases in which the stress resisting vertical fracturing is significantly less than that resisting horizontal fracturing, vertical fractures should result, notwithstanding horizontal plane weaknesses. We agree that breakdown pressure will be less if the fracturing fluid penetrates the formation. In Appendix HI of our paper it is shown that leak-off reduces the pressure necessary to initiate either a horizontal or vertical fracture. It would be difficult to attempt to

Jan 1, 1955

By F. A. Crossley

The titanium-rich end of the Ti-A1 system has been investigated up to 35 at. pct A1 (23 wt pet). One conzpound Ti3Al was found to occur between primary a and TiAl. It is ordered hcp with DO19 structure, it has virtually no solid-solubility range, and it has a closed maximum at about 875°C. OIL either side of the compound are a +Ti3Al two-phase fields. The limiting a1uminum solubility in primary a at the titanium-rich end is indicated to be 7.5 at. pct A1 (4.4 wt pet) at 550°C and about 6.8 at. pct Al fl wt pct) at 500°C. Quenching alloys from above the a + Ti3Al two-phase field produces the following structures with respect to alloy composition: Up to 13 at. pct A1 (7.8 wt pet), a solid solution; from 15 to 18 at. pct A1 (9 to 11 wt pct), shear transformation product or martensite; from 19 to approximately 30 at. pct (11 to 19 wt pet), submicro-scopic coherent Ti3Al in an a malvix. The twin hcp phase fields reported in the literature are the result of nonequilibrium corzdztions. Ti-A1 alloys, once partitioned by dwelling- in the a + ß phase field during either hot working or heat treatment, are extremely difjicult to homogenize at temperatures below 1000°C. Such partitioned alloys exhibit the characteristics or symptoms of two-phase materials, and may be said to suffer the "twin-phase syndrome". THE earliest investigations of the Ti-A1 system by Ogden et al.1 and Bumps et al.2 reported wide solubility of the primary solid solutions. Aluminum was reported soluble in the low-temperature allomorph to the extent of 37 at. pct (25 wt pct), and the first intermediate phase was reportedly TiA1. Somewhat later Kornilov et al.3 reported a similar diagram with phase boundaries displaced towards lower aluminum contents and higher temperatures. Beginning about this time (1956) reports in the literature made it very clear that one or more intermediate phases occurred at lower aluminum contents than TiAl.4-17 These reports included five major investigations of the titanium-rich end of the Ti-A1 diagram.4,12,14,16,17 Three of these diagrams show two two-phase fields below 37 at. pct Al, while two of them show a single two-phase field. The existence of the phase Ti3A1 is firmly established and is included in each of the diagrams, except one—that of Sato and Huang.12 The new phases are reportedly hcp and differ from primary a only slightly when disordered, and when ordered the "a" parameter is approximately one,4,12,15 two, 6-10,13,14 or four14 times that for primary a. Beyond this, however, the diagrams are remarkable for their lack of agreement. Two tacit assumptions are usually made in phase-diagram determinations of metal systems. These are: 1) equilibrium anneals bring the alloy to equilibrium or to indistinguishable closeness to it, and 2) equilibrium conditions established at elevated temperatures are either "frozen" by rapid quenching for evaluation at room temperature, or quench-transformation products are recognized as such. In the current investigation evidence was obtained that over substantial composition ranges neither of these two conditions was met in any of the more recent major investigations. I) MATERIALS, METHODS, AND TECHNIQUES The alloys of this investigation were prepared by nonc on sum able electrode arc melting. Materials used in the preparation of the alloys are summarized in Table I. The investigative tools employed were: optical and electron microscopy, differential thermal analysis (DTA), disatometry, X-ray diffraction, electron diffraction, and resistometry. Alloys for microscopic and X-ray investigations were prepared as 15-g melts. Alloys containing from 7 through 11 at. pct A1 were hot-rolled out of a furnace at 900°C, from 12 through 15 at. pct out of a furnace at 1000°C, and from 16 through 18 at. pct out of a furnace at 1125°C. Alloys containing more than 18 at. pct A1 could not be hot-rolled. The ingots were covered with Markal coating prior to hot rolling to minimize atmospheric contamination. After hot rolling, alloys containing up to 15 at. pct A1 were ground and pickled to remove 7 mils from each surface; alloys containing 16 and 18 at. pct A1 were skinned to a

Jan 1, 1967

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Jan 1, 1938

Discussion of the paper of PHILIP W. HENRY, presented at the New York meeting, February, 1915, and printed in Bulletin No. 97, January, 1915, pp. 23 to 30. C. ,E. GRUNSKY, JR., San Francisco, Cal. (communication to the Secret ary*).-The difference of opinion of engineers on the subject of depreciation is justified, to some, extent, by the many viewpoints from which such a subject can be considered when applied to properties of widely differing physical characteristics. Mr. Henry takes up the subject of depreciation as applied alone to oil properties, and in doing so makes a clear distinction between amortization and the repair or renewal requirement. After carefully reading the paper the purpose of its author in suggesting a "depreciation" reserve or fund to keep the property at a constant value does not seem clear. As shown by Mr. Henry, the depreciation element as compared with the operating expense per gallon of oil is very large because of the comparatively short life of the oil properties, and it is also evident that such a "depreciation" fund would in a short time contain a very large amount of money. It seems desirable and permissible in income-tax matters to take into account the depreciation of property and to separate from the income such money as is necessary to amortize the invested capital to an extent warranted by the depreciation. However, it does not seem either necessary or desirable to create a large fund which will in time approximate the entire invested capital and which may he invested by the directors, as Mr. Henry suggests, "in the property itself or in outside securities." This method seems cumberome and liable to offer in the reinvestment of such a fund temptations to a directorate that might show more devotion to its own interests than that of the stockholders. A reserve fund is very frequently desirable in mining and oil operations, but to keep up a fund of this magnitude or to reinvest it in outside securities does not seem desirable from a stockholders' standpoint. The. money withdrawn from the receipts to be placed in the depreciation fund (not including replacements and the drilling of new wells necessary to keep up the productiveness) should he returned to the stockholders of the company, not as dividends, but as an amortization of the invested capital.

Jan 5, 1915

By AIME AIME

BOB VAN PELTS boyhood days in the mining atmosphere of Colorado apparently influenced him to direct hip college education first towards geology, at. Cornell College, then to mining at Michigan College of Mines and finally to graduate work in geology at the University of Chicago. He taught geology at his alma mater. Cornell College. served as geologist for the National Park Service then was on the staff of the Illinois Geological Surrey investigating and mapping mineral resources, and in 1928 because chief of the department of geology, mining, and metallurgy in the newly organized Rosenwald Industrial Museum (now the -Museum of Science and Industry,). He was made assistant director of the Museum in 1931.

Jan 1, 1941

CONTENTS PAGE Ross, J. M. and WAYLAND, R. G.-Mining Methods at the Homestake. Discussed by Benjamin F. Tillson, R. M. Raymond, I, H. Barkdoll, Arthur Notman 1 EMMEL, RUDOLPH.-Mining Methods in Zaruma District, Ecuador. Discussed by Rudolph Emmel, George A. Packard, Fred Hellmann, R. M. Raymond, Arthur Notman 2 MISHLER, R. T. and BUDROW, L. R.-Methods of Mining and Ore Estimation at Lucky Tiger Mine. Discussed by Fred Hellmann, Benjamin F. Tillson...4 PARK, JOHN FURNESS.-Mining Methods of Jarbidge District. . Discussed by R. S. Lewis : 5 Mining Methods at the Homestake Discussion of the paper of A. J. M. Ross and R. G. WAYLAND, presented at the New York Meeting, February, 1925, and issued, as Paper No. 1408-A, with MINING AND METALLURGY, February, 1925. BENJAMIN F. TILLSON, * Franklin, N. J.--It requires a good deal of timber to operate, especially the caving systems. An important function in mine operations now seems to be forestry and to operate a timber tract in order to insure a supply of timber. By depleting the timber resources of your section, you only increase your costs later when timber must be brought in from the Pacific Coast or the East. Our experience has been that any square-set method of timbering is the most expensive. Timbering for stoping systems, top slicing if you will, although it uses a large volume of time does not require as expensive timbering as the square-set method; also it does not require as long a life for-the timber, so that such wood as birch and maple, which have strength but not long life under the fungus conditions in the mine, may be used. All the drifts in the orebody are thoroughly timbered, and it has required a lot of timber. If we had it to do over, we would spend a lot more money in the development of rock drifts; as a matter of fact, we must ultimately come to that any way.

Jan 6, 1925

By Robert Lepsoe

It is generally realized that in cyaniding the precipitation efficiency of zinc dust is due to the fine division or extended surface of its metallic particles; but frequently it is thought that the presence of other metals, say 2 to 3 percent. lead, is advantageous, causing more complete precipitation. The results of testing about fifty brands of commercial zinc dust have led to the conclusion that there is a distinct relation of precipitation efficiency to fineness and that the effect generally can be estimated by examining the size of metallic particles. The presence of lead was not found to be of any importance. Specifications Generally the term "97 per cent. to pass a 350-mesh screen, 95 to 97 per cent. uncombined metallic zinc" is used by the leading European exporters. Among the many methods of determination of metallic zinc, I have found the iodine test (iodine in potassium iodide) very satisfactory and rapid. It has been controlled by the other methods, samples of the same product having been sent to three different analysts: Iodine method................... 98.11 per cent. metallic zinc, Volumetric method.............. 07.63 per cent. metallic zinc, Ledoux Co., New York Bichromate method.............. 08.16 per cent. metallic zinc, Watson Gray, Liverpool. England Bichromate method.............. 98.20 per cent. metallic zinc, Norway Inst. of Tech., Trondhjem Determination of Precipitation Efficiency The method devised by W. J. Sharwoodl was used. The method proved to be satisfactory, the tests being merely for the comparison of different samples, hence the personal factor in manipulation was eliminated. As nearly all tests showed more zinc in solution than was accounted for by the silver precipitated, the term "dissolved zinc" was introduced—it means zinc dissolved by the action of cyanide and oxygen: Zn + 4KCN + H2O + O = K2Zn(CN)4 + 2KOH; or more probably, resolution of its equivalent precipitated silver, as tests stirred two hours showed more "dissolved zinc" than those stirred

Jan 1, 1925

By A. H. Larson, R. J. McClincy

The activity of Sb,03 in PbO-Sb,03 slags containing less than 50 mol pct Sb,03 was determined by the inert-gas saturation method at 700°C. In this composition range, the activity gf SbzO3 shows a strong negative deviation from ideality. The activity of PbO in these slags was calculated by application of the Gibbs-Duhem iniegration to the Sb203 activity data. The calculated activity of PbO in slags containing more than 63 mol pct PbO was found to deviate in a positive direction from ideality uthile a negative deviation was found for slags containing less PbO. The standard Gibbs free energies of formation of Sb,03 and PbO. Sb203 have been calculated and compared with existing data in the literature. The activity of Sb203 in PbO-Si0,-Sb203 (PbO/SiO, = 2) slags containing less than 25 mol pct Sb,03 was also determined by the inert-gas saturation method at 700°C. In this composition range, the activity of Sbz03 shows a very large negative deviation from ideality. VERY little experimental work has been published in the past to determine equilibrium data in the oxide systems connected with the refining of lead. These data are of value since impurities such as antimony, arsenic, and tin must be removed from lead and recovered for further treatment. Equilibrium studies on antimony and arsenic systems are also of interest for the design of new processes for lead refining and lead dross treatment. Maier and ~incke' first determined the liquidus curves for the PbO-Sb203 system and identified the compound PbO . Sb203. They found the phase diagram for this system to be two simple eutectics located on either side of the congruent melting compound. They also determined a very limited amount of vapor pressure data for Sb406 abbve PbO-Sba3 melts at 697"~. A second phase diagram investigation on this system was reported by Hennig and Kohlmeyer' who confirmed the existence of the compound PbO . Sb203 as well as the form of the diagram. A disagreement was noticed, however, in that their liquidus temperatures over nearly the entire composition range were higher than those reported by Maier and Hincke. Barthel~ and pelze14 redetermined the liquidus curve at the PbO-rich end of the PbO-Sb@, system and agreed very closely with the results of Maier and Hincke. None of the investigators mentioned above reported any mutual solid solubility in the PbO-SbD3 system. Zunkel and Larson5 have determined the phase diagram for the PbO-rich end of the PbO-Sb203 system by slag-metal equilibrium studies in the Pb-PbO-Sb203 system and by thermal analysis studies in the PbO-Sbz03 system. A maximum solid solubility of 5.6 mol pct Sb203 in PbO was observed at the eutectic temperature of 604°C. Their results for the phase diagram agree favorably with those of Maier and Hincke. The vapor pressure of Sb2O3 in the temperature range from 470" to 800°C has been determined by Hincke, using a modification of the transportation meth~d.~ His results for temperatures below the melting point of Sba3 are the only data reported in the published literature. The predominant vapor species has been shown to be Sb,06 by Norman and staley.? Myzenkov and Klushin,8 using the boiling-point method, have determined the pressure of Sb406 above liquid SbD3 in the temperature range from 715" to 1025°C. The agreement between these two studies is not very close. A portion of the discrepancy lies in the fact that Hincke used silica crucibles, which were attacked by the liquid Sbz03 at high temperatures. This fact does not account, however. for the large difference observed at the melting point. ~aier' gives a brief summary of vapor pressure data for Sb,O, above pure liquid Sbz03 which agree quite well with the data of Myzenkov and Klushin at temperatures near the melting point. This paper describes the determination of Sb2O3 activity data in the PbO-Sb203 and Pb0-Si02-Sb203 (PbO/SiOz = 2) systems by the inert-gas saturation method. These activity data are compared with the data calculated by Zunkel and arson. EXPERIMENTAL Materials. The materials used in this investigation were analytical reagent-grade PbO (99.8 pct PbO, 0.14 pct insoluble in CHsCOOH, 0.02 pct not precipitated by HB, 0.1 pct CaO, and 0.08 pct SiOz), Sbf13 (99.6 pct Sb203, 0.004 PC~ C1-, 0.005 PC~ SO;-, 0.15 p~t AS, 0.001 pct Fe, and 0.03 pct other heavy metals such as Pb), and SiOz (chromatographic grade). Apparatus for Vapor Pressure Determinations. The apparatus used in this investigation consisted of a transportation reaction system with two separate gas trains. The argon transporting gas was first mixed with a small amount of hydrogen, metered, and dried by passage through silica gel and anhydrone drying tubes arranged in series. After this preliminary drying, the argon was passed through copper wool at 500°C to convert the residual oxygen to water vapor which was removed by three anhydrone drying tubes. A second stream of argon was metered and dried and then passed around the outside of the alumina reaction tube to flush away the volatile species to pre-

Jan 1, 1970

By F. W. Schremp, J. W. Chittum, T. S. Arczynski

This paper describes a laboratory study of causes of external casing corrosion and the test work that led to the use of oxygen scavengers to prevent this attack. External casing failures are classified as water-line, casing-casing, collar and body failures. A corrosion mechanism based on principles of differential oxygen availability is developed that is consistent with facts known about each kind of failure. The field use of oxygen scavengers is depicted as a direct result of the laboratory study. A part of the paper is devoted to reporting on the field use of hydra-zine to control external casing corrosion. Results of field measurements made over a period of several years are presented as evidence of the efectiveness of the hydrazine treatment. The first conclusion reached is that the use of hydrazine materially reduces the cathodic protection requirements for treated wells. This result is interpreted to mean that a reduction is taking place in the amount of corrosion on the casing. Results indicate also that hydrazine shows its greatest usefulness within the first 12 to 18 months after a well is completed when pitting corrosion is likely to be most active. INTRODUCTION According to surveys sponsored by the National Association of Corrosion Engineers,' the cost of repairing casing leaks caused by external corrosion may exceed $4 million per year. In addition, well damage and lost production resulting from casing leaks probably costs the petroleum industry an additional $5 to $6 million per year. Concern about the cost of external casing corrosion led to an extensive laboratory study of factors causing this external corrosion and to the development of a new approach to its prevention. This paper presents a discussion of various causes of external casing corrosion, details of laboratory studies and the results of the field use of an oxygen scavenger in well cementing fluids to prevent the external corrosion of oil-string casing. Measurements on test wells over a period of several years show that cathodic-protection current requirements are greatly reduced when hydrazine is used in cementing mud. Reduction of current requirements can be interpreted to mean that removal of oxygen by hydrazine has greatly suppressed corrosion cells on the external surface of the casing and thereby, has reduced corrosion. To date, hydrazine has been used by the Standard Oil Co. of California in more than 200 well completions. KINDS OF CASING FAILURES A survey of a large number of casing leaks disclosed four types of external casing failures — water-line, casing-casing, collar and body failures. These types are identified largely by their location on the casing. Water-line failures are found just below the surface of water or mud in the casing annulus. Casing-casing failures occur on the oil string just below the shoe of the surface string. Collar failures are found in the threaded ends of casing joints where they are screwed into casing collars. Body failures may occur at any point on the body of a casing joint. Ex- amples of each kind of failure have some of the general characteristics that are shown in Fig. 1. Water-line failures usually result in the circumferential severance of an oil-string casing. The corrosive action causing a water-line failure usually is sharply defined and is limited to a short length of the casing. Casing-casing failures usually are accompanied by pitting corrosion distributed around the oil-string casing for distances up to 100-ft below the shoe of the surface string. Casing-casing failures may also sever the casing. Collar failures seem to start on the first thread at the bottom of recesses between collar and casing joint. Corrosion proceeds across the threads by what appears to be a normal pitting mechanism. Both casing and collar are severely attacked. Body failures are the result of highly localized pitting at any point on a casing wall. Besides the pit that perforates a casing, a large number of other pits usually are found along one side of the casing joint. The pits occasionally are filled with corrosion products consisting largely of oxides and sulfides.' Frequently, the mill scale is largely intact on the rest of the casing. Examination of a casing failure does not always reveal the cause of the failure. Frequently, the necessary details are destroyed when the failure occurs. For example, formation water flowing through a perforation at high velocity may enlarge the hole and destroy any remaining evidence of the cause of the failure. One way to obtain undistorted information about a failure is to study the nature of other pits on the casing in the vicinity of the failure. A study of such pits frequently suggests that they are characteristic of an attack resulting from the differential availability of molecular oxygen.

A New Microscopy and Its Pntentialities. By Charles S. Barrett. (Metals Technology, .April 1945) Physical Metallurgy Fundamental Principles Involved in Segregation in Alloy Castings. By R. M. Brick. (Metals Technology, Sept. 1944) A Stud) of Age-hardening Using the Electron Microscope and Formvar Replicas. By D. Harker and MI J. MurphyI (Metals Technology, June 1945) Standards for Identifying Complex Twin Relationships in Cubic Crystals. By C. G. Dunn. (Metals Technology, April 1945) Orientation Changes during Recrystallization in Silicon Ferrite. By C. G. Dunn (Metals Technology, April 1945) The Orientation Texture at the Surfacr of Cast RIetals. By Gerald Edmunds. (Metals Technology, Jan. 1945) (With discussion) Internal Friction of Single Crystals of Brass, Copper and Aluminum. By George H. Found. (Metals Technology, June 1945) (With discussion) Recrystallization of Aluminum in Terms of the Rate of Nucleation and the Kate of Growth. By W. A. Anderson and R. F. Mehl (Metals Technology, Feb. 1945) (With discussion) Copper and Copper-rich Alloys Textures, Anisotropy and Earing Behavior of Brass. By F. H. Wilson and R. ill. Brick. (Metals Technology, June 1945) (With discussion) Phantom Laminations in Brass. By Daniel R. Hull, H. F. Silliman and John R. Freeman, Jr. (Metals Technology, Jan. 19451 (With discussion) Corrosion of Yellow Brass Pipes in Domestic Hot-water Systems—a Metallographic Study. By E. P. Polushkin and Henry L. Shuldener. (Metals Technology, Oct. 1944) (With discussion) The .Alpha Solid Solution Field of the Copper-manganese-zinc system. By R. S. Dean, J. R. Long, T. R. Graham and .%. H. Robersori. (Metals Technology, June 1945) A White High-manganese Brass. Ry K. S. Dean, J. R. Long, T. R. Graham and C. W. Matthews. (Melds Technology, June 1945) Magnesium Alloys Oxidation Inhibitors in Core-sand Mixtures for Magnesium Castings. By O. Jay Myers. (Metals Technology, Feb. 1945) (With discussion) Grain Size and Properties of Sand-cast Magnesium Alloys. By R. S. Dusk and C. R. Phillips. (Metals Technology, Feb. 1945) (With discussion) Rater Quenching of 'Some Typical Magnesium Casting Alloys. By R. S. Busk and R. E. Anderson (With discussion)

Jan 1, 1945

By R. W. Raymond

At the Pittsburg meeting of the Institute, in March, 1910, the death of Mr. Metcalf was announced, and Col. H. P. Bope, of Pittsburg, delivered in memory of him a brief but eloquent address, which, though expressing the Norrow of Mr. Metcalf's friends, and, in general terms, the debt of gratitude which his country and his profession owed to him, was not intended to be a complete survey of his life and work. With Colonel Rope's permission, and with the aid of some of the statements in his address, I substitute for it, in our Transactions, this more cletailed, yet still inadequate memorial. As Colonel Rope justly remarked, the men who could do justice to this theme—such men as Jones, Chalfant, Bennett, Oliver, and Park—passed away before Metcalf, leaving him almost the last of the generation of great steel-makers who made Pittsburg the Sheffield of America, and who discharged a double function, as the rearguard of the old metallurgical practice and the ran-guard of the new. I knew many of them; would that I were better qualified to celebrate their achievements. William Metcalf was born Sept. 3, 1838, at Pittsburg, Pa., where his father, Orlando Metcalf, was an eminent member of the bar. At the age of sixteen, he entered the Rensselaer Polytechnic Institute, at Troy, N. Y., from which he was graduated in June, 1858: Returning to his native city, he became an assistant engineer in the Fort Pitt Foundry, of which he was made Superintendent in 1859. This famous establishment produced the largest castings and heaviest machinery then knolrn in the United States; and Mr. Metcalf's intense, intelligent and incessant study of his business prepared him as a manufacturer for a patriotic service to his country more valuable than, as a soldier, he could have rendered in the field. The call came in 1862, when the government needed, above all things, a supply of field, naval and siege artillery. The

Jan 1, 1911

By Gustav Egloff

THE treatment of emulsified crude oils has been a problem for years in the petroleum industry. Until comparatively recently various methods of settling with and without chemicals have been used with indifferent success. Since wells have been acidized, emulsified crudes have been produced, which acid considerably to the need for improved methods of treatment. The presence of large quantities of salts in the oils from acid-ized wells has presented several new problems to the industry. The use of hydrochloric acid in well treatments has caused the formation of sodium, magnesium, and calcium chlorides by interaction with the dolomitic limestones. These salts dissolve readily in the water normally present in the oil, and they are therefore incorporated in the emulsified oil. The crystalline salts have also been found in the oil, prevented from settling by protective films such as asphalt, resins and petrolatum. In, some instances, free hydrochloric acid has been found. The salts may be present in crude oil in amounts ranging from a few pounds to over 1000 lb. per 1000 bbl. of oil. The transportation of these crude oils through pipe lines has led to erosive and corrosive effects, which are a tremendous cost to the petroleum industry. During 1937, more than 1,200,000,000 bbl. of crude oil was transported through pipe lines. Of this quantity approximately 10,000,000 bbl. was water, and considerably more than 50,000,000 lb. was salts. If the crude oils were treated at the producing wells to remove the salts and water, an appreciable saving would be possible. The treatment would not be difficult, since it has been found that these oils are more easily treated at the time they are produced than after the aging and more stable emulsification that occur before they reach the refinery. Since great costs are involved in the use of these salty crudes at the refinery, treated oils essentially free of salts would naturally command a premium.

Jan 1, 1938

By Thomas L. Kesler

IF all of the 14 million tons of kaolin produced in Georgia through 1949 had been mined from a single deposit 20 ft thick, it would represent a mined-out area of less than 1 sq mile. This measure of depletion is in such strong contrast with the large size of the area shown in Fig 1, from which most of the output has come, that there has developed an optimistic outlook concerning future supplies of kaolin equal in grade to that already mined. Dozens of mines are widely but not evenly distributed within this area, which consists mostly of parts of Twiggs, Wilkinson, and Washington Counties. There are scarcely half a dozen kaolin mines elsewhere in the state that have been appreciably productive, although the areas in which they occur may have good possibilities. Most of the efforts to find new deposits, therefore, will be confined to the area shown in Fig 1, for it is here that the industry has established its facilities, and prospecting is encouraged by relatively numerous outcrops and opened deposits of kaolin. The Ounulgee and Oconee Rivers drain the area. They and their main tributaries have flat, swampy flood plains covered with alluvium and for the most part thickly wooded. The divides between these streams are intricately dissected by the minor tributaries, which are spring-fed in steep-walled, cirquelike heads. Gullying and undercutting are active along the streams but are partly controlled by unusually dense vegetation. The mean altitude is about 350 ft above sea level and locally the relief ranges up to 200 ft. The area is near the inner margin of the Coastal Plain, and is underlain by Cretaceous and Tertiary formations. Its general location is shown in black in Fig 1. The division between the Coastal Plain and the Piedmont Plateau is actually a very sinuous line, the fall line, marking the extent to which erosion has stripped the Coastal Plain sediments from the southeastward- sloping surface of the crystalline rocks of the Piedmont.

Jan 1, 1952

By J. M. GARZA

The production of oil in Mexico during 1945 was 43,402,852 bbl., or a daily average of 118,912 bbl. In March 1938 most of the oil properties were taken over by the Mexican Government and since then have been operated by Petroleos Mexicanos, an agency of the Government. The variations in the production during the last 8 years have been due to- fluctuations in the export market demand for Mexican oil rather than to the productive capacity of the fields. Since 1937, yearly production, to the nearest hundred thousand barrels, has been as follows :

Jan 1, 1946

By Ross C. Anderson, William I. Weisman

THE manufacture of anhydrous sodium sulphate or salt cake from natural deposits in the United States has been in general somewhat of a marginal undertaking. Competition from foreign sources and from large quantities of byproduct sodium sulphate produced domestically in the manufacture of hydrochloric acid and other chemicals has existed and continues. For example, most of the sodium sulphate produced is a byproduct or co-product in the manufacture of hydrochloric acid through the reaction of sodium chloride with sulphuric acid. In recent years, many manufacturers of rayon have installed equipment to recover sodium sulphate from waste spin bath liquors; today this is an important source. Before World War II large quantities of sodium sulphate were imported from Germany. In 1949 imported material from Europe again appeared on the domestic market. Natural sodium sulphate from Canada in substantial quantities also enters the United States markets. Despite this kind of competition, numerous attempts have been made to exploit various natural deposits of sodium sulphate in this country, but only a very few of these have survived economically over a period of years. One of these few operations is the plant of the Ozark-Mahoning Co. located 13 miles south of Monahans in West Texas. Several factors contributing to the successful life of this plant may be summarized as follows: 1—Geographical location. Monahans is reasonably close, freightwise, to the Kraft paper mills in Texas, Arkansas, and Louisiana; the Kraft paper industry is the greatest consumer of sodium sulphate in the United States. 2—Availability of natural gas as low cost fuel. Proximity of the natural gas fields of West Texas has been a tremendous asset, as the availability of low-cost natural gas is to all industry throughout the Southwest. 3—The nature of the deposit. The occurrence of sodium sulphate brines in southeastern New Mexico and West Texas has been very well described by Lang,' who writes that the brines are found in the Castile formation of the Delaware basin. Here weathering has altered the anhydrite so that a relatively porous gypsiferous zone overlies a dense impervious mass of anhydrite. This porous zone provides traps where percolating ground waters that have picked up soluble salts may lodge. These traps or pockets are the natural brine reservoirs exploited at Monahans. Although several hundred wells have been drilled, currently some 25 wells serve to supply brine to the plant. All are within 1 1/2 miles of the plant and are conveniently tied together by an electric power system serving electric motors driving the pumps. Having the raw material in the form of a brine which can be pumped from shallow wells makes possible much simpler and more efficient handling than if it were in form of solids. By contrast, other deposits of sodium sulphate, such as those in Arizona, Nevada, and North Dakota, are in the form of the solid minerals, thenardite and mirabilite, which present somewhat more of a mining and mineral dressing problem.' The largest producer of sodium sulphate from natural sources in the United States is at Searles Lake, Cal., and there a brine also is utilized. 4—Water. Substantial quantities are needed for cooling towers and for operation of gas engines. An area underlain with brine is not a promising source of fresh water, but fortunately, after a long search, an adequate supply was found nearly two miles from the plant. It may be appropriate to discuss briefly the grades of sodium sulphate offered on the market. Salt cake is the name usually applied to the grade of sodium sulphate used by the Kraft paper industry. It may be a low analysis byproduct, 95 to 97 pct sodium sulphate, with as much as l 1/2 to 2 pct residual acid, or it may be a natural product. Usually salt cake is considered a low grade product, but a great deal of a higher grade of material is marketed under this name. The specifications for glassmakers' salt cake are somewhat higher than those of the paper industry, usually requiring 98 pct sodium sulphate. Technical anhydrous sodium sulphate is a high grade material and usually exceeds 99 pct sodium sulphate. It finds the biggest market in the textile industry and is used as a builder in some synthetic detergents. Glauber's salt, Na2SO4. 10H20, is usually of high purity. Preferred for some uses, it normally has been recrystallized from an anhydrous salt. A unique manufacturing process has been developed at Monahans. This process results in the production of an exceptionally high grade of salt cake, and qualifies for nearly all uses, including many which specify the technical anhydrous grade. All of the finished product, which is very white, passes a 10-mesh U. S. Standard screen, and is retained on a 200-mesh U. S. Standard screen. It is over 99 pct Na2SO4 with main impurities being sodium chloride and magnesium sulphate. Iron content is less than 0.01 pct. As mentioned, the raw material at Monahans is a brine drawn from wells. Attention was first attracted to this location because a so-called alkali

Jan 1, 1954

By Robert K. Waits

The resistivity, temperature coefficient of resistance, stability at 200°C, and structure of annealed Si-Cr films have been studied as a function of film cowposition. Colorimetric analyses of the films indicated 20 to 60 wt pct Cr over the resistivity range of 1 to 10-4 ohm-cm. Resistors were delinealed by photolithographic etching, contacted with aluminum, and mounted in TO-5 packages. The average temperature coefjicient of resistance, a, between 50" and 150°C is related empirically to the film resistivity, p, by a (ppm per °C) = -2900 - 1140 log p (ohm-cm). Near a resistivity of 3.5 x 10-3 ohm-ctn (30 at. pct Cr) , the temperature coefficient changes from positive to negative. The temperature at which the resistance maximum occurs increases with decreasing resistivity. The conduction mechanism between —196" and +360 C appears to have two components—an activated conduction dominating a1 lord chromium content and higher temperatures and a linearly temperature-dependent conduction dominating at high chromium content and low temperatures. Electron diffraction patterns showed the films to consist of Cr3Si and amorphous silicon. Resistors thicker than 150A typically drifted less than 0.6 pct after 5000 hr stor-age at 200°C. Resistors as thin as approxinzately 80°A that had a silicon overlayer showed similar stability. Such films had sheet resistivities of 10 kilohm per square and temperature coefficients near —600 ppm per "c. THIN-FILM resistors having sheet resistivities greater than 1 kilohm per square are required in many integrated circuit applications. A process for producing such resistors should be compatible with present circuit fabrication and packaging, including exposure to maximum temperatures of 300° to 570°C during contact alloying, die-attach, and ceramic package sealing. In addition, the resistors should have a low temperature coefficient of resistance and change less than 1 pct during storage at temperatures up to 200°C for 1000 hr or longer. Reports of work beginning in 1949 at Battelle Memorial Institute indicated that, among other materials, the silicides of chromium, Cr-Co, molybdenum, tantalum, and tungsten were worthy of further study as high-stability resistive films. Later work on Si-Cr films vacuum-deposited by flash evaporation from a tungsten boat1 led to the conclusion that Si-Cr had "excellent properties for development into high- stability ... resistive elements". More recently, Si-Cr films have been vacuum-deposited from a tantalum boat,2 from a BeO-coated tantalum boat,3 by flash evaporation from an electron-beam-heated tungsten Source,4 and by triode sputtering.5 This paper describes a study of Si-Cr films vacuum-deposited by electron-beam heating of Si-Cr alloys. Resistivity, temperature coefficient of resistance, stability at 200°C, and structure were studied as a function of film composition. EXPERIMENTAL PROCEDURE Film Deposition. Si-Cr sources were prepared by vacuum-melting pressed pellets of mixtures of powdered chromium (99.9 pct, -100 mesh, Lunex Co., Pleasant Valley, Iowa) and silicon (99.99 pct, -325 mesh, United Mineral and Chemical Co., New York). A permanent-magnet focused electron-beam gun having a water-cooled copper crucible was used to heat the pellet until complete melting occurred. The mixture was then kept molten for an additional 5 min. The alloy thus formed was used as the source material for Si-Cr film depositions using the same electron-beam gun. Pieces of semiconductor-grade silicon ingots were used as sources for silicon depositions. The films were deposited within a standard vacuum evaporator having an 18-in.-diam Pyrex bell jar, a liquid-nitrogen-cooled trap, and a Consolidated Vacuum Corp. 6-in. diffusion pump containing Dow Corning DC-705 pump fluid. The substrates were thermally oxidized* silicon wafers 2 to 2.5 cm in diam. The

Jan 1, 1969

By J. Alfred Berger, O. M. Katz

The Zv-Hf-H ternary system was studied between 500° and 900°C at pressures less than 1 atm of hydrogen gas between 1 and 60 at. pct H. A new and unique microgravimentric apparatus was used. Cizanges of slope on pressure-hydrogen composition isothernis delineated phase boundaries. These boundaries separatecl the three regions, a, 0, and y—so designated to correspond to the regions of the Zr-H binary system—from the multiphased areas between them. A eutectoidal decomposition was found with the ß region phase or phases decornposing into a lamellar product on quenching to rool ter,zperatuve. Reproducible decomposition-pressure hysteresis occilrved lnainly at lower hydrogen cornpositions and at lower temperatures across multiplzase vegions between a and 0 and a and y. Tire effects of hqfniur7z on the hydriding charactevistics of zirconiurrz weYe as follows: 1) stabilization of the a and y vegions while destabilizing the 0 region; 2) a/?preciable elevation of the decomposition pressrkres in the multiphase region between the a and /3 field; 3) ~nouenzent of the eutectoid reaction to high te~nperatures; 4) reduction in the total qiiantity of hydrogen absorbed under one atmospheve of Hz p7-essure; and 5) introduction of a split deconzposilion at the eiitectoiclal poinl in pa?? of the ternavy. Assuru~ptions based on an ir-2terstitial vandonl-solulion rtioclel 0.f hydrogen in metals slzowed that the bindit~g energy between solute sites prednnzinatecl at low /i?!dvogen concentrations. However, at high hydrogen contents the entropy was the predorninatlt factor in determining the stability of the Zr-Hf-H al1o.s. This was interpreted to mean a scarcity of filrtlzer itltevslilinl solute sites caused by hydrogen-hydvogen intet-actions in the metal lattice. INTEREST in the reaction of hydrogen with metals has increased in the past few years for the following reasons: 1) the formation or use of high hydrogen potential environments in nuclear reactors; 2) the reaction of hydrogen with alloys in nuclear reactors with the accompanying deleterious effects on the mechanical and corrosion properties; 3) the theoretical implications of thermodynamic data on the theory and rules of alloy formation in the metal-hydrogen systems; 4) the use of hydrogen-containing fuels in rocket engines; 5) the need for a process of making fine metal powders of high-melting reactive metals; and 6) the beneficial impregnation of superconducting alloys with hydrogen. In nuclear pressurized-water reactors, the problem exists of limiting the hydrogen pickup of zirconium alloys which are utilized as fuel cladding, heat shields, and support members. In general, zirconium alloys have good mechanical and corrosion-resistant properties in high-temperature water. However, hydrogen is absorbed from the corrosion reaction between metal and water, greatly accelerating the formation of the corrosion product ZrOz as well as mechanically embrittling the underlying metal. In addition, recent observations1 at zirconium to hafnium welds showed that secondary elements in zirconium can have an appreciable, and somewhat unexpected, effect on hydrogen absorption. This paper lists the thermochemical data in the range 500" to 900°C for the equilibrium reaction of four high-purity Zr-Hf alloys with hydrogen. Phase boundaries and thermodynamic functions are determined while the structural data will be presented in a future paper. In general, the Zr-Hf-H system approximates the well-known, eutectoidal, Zr-H diagram2,3 with modifications introduced through the behavior of hafnium.4,5 The Hf-H system,' published while this work was in progress, provided a consistent trend with the Zr-Hf-H data. PREPARATION OF Zr-Hf ALLOYS Table I presents a complete flow chart of the preparation procedure. The zirconium and hafnium crystal bars were completely immersed in high-purity kerosene and slowly cut into thin wafers. Wafers were then cold-sheared into approximately 1-g pieces, thoroughly cleaned, weighed, and inserted into the furnace. The alloys, B-2, B-4, B-6, and B-8, were then nonconsumable arc-melted under 500 mm of purified argon. Additional purification of the argon was accomplished by melting a large titanium button each time an alloy was re-melted or a different alloy melted. Each alloy button, which weighed 25 g, was remelted four times in an approach to complete homogeneity. Material losses were less than 0.02 wt pct. Alloy buttons were alternately cold-rolled and vacuum-annealed into 10- and 20-mil sheets. Table I1 gives the composition of the four alloys used. Very little elemental segregation existed be-

Jan 1, 1965

By C. S. Matthews, H. C. Lefkovits

Drilling progress is often delayed by sticking of the drill string. The development of preventive and remedial methods has been hampered by incomplete understanding of the sticking mechanism. A recent lahorntory investigation hns indicated that one type of sticking may be attributed to the difference in pressure between the borehole and formation. This paper shows, by means of soil mechanics, that the primary cause for differential pressure sticking is cessation of pipe movement, whereas diflerential pressre and stanrtding time determine the severity of the sticking. The analysis stresses the importance of using low-weight muds with low solids content and low water loss to alleviate diflerential pressure sticking and describes why packed hole drilling, long strings of drill collars, and a large deviation from the vertical are conducive to sticking. Finally, preventrve and remedial methods ore evaluated, and a theory is presented on the release of stuck pipe by spotting oil. INTRODUCTION Since drilling with long strings of oversize drill collars has become standard practice in many areas, the incidence and severity of the stuck pipe problem has increased. It has been noticed that in the majority of these cases the sticking could not possibly be attributed to key seating or caving of shales. It appeared that, due to the differential pressure between the mud column and the formation fluid, the collars were pressed into the wall and so became "wall stuck". Points to note about differential pressure sticking are: (1) sticking is restricted to the drill collars, (2) the collars become stuck opposite a permeable formation, (3) the sticking occurs after an interruption of pipe movement, (4) circulation, if interrupted, can be restarted after the sticking is noticed, and (5) no large amounts of cuttings are circulated out after restarting circulation. Helmick and Longleyl investigated pipe sticking by differential pressure in the laboratory and found an empirical relationship between the differential pressure, the sticking time and the required pull-out force. In this paper an explanation of the mechanism is given based on Terzaghi's theory of clay consolidation. A qualitative description is given in the following paragraphs while the derivation of fonnulas is given in Appendices. This paper is a first attempt to explain pressure differential sticking and many points will require additional theoretical and practical investigation before the problem can be fully understood. PRESSURE DIFFERENTIAL STICKING AS A CONSOLIDATION PROBLEM In any borehole, where the mud pressure is higher than that exerted by the formation fluids, a mud cake is formed opposite the permeable sections of the hole and a continuous flow of filtrate takes place from the mud, through the cake and into the formation. This radial flow pattern requires a certain distribution of the hydraulic and the effective (grain-to-grain) stresses inside the mud cake. Any quantitative or qualitative change in the external pressure conditions will produce a change in the flow pattern and, consequently, also in the internal stress distribution inside the cake. In view of the low permeability and the high compressibility of a clay mud cake, the adjustment of the internal stress distribution is slow and is accompanied by a change in volume. Time dependent stresses are thus created which gradually diminish as the new state of equilibrium between internal and external pressures is approached. Some 30 years ago, Terzaghi developed his "Theory of Consolidation" to account for the time-dependent stresses and settling of clay formations under the influence of external loads. He derived a differential equation by which the time-dependent hydraulic stress and the consolidation can be computed for any point inside the layer during the consolidation process. His theory is based on the assumption that the change in stress is solely due to a change in water content and it may only be applied to one-dimensional consolidation phenomena. Other investiga-tors5,10 have expanded his theory to include processes of more than one dimension. The difference between the external pressures on the mud cake before and after sticking is a qualitative one (isolation of part of the cake by the static contact with the drill collars after pipe movement has been stopped)', and the time-dependent stresses thus created may be investigated by means of Terzaghi's theory. By this analysis the changes in the nature of the contact surface between the drill collars and the mud cake during the sticking can be explained; and the friction force between the two may be computed as a function of the sticking time, the borehole dimensions and the mud cake characteristics.