DECEMBER, 1936
INDUSTRIAL AND ENGINEERING CHEMISTRY
mere speculation, for there is definite experimental evidence to this effect, obtained by delicate electrochemical measurements, in which the current passed through the cell was always so small (10-'6 ampere) as t o obviate any secondary effects which might obscure the behavior in which we are primarily interested. This direct experimental evidence may for the present purpose be summarized by t h e following statements. When a steel initially immersed in a very dilute (0.001 N ) solution of dichromate is, by gradual addition of a very dilute hydrochloric acid, exposed to an increasingly corrosive environment, the observed potential is, at first, that characteristic of the oxide film and begins t o change slowly and continuously. This goes on over a certain range of acidity, which is characteristic of the type of steel; beyond this range the potential begins t o exhibit fairly regular oscillations, the magnitude of which increases with increasing corrosiveness of the solution. In this region the film is presumably breaking and healing again, and the potential oscillates between that of the film and that of the metal itself in the solution in question. Finally, with increasing acidity t h e film is unable to heal itself, and the potential, now that characteristic of the metal in the solution, again changes continuously. The sequence of these phenomena appears to be the same for all ferrous alloys; the breakdown of the film begins at an acid concentration which seems t o be characteristic of t h e film, hence of the type of steel. Here again the order of increasing resistance to breakdown is: ordinary irons and carbon steels, copperbearing steels, Cor-Ten, and, with a large interval, the group of stainless steels. The results, many of them still unpublished, which have been merely outlined here, are encouraging because it may be possible to develop on a scientific basis a type of test which, by yielding information on the nature of the protective film,will be a more reliable indicator of effective resistance to corrosion than the empirical tests now commonly used. We need not enlarge upon the desirability of such a test; i t would throw light on t h e question of just what constitutes a successful barrier film, and quicken greatly the search for steels more resistant t o corrosion in specific environments. I n some chemical engineering applications the metal must withstand continued stress at high temperatures, as well as corrosion. It would lead too far to do more than mention this matter of so-called creep, except to state that present knowledge of this phenomenon is far from satisfactory and that its elucidation will, from t h e nature of the case, be slow; it is fortunate that the alloys most resistant to corrosion happen to be also more resistant t o creep.
e
IN CONCLUSIOX, may we point out that recent developments in steels have been in the direction not only of selecting modified, or new, compositions, but equally, or perhaps even more, of learning better how t o make and treat all steels so as to bring out the optimum properties inherent in each composition. This is a very slow process, largely because of lack of tests which will furnish reliable, unambiguous information on whether a given mechanical or other property of the metal is, or is not, enhanced b y a specific change in composition or treatment. The investigator is therefore forced to make use of time-consuming tests on a large number of specimens, and is always faced with the difficulty of interpreting such test results without being confused by the many variables which entered into them. Moreover, one must be in position t o supply information on as many varied proGerties of a new steel a5 is available on steels known for a long time -lor iristanue, uii strength, ductility, resistauce to impact, a t temperatures ranging from refrigerating temperatures to a bright red heat, and on corrosion resistance and weldability,
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properties which are somewhat indefinite a t best, and hence can be evaluated with reasonable accuracy only by expenditure of a considerable amount of time-and effort. RECEIVED October 16, 1936.
Discussion T. S. FULLER General Electric Company, Schenectady, N. Y.
T
HE stability of any metal against any environment may be said to be a function of the nature of the film which forms on the metal. The oxide layer which is present on an iron surface into which aluminum has been diffused is resistant to high-temperature oxidation but not to oxidation in the presence of liquid water. The surface film present on many of the chromium-containing steels is resisbant to both conditions. Fenwick and Johnston have cautioned the reader against too literal use of the electromotive series in drawing conclusions in regard to the relative stability of two metals. This is a point which is well taken, is frequently overlooked, and which merits additional emphasis. Data resulting from the work of Subcommittee VI11 of American Society for Testing Materials Committee B-3 [Proc. Am. S. 2'. M., 35, Part I, 167-75 (1935)l on metallic couples exposed for a period of three years to the atmospheres of various test locntions throughout the United States have shown that in many instances the influence of one metal in contact with a second metal upon the rate of corrosion of the second metal is not even in the direction predictable by the electromotive series. Fenwick and Johnston's method of film study is ingenious and is certain to lead to fundamental facts of great value to students of corrosion. RECEIVED October 9, 1936,
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Discussion of New Ferrous Alloys for the Oil Industry L. W. VOLLMER AND BLAINE B. WESCOTT Gulf Research and Development Company, Pittsburgh, Pa,
P
ROBABLY no other single industry is faced with the widely variant and extensive corrosion problems normal to the various phases of the oil industry, the damage from which totals many millions of dollars annually. The drilling for and production of crude oil too frequently entail severe damage to drilling and pumping equipment by the action of the complicated phenomenon, corrosion fatigue. Pipe lines for transportation of oil suffer external attack from corrosive soils and internal perforation through the combined action of sour crude oil containing hydrogen sulfide and the small amount of brine that inevitably accumulates along the bottoms of the lines. The life of lease and storage tanks in the sour crude areas is often counted in weeks and months in sharp contrast with the decades usual in sweet crude areas. In the reiinery, sour crude oil is equally difficult to handle and necessitates the use of special alloys for heat interchangers, hot oil pumps, still tubes, valves, and other distillation and cracking equipment. There is little likelihood of subsidence in the corrosion difficultiea of any of the three major branches of the petroleum industry -production, transportation, and refining--for the largest provrd crude reserves are known to be sour and their eventual exploita-
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INDUSTKIAL AND ENGIXEEt1IA-G CHEMISTl1k
tion probably will be attended by an increase in the severity of the present conditions. Solution of the major problems has been slow because of the large tonnage of metal involved, and remedial measures have been coniined in most instances to the intermediate alloy steels for obvious economic reasons. The most significant progress in combat'ing corrosion in production operations has been made in the steels for sucker rods. These rods, which are the connecting link between the surface power unit and the plunger pump a t the well bottom, are subjected to heavy fluctuating loads and the corrosive action of concentration brines frequently containing large amounts of hydrogen sulfide. Failure of ordinary rods made of 0.30 to 0.40 per cent carbon steel, containing either approximately 1.25 or 1.75 per cent (S. A. E. T1340) manganese, is extremely rapid under these conditions and can be explained by the fact that the endurance limit of the 1.75 per cent manganese steel is reduced from 56,400 pounds per square inch in air to 12,100 pounds in oil well brine saturated with hydrogen sulfide. This drastic reduction in fatigue resistance is caused by concentration of stresses a t corrosion pits plus embrittlement of the steel by the hydrogen generated from the reaction of hydrogen sulfide and iron. A decided improvement has been effected through the use of heat-treated, low-carbon, nickel-molybdenum steels, containing in one type approximately 0.20 per cent carbon, 1.75 nickel, and 0.25 molybdenum (S. A. E. 4615 or S. A. E. 4620), and in another, 0.05 per cent carbon, 3.50 nickel, and 0.25 molybdenum. These two steels exhibit a much lower susceptibility to sulfide embrittlement and a high comparative resistance to corrosion and sulfide corrosion fatigue although their air endurance limits do not exceed those of the ordinary sucker rod steels. Thc air fatigue value for the 1.75 per cent nickel steel is 56,100 pounds per square inch, but the high value of 23,100 pounds obtains under sulfide corrosive conditions. With this 100 per cent increase in corrosion fatigue characteristics. there has been as much as 1000 per cent improvement over ordinary rods in actual service. Better corrosion resistance accounts for part of this gain, but the principal effect is attributed to exceptional ductile properties of the nickel-molybdenum steels, which permit local yielding and redistribution of stresses concentrated at small corrosion pits. The application of these steels to sucker rods marks an entirely new industrial use for the S. A. E. 4615 and S.A. E. 4620 steels and the first industrial utilization in quantity for the 3.50 per cent nickel-molybdenum steel. Where the poundage is small, various types of stainless steels are being used successfully for production work. Stainless 18-8 and the inverted type containing 18 to 20 per cent nickel and 8 per cent chromium are economical substitutes for common cold-rolled steel polished rods; here again corrosion fatigue, intensified by atmospheric oxygen, is the condition to be overcome. High-chromium nickel (18-8) working barrels for oil well pumps are used to some extent in corrosive areas, but the service is sometimes poor, probably because the abrasive action of the pump plunger constantly removes the protective oxide film. Pump, ball and seat valves made of the hardenable 12 to 17 per cent chromium steels pay for themselves many times over by greatly reducing the frequency with which laborious replacements must be made with regular steel valves.
VOL. 28, AO. 12
Alloy additions to cast iron and the ability of these alloy irons to respond to heat treatment have widened the use of cast iron in services where resistance to corrosion and wear is prerequisite. Cast irons containing 1.00 to 4.00 per cent nickel and 0.50 per cent chromium or molybdenum are being used to advantage as liners for working barrels, slush pumps, and the like; a white cast iron containing 1.00 per cent boron, which develops extreme hardness, is particularly well adapted to slush pump and working barrel liners that suffer from sand abrasion. The hydrogen sulfide corrosion problem in the refinery is met more or less satisfactorily by three alloy steels: (1) Cast nickelchromium steel with 0.30 per cent carbon, 1.25 nickel, and 0.60 chromium (S. A. E. 3130) is used extensively for pumps, valve bodies, and accessories on stills running the less corrosive crudes; (2) 18-8 stainless and (3) 4 to 6 per cent chromium steel, usually with 0.5 per cent molybdenum or 1 .OO per cent tungsten, in thc rolled or cast form are required for still, condenser, and heat exchanger tubes, return bends, headers, valve bodies, etc., that handle sour crude. The 4 to 6 per cent chromium steels are by far the morc commonly used because of their relatively low cost; their corrosion and oxidation resistances are greatly improved over those of plain carbon steel, but corrosion remains a problem particularly in the ends of still tubes. Frequently the body of t h e x tubes is perfect after long service hut the ends arc so l~ttdly attacked by erosion and corrosion that replacement murt be made. But for the air hardening characteristics of 4 to C, pcr cent chromium steels, such tubes could be economically reolainied a t the r e h e r y by welding-on new tube ends. Unfortunately thc heat of welding produces a hard, brittle condition in 4 to 6 IJer cent chromium steel, rendering it unsafe for high temperature and pressure service without subsequent heat treatment whicli ordinarily cannot be performed conveniently in the refincry. The addition of 1.0 per cent aluminum to this alloy eliminatcb the air-hardening property, but the aluminum is partially oxidized during melding and some hardening still persists. Columbium additions, which also prevent hardening, are not its readily lost as aluminum during welding and offer attractive possibilities for future development not only in the reclamation of used tubes but also for the manufacture of overlength tubes for special rcfining processes. 3.0 per cent rhromium-0.5 prr ccnt molybdenum sterl and :t 9.0 per cent chromium-1.5 pcr ccnt molybdenum steel arc 1 1 0 1 ~ available as alternates for 4 to 6 per cent chromium steels in onc case 2nd 18-8 in the other; they closely approach the corrosion resistances of the respective latter steels and, of course, are much cheaper. .\ carlion steel containing 0.5 pvr ccnt nio1yl)denum has become popular as a subbtitutc for plain carbon s t f d in still tU)Jes wherr corrosion is not serious; its improved creep strength assures a service life decidedly longer than that of plain carbon steel at the high temperature and pressures of present-day cracking opcrations. I t is hoped that the steady progrehs being made in the development of new steels for all branches of the oil induatry will continue unabated. A\
RECEIVED October 0. 1936.
