INDUSTRIAL AND ENGINEERING CHEMISTRY
1374
a t higher temperatures, by treating the metal before casting (for example, with calcium silicide), or by the me of alloys. Any reduction in the growth of cast iron is also frequently accompanied by less scaling and corrosion. The most effective method is to use alloys. Well-known compositions contain from 1.5 to 2 per cent nickel and about 0.5 per cent chromium. Other effective compositions contain 1 or 2 per cent chromium and 0.5 to 0.8 per cent molybdenum, the chromium improving resistance to scaling and the molybdenum aiding the resistance to deformation at elevated temperatures. Cast iron of exceptionally high tensile strength can now be made without the use of alloys by holding the carbon content to 2.60-2.95 per cent. Nickel, chromium, copper, and molybdenum, either singly or in combination, are effectively used t o increase the strength of the ordinary higher carbon iron or the low-carbon high-test cast iron.
VOL. 28, NO. 12
Use of the austenitic cast iron containing about 15 per cent nickel, 6 per cent copper, 2 per cent chromium, 1 per cent manganese, 1.5 per cent silicon, and 2.75-3.1 per cent carbon, developed several years ago, has now become common and has proved to be a most advantageous material for use at high temperatures and for extremely corrosive conditions. In the nonferrous field, one fairly recent development deserves notice. Advantage of the high thermal conductivity of copper, which is about nine times that of iron or steel, has not normally been taken because of the relatively poor strength properties of copper. A copper alloy containing about 2 per cent chromium. heat-treated, to cause precipitation hardening by the chromium, can now be produced which has highly increased strength and hardness, and yet retains 80 to 90 per cent of the thermal conductivity of pure copper. RECEIVED October 9 , 1936.
STEELS RESISTANT TO SCALING AND CORROSION FLORENCE FENWICK AND JOHN JOHNSTON
United States Steel Corporation, Kearny, New Jersey
T
HE newer f e r r o u s alloys developed in recent years have, in general, been characteiied by both increased strength and enhanced resistance t o corrosion. These properties are not necessarily related, yet for many purposes increased strength is of little practical advantage unless the metal is more resistant to corrosion than plain carbon steels. For in service the same depth of corrosive attack would endanger the thinner sections made possible by greater strength more than the thicker sections of ordinary steel used for the same purpose, because it would cause a proportionately greater lessening of the safe load which the member could support. Consequently corrosion resistance is, in perhaps the majority of everyday applications, almost the controlling factor, even when higher strength is the primary consideration; this fact, together with the circumstance that enhanced resistance to corrosion is harder to secure at a reasonable cost than is increased strength, directs our attention at this time to some of the many questions involved in the scaling
Courtesy, U.S. Steel Corporation Subeidiaries Uary Sheet Mills, and Chicago-Illinois Steel Corpirotion
CHARQINQ STEELSHEETSINTO
THE
HEAT-TREATING FURNACE
This treatment at 1300' F., usually for 50 hours releases the strains set up in the steel during ooldworking, and i m p a d s ductility and softness.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
and corrosion of ferrous materials. Moreover, until the corrosion resistance of a new alloy has been ascertained by direct exposure tests over a period of some years, its use in any place where it is subject to corrosion must be regarded as an experiment to be watched carefully. In considering this subject, the word “corrosion” is used in the broad sense of any reaction between the metal and its environment, whether that environment be a flame or a hot gas, the atmosphere, a pure liquid, or a solution. For, in spite of considerable difference in detailed mechanism, each case of corrosion is to be regarded as a reaction between the metal and its environment; and thus the degree of permanence of a metal or alloy depends not only on the properties of the metal itself but also upon the precise environment to which it is exposed. As an illustration of this point, careful observations, under the auspices of the National Bureau of Standards, of various ferrous metals buried in a wide variety of soils over a period of ten years have shown that the extent of corrosion depends much less upon the kind of ferrous metal than upon the chemical and physical characteristics of the soil and the soil solution in immediate contact with it. T H E study of corrosion therefore involves what is per0 haps the central problem of chemistry: How, under what conditions, and to what extent do two substances react?
A partial answer to these questions is given by thermodynamic considerations, namely, that two substances tend to react spontaneously so long, and only so long, as the reaction is accompanied by a decrease in the free energy of the system; in other words, the initial state of the system as a whole must be less stable, or less probable statistically, than the final state. To take a specific instance, in the system ironoxygen, ferrous oxide tends to form because i t is more stable than iron and oxygen existing separately, when the oxygen is a t atmospheric temperature and pressure. Nevertheless, if iron were heated above 3000’ C., there would be a temperature a t which the dissociation pressure of ferrous oxide would just exceed the pressure of oxygen in the atmosphere, and iron would cease to corrode in air; for the same reason i t would cease to unite with oxygen at ordinary temperature provided that the partial pressure of oxygen in contact with it was of an order smaller than about 10-40 atmosphere. If these temperatures and pressures seem too extreme to be real, we may take the completely analogous system palladiumoxygen; direct measurement of the equilibrium pressure shows that palladium does not oxidize above about 790” C. in air or above 860” C. in pure oxygen. In these metal-metal oxide systems the difference between the prevailing actual pressure of oxygen in the environment and the equilibrium pressure of the oxide in contact with metal is a direct measure of the tendency of the oxygen at that temperature and pressure, to react with the metal, or conversely, of the oxide to decompose spontaneously. An alternative, entirely equivalent measure is furnished by the electromotive force of an appropriate reversible cell. But whether we measure, and express, the departure from thermodynamic stability in terms of free energy change or of electromotive force is a matter merely of convenience; the essential point is that the difference in energy between the initial and final state of the system is a direct measure of the “driving force’’ of the reaction, no matter how we happen to observe it. This driving force obviously varies with the environment to which the metal is exposed, so that in speaking of corrosion resistance the precise medium to which the metal is resistant must be specified. For not only is the range of stability of any one metal or alloy different in different environments, but the order of corrosiveness of a series of reagents may change with temperature or with the concentration of the
1375
Steels of higher strength can in many applications be used to advantage only if they have a greater resistance to corrosion than ordinary ferrous materials; and enhanced corrosion resistance is usually a primary requirement in metals for use in structures where lightness is desirable. The corrosion resistance of a metal depends upon the extent to which the product of its reaction with the environment isolates the metal from the environment ; it thus depends upon the environment as ’well as upon the metal. The isolating I agent is a film which may in many environments be protective against further attack, as in the stainless steels, or moderately impervious, as in some of the newer rust-resistant steels, or relatively pervious and unstable, as in ordinary ferrous materials. The difference is thus in degree rather than in kind, and appears to be associated with differences in the ability of the film to resist breakdown or to heal itself if broken. This view, which is supported by experimental evidence, is used as a basis for the discussion of the )range of usefulness of some of the cor(rosion-resistant alloy steels.
reagent. Clearly a minute specification of every environment is not practicable, and some generalization is necessary if the bulk of available information is to be reduced to a form which can be conveniently handled ; but broad generalizations which purport to give a complete picture of the general corrosion resistance of a group of metals or alloys should be regarded with suspicion. For instance, we may not safely conclude, because metal A dissolves in dilute sulfuric acid or in boiling concentrated nitric acid less rapidly than metal B, that metal A will have a longer useful life in the atmosphere; the circumstances are too unlike to permit of a valid comparison. An outstanding example of a broad generalization which is not always used with proper caution is the so-called electromotive series. This series lists the elements in the order of their standard electrode potentials-that is, the potential of the element in a pure, physically uniform state in contact with a solution containing active ions of the element a t an effective concentration of unity. The condition that both the reference solution and the electrode are rigidly specified is frequently overlooked, as is the fact that any departure from unit activity in either case may alter the relative position of an element in the list. It is therefore quite unjustifiable, and often unsafe, to draw from this series conclusions as to the relative stability of two metals, even of two pure metals, in any environment whatever. For instance, in the electromotive series chromium metal is less noble than iron, yet far more resistant to atmospheric corrosion than iron. Still less can we make use of ordinary data on the potential of a steel in contact with a solution to assign to it a position in the series, because the activity neither of its constituents nor of
INDUSTHIAL A S D ENGINEERING CHEMISTH L
1376
the several ions is unity, and further because the measured electromotive force is usually a characteristic of the oxide film on the metal rather than the true reversible potential of the metal itself. An example in the literature includes in a position more noble than iron in the series the most common type of stainless steel, that containing 18 per cent chromium and 8 per cent nickel; this alloy appears to be more noble if the two metals are in contact with a solution of nitric acid but is less noble in hydrochloric acid. Obviously, therefore, in SO far as tendency to corrode is concerned, corrosion is not a simple problem but, even for a single metal, comprises a great variety of problems by reason of the great variety of conditions to which the metal may be exposed. This point, which should need no emphasis, is stressed because statements implying the opposite are not infrequent even in technical papers and are common in advertising and promotional statements.
e
SO FAR only the tendency of a reaction to proceed
under given circumstances has been considered. The other dominant factor-in many practical cases the predominant facto-is the rate of continuance of the reaction. For if, under the circumstances, the effective rate of the process can be made to be substantially zero, corrosion will not proceed no matter how large the driving force may be; likewise, if the driving force is zero, there will be no reaction no matter how great the inherent rate may be. At present, the rate of reaction can be predicted in so few cases, and these under such specialized circumstances, that it can be ascertained only by trial under precise and specific conditions; and any departure from these conditions may affect the rate so enormously (the phenomenon termed catalysis) that no generalization or extrapolation is permissible. It is not even true that reactions with a great tendency (large decrease of free energy) necessarily go faster than those with a much smaller tendency. As an illustration, the tendency of the reaction Hg 0 +HzO
+
is very great, yet hydrogen and oxygen refuse to react at ordinary temperatures. On the other hand, under similar conditions the reaction NO +O--+NOn which is accompanied by a smaller change in free energy, goes readily, The only general statement that is valid is that the inherent rate of a given reaction for a given departure from equilibrium always increases, and increases rapidly, with temperature. On the other hand, reaction can proceed only as fast as the actual reacting units are brought together; if the rate at which the reactants can meet is slow compared with the inherent rate of reaction under the circumstances, the net rate of the process as a whole is determined by the diffusion rate. If, therefore, we can interpose some hindrance to diffusion, the barrier so set up is usually the predominant factor in slowing up, or even preventing, the continuance of corrosion. And this is the only way to lessen the rate of corrosion of the metals and alloys commonly used by engineers, since all of them are inherently unstable in contact with the atmosphere or with water. This behavior is clearly brought out in investigations of the rate of scaling of metals-that is, the rate of continued corrosion of the metal by oxygen at high temperature. This, the simplest case of corrosion, illustrates directly the main principle here emphasized-namely, that we can lessen corrosion only in so far as we can effectively keep the corroding atoms away from the metal. A clean iron surface, exposed to air a t high temperature, scales rapidly at first in accordance with the fact that the rate of reaction between iron and oxygen
VOL. 28, N O . 12
is then very great; but, as soon as a thin film of oxide has covered the surface, iron and oxygen can meet only as they diffuse through this film. Consequently the over-all rate of scaling is limited by the speed of this diffusion, and the rate gradually falls off as the thickness of the scale increases, as long as the scale remains continuous and adheres to the metal. This diffusion rate increases with increasing temperature but not nearly so fast as the inherent rate of reaction itself increases. Moreover, when scaling has once started, there is little difference in rate, whether the gas be air, steam, or carbon dioxide, which shows again that the dominant factor is not the driving force but the rate of diffusion through the scale; such differences as exist are due to differences in constitution or structure of the scale formed in the several gases. As the driving force of the oxidizing reaction in air is beyond our control, it is clear that we cannot prevent the continuance of the scaling reaction unless we can somehow lessen the perviousness of the scale film or layer and thus hinder the meeting of iron and oxygen atoms. The extent to which this can be done is largely a matter of experiment because at the moment little can be predicted as to what will constitute a successful impervious film. It is now well known, however, that the presence of chromium in steel lessens the rate of scaling in air at high temperatures. The explanation of this behavior is not completely understood but there is no doubt that chromium is capable of forming an adherent, highly impervious, oxide film even when the majority of the atoms on the initial metal surface are iron. As an illustration of this fact, the protective scale formed on a steel containing 12 per cent chromium contained three times as much chromium as iron, whereas the underlying metal contained over seven times as much iron as chromium. The protection is evidently due to the selective oxidation of chromium, the more easily oxidizable (less noble) metal. Indeed, experience shows that an added element does not enhance the resistance of steel to scaling unless that element oxidizes more readily than iron. The beneficial effect of chromium is evident even with small percentages of chromium in the matrix but becomes much more pronounced as the chromium content is increased, the rate of scaling at 1200' C. of an alloy containing 27 per cent of chromium being only about 1 per cent of that in the absence of chromium. These alloys are therefore finding wide application as heat-resisting materials and are available in standard compositions covering the range 5 to 27 per cent chromium; the greater the net chromium content of the matrix, the longer is the useful life of the alloy at a given temperature, and the higher the temperature which it will withstand. The chromium-nickel steels also resist oxidation at high temperatures and have certain advantages over the chromium-iron alloys because of the desirable mechanical properties imparted to the alloy by the presence of nickel. Addition of silicon or aluminum, which are also easily oxidizable, likewise lessens the rate of scaling of steel, but in practice this effect is less significant because of the adverse influence of more than a small percentage of these elements upon the useful mechanical properties of the alloy; for with iron they form brittle compounds whereas chromium and iron form a tough stable solid solution over a wide range of concentration of chromium. Nevertheless these elements are being used, and will probably be more widely used, in ferrous alloys containing other constituents which, by increasing the solubility of silicon or aluminum in the matrix, permit their addition in larger proportions.
@ IN AIR
at high temperature, the effective barrier is an oxide more or less efficacious depending upon the metal or alloy itself, on the precise way in which the film was formed, and on the temperature, which affects the rate of diffusion
tiirougli, lienee the rate d gniwtlr of, a given film. I n dry air at atmospheric temperature the rate of dilfnsion is so slow that when once a film is formed corrosion progresses at a rate which soon becomes vanishingly small. Such oxide films are itsually invisible but their existence may be demonstrated by a simple experiment. If a piece of steel is broken under mercury, the surfaces a t the fracture amalgamate so that, when the steel is removed, a droplet of mercury clings to them; in other words, mercury corrodes iron if the atoms of mercury can meet those of iron. On the other hand, if the steel is broken in air and then dipped into mercury, 110 matter how quickly, no amalgamation occurs; that is, nrcrc.ury is now prevented from coming into contact with iron. The conchision to be drawn from this behavior is obvious; when a fresh iron snrface is exposed to oxygen at room temperatiire, air oxide film is formed almost instantaneously (just as at liigli temperature) but a t atmospheric temperature the rate of ditrusion of iron and oxygen (and mercury) through this film is so small that no further corrosion (low-iemperature scaling) occurs. Samples of iron which do not amalgamate 1vit.h mercury, a i d hence must be covered with an oxide film, can be kept, in dry air for years vitliniit showing visible sians nf tarnish.
I n the atmosphere --that is, in mtiist nir-~~-~the fihn on iron is liewiso primarily an oxide; but in this case it may be altered by partial hydration or carbonation (reactions which (.annot go on at high temperatures) which may break up the cont,iniiity of the fdm and thus lessen or destroy its protective pouw. The effect of moisture is not marked in the absence of liquid water on the surface-that is, so long as the surface is a t a temperature above the dew point of the atmosphere in immediate contact with it; but the presence of water-soluble material, or dust particles from the atmosphere, lowers the effective dew point at the metal surface und may in this way cuhance the rate of corrosion. I n service the rate of rusting of iron and st.ee1 is negligible so long as the relative humidity of the at.mosphere in contact with it is less than about 70 por cent,. The fact that under at.mosp1ieric conditions liquid wat,i>ris likely to be present intermittently raises other complicutinns. For instance, a film to be successful must now he practically insoluble in water; otherwise it WiJdd wash away, as happens with metallic sodium. Indeed we conld not make use of any of t,he common metals of construction but for the cireiimstance that their oxides are quite insohhie in water and in most neutral solutions. Again, the alternate wetting and dryinx may, by altering the film, make it less protective; moreover, particularly in industrial atmospheres, the water fihn m i ~ ybe distinctly acid, with pronounced effects upon the permanence of the oxide fihll. The rate of atmospheric corrosion of all ordinary irons and carbon steels is practically independent of the carboncontent, and of the commonly occurring siIicon and manganese content. But the addition of copper up to about 0.20 per cent brings about a marked increase in resistance to atmospheric corrosion, the useful life of such a copper steel in an industrial atmosphere being at least twice, sometimes three or four times, as long as that of the same steel without copper. Indeed, copper is themain, if not thesole, effective constituent in a n u m b e r of steels widely advertised under various trade names; these copper steels are s u p e r i o r in resistance to atmospheric corrosion but not necessarily to underwater corrosion. The precise explanation of this added protection against atmospheric attack afforded by copper is not yet certain; all that can be said i s that the rust coat on copper-bearing steel differs from ordinary rust in color, in eoherence, and in continuity. I n any a t mospheric exposure the group of copper steels rusts initially as fast a8 o r d h r y steels, but the rust coat is a better p m Lector against continued attack on the metal. standpoint of the invea9FROM . .the. unfortunate that we cantigator I t IS
not nowpredict just what will beasuceessful protective film; and so we are forced to proceed by trial and error, making up series of compositions, rolling into sheets, exposing them to the atmosphere (preferably in more than one Iocation) for a period of several years, and comparing the results. The oiiterime of a seriesof com-
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INDUSTRIAL AND ENGINEERING CHEMISTRY
parisons carried out as just outlined, on a large number of compositions, has resulted in the development and commercial production of a chromium-copper-silicon-phosphorus steel, which proved to have greater resistance to atmospheric corrosion than any other steel of comparable cost in the group examined, and to have, moreover, high strength with adequate ductility and satisfactory weldability. It has been named Cor-Ten, and is being widely used on railroad and other equipment where lightness is advantageous; its useful life in the atmosphere is, according t o present experience extending over four years, fully twice that of a copper steel and about five times as long as that of a plain carbon steel. Some recent test results show that Cor-Ten is decidedly more resistant to corrosion than ordinary steels when immersed in fresh or salt water or in certain mine waters; the initial rates are about the same, but after a year’s immersion its rate of corrosion has fallen to about half that of an ordinary steel, and gives every sign of continuing to decrease, provided the protective rust coat re mains intact. This alloy may be classed as a rust-resisting steel, this term implying, not that it does not rust, but that its rust coat, by hindering further rusting, prolongs its useful life and so enables it to be used in the thinner sections made possible by its higher strength. Its superior resistance to atmospheric corrosion could not have been predicted from its composition, for the combined effect of the alloying elements, in proper proportions, proves to be considerably greater than the sum of the effects of the several elements separately. This steel is mentioned as being representative of a class of rust-resisting steels put on the market within the last year or two, because it i1lustrat)es the point we are stressing-namely, that the search for a corrosion-resistant alloy is a search for one which, under prevailing conditions of service, readily forms a selfhealing, protective film. This search will become more direct as specific knowledge is gained of the basic properties of films on metals and wherein a successful film differs from an unsuccessful one; delicate electrochemical and other methods are already furnishing significant indications, but further B-ork will be required to establish the basis vf these differences. The rust-resisting steels, a t their present stage of development, will probably be useful to the chemical engineer only in his subsidiary equipment, since they will not, in general, be sufficiently resistant to the solutions with which he deals. Under his conditions the rate of corrosion is subject t o the influence of many factors in addition to those already mentioned. For instance, the rate of attack in an aqueous medium depends upon the kinds and concentration of the ions present, which affect the constitution and solubility of the possible film-forming substances; upon the concentration of gases such as oxygen, carbon dioxide, or sulfur dioxide; upon local differences of potential, whether due to local differences a t the metal surface or in concentration of the solution in immediate contact with the metal, and so on. With these complexities the problem of interposing an effective barrier between metal and environment is much more difficult; and no general statement is possible, except that those alloys which most successfully resist atmospheric attack are also the more resistant in many (but not in all) liquid environments. The steels most serviceable for chemical engineering processes are those which owe their resistance primarily to a substantial percentage of chromium. For some purposes an alloy with about 5 per cent chromium has proved to be economical in service; but in general the steel is of the socalled stainless type, of either the ferritic or austenitic class. The ferritic class comprises iron alloys carrying 12 per cent or more chromium, with little or no nickel, whereas the austenitic alloys contain nickel sufficient t o make the alloy persistently austenitic at ordinary temperature. In general, the carbon content is preferably low, 0.1 per cent or less, and
VOL. 28. NO. 12
the less the better; and the alloy may carry small proportions of other elements such as molybdenum or titanium. These alloys are not inherently noble, but only appear so because in any oxidizing environment they readily form an oxide film, practically invisible yet protective. This is readily demonstrated, for the film is penetrated by hydrochloric acid thereby exposing the metal to attack; when thus deprived of its protective film, it will displace copper from a copper nitrate solution, though a very brief contact with the atmosphere, or other source of oxygen, suffices to re-form the film and so to prevent reaction with the copper ions. On the other hand, the film formed in nitric acid is such an effective barrier to further attack that the metal can be used as a container for nitric acid. There are, therefore, differences in effectiveness of the film, depending upon how it was made and with what it is in contact; and it is indeed easily possible, by a kind of electrometric titration, to determine the precise concentration of hydrochloric acid, or of a chloride, which just begins to break it down. Such titrations show that this concentration is quite small for all ordinary steels and irons, somewhat greater for copper steels, still greater for Cor-Ten, and very much greater for the stainless steels, this order being identical with the order of the resistance of these types of steels to at,mospheric corrosion; moreover, as between different stainless steels, or specimens of the same alloy differently heat-treated, the order of this breakdown concentration seems likewise t o be about the same as that deduced from exposure tests. Further comparisons will be required, however, before we shall know the precise extent t o which the results of such electrometric titrations of the film correlate with the useful life of the metal when exposed to the atmosphere. This brings us to a second point-namely, that the best results will be obtained from a stainless steel of the austenitic type only if it has been heat-treated properly; otherwise it may show the troublesome attack known as intergranular corrosion. This is due to the precipitation of a chromiumrich carbide a t the grain boundaries, a process which readily occurs within a certain high-temperature range; the formation of this carbide (probably approaching CrrC) brings about a local lessening of chromium content in the metal a t the grain boundaries with a concurrent lessening of its corrosion resistance. This impoverishment may go so far that the metal, after exposure to a corrosive medium, has completely lost its metallic ring and can be crumbled between the fingers, though in appearance it remains unchanged. The danger of intergranular corrosion may be removed by proper heat treatment, provided the metal is not to be used in the dangerous temperature range or is not to be welded by certain commercial processes (in which case, some portion of it adjacent to the weld is bound to have been in the dangerous range), The susceptibility to intergranular attack may be removed by keeping the carbon content below about 0.02 per cent, which is not yet commercially practicable; or more readily by the addition of titanium or of columbium, in slight excess of the quantity chemically equivalent to the carbon content, followed by a proper heat treatment to fix the carbon as a less soluble carbide of titanium or columbium, respectively, and thus to prevent it from fixing chromium.
TO RETURN now to the question of protective films, it @ is useful to regard the film on different steels, or on a single steel in different environments, as differing in degree rather than in kind, Moreover, they are preferably regarded, not as permanent or static, but as the momentary resultant of two opposing tendencies, just as in the case of any chemical reaction. In the formation of a successful film there may be a number of false starts and local failures; and in many environments it may break and heal repeatedly. Nor is this
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 the 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 the 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 the 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; it would throw light on the 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 the 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 by 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,
1379
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,
* *
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-
