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 The results, many of them still unpublished, which have been merely outlined here, are encouraging because it may be possible to develop on a scientific basis a type of test which, by yielding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.
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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,
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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-
