IiVDLTSTRIAL AND ENGINEERING CHEMIXTRY
426
Vol. 15, No. 4
NOTES AND CORRESPONDENCE The Mechanism of the Corrosion of Iron Edator of Industrial and Engineerzng Chemistry: Iron implements have been used from prehistoric times, one of these found in a pyramid of Egypt being probably five thousand years old. Possibly, even as long ago as that people were interested in the corrosion of iron, although a study of the literature of the last twenty years would almost make one believe that the problem is a new one. Recently, hundreds of papers and several books have been written on this subject. Most of these papers are concerned either with the description of some personal experiences with corrosion or with work done in attempting to clear up the understanding of the mechanism of corrosion in the light of modern chemical theory. Perfect agreement with regard to any theory is hardly to be expected, and it is not surprising that there should be marked disagreement with respect to the theory sf corrosion. According to J. W. Mellor,’ “Rusting is a complex process and workers are by no means agreed on the simple facts. Dry iron in dry air does not rust; moisture must be present before rusting can occur. Some deny and others affirm that the presence of an acid is necessary.” It is interesting to note that Mellor then proceeds to outline the so-called “carbonic acid” theory, and refuses to say a word about the “electrolytic theory” beyond stating that “several other hypotheses have been suggested and the subject is still sub judice.” Just as this prominent English author utterly ignores the large amount of literature that has been published, especially in the United States, in support of the electrolytic theory suggested by Whitney,2so in many recent articles i t has been assumed here that this theory of Whitney has been adopted quite generally, and but little attention is paid to the conflicting testimony of some very capable ,English observers. Thus, in a recent article R. E. Wilson3 writes, “There can be no real doubt but that the fundamental reaction in the corrosion of iron is essentially Fe 2H+*Fe++ 2H.” Expressed in this form, the first stage of iron corrosion is the action of hydrogen ions upon the metal. This corresponds just as well to the carbonic acid theory of Moody as to the views of Whitney, Walker, Cushman, and others. These latter, however, have assumed that water itself furnishes enough hydrogen ions to start the process. Wilson’s assumption that all chemists unite in agreeing that this is the fundamental reaction of iron corrosion is not quite true. Probably the only thing that has not been contradicted by different writers is that iron rust contains ferric oxide, so both iron and oxygen must have something to do with its formation. Everybody knows that iron under certain conditions can decompose water with liberation of hydrogen. Walker and Cushman have undertaken to show that this decomposition takes place to an appreciable extent a t room temperatures, but English observers have doubted whether all traces of carbon dioxide and dissolved oxygen were removed from the water. Walker and Cushman admitted that the water itself could not account for the formation of rust, because if iron does dissolve a t room temperature in pure water there is not enough ferrous hydroxide formed to give a precipitate, and no hydrated ferric oxide is formed until the solution is brought into contact with air.
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“Modern Inorganic Chemistry,” 1919, p. 486. J . Am. Chem. Soc., 25 (1903), 394. THISJOURNAL, 1 6 (19231, 127.
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The rusting of iron is unquestionably an oxidatioiz process. -A study of rusting, therefore, suggests a study of the oxidation potentials involved. Few chemists appear to realize that the solution tension of a metal is merely another name for the tendency of the element to be oxidized. Just as we can measure the relative tendencies of the different metals to be oxidized, in the same way we can ineasurg the relative readiness with which all reactions of oxidation or reduction take place, and a complete potential series will show, not only the relative activities of the metals and nonmetals, but also the tendencies of reactions such as the reduction of permanganate or dichromate and the oxidation of stannous, ferrous, or titanous iron. When iron dissolves in an acid the reaction emphasized by Wilson undoubtedly takes place. Interpreted in terms of the modern theory, metallic iron is oxidized and hydrogen ions are reduced. Chemists are so used to thinking of hydrogen as a reducing agent that they hesitate to think of this reaction as expressing an oxidation of iron and a reduction of hydrogen ions, but this is exactly true. In the potential series, the oxidation of metallic iron to the ferrous condition when measured against a molal solution of ferrous ions is about 0.43 volt above the potential of hydrogen in a normal solution of hydrogen ions. Consequently, a t room temperature metallic iron is oxidized, or dissolved, by any appreciable concentration of hydrogen ions. As to whether enough hydrogen ions are present in pure water to cause an appreciable oxidation of irdn, is not worth arguing about. Everyone will agree that theoretically there is a slight reaction, and practice shows that a t room temperature the action proceeds only to a slight extent just as the theory demands. In this respect we fully agree with the conclusions drawn by Wilson. The oxidation of ferrous iron to the ferric condition is an entirely different matter. It is harder to oxidize ferrous iron than it is to dissolve copper. The oxidation potential of copper against a molal solution of cupric ions is about 0.34 volt below that of hydrogen, but that of ferrous iron to ferric iron is 0.75 volt below hydrogen or 1.28 volts below the oxidation potential of metallic iron. It requires a much stronger oxidizing agent, therefore, to oxidize ferrous ions, and a t room temperature hydrogen ions and certainly water are incapable of exerting such an oxidizing effect. Such an oxidation can be accomplished by dissolved oxygen, as Cushman, Walker, and many others have shown. The astonishing thing about this whole matter is the fact that few investigators have realized that if dissolved oxygen is capable of oxidizing ferrous ions it will be even more effective in oxidizing iron from the metallic to the ferrous condition. I t is just as absurd to assume that the fundamental reaction in the corrosion of iron is always the oxidation of the metal by hydrogen ions as it is to assume that the electric current decomposes a solution of sodium chloride setting sodium free a t the cathode, which then reacts with water to form sodium hydroxide. LeElanc pointed out the absurdity of such an assumption about thirty years ago. In the presence of dissolved oxygen it seems reasonable to express the stages in the oxidation or rusting of iron as follows: 2Fe 0 2Hz0-2Fe+T -4- 4 0 H (1) Oa 2H20----t4Fef++ 40H(2) 4Fe+T and then 2Fe+++ 6OH- --fFeaOs 4- 3Hz0 (3) It is clear that such an explanation of the corrosion of iron is just as plausible as that given by Wilson. I t agrees with his observation that a t concentrations of hydrogen ion corresponding
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I S D U S T R I A L A Y D ESC;‘IiYEERI,VG CHEMISTRY
April, 1923
to pH = 1 1 the reaction between iron and the hydrogen of water is extremely slow and that a t higher acidities it becomes appreciable. It agrees perfectly well with the observations which he cites that were made a t the laboratories of the National Tubecompany, and it agrees with the observations of Walker‘ and others, that “the rate of corrosion is directly proportional to the oxygen content of natural waters.” This explanation does away with all hypothetical deductions with regard to films of hydrogen, etc. If the hydrogen-ion concentration becomes higher, eventually there will be so many more hydrogen ions available that the reaction Fe 2H++Fe++ Hz will take place, but this is no argument for assuming it to be the fundamental reaction of corrosion. Hydrogen ions may cause the first stage of corrosion, but only when the reduction potential of the hydrogen ions due t o @creased concentration becomes greater than that of dissolved oxygen. This view nearly, if not quite, reconciles the differences between the interpretations of corrosion by Bmerican and English chemists. WM. T. HALL
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MASSACHUSETTS INSTITUTE O F
TECHNOLOGY
CAMBRIDGE, MASS. February 6, 1923
Editor of Industrial and Engineering Chemistry: The writer is in substantial agreement with the views of Professor Hall regarding the corrosion of iron, with the exception of the theory advanced in his closing paragraphs. Here he states: The astonishing thing about this whole matter is the fact that few investigators have realized that if dissolved oxygen is capable of oxidizing ferrous ions it wiU [italics are mine] be even more effective in oxidizing iros from the metallic to the ferrous condition. I t would be strange if no one had realized that this might be the mechanism of corrosion, but any electrochemist can testify that to reason from data on oxidation potentials that any given reaction will take place in a given manner is a distinctly dangerous proceeding. In this case many chemists have unquestionably considered the possibility of a direct reaction between dissolved oxygen and metallic iron, and promptly rejected it because it entirely failed to account for many of the established facts regarding the rate of corrosion of iron-for example, that: 1-While the rate of corrosion in natural waters is proportional to the total area of iron exposed, all the corrosion may be localized in a comparatively small area. 2-When corrosion is thus localized there is a net decrease in H + ions (or increase in OH- ions) a t points where the iron is not going into solution (but where hydrogen is being oxidized). 3-Corrosion is accelerated by contact with any less positive electrical conductor, and roughly in proportion t o the area thereof within reasonable distance. 4-Iron in a solution containing n o oxygen will corrode if connected in an electrolytic circuit with a metal, such as copper, in a solution which does contain oxygen. 5-Any localization of corrosion is accompanied by the flow of electricity through the conductors in the direction predicted by the electrolytic theory. 6-Corrosion practically stops in strongly alkaline solutions. None of the foregoing facts seem to be explainable on the basis of the direct reaction between dissolved oxygen and iron postulated by Professor Hall, but they would all be predicted on the basis of the fundamental reactions emphasized by the writer: Fe 2H
+ 2H+ +Fe++
+
l/202
+HzO
+ 2H
(polarizing film)
(1) (2)
since the two parts of Reaction 1 (Fe--+Fe++and2H++2H) can obviously take place a t separate points if provision is made for the accompanying flow of electricity. 4
Walker, Ccdcrholm, and Bent, J. A m Chcm Soc., 8 9 (1807).1251.
427
Furthermore, the deductions regarding the presence of polarizing films of hydrogen are far from being “hypothetical,” as claimed, for their actual existence has been proved by a variety of methods. Professor Hall cites the following statement of the writer (including the first part of the sentence from the original article) as evidence that he paid little attention to the testimony of some English observers who do not subscribe to the electrolytic theory: Regardless of the school of corrosion theory to which we may give our personal allegiance, there can be no real doubt but that the fundamental reaction in the corrosion of iron is essentially Fe 2H--+Fe++ 2H.
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This inference appears inconsistent in view of Professor Hall’s admission, in the second sentence following, that “This (reaction) corresponds just as well to the carbonic acid theory of Moody as to the views of Whitney, Walker, Cushman, and others.” Within the scope of his journal article the writer could obviously not outline even a tithe of the voluminous literature on corrosion, but after a careful study of most of the recent work in both America and England he is more than willing to stand back of the foregoing statement regarding the fundamental reaction of corrosion. ROBERT E. WILSON Cmcaco, ILL. M a r c h 7 . 1923
‘‘America First ’ ’ Editor of Industrial and Engineering Chemistry: When I first read Dr. Noyes’s article under the above caption I was so impressed with his attitude of scientific fraternity, which every right-thinking scientific man desires, that I failed to grasp his major premise that “ ‘America First’ is so like ‘Deutschland iiber Alles’ that it is hard to distinguish the spirit of the two slogans.” The increasing elements of unrest in Europe which give promise of renewed war led me to reread the article. Dr. Noyes ascribes the similarity of the two slogans to nationalistic and capitalistic tendencies. I admit that there is a large body of our chemists engaged in the coal-tar industries who are not particularly happy over the suggestion that Germany pay us for the upkeep of our army on the Rhine by sending us $350,000,000 worth of coal-tar dyes. My own impressions of “America First” are not those entertained by Dr. Noyes. I think it is not only the duty but the privilege of the citizens of every country to love their own country first. In the light of recent events I think there is a distinct feeling in this country, even among chemists, that Germany would not be second in our choice of nations. I, for one, would much prefer France or England as my second choice. I fear I shall have to be reeducated, if not reborn, before I can have equal regard for the land of Washington, Lincoln, and Emerson, and the land of Bismarck, Emperor William 11, and,Nietzsche. If Dr. Noyes will read the memoirs of Bismarck and Moltke he will no longer believe that France is just as culpable as Germany in initiating the Great War. I cannot accede, moreover, to Dr. Noyes’s idea that the “socalled reparations,” now the base of the disturbance in the Ruhr, are intended to provide pensions in France. Anyone acquainted with the terrible devastation of the German occupation of France fully realizes that if France could receive the total amount of reparations awarded her by the Versailles treaty it would fall far short of the payment of the damage done. There are other reasons which lead me to object to the equality of “America First” and “Deutschland iiber Alles.” One of our national anthems a t least is not based upon the idea of military
