April, 1925
IXDUSTRIAL AND ENGINEERING CHEMISTRY
335
CORROSION SYMPOSIUM Pauers t o be presented before the Division of Industrial and Engineering Chemistry a t t h e 69th Meeting of the American Chemical Society, Baltimore, Ald., April 6 to 11, 1925
I
N PRCPARIXC; this SynipoLiuni, the Corrosion Committee of the ERICAS AS CHEWLAL SCiCrETY has
attempted t o give a balanced cross section of present corrosion thought. Thiq is the first step in forwarding the study of the theory of corrosion, vhich is this committee’s field in the general program of the Sational Research Council Corrosion Committee. This cross section is sufficiently complete for any one Tvho reads it t o grasp the status of knowledge. It is, of course, not perfect. Certain laboratories have been unable t o present data, and in some useful lines new data are not available. We believe these gaps will be largely filled by discussion of papers when presented. The last few years have been prolific producers of useful data. Worth-n-hile new alloys have been developed, and our knowledge of older metals and the factors of corrosion exterior to the metal has increased. thereby opening new channels for research which promise to be of extreme value. The striking corrosion-resisting properties of some new alloys are still unexplained. Uncovering of the laws governing this resistance to corrosion, with the resulting development of better alloys, does not seem remote. Additions to electrochemical theory have removed part of the past difficulties in applying electrical considerations t o predict the course of corrosion and explain present experience. The basic cause of pitting is clearer than ever before. Pitting seems to be due more to differences in oxygen supply than in the metal. Investigators differ as to the exact mechanism of the action. This matter can be cleared up by further study, and from the results a method of preventing pitting might be expected. Quantitative work on the effect of dissolved oxygen is getting a t the basis of variations in corrosion rate in the same solution. This work applies to almost all other corrosion study, and with these quantitative data the other work can proceed where it has been a t a standstill. The production and properties of films have been studied. Knowledge of how films may affect the course of corrosion has been obtained in special cases, but the acquisition of quantitative data has only begun Microscopic study of the progress of corrosion has produced interesting results, and the possibilities for new results in studying film formation and electrochemical effects are great. The phenomenon of passivity is still unexplained, but the progress along other lines is so rapid that we can reasonably expect it, if continued, to lead to a solution of the passivity conundrum. It is hoped that the progress of research on corrosion will not be interrupted. Unfortunately, the subject of corrosion is of such a nature that research on it tends to be spasmodic. Although many people are interested, few organizations have hope of any direct return, and therefore few will spend the time and money to produce enduring results. At present, it appears that comparatively little work along certain lines would produce results of much utility. il more definite centralization of theoretical work than the present committee organization might be worth while, possibly along the lines suggested in this symposium. It seems particularly feasible since the present trend appears to be towards agreement of the most advanced workers. This agreement on fundamental considerations offers a sound basis for further progress.
The Corrosion Situation and Its Relation to the Work of the American Chemical Society By W. M. Corse NATIONAL RSSEARCH COUHCIL,
\v.4SHINGTON, D.
C.
The present-day corrosion situation is an awakening to the need for intensive study. Recent research progress explains many formerly disputed questions. The American Chemical Society should produce further progress along theoretical lines based on these recently strengthened foundations. This work should parallel and fit in with the work of committees of other national organizations sponsored by the Corrosion Committee of the National Research Council.
HE present-day corrosion situation may be described as an awakening to the need for an intensive study of corrosion, and as a coming together of the various divergent views on its theory and mechanism. Let us consider for a moment the first part of this statement. Corrosion has been known for thousands of years. The term “rust” has been used for centuries to describe something that is harmful and an economic loss. The practical man has used such preventives as paint for a long time, but a t the best the loss has amounted to millions of dollars annually. When noble metals were first known, probably the loss from wear and abrasion exceeded that from corrosion and was the more important factor. With the discovery of iron the subject of rust or corrosion assumed large proportions, and its prevention has been of increasing importance as the quantities increased by leaps and bounds. The scientific study of alloys, one property of which is usually to withstand corrosion, is hardly more than twenty-five years old, so that until we knew about the metals themselves we could hardly expect t o study their decomposition products. This awakening interest in corrosion is well described by Evans in his recent book, “The Corrosion of Rfetals,” in which he says, “The study of Metallurgy-the ‘Making of Metals’-appeals directly to a comparatively limited number of persons, namely, those connected with the manufacture of metallic materials; but the study of Corrosion-the ‘Unmaking of Metals’-has an interest for all who use metals, a far larger class.” The general interest in this subject was recognized by the National Research Council when it appointed a few years ago its committee on corrosion. Through the members of the Council-the national technical societiesit has stimulated researches on corrosion which have suggested such symposiums as this one and those held under the auspices of the American Electrochemical Society and the American Society for Testing Materials. All of these agencies have awakened a new general interest in the subject, which indicates a healthy growth of knowledge. This popular interest is warranted by the economic value of the loss entailed, as is shown by the reports of the Federal Govern-[ROBERT J. ~ I c K A PChairman, , Corrosion Committee] ment through its Department of Commerce.
T
336
INDUSTRIAL AND ENGINEERISG CHEMISTRY
I n order to make real progress and use this awakened popular interest, certain fundamental things must be accomplished. I n the past the theory and mechanism of corrosion were so indefinite and apparently complex that the scientist almost gave up in despair. This led the practical man to attempt tests of an engineering nature, without much regard to theory, in an effortto get some results quickly. As might be expected under such conditions, much time and money were spent in order to obtain a relatively few facts. Happily, during this period, progress was being made on the scientific side of the question, both in this country and abroad. Such men as Bancroft, Bengough, Evans, Aston, McKay, Fink, Speller, and Whitman-all added their contributions to the theory, while valuable data have been obtained and correlated by Calcott. Hatfield, Turner and Hamlin, Whittaker and Van Patten, and, although a few years ago many widely divergent views were held, it is a pleasure to report agreement in almost all questions a t the present time. The tests by the practical men mentioned above were brought about partially by the fact that no central agency in which chemists and engineers had representation was in existence. The AMERICAN CHEMICAL SOCIETYnow has a committee on the subject, whose aim is to study the theory and mechanism of corrosion in collaboration with a similar committee of the American Electrochemical Society. No other societies are a t present studying this phase of the subject, and it seems eminently proper that it should remain under these auspices. We have, then, a definite plan to follow in order to get the fundamental data on which to build our theory structure, and has been assigned to sponsor the work. this SOCIETY Let us look a t some of the outstanding things that have been accomplished on the theoretical side of this question. Speller in his new book on the subject, will list the following theories in addition to the electrochemical theory: (1) the acid theory, (2) the colloidal theory, (3) the theory of direct chemical attack by oxygen, (4) the biological theory, (5) the film protection theory, (6) the peroxide theory; and will point out that none of these have given even an approximately general explanation of observed facts. Evans includes even the earlier forms of the electrochemical theory in this category. It is well known that until recently some of the most common and striking phenomena, such as the formation of protective films in some cases and autocatalytic films in others and the prevalence of pitting of even pure metals under certain conditions, were unexplainable by any known theories. The older electrochemical theory did not explain the cause of pitting. In 1916, J. Aston showed that an electrolytic cell could be set up by bubbling air around one of a pair of submerged iron electrodes and not around the other. The aerated electrode was the cathode, and this showed that the reason wet rust promoted further rusting was not that it acted as a cathodic contact material, but that it acted as a diaphragm, screening the underlying metal from the direct access of oxygen. I n 1922, McKay iirst applied the well-known theory of concentration cells to corrosion, and pointed-out that “corrosion may be produced by cells due to differences in the concentration of acids, concentration of dissolved oxygen or hydrogen, concentration of dissolved oxidizing or reducing salts,” in addition to the copper-ion concentration cells which were the subject of his study. This probably furnishes the theoretical key explaining Aston’s results. Closely following this, Evans elaborated on the effect of “oxygen-variation” cells in the corrosion of iron and zinc, and showed their striking properties in many interesting laboratory experiments. He has pointed out how these cells explain “the local and apparently capricious nature of corrosion.”
Vol. 17, No. 4
In 1923, Pilling and Bedworth published a paper on the oxidation of metals in air a t high temperatures, which was supplemented by the work of Tammann upon the action of halogens on metals a t low temperatures. Both of these papers helped to explain the formation of films and described the different varieties and the effect of each on corrosion. In this work the microscope has played an important part, and such work as that by Desch and Whyte on the microchemistry of the corrosion of brass is very valuable. In 1924, Bancroft prepared a report for the Corrosion Committee of the National Research Council, which describes the electrolytic theory and comments on the present status. He quotes liberally from all the workers on the subject so that this work is really a reference book. Bengough and Stuart, in seven long reports to the Corrosion Committee of the British Institute of Metals, have presented an exhaustive study, principally of the corrosion of condenser tubes, and have commented on the theories involved. They have emphasized the value of the ‘‘newer electrochemical theory.” Whitman and Russell have shown important quantitative relation between the supply of oxygen to steel surfaces and the rate of corrosion, and McKay and Thompson have demonstrated the same relations on nonferrous alloys. The concensus of opinion seems to be in favor of the electrolytic theory of corrosion as amplified by the later workers, and there is sufficient agreement now so that a program to carry the work further should be a distinct possibility. As was stated earlier, the AMERICAN CHEMICAL SOCIETY has an important part to play, and it is hoped that its committee will further this plan and act to play this part in a manner most creditable to the SOCIET?.
Corrosion in Aqueous Solutions By Wilder D. Bancroft CORNELL UNIVERSITY, ITHACA, N. Y.
The important points brought out in corrosion study during approximately the last twenty-five years are discussed in their relation to the present status of corrosion theory. It is shown that all corrosion reactions are electrochemical. The agreement of these theoretical points with experiments in the author’s laboratory is discussed. The importance of further research along such linese. g., the rate of oxygen supply to the metal, the development of a useful accelerated test, the formation and effect of films, and the other matters affecting electrolytic corrosion-is pointed out.
T
H E most striking characteristics of an electrolytic reaction1 is that it occurs a t two places-at the anode and the cathode. This peculiarity can be made less marked by bringing the electrodes nearer and nearer together. When the distance between them vanishes, we usually speak of the reaction as a chemical one and not as an electrochemical one. This is not a sound distinction. Any chemical reaction which can be split into an anode part and a cathode part2 may be an electrochemical reaction. When zinc dissolves in hydrochloric acid, the formation of zinc chloride is the anode reaction and the evolution of hydrogen the cathode reaction. When copper dissolves in dilute nitric acid, the formation of copper nitrate is the anode reaction and the reduction of nitric acid a t a copper cathode is the cathode reaction. 1 2
Bancroft, T r a m . A m . Electrochem. Soc., 9, 13 (1906). Traube, Ber,, 26, 1473 (1893); Haber. Z.fihrsik. Chem., 34,514 (1900).
