IND UXTRIAL AND ENGIAVEERIhTGCHEMISTRY
904
and elementary oxygen. Thus, if A be the autoxidizable substance and B be the inhibitor, the sequence of reactions would be A 0 2 = A ( 0 z ) ; A(02) B = A(O) B(O); A(0) B(0) = A B 0 2
+
+
++ + +
or alternatively
Certain data already accumulated lend a degree of credence to this formulation. It is doubtful, however, whether it is applicable in more than a few cases; it would need entirely recasting to cover the general case of negative catalysis, which, as we have seen, includes reactions not autoxidant in nature. -4n attempt to formulate a more general theory of negative catalysis has recently been made by the writer
Vol. 15, No. 9
independently of the work of the French a ~ t h 0 r s . I ~ The theory accounts for inhibitory power by assuming interaction between one of the reactants and inhibitor to form a molecular compound, as an alternative to reaction between the two or more reactants. The extent of the inhibition is determined by the degree and velocity of compound formation. The mechanism of Moureu and Dufraisse would be a special case of this more general formulation. The general theory would suggest other possibilities of inhibitory mechanism and thereby a wider range of reactions sensitive to negative catalysis and a more extended series of inhibitory materials. It should be very evident, therefore, that catalysis, both positive and negative, is a subject that the chemical organization cannot ignore; the theoretical chemist can surely find therein problems that will delight his soul. 1:
J. P h y s . Chem., 27, 322 (1923).
T h e Mechanism of Corrosion By John Johnston YAJ,E
UNIVERSITY,
h’EW
HAVEN,C O N N .
F T H E great number of publicathat in discussing this problem it is almost tions dealing with the general impossible to avoid dogmatic statements, topic of corrosion, few add much unsupported by convincing evidence, or to the reader’s positive knowledge. Ingeneralizations too broad to be useful. Points of similarity in the diverse condeed, by their lack of agreement, many ditions have led different writers t o atof the observations recorded in the literature are likely to confuse and weary tribute the effects to various factors. Adequate emphasis has not been given to one him. This appears to arise in part from of these, namely, that in all cases of corthe desire of an author to show that other rosion we are dealing with effects a t a surproducts are worse than the one whose face between at least two, and possibly merits he is championing and in part three, phases. Therefore more attention from the fact that many investigations should be directed to what may be exhave been limited to a specific set of pected to happen at such a surface of conditions which have not been defined and controlled with sufficient care. The separation, and the formation and behavior of a film or layer (using these words most important cause, however, is an in a general sense) a t a metal surface oversimplification of the problem in an under various conditions should be atterhpt to discover a single primary studied. It has long been recognized that factor to the operation of which all phein a general way the character of the nomena of corrosion may be ascribed. metal surface is of importance. For inThe protagonists of several such factors, Harris and Ewine stance, polishing lessens the rate of corassumed to be primary, bring forward JOHN JOHNSTON rosion. and txotection from corrosion is evidence in favor of their particular beliefs and give reasons why any other explanation is un- achieved if the surface layer be impervious to the corrodsatisfactory. This does not mean that evidence has been ing agent. But little is known definitely as to why there deliberately manufactured or ignored, brit rather indicates should be such large differences in the protection afforded that the problem is more complex than many have assumed by such coatings, even when formed under conditions which it to be. In other words, a complete description of the proc- appear to be nearly identical. A clue to the cause of these differences in some cases may be esses of corrosion necessitates the consideration of several factors, any one of which may under some circumstances given by the following considerations. When a metal oxide be dominant,. ThaLit could not well be otherwise is evident or Eydroxide is shaken with water in the presence of carbon from the wide range of conditions under which the term dioxide until the system is in equilibrium, the stable solid “corrosion” is applied-in water, in salt or acid solutions, phase will be hydroxide or carbonate (excluding, for the cold and hot, in the atmosphere, dry and moist, clean and moment, the possibility of a basic carbonate) , according t o dirty, in air or gases a t high temperature. And the medium whether the partial pressure of carbon dioxide a t equilibrium may be stagnant or in rapid relative motion. Further- is below or above a certain limiting value. This limiting more, we may be dealing with a single*metalfrom any part value, or transition pressure, characteristic of the particular of the electrochemical series, pure or impure, with a metal metal, is determined largely by the relative solubility of its made up substantially of a single kind of crystal or containing hydroxide and carbonate, so that it depends upon temperaseveral kinds of crystal, and the metal may be acting as ture. At present it is not known for any metal except magelectrode and under electrical or mechanical stress produced nesium, for which it is about 0.0005 atmosphere of carbon by external causes or internal strains. It is clear, therefore, dioxide, a concentration slightly greater than that in outside
0
September, 1923
INDUSTRIAL AND ENGINEERING CHEhYSTRY
air but less than that commonly present in a room. ilccordingly, if conditions are such that equilibrium is attained, in a laboratory we might get magnesium hydroxide when all the windows are open but magnesium carbonate with less ventilation. If equilibrium is not attained, we would get a mixture of the two, or even alternate layers of hydroxide and carbonate. Thus, a piece of magnesium immersed in water exposed to the atmosphere might become coated with hydroxide, with carbonate, or with both. Moreover the structure of the film might be different, according to whether the carbonate had been formed from hydroxide or precjpitated directly. It is possible therefore that the different coatings, which may thus be produced through a relatively small variation of conditions, differ largely in the protection which they afford to the metal. This is a case of a possible factor which has been left out of account as being negligible, but it may in some cases be important and responsible for the lack of consistency of results obtained under apparently identical conditions. For the metals in which we are more interested as subject to corrosion this transition pressure oi carbon dioxide is not known, but that in some cases it should be not far from the proportion commonly present in the atmosphere is not improbable. Incidentally, these considerations lead to the questionare so-called basic carbonates definite compounds and not merely mixtures of hydroxide and carbonate which appear to be definite only because they are usually made under substantially identical conditions? To what extent the partial pressure of carbon dioxide is a factor in the process of corrosion can be determined only by careful work to ascertain what may be expected to occur in the systems RO-H~O-COZ (R denoting a metal) and whether films formed within an appropriate range of concentrations of COz differ in protective power. This leads to the question of the criteria which can be used to characterize films on metallic surfaces. Films differing in protective power may look alike, even under the microscope, as they are composed of extremely fine particles. Chemical reactions are generally not very helpful, for the quantity of material is small and the observations cannot be interpreted unambiguously. It would therefore seem necessary to resort to physical methods, particularly sensitive electrical methods, as a means of estimating the degree of difference of films produced under different conditions. The electrical resistance and capacity of the films are possibilities and preliminary investigation has indicated substantial differences on making such measurements on films made under different conditions upon the surface of a piece of metal. It is possible also that measurements of electromotive force, thermionic emission, photoelectric effect, catalytic activity, or gas absorption would prove useful in some types of corrosion. Along these lines, however, one could not hope to achieve much except by taking the risk that the systematic investigation which would be required would not be worth the time and effort involved. Nevertheless, it looks as if our knowledge of the mechanism of corrosion will be unsatisfactory until we have learned how to characterize our surfaces and films and to distinguish one from another. A study of the structure of metal surfaces and of the reactions which may occur at those surfaces is of value also as a means towards a better knowledge and control of the processes of electrodeposition which are still far from being well understood, for the surface of the base affects the properties of the electrodeposit, for instance, its adherence and continuity, and hence its protective power. Thils the problems of corrosion and electrodeposition are closely connected and can with advantage be investigated together, as it would seem that in both processes an important factor
905
in the result is the nature and structure of the first film formed, that is, of the phase which is in immediate contact with the metal. If hydrogen is passed over pure copper oxide, reduction may not start until a temperature higher than that which suffices to Faintain the progress of reduction when once it has started has been attained. Moreover, the reduced area spreads out from the point or points a t which the copper appeared initially. This reluctance of the process to start, however, may be obviated by the presence in the oxide of a few particles of copper which serve as nuclei about which the reaction proceeds. This suggests that there is much more likelihood of reaction at a place in the surface where we already have the two solid phases concerned in the reaction, or, a t least, when there is some discontinuity in the regular structure, and that the causes of the reaction may be dependent upon contact between the pair of solid phases concerned. This is analogous to the fact that a perfect crystal of a substance which normally would effloresce may remain unchanged but begin to effloresce as soon as its surface is scratched. We meet the same phenomenon in cases more usually regarded as cases of corrosion, corrosion being much more likely to occur a t irregularities of any kind in or at the surface of the metal, even if the irregularity is a grain of so inert a substance as sand resting on the metal. This leads one to wonder if we should not look upon the break in regularity of the surface as the occasion both of the corrosion and of local potential differences, upon the latter as a symptom rather than as a cause of the disease. To the writer it seems advantageous to refer the phenomena directly to the surface even though this, in a sense, is to try to explain one unknown in terms of another unknown, At least, it emphasizes the necessity of further and more thorough investigation of surface films on metals. As to the value of more definite knowledge of the mechanism of corrosion, little need be said. We are hardly likely to be able effectually to minimize the losses due to corrosion until we have a better understanding of its mechanism. This loss is enormous, for an appreciable fraction of the metal produced each year is used to replace metal rendered useless by corrosion. Consequently, the return from even a partial answer would soon repay the cost of investigation. There is an increasing willingness to consider the problem as a whole, instead of merely attempting to find a way of obviating some particular difficulty, and recent work has thrown light on many places which were dim or even dark. Many committees are interested in different aspects of the matter, and an effort is being made to bring together these various viewpoints. There is, however, no lack of opportunity for the investigator who has an idea and opportunity to try it out.
Society Meetings with Exposition 1
The Chemical Exposition this year is being held as usual during the week following the annual meeting of the AMERICAN CHEMICALSOCIETY at Milwaukee. Immediately after the close of the meeting its members will entrain for New York in order t o spend the week of the 17th a t the exposition. T h e usual meeting of the American Ceramic Society will be held in conjunction with the exposition in the Grand Central Palace on September 19, probably extending over several days. T h e annual banquet and election of the Salesmen's Association of the American Chemical Industrv will also be held during this week, probably at one of the P-ershing Square group of LotGls near the Grand Central Palace. The regular dinner of the American Institute of Chemical Engineers will be on Wednesday evening, September 19, at the Park Avenue Hotel, reservation for which should be sent t o John C. Olsen, secretary, 85 Livingston'St., Brooklyn, N. Y .
