Courteev, American Sheet and T i n Plate Co.
SCRAPSTEELIN CHARGIIVG BOXESov ITS W a y
TO THE
OPES HEARTH
Corrosion Problems JOHNJOHNSTON,United States Steel Corporation, Kearny, N. J.
A
An endeavor is made to outline a more cornoxygen; direct measurement of prehensiz,e of the of corrosion the equilibrium presaure shows rode in a given environment only in so far as that palladium doe- not oxidize than appears to be held by many of those who it can react with some constituabove about 7 9 0 ~ in air or above write about this subject. I n such a CWdensed 860" C. in p u r e o x y g e n ; for ent of that environment; if reaction is inherently impossible treatment of SO large a topic, there is danger of silver or mercury the correspondmaking ex cathedra pronouncements, for it m a y ing temperatures are still lower. under the conditions, there can not be possible to convey all of the qualifications Indeed a t any t e m p e r a t u r e be no corrosion. The general the difference between the preproblem involves, t h e r e f o r e , strictly necessary; yet the writer i s willing to vailingactual pres.ure of oxygen what is perhaps the central probrisk this and will u'elcorne any real et'idence, as in the e n v i r o n m e n t and the lem of chemistry: How, under what conditions, and to what exopposed to mere assertions, not in accord with his e a u i l i b r i u m Dressure in the syqtem metal-metal oxide is a tent, do two substances react general statements. direct measure of the tendency with each other? A p a r t i a l of oxygen, a t t h a t p r e s s u r e answer is given by thermodynamic considerations-namely, that two substances tend to and temperature, to react with the metal, or conversely for react rpontaneously so long, and only so long, as the reaction is the oxide to decompose spontaneously. An alternative, enaccompanied by a decrease in the free energy of the system. tirely equivalent measure is furnished by the electromotive This loss in free energy by the system may be evident largely as force of an appropriate reversible cell; but this method is, heat, as in ordinary combustion; or partly as mechanical vork, in effect, a special case, restricted to a narrow range of atpractice almost to the single as in an explosion engine; or partly (wholly, in the special case mospheric temperatures-in of a perfectly reversible cell) as electrical work; or partly as temperature 25" C. Whether we proceed by one way or the other forms of energy. Or it may be evident as the equiva- other to measure, and express, the departure from thermolent amount of work required to make the reaction reverse, dynamic stability in terms of free energy change or of electroas in the reduction of iron oxide to iron. The essential point motive force, is a matter merely of convenience; free energy is not the form in which the energy loss of the qystem is ap- considerations have the general advantage that they are apparent; it is that the initial state of the ,system is less stable plicable to a wide range of conditions, for they enable us to (or less probable statistically) than the final state. To take check and correlate equilibrium measurements made a t very a specific instance, in tbe system iron-oxygen, iron oxide different temperatures when the appropriate thermal data tends to form because it is more stable than iron and oxygen, (principally the specific heat of each substance concerned) separately, when the oxygen ii: a t atmospheric temperature are available. The essential point is that the difference in and pressure; this conclusion is evident from the fact that energy between the initial and final state of the syatem is a we must supply energy in order to re-produce iron from iron direct measuce of the "driving force" of the reaction, no oxide. Nevertheless, a t some temperature (probably about matter how we happen to observe i t ; in other words, this net 2000" C.) a t which the dissociation pressure of iron oxide difference measures the tendency of any chemical reaction would just exceed the presqure of oxygen in the atmosphere, to proceed spontaneously under given conditions. It is iron would cease to tend to corrode in air; and for the same therefore beside the point to speak of the "electrochemical reason it would not tend togunite with oxygen a t ordinary theory" of corrosion (and still more to doubt its essential, temperature provided that the partial pressure of oxygen in validity) merely because the electrochemical potential is, contact with it was of an order smaller than about 10 40 under some circumstances, a convenient way of ob-erving, atmosphere. If such a pressure seems too small to be real, and measuring, this fundamental energy difference, particularly where, as is frequently the case, it is not quite uniform we may take the completely analogous system palladiumSUBSTANCE can cor-
1238
7
December, 1934
INDUSTRIAL
AND E N G I N E E R I K G C H E M I S T R Y
over the metal surface. We might as well speak of an "electrochemical theory" of combustion (corrosion) of hydrogen and oxygen, for we can likewise determine its tendency to proceed by measuring the electrochemical potential of a reversible hydrogen-oxygen cell. Whether a substance is really stable or not--that is, does not or does tend to corrode-depends therefore not only upon its own inherent properties, but also upon the precise environment to which it is exposed. It is true that gold and the other so-called noble metals are entirely stable over a wide range of ordinary conditions, but under some circumstances they cease to be noble and are attacked. On the other hand, iron is under some conditions, as has been pointed out, noble with respect to oxygen-namely, at very high temperatures and a t low pressures of oxygen. Moreover, the range of stability of any one substance, when brought into contact with different substances, is different; even the order of relative corrosiveness of a group of reagents varies, in general, from one metal to another, and may vary indeed with change in the reagent concentration used in the comparisons. It is obvious therefore that, in so far as tendency to corrode is concerned, corrosion is not a single 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 is a point which should need no emphasis; yet statements are not infrequent, even in technical publications, which imply the opposite.
RATEOF REACTION So far the tendency of a reaction to proceed under given
circumstances has been considered and this driving force will be discussed later. The other dominant factor-and in many practical cases, the predominant factor-is the rate of reaction. For, clearly, 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, of course, if the driving force becomes zero, there will be no further reaction no matter how great its inherent rate may be. The rate of a reaction cannot be predicted; it can be ascertained only by trial under precise and specific conditions, and a n almost imperceptible change in conditions may affect the rate enormously. This phenomenon-catalysis-is largely unpredictable and still far from being thoroughly understood. It is not even true that reactions with great tendency (large loss of free energy) necessarily go faster than those with only small tendency. As a n illustration, the tendency of the reaction H P 0 +H20is very great, yet hydrogen and oxygen refuse to react at ordinary temperatures; they begin to react st higher temperatures, particularly in the presence of a so-called catalyst such as platinum black, which increases the speed but is without influence on the tendency of the reaction since i t itself remains substantially unchanged a t the end of the process. On the other hand, under similar conditions the reaction KO 0 NO,, which is accompanied by a smaller change in free energy of the system, goes readily. Another instance from a somewhat different field is the transformation of a-iron (ferrite) to y-iron (austenite) which in a steel containing 7 per cent chromium occurs at about 850" C.; yet the rate of transformation of the reaction a + y is a t least one hundred times greater than that of y +a , for a transforms completely before i t is heated a couple of degrees above the transformation temperature, whereas y may be undercooled many degrees before .any sign of transformation can be observed. There are analogous enormous differences in rate as between the several crystal transformations of the substance silica although the respective driving forces differ very little. The only general statement that is valid is that the in-
+
+ *
1239
herent rate of a given reaction, for a given driving forcethat is, a given departure from the equilibrium state-always increases, and increases rapidly, with increase of temperature. On the other hand, many processes comprise several steps, and the over-all rate of the process as a whole is limited by the speed of that step which under the conditions is slowest. One of these steps in many cases-especially in the attack on a solid by a gas or liquid, as in corrosion-is a relatively slow diffusion process a t the interface which, by hindering the coming together of the reacting atoms, determines the net rate of the process. The barrier thus set up is usually the predominant factor in slowing up or preventing the continuance of corrosion. This is brought out clearly by some recent careful investigations of the rate of scaling of iron, nickel, and copperthat is, of the rate of corrosion of the metal by oxygen a t high temperatures. A clean iron surfaee, exposed at a given temperature, say 1000" C., scales rather rapidly a t first, in accordance with the fact that the inherent rate of reaction between iron and oxygen a t 1000" C. is very great; but, as soon as a thin film of oxide has covered the surface, iron and oxygen atoms can meet only as they diffuse through this film, and consequently the over-all rate of scaling is then limited by the speed of this diffusion. Thus the rate gradually falls off as the thickness of the scale increases, so long as the scale remains continuous and adherent to the metal; and mathematical analysis of the curve representing the amount of scaling us. elapsed time at constant temperature enables us to calculate the constant of diffusion for a particular type of scale. This diffusion constant, which is a measure of the rate a t which iron and oxygen atoms can meet through a scale layer of given thickness, increases with increasing temperature, but not nearly so fast as the inherent rate of reaction itself increases. Actually, if W represents the gain in weight in grams per square centimeter surface of the specimen, t elapsed time in minutes, and T absolute temperature in O C., W = k ' d t at constant T W = k"e-b/* at constant t (or In W or by combination W = k d / t e - b / T and
=
In k''
- b/T)
For commercially pure iron and pure copper over a range of constant temperatures in air it was found that the experimental results were well reproduced by the expressions, W
=
6.30
dce-90QO/T
and W = 12.8 f i e - 1 Q @ O / T
respectively.' Moreover when scaling has once started, there is little difference in rate whether the gas is air, steam, or carbon dioxide. This shows again that the dominating factor is the rate of diffusion through the scale and not the driving force of the primary reaction; such differences as there are are due to differences in structure or constitution of the scale formed in the several gases. Since the driving force of the oxidizing reaction is beyond our control, except in so far as we can reduce the oxygen content, i t is clear that we cannot lessen the scaling of a metal in air unless we can somehow lessen the perviousness of the scale film or layer and thus hinder the meeting of oxygen and iron atoms. To find out to what extent this can be done is a matter of experiment, because at the moment little can be predicted as to what will constitute a successful impervious skin. It is now well known that the presence of chromium in a steel lessens the rate of scaling to such an extent that the rate of scaling a t 1200' C. of a steel cowaining 27 per cent chromium is only about 1 per cent of that in the absence of chromium; but it is difficult to give an explanation for this behavior 1 Lareen and Heindlhofer, Trans. Am. SOC. Steel Treating, 21, 865-95 (1933).
(b,r i!liroiiiiiim is riot a i i o l ~ l ciiietal with reapct t u oxygen) excelit in geiir:ral terms to the efitict tlint cliromioni dues furni a n oxide f i l i i r which is ndlmeiit and highly impervious, nrid can do this evm when a majority of the iitorrrs at tlic irriginal surface arc iron atoms. Apparently, Irowever, the addition of clironiimn is n r i ~ c hniore effective in reducing liigli temjx:ratim \\-licii :it I
