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Mar., 1953 :PHENOMENA AND THERMODYNAMICSOF IRREVERSIBLE PROCESSES 275 by the loss of active sites by the readsorption of im- Further attention would have also been given to purities and other species. the (‘stripping off” of adsorbed hydrogen through When the ultrasonic waves were turned off, the cavitation except that such an effect would modify overvoltage did not recover its original value. This the slope of the overvoltage-log I plot. The only can be explained on the basis that impurities and apparent way such a meqhanism could be a major other particles adsorbed irreversibly on the elec- factor without influencing the Tafel slope would be trode surface prior to the overvoltage measure- for the stripping action to be proportional to the ments were not readily readsorbed at the more square of the surface concentration which is un- cathodic potential of the electrode when polarizing likely. Explanations involving the formation of current was being passed. This hypothesis is sup- electroactive materials such as free radicals through ported by the fact that the overvoltage regained cavitation have been given little consideration its original value when the polarizing current was since the effect of such depolarizers would be de- turned off for a short time. Under these circum- pendent on current density, Likewise, it does not stances the potential of the electrode decayed to a seem feasible to explain the effect in terms of voltage approaching the original value before pass- changes in the average at the electrode age of polarizing current. .because such effects would be very small in a well The ability of ultrasonic waves to strip off ad- thermostated system and would not persist after sorbed materials o~ the electrode surface can be the radiations have ceased. readily appreciated if consideration is given to the Thus, the two explanations involving the factor fact that high intensity ultrasonic waves (10 watts/ H in equation (14) and N in equations (6) and (7) cm.2) have been used (in this Laboratory) to pro- seem most feasible in terms of the experimental re- duce suspensions of metals such as copper and alu- sults. Experimental is in progress at the minum by direct irradiation of these metals as present time in an. attempt to further substantiate foils in liquids. An increase in the surface area these hypotheses . through the erosion of the electrode surface might Acknowledgments.-The authors are pleased have been considered as a direct explanation for the to acknowledge the assistance of Dr. D. N. Stai- decrease in polarization produced by ultrasonic copoulos who helped in the design and construction waves if it were not that the original values for the of the ultrasonic propagation system and Dr. John polarization are obtained after the current has been Yeager who helped in the design and constructed interrupted for a short period. the ultrasonic generator used in this investigation.

ELECTRODE PHENOMENA AND THE OF IRREVERSIBLE PROCESSES1 BYPIERRE VAN RYSSELBERGHE Department of , University of Opegon, Eugene, Oregon Received Norember 17, 1966

The thermodynamics of irreversible procc~ssesdevelopcd by De Donder, Prigogine, de Groot aitd others makes ossible a systematic presentation of electrochemical thcrnmodynaniics in which reversible electrode processes, reversible ce??ls, states of electrochemical equilibrium, etc., appear as limiting cases in a more general treatment. A rational thermodynamic theory of polarization can thus be constructed. The concepts of uncompensated , power of irreversibility, production and affinity of irreversible chemical reactions are now extended to irreversible electrochemical processes. The concept of power of polarization is introduced and its connection with electrochemical affinity and overvoltage is developed and discussed. In addition to complete cells one treats single elpctrodes which are the seats of one or of several half-reactions. In the case of mixed electrode processes occurring not too far from reversibility, linear relations between reaction velocities (or currents) and electrochemical affinitiefi (or overvoltages), with application of Onsnger’s reciprocity principle between “drag coefficients,” are used. Prigogine’s theormi concerning stationary states of minimum is extended to electrochemical processes. These theoretical considerations suggest experimental investigations, in particular in the ficld of corrosion. Introduction who are conceriied with the whole range of degrees A rational thermodynamic t,reat men t of electro- of irreversibility of chemical reactions ( and chemical systems (galvanic and electrolytic cells, temperature being uniform and chemical potentials single ) requires the use of the thermody- corresponding to the ordinary statistical distribu- namics of irreversible processes. Although by no tion functions of the various components) and the means new this discipline is still unfamiliar to the authors. who are mainly concerned with the neigh- majority of physical and electrochemists. borhood of equilibrium and with the linear relations Moreover, has not yet been given between reaction velocities and affinities (or more more than scant attention by the main contributors generally between flows and ), the‘phenomeno- to the field of irreversible thermodynamics. Among logical coefficients obeying Onsager’s reciprocal these we should distinguish between the authors relations.2 The first group of authors consists (1) Paper presented at the Symposium on Electrode Processes held chiefly of De Donder and his collaborators, often by the Division of Physical and of the American designated as constituting the “Brussels or Belgian Chemical Society at the Atlantic City, N. J., meeting, September 14 to 19, 1952. (2) L. Onsager, Phys. Ren., ST, 405,2265 (1031). 276 PIERREVAN RYSSELBERGHE Vol. 57 school.” A first presentation of electrochemical usually found in the discussions of these irreversible thermodynamics was included in the 1936 mono- phenomena, a quantitative treatment such as that graph on affinit~.~Prigogine4 also devoted a chap- available through irreversible thermodynamics ter to electrochemistry in his monograph of 1947 in appears highly desirable. which the Onsager relations and their implications The electrical potentials we shall be dealing with are systematically added to the De Donder type of are the internal ones which have been clearly de- presentation of chemical thermodynamics. In de fined by Lange.g We shall designate them by the Groot’s recent book6 will be found a very brief dis- Greek letter p. The differences of internal poten- cussion of electrochemistry which does not go be- tials between two phases are Galvani potential yond the scope of Prigogine’s presentation, Sev- diferentes. The external electrical potentials $ are eral other authors have made some use of irreversi- those taken at a distance of the order of cm. ble thermodynamics in their electrochemical stud- of the external geometrical surface of the phase. ies. Among these we wish to mention particularly Differences of 6 potentials are Volta potential Piontellis and Pourbaix.’ In a monograph now diferences. Between the p and $ potentials per- nearly completeds we present a detailed and system- taining to the same phase we have the relationship atic treatment of electrochemical systems and . k electrode processes by the methods of the theory of P’rl.+X (1 1 affinity of De Donder and taking into account the in which x designates the surface potential differ- later developments contributed by Prigogine and ence. others. In the present communication we wish to Our electrochemical systems will be of one or an- outline the essential steps of this treatment which other of the types represented on Figs. 1 to 4: an we believe to be of fundamental importance to the electrode of metal CY and one of metal cy’ dip in the progress of our understanding of electrode processes. ‘ same p - p‘ (Fig. 1) or in separate solu- Time and space limitations us to postpone to tions /3 and p‘ connected by means of a siphon con- later communications certain important aspects of stituted in such a manner as to minimize liquid the subject and in particular the discussion of junction potential differences (Fig. 2). To metal thermo-elec trochemistry, Pel tier , e tc. CY we attach a piece of metal a” identical with a‘. The systems we shall be considering are cells, This precaution does away with several complica- galvanic or electrolytic, and half-cells or single tions to be discussed later. One may also attach electrodes. In addition to heat and work against pieces of a common metal to a and CY’(Fig. 3). the external pressure a exchanges electrical work The external circuit may consist of a simple resist- with its surroundings through the passage of an elec- ance or it may include one or several generators of trical current from’one terminal to the other in an current (Fig. 4). Our thermodynamic “closed” external circuit. Inside the cell this current causes system is (~~”app’a’),the external circuit being a electrode reactions to occur in accordance with portion of the surroundings. During the lapse of Faraday’s laws of electrolysis. At the same time time dt we have dne- moles of electrons transported electrolytic migration and various polarization from terminal a” to terminal a’. The positive cur- phenomena will occur in the system, diffusion rent I flows in the opposite direction and we have phenomena will occur at liquid junctions, etc. All I=F-dne- these phenomena are irreversible. Perfect reversi- dt (2) ble behavior is exceptional and is theoretically pos- in which F represents the Faraday. The layers of sible only for zero current. Independently of po- solution p and p’ are taken at sufficient distances larization, diffusion, etc., the passage of current will from a and cy’ to maintain from @ to 8’ a composi- result in the production of the Joule heat R12 which tion identical with that before the passage of the is an ever-present item in the various contributions current. The. various phenomena resulting from to the total uncompensated heat. It is clear that, the passage of the current (polarization, migration, beyond the inequalities and qualitative statements diffusion) involve the layers between a and p or a’ (3) Th. De Donder, “L‘AffinitB,” now presentation by P. Van and p’. Rysselberghe, Gauthier-Villars, Paris, 1936, see Chapter XVI, pi). If we consider dn,- as positive we have at elec- 123-137. Th. De Donder and P. Van Rysselberghe, “Thermo- dynamic Theory of Affinity.” Stanford University Press, 1936, see trode a the anodic reaction Chapter XVI, pp. 123-137. X- = X +‘ e-(a) (3) (4) I. Prigogine, “Etude Thermodynamique des PhBnomdnes Irrkversibles,” Dunod, Paris, and Desoer, LiBge, 1947, see Chapter and at electJrodea‘ the cathodic reaction 111, PP. 29-47. Y e-(a‘) = Y- (5) 8. R. de Groot, “Thermodynamics of Irreversible Processes,” + (4) Interscience Publishers, Ino.. New York, N. Y., 1951, see Chapter The symbols X and I‘ represent one or several spe-. IX, PP. 181-184. cies corresponding to the oxidized forms, the sym- (6) R. Piontelli, see various articles and further referenccs in “Proceedings of the Second and of the Third Meetings of t.he Inter- bols X- and Y- one or several species correspond- national Committee of Electrochemical Thermodynamics and ing to the reduced forms of the couples (X, X-) and Kinetics.” Tamburini, Milan, 1951; Manfredi, Milan, 1952. (Y, Y-). It is convenient to write all electrochem- (7) M. Pourbaix, “Thermodynamiqbe des Aqueuses ical half-reactions in terms of one single electron. DiluBes-RBle du pH et du Potentiel,” Meinema, Delft, and BBranger, Paris, 1946. “Thermodynamics of Dilute Aqueous Solutions- In addition to the electrode processes (3) and (4) with Applications to Electrochemistry and Corrosion,“ Longinans- we have the transfer of electrons from a to a” Green, New York. 1949. Articles and further references in “Pro- oeedings, etc.”8 e-(a) = e-(a”) (5) (8) P. Van Rysselberghe, “Electrochemical Affinity-Studies in (9) E. Lange, “Elektrochemie des Phasengrenzen” in “Handbuch Electrochemical Thermodynamics and Kinetics,” a monograph in des Experimentalphysik,” Vol. XII, Part 2; also, ref. 6, 1951, 6ee Pp. preparation. 317-343. Mar., 1953 ' ELECTRODEPHENOMENA AND THERMODYNAMICS OF IRREVERSIBLE PROCESSES 277

Adding processes (3)' (4) and (6) me get the cell re- di? dn A c- action 5- x-+ Y = x+ Y- (6) which, through addition on either side of ionic spe- cies not involved in the reaction, can always be n written as a reaction between neutral species. This will of course be possible only if process (5) is added to (3) and (4). We have here one of the several reasons for making the terminals of the cell chemically identical. Another reason which will be immediately apparent is that in this manner there is no work of chemical extraction of the elec- trons to be considered in the total work done by the II svstem.-., - I- First Law of Thermodynamics for Electro- ul I chemical Systems.-During the time dt the of the svstem is increased bv dE, the heat received Fig. 1. is dQ, tke work done is dw. " We have dE = dQ - dw (7) The work against the external pressure is p dV and the work corresponding to the transfer of dne,- moles of electrons from CY" to CY' is dwel = F(pa' - Va")dne- (8) The energy balance of our system is thus dE = dQ - p dV - F(va' 'pa")dne- - (9) 3 /3' d' In the particular case of a short-circuit being es- tablished between the terminals, making pa' = 'pa", we have the physical chemical change of state of the system remaining the same as in (9) dE = dQ. - pdV (10) 1 in which the subscript s designates the condition of short-circuit. We have also Fig. 3. Fig. 4. dQ, = dQ - F('pa' - cpa")dn.- (11) All this applies to the case of short-circuit. With Second Law of Thermodynamics for performance of electrical work by the system the Electrochemical Systems entropy production is decreased. The uncompen- Introducing, in accordance with De Donder's sated heat, instead of being given by (12) is now method12the uncompensated heat dQ', the entropy given by (see (11) and (12)) increase dS, the affinity of the reaction A and its dQ' = A d4 - F('pa' - van)dne- 2 0 (16) progress dl during the time dt, we have in the ab- or, in terms of the current defined in (2) sence of electrical work, ie., for the condition of short-circuit in the case of our electrochemical sys- dQ' = [A/F - ('pa' - 'pa711 dt 10 (17) tem Introducing the corresponding value of dQ in (9) dQ.' = T dS - dQB = A d 5 2 0 (12) we obtain The affinity A is that of reaction (6) and is related dE = T dS - p dV - dQ' - P(pa' - qa")dne- (18) to the free G = E - TS + pV of the sys- or also tem by the relationsJO dE = T dS - pdV - A d4 (19) ($> =-A a formula which is identical with that holding in the RT absence of electrical work.3 Our reasoning is thus Here we have d.$ = dne-. The ratio dQ',/dt has seen to be equivalent to that of Prigogine4and of de been called the power of irreversibility,a while the Groot6 which is based upon the hypothesis of the ratio of this quantity over the temperature T is the validity of Gibbs' fundamental formula for the dif- entropy production (see particularly Prigogine4). ferential of entropy outside of equilibrium. Intro- We have ducing the electrical power produced by the system Pel we see that the power of irreversibility is given by the formula The total change in entropy of the system is such P AV - Pel (20) that in which v is the reaction velocity dt/dt or dn,-/df. Entropy Production Near Electrochemical Equilibrium and Entropy Production Due to the (10) P. Van Rysselberghe, Chem. Reus., 16, 29, 37 (1935): J. Chem. Joule Effect.-The difference A - F(@' - pa? be- Educ. 16, 476 (1939). ing such that its product by the current is positive 278 PIERREVAN RYSSELBERGHE VOl. 57 '

can be regarded as the cause of this current and, in tials or sums of electrochemical potentials. Intermetallic contacts are known to be unpolarizable and therefore equa- the range of small values of this difference, the cur- tion (35) will always hold. On the other hand the electro- rent will be correspondingly small. The two quan- chemical affinities of the cathodicand anodic processes will be tities can then be regarded as proportional to each different from zero whenever there is polarization. other. Representing the difference A - F(rpa' - Causes of Irreversibility Other than the Joule Eff ect-Po- larization-Overvoltages.-When the electrodes are po- pa") by3 we have larized the power of irreversibility includes, besides the R12 v = LAorI = Fv = FLx of formula (25), a term P, which we shall call power of po- (21) larization and we have in which L is the phenomenological coeficient2s5 connecting reaction velocity and affinity. The power of irreversibility and the entropy production are then such that From (24) and (31) we deduce [W - (pa' - $06') - ('p6 - 41 I = P, 2 0 The quantity 1/F2Lcan be considered as a general- (37) c ized resistance r and we have or, introducing separate cathodic and anodic powers of polarization P = rI2 (23) A Prc pa' - (@'= -0 - - Let us nom consider the case in which the only Fl (38) cause of irreversibility is the internal ohmic re- +,@- A Pm sistance of the cell. More precisely the only re- FI (39) sistance to be taken into account is that between The ratios A,/F and A,/F also appear in formulas (29) and layers p and p'. We have then (30) which show them to be equal to the reversible values of A the corresponding potential differences. Introducing again 'p@- @' = RI and 'pa' - pa" = - - RT (24) the electrochemical affinities one finds that formulas (38) F and (39) can be written and P,, d F~= (96' - pa') ('p6' - &)rev = qc > 0 --I - (40) -=-=P,, La The net amount of heat received during the time dt I ('pa - @) - ('pa - &rev = qs > 0 (41) is in which the familiar overvoltages qc and qn are related with dQ = T dS - Ri2dt (26) the thermodynamic concepts of power of polarization and electrochemical affinity. while the uncompensated heat is Single Electrodes-Anodic and Cathodic Currents- dQ' = RP dl (27) Relative Electrical Potential Diff erences.-The overvolt- ages defined in (40) and (41) are necessarily positive be- Decomposition of a Cell into Pairs of Phases in Contact cause thermodynamics requires the corresponding powers . Case of Reversible Electrodes.-On the basis of formulas of polarization or uncompensated heats or entropy pro- (24) we have ductions to be positive. It is, however, convenient, when- ever one studies a single electrode whose operating potential ('pa' - d') (P@- 'pa) (pa 'pa") = A/F (28) + + - difference from metal to solution is measured by means of when the only cause of irreversibility is the Joule effect. the method of the Haber-Luggin capillary, i.e., relatively Decomposing A into a sum of three terms corresponding to to a standard reference electrode, to give a sign to the over- the three proc,esses (3), (4) and (5) and introducing the voltage and to the corresponding current. Calling E the chemical potentials or sums of chemical potentials for the metal and 6 the solution one would always write, whether X, X-, Y, Y- species and for the electrons, we find the electrode functions as anode or cathode qa = (pe - - ((0, - d)W (42) or qe E,' - E, (43) in which E,' designates the relative potential difference under current and E, the reversible values of this relative difference. Both E,' and E, are so-called "reduction po- tentials." If one were to prefer to use "oxidation poten- tials'' Ea' and Ea one would write C These equations are the equilibrium conditions for the corre- sponding processes. Let us introduce the electrochemical q, = E8 - Ea' (44) potentials by adding to each the product with ziFa corresponding to the molar charge ziF of reactant or product i and the electrical potential of the phase where this Ea = -E, and Ea' = -E2 (45) species i is reacting or produced. Let We have discussed elsewhere" the problem of nomenclature and conventions for these potentials, It appears preferable Pi = Pi + ziFa (32) always to use potential differences from electrode to solu- One easily finds that the equilibrium conditions (29) to (31) tion and to speak of reduction or of oxidation potentials can then be written according to whether the particular reaction under consid- - -, - eration occurs as a reduction or as an oxidation. A, = PY + P:- - PY- = 0 (33) Since the second law requires that the power of polariza- ~I tion be positive we have = PX ,u:- = 0 4, ;x- - - (34) Pr, = TfIf 2 0 (46)

I "a "a" e A. = pe- - p,,- = 0 (35) In the case of an anodic reaction both qf and I, are positive, in which Ac, and 2, are the electrochemical affinities of (11) P. Van Rysselberghe, ref. 6, 1951, see pp. 315-316; also, the three processes, jix-, etc., single electrochemical poten- ref. 6, 1952, see pp. 409-415. Mar., 1953 ELECTRODEPHENOMENA AND THERMODYNAMICS OF IRREVERSIBLE PROCESSES 279 in the case of a cathodic reaction they are both negative. or, in terms of the total current I . Introducing E,’ and E, in (46) we obtain Pourbaix’s for- mulae (E,‘ - Er)Ie 2 0 (47) Plots of E,’ against I, are polarization curves. It is par- LlLZ - L1Z2 (E1 - Ez)* (60) ticularly convenient to use the following system of coordi- F2 L1 + Lz + 2LlZ nates: E,’ is p1ott;ed vertically with the nobility, measured The existence of drag coefficients LIZ different from zero by increasing E, values, augmenting upwards; anodic would mean that polarization curves of single half-reactions currents are plotted horizontally toward the right and are not additive when these reactions occur simultaneously cathodic currents horizontally toward the left. at the same electrode. This additivity is often assumed in Simultaneous Half-reactions at the Same Electrode.- spite of considerable experimental evidence against its So far we have considered the case of a single half-reaction validity. Not only is the mutual influence corresponding occurring in the anodic or in the cathodic direction at the to the LIZcoefficients a theoretical possibility but its magni- electrode e or at each of the electrodes CY and CY’of the cells tude and direction might be such as to make one of the re- represented on Figs. 1 to 4. If several half-reactions actions occur in its otherwise non-spontaneous direction. In other words the coupling of an electrochemical half- Xp- = Xp+ e- (p = 1, 2, . . .) (48) reaction by one or several others is possible and should be oceur simultaneously at the same electrode (all in the same looked for experimentally. Its existence could have con- direction or some in the anodic direction and the others in sequences of very great practical interest. the cathodic direction) the net current carried by the elec- States of Minimum Entropy Production .-A single elec- trode will be trode carrying a current I at a certain relative potential dif- ference E‘ constitutes an open system in which several elec- (49) trochemical affinities could be kept constant by means of suitable additions or removals of the various reactant and The power of polarization is now given by product species. Let us still consider the case of two si- multaneous reactions and let us maintain the electrochemical Pm = ZqpaIpc = Ee’Ie - ZEpJpe 2 0 (50) affinity of reaction 1 constant by maintaining E‘ constant in which qpcis the overvoltage of reaction p, I,, the partial and the chemical affinity A1 constant. There will then be a current of that reaction, E,’ the common reaction potential possible state of the system for which the power of polariza- difference for all reactions and Epe the reversible potential tion or the entropy production will be a minimum. On the difference of reaction p. Interesting developments into basis of Prigogine’s theorem12 on steady states of minimum which we shall not enter here involve the introduction of entropy production, which we are here extending to electro- mixed overvoltages for the various anodic and cathodic re- chemical systems, the reasoning is as follows: we differen- actions, of average reversible potential differences, etc. tiate P,, given by formula (59) with respect to EZ at E’ Let us consider only the simple case of one anodic and one and E1 = AI/F constant and obtain cathodic reaction. Formula (50)becomes then Pre = Ee’Ie - (EJa + EcIc) (51) with le = Ia + Io (52) Setting this derivative equal to zero we see that P=cwill be Za being positive and I, negative. an extremum when In the particular case of an isolated electrode carrying no net current we have I, = -Ia and hence Pre = (Eo - E&a (53) This extremum is a minimum since The difference E, - E, is equal to the affinity of the re- sultant purely divided by F and we are (a$)E,,E, = 2F2Lz > 0 thus led back to the non-electrochemical power of irreversi- bility The value of the minimum is A P = jj XI, = AV 2 0 (54) Further considerations of this type can be shown to bring a great deal of order in the electrochemical theory of corro- For this particular state of the system the currents are sion phenomena. The Neighborhood of Equilibrium-Use of Onsager’s Il = FZ(L1 - Z)(E’ - El) 12 = 0 (65) Reciorocd Relations.-Although the introduction in this therhodynamic theory of kineFic information such as that During the transition of the system from its original state provided by Tafel’s equation connecting overvoltage and to that of minimum entropy production the electrochemical ’ current density is of very great interest we shall limit our- affinity of reaction 1 remains constant. That of reaction 2, selves here to the neighborhood of equilibrium, Le., to the however, will vary until the value range of validity of the linear relations connecting reaction LIZ velocities and affinities. Let us consider two half-reactions Ai = F(E’ - Ea) = -F -(E‘ - El) (66) 1 and 2 occurring in the same direction or in opposite direc- LZ tions at the same electrode. Their velocities will be linear is reached. functions of both electrochemical affinities provided the re- Prigogine’s theorem is easily generalized to the case of sev- versible potential differences of these reactions are sufficiently eral simultaneous reactions. If, out of a total of n reac- close to each other to allow the system to function in the tions, the affinities (or here electrochemical affinities) of k close neighborhood of equilibrium. We then have reactions, are kept constant the system may reach in the course of time a state of minimum eatropy production in VI = Llk, + LIZ& (55) which the velocities of the n-k reactions which are not con- v2 = LIZ21 LZA2 (56) trolled become equal to zero (see de Grootls). These sta- + tionary states of minimum entropy roduction can be shown or, in terms of currents and overvoltages to follow a principle analogous to tiat of Le Chatelier con- I1 = FZ(Liqi Lnqz) (57) cerning the “moderation” of disturbances. + It would be highly interesting to explore experimentally Iz = F2(L1z71 + Lzd (58) the possible applications of these theoretical considerations formulas in which the Onsager reciprocal relation LIZ= LLIhas in the field of electrochemistry. In corrosion, for instance, been taken into account. The power of polarization is then the establishment of stationary states of minimum entropy

Pre = F2[Lj(E’- El)’ + L2(E’ - Ez)~+ (12) I. Prigopine, Ref. 3, pp. 55-59. 2LdE’ - EI)(E’ - Ez)] (59) (13) S. R. de Groot, ref. 4, pp. 195-207. HERBERTH. UHLIGAND GLENNE. WOODSIDE VOl. 57

production might to the blocking of destru'ctive reac- of the corrosion of iron by nitrites, chromates or even oxy- tions through the device of maintaining other reactions at gen when present in sufficient amount lends itself to interest- constant electrochemical affinities. It seems likely that new ing Further study on the basis of irreversible thermody- light could thus be thrown on the problem of corrosion in- namics. Let us note in this connection that, through the hibition. For example, the electrochemical mechanism approximate procedure of linearization of polarization which we have proposed with Pourbaix14 for the inhibition curves the practical range of usefulness of the above con- siderahons can be considerably enlarged. We ho e to re- (14) M. Pourbaix and P. Van Rysselberghe, Corrosion, 6, 313 turn to this and other topics of theoretical and appied elec- (1950). trochemical thermodynamics in later communications.

ANODIC POLARIZATION OF PASSIVE AND NON-PASSIVE CHROMIUM-IRON ALLOYS BY HERBERTH. UHLIGAND GLENNE. WOODSIDE Corrosion Laboratory, Department of , Massachusetts Institute of Technology, Cambridge, Massachusetts Received November 86, 1868

Anodic polarization data for 0-16,770 Cr-Fe alloys in 3y0 NalSOl are reported. Critical current densities for passivity accompanied by sudden reduction of anodic dissolution and a pronounced noble potential range from 150 ma./cm.* for iron to 0.05 ma./cm.Z for 11.6% Cr. At 12% Cr and above, a critical current no longer exists; instead any small polarizing current produces a marked shift of potential in a noble direction. Small corrosion currents, therefore, can equipotentialiae the surface and stifle the corrosion process in accord with the electrochemical mechanism of corrosion. Since 12% Cr and above mark the stable passive compositions (stainless steels) in solutions like NazSOd, it appears that the property of so- called passive alloys to equipotentialiae themselves is an important characteristic of passivit and perhaps an essential condition to their remarkable resistance to corrosion. Coulometric measurements show that atout 0.014 coulomb/cm.z or less is concerned with the passivation process for the 9.2-16.770 Cr alloys and probably for other compositions as well. This corresponds at most to a few monolayers of subatance on the passive surface. Thick oxide films, therefore, cannot possibly be the primary cause of passivity.

When iron is polarized as, anode at moderate A tubulus, sealed at each end with asbestos fibers and filled with the same solution is mounted on one side of the anode current densities in dilute sodium sulfate or sulfuric close to the metal surface. The opposite end is immersed , the major reaction is Fe -+ Fe++ + 2e. As in a 0.1 N KC1 solution into which an Ag-AgCl electrode the current is increased, a critical value is reached also in 0.1 N KCI is immersed. at which the potential suddenly changes to a value Potentials were measured using a vacuum tube potenti- ometer. No correction was made for liquid junction po- more noble by about 2 v., and the anodic reaction tentials, since these were assumed to be approximately con- changes to stant throughout the measurements and, in any case, were 20H- -+ 1/z02+ HzO + 2e small. Current was supplied usually by a high-vqltage B battery through a high resistance, thereby maintaining con- accompanied by the reaction stant current, actual values of which were read using a pre- Fe -+ Fe+++ 3e cision (f l/,%) milliammeter and also, for very small cur- + rents, by the potential drop across a precision 100-ohm re- Iron is said to become passive, both because it as- sistance. sumes a more noble potential and because oxygen For many measurements, the sodium sulfate solution was first freed of chlorides by adding silver sulfate and removing evolving at its surface resembles the behavior of a excess silver by electrolysis for several hours between plati- noble metal electrode. Also, the dissolution rate is num electrodes. Contaminating heavy metals were re- appreciably less than that of an active iron anode. moved simultaneously. The pH was then adjusted to 7 Soon after the polarizing current is stopped, the by adding sodium hydroxide. This urification procedure electrode again assumes the active state. If the did not appear to be critical, since vaLes of potentials were ' the same within the experimental error whether or not pre- current is gradually decreased, the active state is electrolysis was carried out. achieved at a current density somewhat lower than Electrode materials consisted of electrolytic iron, and of is necessary to achieve the passive state when ap- laboratory melts of chromium-iron alloys. Alloys col- lected from three sources gave consistent results indicating plying progressively higher currents. that small differences in impurities are not important. Ther- Conditions that apply for achieving anodic pass- mal history, however, was felt to be a significant factor, ivity of iron-chromium alloys substituted for iron and, hence, all the alloys were heated in. helium at 1000° have not yet been studied. Information of this and water quenched. Analyses are gwen In Table I. Before measurements were made, the solutions were de- kind should be of value to a better understanding of aerated about one hour by bubbling nitrogen through them. passivity, particularly since passivity is established Nitrogen was previously purified by passing it over copper in a more stable form when the chromium content turnings maintained at 400'. The gas entered the cell reaches 12% or more, This composition estab- through a sintered glass disc providing a fine dispersion of gas bubbles, but during measurements the gas was by- lishes the basis for the modern stainless steels. passed through a glass tube at the side of the cell, so that bubbles did not impinge on the anode. This was necessary Experimental Arrangement because stirring had some effect on the critical current for Polarization data for the Cr-Fe alloys were obtained using passivity, greater current being necessary the higher the the cell shown in Fig. 1. This was placed in an air thermo- stirring rate. stat maintained at 25 f lo. Two circular platinum cath- Preparation of Electrodes and Procedure.-The anode odes 3.1 cm. in diameter are located 1 cm. from either side was attached to a nickel wire by spot-welding and then of the alloy anode. The latter measures approximately 1 sealed into a glass tube using de Khotinsky Cement to cover cm. square by 2 mm. thick. The electrolyte IS 3% NazSOd. the exposed wire and the spot-welded area. The electrode