Sept., 1957
STRAINELECTROMETRY AND CORROSION
the polymer contained anionic groups with a similar spacing so as to allow close fitting of opposite charges. There is no clear indication of ethylenediamine binding in the results obtained at the higher concentration and the data for the lower concentration are subject to the same uncertainty as discussed above for the ammonium ion. The results of the present investigation show clearly that there is no correlation between the ability of individual carrageenin molecules t o bind counterions and the effectiveness of these counterions to produce, under favorable conditions, the precipitation or gelation of the hydrocolloid. Stoloffl gives the order of effectiveness of cations in the gelation of carrageenin as K + > Ca++ > Mg++ > Na+ while the dialysis equilibrium data show no difference in the Dolvmer interaction with K + and Na+ and the rev&se”order of affinity with
1179
the alkaline earths. It must be concluded that gel formation involves the counterions in a different manner than interaction with the isolated macromolecules. This interaction could be either in the nature of “salt bridges” or the formation of microcrystallites which might be able to accommodate only cations of a certain given size. A similar interpretation of the selectivity of carrageenin between sodium and potassium was proposed by Bayleyi3 on the basis of X-ray diffraction data. Acknowledgment.-The authors wish to express their thanks to E. Borker and A. Stefanucci of the General Foods Research Laboratories who carried out all the analyses required for this study and to J. Olsen and M. Glicksman for valuable technical assistance. (13) s. T.Bayley, Biochim. Biophys. Acta, 17, 194 (1955).
STRAIN ELECTROMETRY AND CORROSION. 11. CHEMICAL EFFECTS WITH COPPER ELECTRODES BY ALBERTG. FUNK,J. CALVINGIDDINGS, CARLJ. CHRISTENSEN AND HENRY EYRING Department of Ch.emistry, Univer.sity of Utah, Salt Lake City, Utah Received ApriE 87, 3867
The potent.ia1 transient induced in a copper electrode through mechanical strain is studied as it depends upon the concentration of various solutions. Thc transients are of about one Recond duration, and the potential chmge is ordinarily about 0.1 volt. The results have been interpreted in terms of the film-rupture model, previously presented by the authors.’ Other possible mechanisms of potential change have been presented, and in certain cases these additional reactions appear to be important.
Introduction A metal electrode in aqueous solution acquires a potential as a result of chemical reactions near the metal-solution interface. When the electrode is plastically strained, a change of the interface chemistry leads to a change in the measured potential of the electrode. This potential change is generally negative (anodic), and is short-lived; a time of about one second is required for its decay. The decay time, as well as the maximum voltage change, depend upon the nature of the electrode and the content of the solution. We have measured these electrode strain transients with several metal electrodes (most extensively with copper), and in many different solutions. The data have been used to study the importance of the several chemical processes contributing t o the voltage change. Preliminary results have been published e1sewhere.l Considerable effort has been directed at understanding the change of electrode potential due to strain, but fewer people have considered the potential transient immediately following strain. However, the work of Dudley, Elliott, McFadden and Shemilt,2like our own, is concerned primarily with the electrode strain transients of copper in various solutions. They record transient voltages as high as 30 mv. in cupric sulfate solutions. Results were
also obtained for solutions of cupric chloride, magnesium sulfate and potassium nitrate. Nikitin3 measured electrode strain transients for copper, as well as for silver and iron. He observed larger potential transients in the case of work-hardened copper than for a soft-drawn electrode. Zaretskii4 used copper, magnesium and aluminum in a series of similar experiments. Gautam and Jha5 found potential transients for copper, but these were of opposite sign to those of references 2 and 3, as well as those reported here. The external resistance in their circuit was so low as to preclude transients of more than a few millivolts. Experimental Procedure
(1) A. G. Funk, J. C. Giddings, C. J. Christensen and H. Eyring. Proc. Natl. Acad. Sci., May (1957). (2) R. 8. Dudley, R. Elliott, W. H. McFadden and L. W. Shemilt, J. Chsm. Phys., 28, 585 (1955).
(3) L. V. Nikitin, J . Gen. Chem. ( U . S . S . R . ) , 11, 146 (1941). ( 4 ) E. M. Zaretskii, ZhuT. Priklad. Khdm., 24, 614 (1851). ( 5 ) L. R. Gautam and J. B. Jha, Proc. Ind. Acad. Sci., M A , 350
The apparatus used to study the effects of deformation and stress on changes in the electrode potentials of metals includes a shielded, constant-temperature cell containing two wire electrodes and the solution. The wire electrodes are inserted through holes in the bottom of the cell and connected to an insulated frame. They are then connected into the amplifier with shielded cables to avoid pick-up of stray electrical fields. One wire is left undisturbed whilc the other is attached to an insulated weight holder. Tension is ap lied by adding designated weights to the weight, holder. $he changes in length are read by a pivoted pointer on a meter stick.
Theory and Results An example of the potential-time behavior of
(1953).
A. G.FUNK,J. C.GIDDINGS,C. J. CHRISTENSEN AND H. EYBING
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b
II
0
I
0.2
0.4
0.6
0.8
(SEQ.
Fig. 1.-The electrode strain transient of copper in distilled water.
metal electrodes is shown in Fig. 1. The stress has been applied to the electrode at the origin of the time scale, for reasons discussed below. The resulting strain causes chemical reactions involving the transfer of charge across the interface t o proceed at different rates, and a potential change results. An equation which approximates this voltage change is'
where C is the capacitance of the immersed electrode, V is its potential, e the charge on the electron, and Zi the number of charges transported across the interface with the occurrence of a single ith process. The anodic reaction velocity (reactions per second per unit area), for V = 0, is vi,, and the reverse (cathodic) reaction has a corresponding velocity, v'io. Equation 1 will in general involve the time implicitly through a dependence of the reaction velocities, v , on time due to changing conditions at the electrode interface. At the point of maximum voltage change, the left side of eq. 1 goes t o aero. The maximum negative voltage change, henceforth called the arnplitude, is approximately
where it has been assumed that for every i, zi = z, a constant value. The reaction velocities before strain are vi0 and v'io, and those afterwards are uio and u t i o . This equation has been derived in greater detail in ref. 1. Results obtained so far indicate that the left side of eq. 1 can be negleched, not only a t the point of maximum voltage change, but, t o a good approximation, everywhere on the transient curve except in the region of rapid growth. This is the steady state approximation for charge transfer, and does not differ in principle from the steady-state approximation for free radical concentrations used in chemical kinetics. The latter approximation is valid when following a perturbation from the steady state, the restoration t o the steady-state concentration is rapid. I n the case of strain electrometry, this relaxation time is a t least as short as the growth time6 (time elapsed between applica(6) The relaxation time, and t h e time needed to oompleta the straining of the wire seem to be of the same order of magnitude for copper. The growth time approximates to the aborter of these two times. Data which concern the relaxation time for copper and other metala will be published in the near future.
Vol. 61
tion of stress and maximum voltage change). Hence the steady state approximation, which leads to eq. 2, is reasonably accurate for the slower decay processes. Equation 2 provides an approximation for the dependence of the observed voltage transients on the various cathodic and anodic reaction velocities. The discussion which follows concerns the nature of some of the important reactions which will have t o be considered in the application of the quantitative theory. The reduction of oxygen through the reaction '/a01
+ HIO + 2e-
--3 20H-
(3)
is one of the principal cathodic reactions to be considered. When the entire surface is covered with oxide film, this reaction may be presumed t o occur somewhere in the film, or a t the film-liquid interface. Under certain conditions the reaction may proceed directly at the metal surface. I n acid solutions, where the film has dissolved, this must be the case. We have postulated that strain ruptures metallic oxide films and, in this case, a metallic surface is presented for the reduction of oxygen. The reduction of the hydrogen ion may be important in some cases, but usually it does not make a sizable contribution to the cathodic current. The combination of metal ions with the surface is another cathodic reaction t o be considered. This reaction is no doubt important in those areas free from the oxide film. This will be discussed presently as a possible contributing reaction (through concentration polarization) to the decay process. The anodic reaction of primary importance is the dissolution of the parent metal to form ions in solution. An oxide film greatly hinders this reaction, since the ions must diffuse through the film before they can cross the electrical double layer, and so contribute t o the potential. I n regions where there is no oxide film, the reaction is rapid. This occurs, as suggested, in acid solutions, and in areas where the oxide film has been ruptured by strain. An additional class of reactions, involving impurities, has been suggested by Gibbs.' Theoretical considerations on the dynamic properties of solids have suggested that impurities, originally in the interior of the solid, are deposited on the surface as the solid is strained. These patches of impurities could react either cathodically or anodically. Thus if the principal impurity is a metal higher on the electropositive scale, the conversion of the metallic atoms to ions would yield an anodic current. In certain cases, negative ions might be brought to the surface as oxides, chlorides, etc., and the passage of these across the double layer would provide cathodic current. Evidence concerning the importance of impurities is found in the measurements comparing the transients of oxygen-free, high-conductivity copper, and copper in which the oxygen has not been removed. Each electrode is strained several successive times, with a period of delay before each additional strain. The initial transient of oxygencontaining copper is more cathodic than the initial (7) Private communication with Dr. Peter Gibbs.
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Sept., 1957
STRAIN
ELECTROMETRY AND COflROSlON
transient of oxygen-free copper, indicating that negative ions are contributing to the cathodic current of the former. After the third to fifth transient, the difference between the two has practically disappeared. This is in agreement with the theory, which predicts that the supply of impurities to the surface diminishes with successive strains. Under ordinary experimental conditions, where the electrode possesses a protective oxide film, the cathodic reduction of oxygen and the anodic dissolution of metal ions, along with the reverse of these reactions, appear to be the important processes contributing to the transient voltage. It is doubtful whether the other reactions alter the general characteristics of the transients, although they may be responsible for detailed differences, as outlined above. It is postulated that strain causes the rupture of the protective oxide film, and with this, the metal ions pass much more readily into solution. Hence the transient is anodic. This has been observed in the case of copper as well as with silver, iron, aluminum, nickel and several alloys. There are several processes able to contribute to the decay of the transient as observed in Fig. 2. Concentration polarization of the anodic areas is probably the most important decay process. However in those cases where the regrowth of the oxide film over the ruptured areas is rapid, this may control the rate of decay. Also, when the reaction of surface impurities is important, the depletion of the impurity deposit will contribute its effect to the decay process. Values for the decay time are reporbed here for a number of experiments, but extensive correlation with theory has not been made. We will now proceed to outline the experimental results in greater detail. Data has been obtained which shows the effect of saturating the electrode cell with various gases. The results show that the electrode reactions are practically unaffected by hydrogen, since the transients with hydrogen and helium are the same. However, the amplitude of the transient is increased with the oxygen content. This is a result of the strong contribution of oxygen reduction to Zv'io, in eq. 2. With hydrogen or helium, the transient amplitude, -AI',, is 40 mv. With air, -AVm = 47 mv. and in oxygen, -AV, = 72 mv. Saturation was obtained by bubbling the gas through the solution for 15 minutes, then simply maintaining an atmosphere of the gas during the experiment. It is doubtful whether all the occluded oxygen in the oxide film is removed by the bubbling hydrogen and helium, so that the oxygen reaction, though diminished, is still significant. Figure 2 shows the effect of pH on the amplitude of the transients. Except for acid solutions, a single anodic peak characterizes the transient. At pH's less than 2, two peaks are observed. The first is anodic and is followed quite suddenly (0.5 sec.) by a cathodic peak. Both peaks are indicated in Fig. 2. I n the intermediate pH regions, the data follow the film rupture model. The rupture of the film enhances the passage of copper ions in both directions across the interface. The combina-
1181
I40
I20
100
-
80
v)
t; 0
60'
2
2 5,
40
>E
-
BASE NOOH ACID HNO3
-
eo
ANODIC AMPLITUDE CATHODIC AMPLITUDE
0
-20
-40
0
I I
I
I
I
I
I
I
I
2
3
4
5
6
7
8
I 9
I
I
1011
1
1
121514
PH.
pH dependence of transient amplitude.
Fig. 2.-The 200
1
I
PYRID'NE
I
10-5
10-4
I 1.50
I
I
AMPLITUDE 0 DECAY TIME
I 10-3
I
I
10-2 C (MOLESILITERL
I
10-1
Fig. 3.-Amplitude and decay time as a function of and pyridine conoentration.
I
",OH
tion of ions with the surface a t these ruptured areas is one of the important cathodic reactions following strain. An increase of hydroxide ion lessens the concentration of copper ion, as deter-
1182
A. G. FUNK, J. C. GIDDINGS,C. J. CHRISTENSEN AND H. EYRING
100
cuso4 {{
1
cuso4
I
2.n
AMPLITUDE AMPLITUDE' ODECAY TIME
8
SODIUM AMPLITUDE TARTRATE E o DECAY TIME 0
';
\
60
-I
1::
- 0
0.4
--
01 10'5
0.2
I
I
I
104
10-4
10-2
10
I
10-1
I
C [MOLES/ LITER 1.
Fig. 4.-Am litude and decay time as a function of the CuS& and sodium tartrate concentrations. 50
25 .-ANODIC AMPLITUDE *-CATHODIC AMPLITUDE
45
0-DECAY TIME OF CATHODIC TRANSIENT
40
12.5
35 -*
m
30
!-
0
3
L i
o
25
w
2 *
5 >E
7
20
15
-12.5 10
5
25 10-5
Below a pH of 2, the acid solution removes the protective film. In any solution in which the oxide film has been removed, we expect smaller amplitudes, since the reactions before and after strain are about the same. This will come up again when working with various complexing agents. It is still necessary to understand the reactions contributing to these smaller transients. Impurities brought to the surface with strain would cause a change of electrode potential. Depending on the type of impurity, this effect could be responsible for either the cathodic or the anodic peak, or both. However, in these concentrated acid solutions, the reduction of hydrogen ion, catalyzed by the active sites and impurities characteristic of the new surface, may be responsible for the cathodic peak. The .increase of the cathodic amplitude with increased hydrogen ion concentration is evidence in support of this mechanism. At high values of the p H the amplitude is again small. In this case dissolution of the film occurs with the formation of the cuprite ion.8 Figure 3 shows the amplitude and decay time as a function of NH40H and pyridine concentrations. The decay time is the time required for the transient potential to decay from its maximum value to half of this. The results for NH40H have been presented e1sewhere.l The amplitude increases with the concentration of these substances for the same reason that it increases with hydroxide ion concentration. These complexing agents are able to remove copper ions from solution just as hydroxide does, thus reducing the cathodic reaction after strain. It was noticed in acid solutions, where there is no protective film, that small amplitudes were obtained. In general, the amplitude increases with the ability of the oxide film in preventing dissolution, through the film, of the metal ions. Thus we are led to expect, in Fig. 3, that a change in the protective film is responsible for the sudden drop of amplitude. Visual inspection of the electrode surface verifies this change for the two examples. In the two minute immersion time that precedes strain, a concentration of "*OH greater than 0.01 M , and a concentration of pyridine greater than 0.1 M , cause a noticeable change in the film, which is the first stage in the complete removal of this layer. These two concentrations correspond approximately t o the maximum on the amplitude curves. F Figure 4illustrates the data for solutions of CuSO4 and sodium tartrate. The two examples are different with respect to the fact that the wire electrodes strained in sodium tartrate solutions were soaked in distilled water for several days before the experiment. The soaking has been used in some cases to obtain more reproducible data and, incidental to this, a larger amplitude is obtained. Apparently soaking causes structural changes in the film which leave it more protective with respect to cation diffusion. The amplitudes of other metals, silver and nickel, are not noticeably affected by soaking. Soaking has been used for the data in Figs. 1, 3, 5 and 7, and the example mentioned
---__
----o-
20 IO
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10-4
10-3
10-2
10-1
I
C ( M D L E S I L I T E R I NaCN.
Fig. 5.-Amplitude
and decay time in NaCN solutions.
mined by the solubility product of the hydroxide. and increases the amplitude This diminishes oi'& of eq. 2.
(8) G . V. Akimov and 14, 1486 (1940).
I. L. Roeenfeld, J . Phyo.
ch0'hent. (U.S.S.R.),
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STRAIN ELECTROMETRY AND CORROSION
1183
50 1 1 IO here. I n all these cases the potentials for M solutions are about equal (near 90 mv.). Such rn AMPLITUDE oDECAY TIME dilute solutions do not usually change the amplitude ,/"- ' significantly from its value in distilled water. The curve for CuSOc shows that, with increasing 8 concentration, the cathodic recombination of metal ions with the surface becomes more important, and so reduces the amplitude. The drop in 7 amplitude for sodium tartrate may be due t o a decrease of the reduction of oxygen, appearing in the Zv'io term of eq. 2. The oxygen on surface sites 6 may be reduced by the tartrate instead of through the electrode reaction 3. The increase of amplitude a t higher concentrations might result from an additional precipitated film of cuprous tartrate. Fig. 5 shows the results for NaCN solutions. 4 Cyanide ion complexes vary strongly with copper, so that the oxide film starts to disintegrate a t dilute concentrations. With the increasing loss of this 3 protective film, the amplitude decreases, as shown. At concentrations of M and greater, a cathodic peak follows the anodic one by about 0.1 see. This 2 behavior is the Same as with acid solutions, presumably because of the complete removal of the I protective film. The decay times are those characteristic of the final, cathodic peak. Figure 6 shows results for two solutions, each 0 containing cupric ion and chloride ion. The amplitude shows the same decreasing trend in dilute C (MOLES/LITERl. solutions as for CuS04 (Fig. 4),and may also be atFig. 6.-Amplitude and decay time data in chloride and tributed to the increased rate of ion combination cupric ion solutions. with the surface. At higher concentrations, a con150 I I 0 I I 2.0 siderable amount of cuprous chloride can be seen t o deposit as a white film on the electrode surface, It is assumed that the inhibitive properties of this film oause the amplitude t o increase a t these concentrations. The decay time in 1 M CuClz is the shortest that \ 1.60 has been observed in strain-electrometry measurements. With such high chloride ion concentration, it is expected that, when the film is ruptured, the cuprous ions passing into solution immediately re-form the protective precipitate of CuC1. The decay time will coincide with the time necessary to form this protective film. I n Fig. 7 the absence of the initial concentration of cupric ion causes a different behavior for the two curves than is observed in the previous example. The initial increase of amplitude with concentration has been seen previously (Fig. 3) with those solutions able to tie up the ions of copper in the neighborhood of the electrode surface. After the amplitude maximum, it appears that the oxide film is being peptized. This occurs more readily than in CuClz solutions, since in the latter there is 30 IS NH4C,(rn ~ O AMPLITUDE E C A YTIME an oxidation from the oxide film. It is probably because of the low concentration of cuprous ions *AMPLITUDE able t o form that a significant protective film of LiCl{ ODECAY TIME CuCl does not seem to form in the case portrayed I I I I O L '0 by Fig. 7. However, the final upswing in the LiCl 10-5 10-4 10-3 10-2 10-1 I curve indicates the possible formation of a CuCl C (MOLES/LITER). film. Fig. 7.-Amplitude and decay time for NH&1 and LiC1. Acknowledgment.-The authors wish to ac- Research, Contract Number N7 onr 45103, in knowledge a grant from the Office of Naval support of this work. I
,
4
I
wl
