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The passivity of zinc in aqueous solutions of sodium carbonate and sodium bicarbonate
Elecrrochlmlca
Acta.
1964.
Vol
9, pp
383 to 394
Pergamon
P~CSS Ltd
Printed
1t1 Northern
Ireland
THE PASSIVITY OF ZINC IN AQUEOUS SOLUTIONS OF SODIUM CARBONATE AND SODIUM BICARBONATE* H KAESCHE Bundesanstalt
fur Matenalpruefung,
Berhn-Dahlem,
Bundesrepubhk
Deutschland
Abstract-The passlvatlon and passlvlty of zmc m 1 M Na,CO* (a), 0 1 M NaHCO, (b) and 0 01 M NaHCOs (c) have been mvestlgated by measurmg galvanostatlc chargmg and dlschargmg curves m the range between 1 ms and 30 mm In (a) passlvatlon 1s fast and brought about by small amounts of oxide, at 20°C by an approxnnately mono-molecular oxide layer In (a) the first traces of reducible oxide are formed mnnedlately after charging of the double layer. In (b) and (c) passlvatlon IS very slow, mvolvmg the formation of thick oxide layers In this case a marked dependence of the amount of oxide necessary for passlvation on the temperature indicates a smular dependence of the number mltlally formed oxide nuclei In (a) current efficiency with respect to oxide formation pnor to passlvatlon 1s high, It 1s very small m (c) due to parallel dlssolutlon of Zn to Zna+ Quasi-stationary current/voltage curves are presented for both (a) and (c) R&sum&& moyen de courbes charge-dkharge, Ctabhes par mesures galvanoplastlques dans l’mtervalle 1 ms B 30 mm, l’on &udle la passlvatlon du zmc dans (a) Na,CO, M (b) NaHCO, 0,lM (c) NaHCO, 0,OlM La passlvatlon raplde en (a) semble due, B 2O”C, il une couche monomol&dalre d’oxyde, dont les premldres traces se mamfestent d&s la charge. de la couche double Par contre, la passlvatlon est lente en (b), (c), ce qm unphque la formation de couches d’oxydes &passes L’mfluence de la tempkrature SW la quantltk d’oxyde co&rant la passlvlt& d¬e alors le rdle des premiers germes d’oxyde Zusammenfassung-Der Mechamsmus der Passlvlerung und der Passlvzustand des Zmks wurden m 0,l M Na,CO, (a), 0,l M NaHCOs (b) und 0,Ol M NaHCO, (c) an Hand galvanostatlscher Ladeund Entladekurven wahrend Z&en zwlschen 1 ms und 30 mm untersucht In (a) wlrd Zmk sehr schnell durch germge Oxydrnengen passlvlert, spenell be1 20°C schon nach Ausblldung emer cu monomolekularen Deckschlcht Dabel treten m (a) die ersten Spuren von reduzlerbarem Oxyd unnuttelbar nach der Umladung der Doppelschlcht auf In (b) und (c) 1st die Passlvlerung em sehr langsamer Vorgang, wahrend dem dlcke Oxydschlchten gebddet werden. Hoer zelgt die starke TemperaturabhFinggkat der Passmerungszelt emen starken Emfluss der Temperatur auf die Zahl der zunachst geblldeten Oxydkelme an In (a) 1st schon vor Emtreten der Passlvltat die Stromausbeute m Bezug auf die Oxydbddung hoch, w&rend m (b) die anodlsche Zmkauflosung emen hohen Stromverlust bewlrkt Sowohl fur (a) als such fur (c) werden die quasistatlonaren Stromspannungskurven mitgeteilt INTRODUCTION
passtvrty of zmc m aqueous electrolytes has been studred by a number of mvesttgators, especrally with respect to the behavrour of the metal m alkaline solutrons I-’ As recently shown by the authors during an mvestrgatron of the mechanism of prttmg of galvanized steel pipes carrying hot water, the well known spontaneous potential reversal of zmc m oxygen-contammg hot water9 can be interpreted m terms of spontaneous passrvatron m dilute soluttons of sodmm bicarbonate. Also rt was shown that the deleterious effect of copper traces in hot water appears to be due to the electrondonor properties of copper atoms mcorporated m the zmc oxide lattice, rendering the passive zinc an efficient cathode for oxygen reductron, which otherwrse m such electrolytes it is not. This corresponds to other finding+ according to which the THE
* Presented at the 14th meetmg of CITCE, Moscow, August 1963, manuscnpt received 8 October 1963 383
H KAEscm
384
electromc conductmty of zinc oxide produced by anodlzmg zinc depends on the excess zinc content and can be varied by varying the expermental condltlons. EXPERIMENTAL
TECHNIQUE
In view of the practical interest attached to the behavlour of zmc m such electrolytes, a closer mvestlgatlon of the passivity and passlvatlon of the metal m weakly alkaline solution was carried out The experimental arrangement was conventional Zmc electrodes (99 99 per cent Zn) were cut from sheet metal m the shape of small flags (surface area ca 4 cm2), sealed mto glass holders by means of polyethylene, cleaned, abraded, etched for 10 s m dilute HNO,-H2S04 solution, rinsed, and lmmedlately immersed m the solution m a glass vessel fitted with ground Joints. The solution was prepared with trlply distilled water and analytically pure salts, deaerated by passing a stream of purified nitrogen through, and thermostated to within fO*l”C of the indicated temperature. The solution was moderately stirred, and the electrodes freshly etched before each single measurement, unless otherwise specified The latter treatment was found necessary m order to keep scatter of experimental results wlthm reasonable limits Electrode-potential measurements were carried out with a Hg/HgO reference electrode (equipped with a Luggm capillary), at room temperature Measured values were recalculated with respect to the standard hydrogen electrode, usmg $0 244 V as the potential of the saturated calomel electrode which was used to control the Hg/HgO electrode, and neglecting temperature differences between the zmc and the reference electrode For polarization experiments, a platmum-gauze electrode was introduced m a compartment of the solution separated from the mam volume by a porous glass disk Potential/time curves at constant polarlzmg current were recorded with an oscdloscope (Tektronix A 535), or with an oscdlograph (Honeywell Vlslcorder), or with a self-balancing slow recorder (Radiometer), depending on the type of measurement. The ohmic resistance between the terminals of reference and zmc electrode was always kept at 210 Ma m the measuring circuit Unless otherwise specified the zmc electrode was always polarized with a cathodic current of - 10 mA/cm2, sufficient for ehmmatmg spontaneous corrosion, except for the time of measurement of anodlc charging curves For these measurements the polarization clrcult was switched wlthm less than 0 1 ,us from cathodic to anodlc currents, or back to cathodic currents, with the aid of a mercury-wetted relay Currents were taken from dry cells m series with resistance decades, and were constant to wlthm *2 per cent of the indicated values For current/voltage-curve measurements an electronic potentlostat (Wenkmg) was used. RESULTS
AND
INTERPRETATION
Behavzour of zznc zn 1 M Na,CO,-solutzon
The behavlour of the zmc electrode was first investigated m deaerated 1 M Na,CO, solution at 60°C The change of potential with time during galvanostatlc anodlc and cathodic polarization 1s schematically represented m Fig 1. At point A the anodlc charging current 1s switched on. In the time between A and B the electrode 1s passlvated, with followmg oxide growth between B and C, and superimposed oxygen evolution beyond C. If the current IS reversed during charging, then between F and G the oxide 1s reduced, and the potential finally returns to the potential of exclusive hydrogen evolution.
Passwty
I fF L_-_-_----. IJ +
385
of zmc m NaZCOs,q and NaHCOsas
hydrogen evolufron
reductron / oxrde G \ \, evokdon \‘z hydrpgen c ---____ T/me
W
FIG 1 Potential/time
curve of zmc m 1 M Na,CO, solution, 6O”C, dunng galvanostatic anodlc charging (sohd curve) and cathodic dnchargmg (dashed curve)
FIG
2
15 :?i3 10 05 Change of potential of zmc m 1 M Na,CO,, 60°C after reversmg the polarlzmg current from 5 &/cm2 to the mdlcated anodlc value
As shown m Fig. 2, the first rise of the electrode potential after reversing the current from a constant cathodic to a constant anodlc value can be Interpreted m terms of chargmg of the double layer capacitance C, where C = 22 ,uF/cm2 independent of the chargmg current density z, After reachmg the first plateau, the potential slowly rises, as shown by Fig 3 (which also shows the dlschargmg curve), until shortly before the change to the lmear increase of potential with time, typical for oxide-film growth, a sudden rise occurs. There 1s little doubt that 7P 1s the passlvatlon time, as the mflexlon at B corresponds to the expected potential behavlour for a change of current conduction through pores of a surface layer to conductlon by lomc transport across a non-porous oxide. lo If the property 7P 1s measured as a function of the charging current density lc, it 1s found (Fig 4) that log 7P 1s a linear function of log l,, with d log rp/d log I, equal to - 1 6 f 0 1. This relation 1s expected to hold m the present case, since other metal/electrolyte-solution systems, as Ag/H,SO, or Cu/HCl, where scarcely soluble salt layers are formed, generally also show a constancy of the product 1,~~~‘~lo l4 As shown elsewhere, lo a theoretlcal explanation other than the quahtatlve
FIG 4. Paswatlon
time Q, as a function of anodlc current density I,, m 1 M NaBCOB, 60°C
that the thickness of the oxide nuclei decreases with mcreasmg current density due to mcreasmg supersaturatlon, IS not possible at present In accordance with ths relation between the passlvatmg current density and the passlvatlon tnne It can be shown that the oxide IS formed at an early stage of the chargmg curve. As seen from Fig. 5, reducible matter 1s present on the electrode shortly after the begmmng of anodlc polanzatlon, and m fact the first traces of such reducible matter can be detected after only 2 ms of anodlc chargmg with 10m A/cm2 At a shghtly later stage, the cathodic reduction curve exhlblts a well-defined plateau, allowmg the determination For a comparison with thermodynamic data, slmllar of the reduction potential measurements were carried out at 2O”C, where the pH of the Na,CO, solution JS 12, and the potential of the reduction plateau (Fig 6) was found to be -1.15 f 0 05 V. As the eqmhbrmm potential of the Zn/ZnO electrode (standard potential -0 42 V)ll IS -l$B V, it IS seen that the reducible matter can be assumed to be zmc oxide In view of the low solublhty of the oxide (ca 1O-s mol/l at 2O”C), and transport of the zmcate ion into the bulk of the solution other than by dlffuslon bemg suppressed due to high salt concentration, an adherent solutlon layer of a tbckness of around 103 cm should indeed be supersaturated after chargmg with several ,&/cm2. Therefore, the behavlour of the electrode m Na,CO, solution departs from the behavlour m more alkaline solutions, where 7p is governed by diffunon-controlled supersaturation of the adherent layer of solutlon.6*7 As shown by Fig 7, temperature mfluences the shape of the anodlc charging curve Wtth increasing temperature, the mflexlon C attributed to oxygen evolution IS dlsplaced to less noble potentials, mdicatmg a decrease of oxygen over voltage. Also, the assumption
Passivity of zmc m Na,CO,.Q and NaHCOasp
387
Time,s
FIG 7 Anodlc chargmg curve of zmc m 1 M Na,CO,, at 20°C and 80°C
z, = 10 mA/cm2 Letters as used m Fig 1
temperature,
“C
FIG 8 Passivatlon time 7p as a function of temperature of 1M Na,COI.
I, = 10 mA/cma gradlent of the hnear portion of the chargmg curve between B and C decreases As this linear portion IS characterlstlc for the growth of non-porous oxide of constant specific lomc conductlvlty u, with transport of ions through the oxide layer, the decrease of the gradient can be interpreted m terms of an increase of cr with increasing temperature. With z, = 10 mA/cm2, d&/dt 1s 4 0 V/s at 20 “C, and 1.7 V/s at 80°C
The numerical value of (T, dependent on the roughness factor and on the density of the oxide, cannot be dzscussed at present Fmally, the passlvatlon time 7P decreases with decreasmg temperature, as shown m Fig. 8 It 1s seen that 7P approaches a lower hmltmg value of 0 1 s, and the product z x 7p therefore approaches a lower value of 1 mC/cm2. This corresponds to an approximately monomolecular oxide layer provided that the current efficiency with respect to oxide formation 1s 100 per cent The current efficency was checked at 60°C by comparing the amount of electriczty consumed durmg charging, ze the product zotc, were tc 1s the time of anodzc charging, with the amount of electnclty regained by oxide reduction, ze the product iDrE, where zD 1s the current density during discharge, and TV the time of oxide reduction (see
388
H. KAESCHE
Figs. 3, 5, 6). The ratio z,tc/zDTRwas found to be
FIG 9 The ratio QR/Qo = tDrB/Q, as a function of the anodlc charge z,to measured m 1 M Na,CO,, 6O”C, /z,l = /z,\ = 10 mA/cme
After passing through the mflexron at C during anodrc chargmg, the cathodic reduction curve becomes more complicated, as shown by Fig 10, mdicatmg the presence of reducrble matter not identical wrth ZnO, possrbly mdicatmg the reduction of absorbed molecular oxygen. The further rise of the potential beyond the second mflexlon at pomt D results m the appearance of a third distmct part of the reductron curve, as shown by Fig 11, and this possrbly Indicates zmc peroxide, the presence of which has been postulated. l3 It should be noted that after prolonged passrvatron a thermodynamic evaluation of potentials measured durmg cathodic reductron is not rmmedlately possible, since reduction of thrck oxrde layers may mvolve an appreciable overvoltage If anodic charging 1s further contmued, then, as shown by Fig. 12, the potential eventually rises to very high values until the external crrcurt can no longer furmsh the constant current Obvrously, the oxrde layer continues to thicken. It has been shown elsewhere1 that by such prolonged polarization oxrde films are obtamed that can be mvestigated by optical and X-ray analysis, ZnO being the only specres found Therefore the assumed presence of ZnO, IS still open to drscussron. As mentroned above, all measurements so far reported have been carried out with electrodes etched before each smgle experrment. If after passrvatron and subsequent reactivation the electrode is again anodrcally charged without mtermittent etchmg, the shape of the charging curve changes This may be seen from the sequence of charging curves A, B and C m Fig 13 A was measured with an etched electrode, B after anodic charging durmg 6 s and subsequent oxide reduction, wrthout etchmg, and C slmrlarly after 36 s of anodrc chargmg It IS seen that the major effect of pre-passlvatlon of the electrode IS a slowing down of the potential rrse, especially after onset of
Passivity of zmc m NaSCOI,a and NaHCO,,
389
FIG 13. Anodtc chargmg curve durmg prolonged polarzatlon m 1 M Na,CO,, 60°C zc = 30 mA/cmsI, with mterrmttent reactivation, without mterrmttent etchmg
20
I
‘!’ I-10
”
-08
”
-06
Potenhal, ” -04
”
V -02
”
0
” +O?
FIG 14 Potentlostatlc quasi-stationary anodlc current/voltage curve of zmc m 0 01 M NaHCO, solutron at 75°C (mcludmg change of curve urlth time of polanzatlon), and ..bZDfi
oxygen evolution. Thts change must be due to the influence of dtspersed metallic zinc In view of the observed on the electrode surface remammg after oxide reduction influence of excess metalhc zinc on the conductivrty of the passive oxide1 and m vrew of the similar mfluence of excess metallic coppers it may be assumed that the shape of curves B and C indicate an increase of electronic conductivrty of the oxide due to mcorporation of excess zinc, resultmg m an increase of the current density of oxygen evolution. However, as there also should be an increase of the roughness factor of the electrode surface, further experimental evidence with respect to the influence of the defect propertres of the oxide 1s necessary The quasi-stationary current/voltage curve of zmc m 1 M Na,C!Os solutron at 75 and 25°C measured with a potentiostatrc circuit, is shown m Fig 14. For measurmg the sequence of points shown at constant potential, at each single potentral the 5
390
H. KAFSCHE
electrode was etched and the cell filled with freshly prepared solution. It 1s seen that a statronary state is reached only very slowly, mdicatmg a considerable thickening of the oxide layer at each potential beyond the potential of passivatron. The latter potential corresponds to the first plateau (between pomts A and B, Fig. 1) of the galvanostatlc anodlc chargmg curve. Behavrour of zmc in O-1 M NaHCO, solutlon
The anodrc chargmg curve of zmc m deaerated 0.1 M NaHCOa solutron (75”C), measured as described above, is shown m Fig. 15 for different anodrc current den&es
-150
0
I
05
10
15 T/me,
mm
20
,?5
30
FIN 15 Galvanostatic anodlc chargmg curves of zmc m 0 1 M NaHCO* solution, 75”C, and cathodic dxhargmg curves Chargmg current as mdlcated, dshargmg current 10 mA/cma Anod~c chargmg mterrupted at $2 25 V
It IS seen that passlvatlon m this case proceeds very slowly. It is also seen that imtrally the potential passes through a distmct peak, returmng afterwards to a lower value and remarmng approximately constant for a time of the order of mmutes untrl the final rise due to passlvatlon. The discharging curves measured at 10 mA/cm2 are also shown m Fig. 15. It is seen that the current efficiency with respect to oxrde formatron is low. There is, therefore, an appreciable current loss due to zmc dlssolutron prior to the onset of passlvation From an mspectlon of the 10 mA/cm2 curve it is evident that approximately 4/5 of the current was not used for oxide budd-up, and the correspondmg amount of anodlcally dissolved zmc IS sufficient to saturate the solution with Zn(OH),. The appearance of flakes of loose white maternal is Indeed observed, and durmg experiments mvolvmg long polarization times, as for mstance at 40°C and I, = 10 mA/cm2, such flakes also adhere to the electrode, but this substance IS not identical with the passlvatmg oxide, and cannot be reduced durmg cathodrc discharge. Also, if chargmg curves are measured m a solution already used for previous passivatlon measurements, the passrvatlon time is the same as measured m a solution mrtlally free from dissolved zmc Smce the length of the potential plateau of the dlschargmg curves represented m Fig. 15 is approximately independent of the current density of the precedmg anodlc chargmg (m each case contmued untrl the potentral reached +2 25 V), the amount of oxide present is independent of the chargmg current density. Compared to the
Passlvltyof zmc m NaaCOs,Qand NaHCOIaa
394
behavlour m NazCOa solution the amount of oxide, correspondmg to an amount of electnclty of cu 300 mC/cma, IS very high, mdlcatmg a comparatively thick passlvatmg oxide layer By measurmg cathodic dlschargmg curves prior to the onset of passlvatlon, that IS, from potentials between the mltlal peak and the final nse of the potential m the region of the extended plateau of the chargmg curve, it IS found that m this case agam there IS reducible matter present on the electrode surface shortly after the start of anodlc charging. Durmg measurements of the passlvatlon at 75°C with zc = 10 mA/cm2 an mtlexlon of the discharging curve correspondmg to oxide reduction was found as early as 0 5 mm after the beginning of charging, where passlvatlon did not occur until 2 mm later After 0 5 mm of anodlc charging the amount of oxide already present corresponded to cu 80 mC/cm 2. Cathodic reduction curves measured at 60°C (pasavatlon time with z, = +lO mA/cm2, 7 mm) 5 mm after startmg the anodlc current are shown m Fig 16 as a function of the discharging current denstty zD It -1-00
-m? -Y-40
T/me FIG 16 Cathodic
swty,
reduction curves mdxatmg presence of oxide pnor to onset of pasmeasured at 6O”C, rc = 10 mA (chargmg curves not shown), cathodic current density as mdwated
IS seen that the plateau of the reduction curve markedly depends on zD, mdlcatmg a correspondmg overvoltage of oxide reduction The dependence of the oxide reduction potential on the dlschargmg current density ZDIS shown m Fig 17, and the unpolarized reduction potential is seen to be -0 9 V. Since the hydroxyl Ion concentration con_ at 60°C m O-1 M NaHCO, IS estimated to be lo4 mol/l, the equlhbrtum potential of the Zn/ZnO electrode should be -0.95 V, and therefore the oxide present on the electrode can be assumed to be ZnO. From the above-described observations It IS to be concluded that m this case again the eventually passlvatmg oxide IS present much earlier than passlvatlon occurs
392
-1.8_
FIG 17 Potential of oxide reduction m 0 1 M NaHCO*, 6O”C, as a fimctlon of dlschargmg current density z6
PIG 18 Chargmg and dlschargmg curves of zmc m 0 1 M NaHCOs as a fun&on the temperature I, = 10 mA/cmB, z = 10 mA/cms.
of
There IS no prmary passrvatmg layer drfferent from zmc oxrde. In contrast to the behavrour m Na,CO, solutrons m thus case the oxrde nuclei are relattvely thtck particles Such particles cannot, however, be observed dn-ectly, smce microscoprc mvestrgatron reveals only the presence of loosely adherent, powdery wlute material which may be either hydroxide or basrc carbonate,l and whtch cannot be Identified with the passrvatmg oxide As shown m Fig. 18, the passrvatron time decreases wrth mcreasmg temperature, m contrast to the observatrons made durmg experiments carrred out m Na&O, solutron Accordmg to the measurements recorded m Frg 18, the passrvatron time decreases from approxrmately 22 mm at 40°C to approximately 1 mm at 9O”C, with z, = 10 mA/cma. As the drschargmg current den&y was always 10 mA/cn?, and the plateau of oxrde reductron shortens considerably with mcreasing temperature, rt IS
393
Passlwty of zmc m NaBCOca and NaHCOJap
seen that the reason for the shortening of the passlvatlon trme 1s a marked decrease of the amount of oxrde necessary for passrvatron. From Fig. 18 the amount of oxide present at +25 V ISestrmated to correspond to 2800 mC/cm2 at 4O”C, and 150 mC/cm2 at 90°C Consrdermg that the presence of dlstmct oxrde particles prror to the onset of passrvrty has been shown already, the observed dependence of the amount of oxrde on the temperature can probably be best explamed by an Increase of the number of oxide nuclei with increasing temperature. Clearly, If the rate of growth of each oxide nucleus
.:. . !L 5_ .. EK 1OW
.
/
- 1:. ’ temperature
0
/
*:
high
temperature
:
*.
*.
I S0lUtfCh-l
.
passwe active FIG 19 Model of growth of passlvatmg oxide, assummg a small constant number N of oxide nucla at low temperature, and a high constant N at hgh temperature
IS approxrmately constant, and the rate of nucleation increases with temberature, then the oxrde layer at the moment of passrvatlon will be thmner the higher the temperature A simple model of this behavrour IS sketched m Frg. 19, assuming the number of oxide nuclei to be constant during the growth of the oxrde layer This model quahtatrvely also explains the observed increase of passrvatlon time with decreasing number of oxide nuclei, E with decreasing temperature, and appears acceptable as a first approxlmatlon. The assumption of a constant number of oxrde nucler would be quantitatively Justified, rf the mrtral peak of the charging curve can be identified with a nucleation overvoltage. Smce anodlc peaks are frequently observed m other metal/electrolyte-solution systems such a detarled explanation needs, however, further experimental evidence. The quasi-stationary potentiostatlc anodlc current/voltage curve of zmc m the more dilute but otherwrse slmllar 0 01 M NaHCO, solution IS shown in Fig 20 The indicated potential values have been measured as described above for the case of 1 M Na,CO,. At 25”C, the electrode is not passive even after polanzatlon for 30 mm At 7X!, passive behavrour IS observed, with a passrvatron potentral of -0.95 V. Due to the low passlvatmg current density, thrs IS close to the unpolarized oxidereduction potential described above It 1s seen that at more positive potentrals the drop of current density with time IS very slow, mdlcatmg a considerable thrckenmg of the oxide layer during attainment of the stationary state. Also rt is seen that after passmg through the passrvatron potential the current becomes cathodrc between -0 8 and -0 9 V. At thrs stage the cathodic partial current of hydrogen deposltron therefore exceeds the anodrc partial current of oxide formatron. Consequently, there
K KAESCHE
394
1
-10 -08
-0.6
-04
-02
0
+02
FIG 20 Quasi-stationary potentlostatlc anodlc current/voltage curve of zmc m 0 01 M NaHCOI at 75°C (mcludmg change of curve with time of polamtion), and 25°C
IS a stable resting potential where the electrode 1s passive w&out externally applied current. It may be noted that a similar behavlour was observed m 0 01 M NaOH
solution. Acknowledgements-The author 1spleased to acknowledge the assistanceof H Gehrke, W Hopfner, and Mrs J Schneiderm the experlmentalwork He ISmdebted to Deutsche Forschungsgememschaft for financial support REFERENCES 1 K HUBER, Helv Chrm Acta 27, 1443 (1944), 26, 1037 (1943) 2. R LANDSBERG and H BARTELT, Z Elektrochem 61.1162 (1957) 3 1 SANGHI and T P RADHAICI~HNAN, S~J~OSJU~. on Ek&OhepOsJtJO~ and Metal Fwshmng, 1, 133 (19571 4 ‘s E s Ei WAKICAD,A M SHAMSEL DIN and H KOTL, J Electrochem Sot 105, 47 (1958) 5 K SCHWABE,Z phys Chem 205, 304 (1956), Electrochrm Acta 3,47 (1960) 6 T J POPOVA, V S BAGOTSKII, and B N KABANOV, Dokl Akad Nauk SSSR 132,639 (1960), Zh FJZ. Chum 36, 1432, 1439 (1962) 7 B N. KABANOV, Electrochlm Acta 6,253 (1962) 8 H KAFSCHE, Herzung, Lueftung, Haustechnrk 13, 332 (1962) 9 P T GILBERT, Pzttsburgh International Conference on Surface Reactrons, p 127 Electrochermcal Society, New York (1948) 10 U F ~ANCK, Werkst Korroston 14, 367 (1963) 11 W M LATIMER, Oxzdntron Potent&s Prentice-Hall, Englewood Chffs (1959) 12 J OS~RWALD,Z Elektrochem 66,492 (1962) 13 H FISCHER and N BUDILOFF, Z Metallk 32,100 (1940)
Acta.
1964.
Vol
9, pp
383 to 394
Pergamon
P~CSS Ltd
Printed
1t1 Northern
Ireland
THE PASSIVITY OF ZINC IN AQUEOUS SOLUTIONS OF SODIUM CARBONATE AND SODIUM BICARBONATE* H KAESCHE Bundesanstalt
fur Matenalpruefung,
Berhn-Dahlem,
Bundesrepubhk
Deutschland
Abstract-The passlvatlon and passlvlty of zmc m 1 M Na,CO* (a), 0 1 M NaHCO, (b) and 0 01 M NaHCOs (c) have been mvestlgated by measurmg galvanostatlc chargmg and dlschargmg curves m the range between 1 ms and 30 mm In (a) passlvatlon 1s fast and brought about by small amounts of oxide, at 20°C by an approxnnately mono-molecular oxide layer In (a) the first traces of reducible oxide are formed mnnedlately after charging of the double layer. In (b) and (c) passlvatlon IS very slow, mvolvmg the formation of thick oxide layers In this case a marked dependence of the amount of oxide necessary for passlvation on the temperature indicates a smular dependence of the number mltlally formed oxide nuclei In (a) current efficiency with respect to oxide formation pnor to passlvatlon 1s high, It 1s very small m (c) due to parallel dlssolutlon of Zn to Zna+ Quasi-stationary current/voltage curves are presented for both (a) and (c) R&sum&& moyen de courbes charge-dkharge, Ctabhes par mesures galvanoplastlques dans l’mtervalle 1 ms B 30 mm, l’on &udle la passlvatlon du zmc dans (a) Na,CO, M (b) NaHCO, 0,lM (c) NaHCO, 0,OlM La passlvatlon raplde en (a) semble due, B 2O”C, il une couche monomol&dalre d’oxyde, dont les premldres traces se mamfestent d&s la charge. de la couche double Par contre, la passlvatlon est lente en (b), (c), ce qm unphque la formation de couches d’oxydes &passes L’mfluence de la tempkrature SW la quantltk d’oxyde co&rant la passlvlt& d¬e alors le rdle des premiers germes d’oxyde Zusammenfassung-Der Mechamsmus der Passlvlerung und der Passlvzustand des Zmks wurden m 0,l M Na,CO, (a), 0,l M NaHCOs (b) und 0,Ol M NaHCO, (c) an Hand galvanostatlscher Ladeund Entladekurven wahrend Z&en zwlschen 1 ms und 30 mm untersucht In (a) wlrd Zmk sehr schnell durch germge Oxydrnengen passlvlert, spenell be1 20°C schon nach Ausblldung emer cu monomolekularen Deckschlcht Dabel treten m (a) die ersten Spuren von reduzlerbarem Oxyd unnuttelbar nach der Umladung der Doppelschlcht auf In (b) und (c) 1st die Passlvlerung em sehr langsamer Vorgang, wahrend dem dlcke Oxydschlchten gebddet werden. Hoer zelgt die starke TemperaturabhFinggkat der Passmerungszelt emen starken Emfluss der Temperatur auf die Zahl der zunachst geblldeten Oxydkelme an In (a) 1st schon vor Emtreten der Passlvltat die Stromausbeute m Bezug auf die Oxydbddung hoch, w&rend m (b) die anodlsche Zmkauflosung emen hohen Stromverlust bewlrkt Sowohl fur (a) als such fur (c) werden die quasistatlonaren Stromspannungskurven mitgeteilt INTRODUCTION
passtvrty of zmc m aqueous electrolytes has been studred by a number of mvesttgators, especrally with respect to the behavrour of the metal m alkaline solutrons I-’ As recently shown by the authors during an mvestrgatron of the mechanism of prttmg of galvanized steel pipes carrying hot water, the well known spontaneous potential reversal of zmc m oxygen-contammg hot water9 can be interpreted m terms of spontaneous passrvatron m dilute soluttons of sodmm bicarbonate. Also rt was shown that the deleterious effect of copper traces in hot water appears to be due to the electrondonor properties of copper atoms mcorporated m the zmc oxide lattice, rendering the passive zinc an efficient cathode for oxygen reductron, which otherwrse m such electrolytes it is not. This corresponds to other finding+ according to which the THE
* Presented at the 14th meetmg of CITCE, Moscow, August 1963, manuscnpt received 8 October 1963 383
H KAEscm
384
electromc conductmty of zinc oxide produced by anodlzmg zinc depends on the excess zinc content and can be varied by varying the expermental condltlons. EXPERIMENTAL
TECHNIQUE
In view of the practical interest attached to the behavlour of zmc m such electrolytes, a closer mvestlgatlon of the passivity and passlvatlon of the metal m weakly alkaline solution was carried out The experimental arrangement was conventional Zmc electrodes (99 99 per cent Zn) were cut from sheet metal m the shape of small flags (surface area ca 4 cm2), sealed mto glass holders by means of polyethylene, cleaned, abraded, etched for 10 s m dilute HNO,-H2S04 solution, rinsed, and lmmedlately immersed m the solution m a glass vessel fitted with ground Joints. The solution was prepared with trlply distilled water and analytically pure salts, deaerated by passing a stream of purified nitrogen through, and thermostated to within fO*l”C of the indicated temperature. The solution was moderately stirred, and the electrodes freshly etched before each single measurement, unless otherwise specified The latter treatment was found necessary m order to keep scatter of experimental results wlthm reasonable limits Electrode-potential measurements were carried out with a Hg/HgO reference electrode (equipped with a Luggm capillary), at room temperature Measured values were recalculated with respect to the standard hydrogen electrode, usmg $0 244 V as the potential of the saturated calomel electrode which was used to control the Hg/HgO electrode, and neglecting temperature differences between the zmc and the reference electrode For polarization experiments, a platmum-gauze electrode was introduced m a compartment of the solution separated from the mam volume by a porous glass disk Potential/time curves at constant polarlzmg current were recorded with an oscdloscope (Tektronix A 535), or with an oscdlograph (Honeywell Vlslcorder), or with a self-balancing slow recorder (Radiometer), depending on the type of measurement. The ohmic resistance between the terminals of reference and zmc electrode was always kept at 210 Ma m the measuring circuit Unless otherwise specified the zmc electrode was always polarized with a cathodic current of - 10 mA/cm2, sufficient for ehmmatmg spontaneous corrosion, except for the time of measurement of anodlc charging curves For these measurements the polarization clrcult was switched wlthm less than 0 1 ,us from cathodic to anodlc currents, or back to cathodic currents, with the aid of a mercury-wetted relay Currents were taken from dry cells m series with resistance decades, and were constant to wlthm *2 per cent of the indicated values For current/voltage-curve measurements an electronic potentlostat (Wenkmg) was used. RESULTS
AND
INTERPRETATION
Behavzour of zznc zn 1 M Na,CO,-solutzon
The behavlour of the zmc electrode was first investigated m deaerated 1 M Na,CO, solution at 60°C The change of potential with time during galvanostatlc anodlc and cathodic polarization 1s schematically represented m Fig 1. At point A the anodlc charging current 1s switched on. In the time between A and B the electrode 1s passlvated, with followmg oxide growth between B and C, and superimposed oxygen evolution beyond C. If the current IS reversed during charging, then between F and G the oxide 1s reduced, and the potential finally returns to the potential of exclusive hydrogen evolution.
Passwty
I fF L_-_-_----. IJ +
385
of zmc m NaZCOs,q and NaHCOsas
hydrogen evolufron
reductron / oxrde G \ \, evokdon \‘z hydrpgen c ---____ T/me
W
FIG 1 Potential/time
curve of zmc m 1 M Na,CO, solution, 6O”C, dunng galvanostatic anodlc charging (sohd curve) and cathodic dnchargmg (dashed curve)
FIG
2
15 :?i3 10 05 Change of potential of zmc m 1 M Na,CO,, 60°C after reversmg the polarlzmg current from 5 &/cm2 to the mdlcated anodlc value
As shown m Fig. 2, the first rise of the electrode potential after reversing the current from a constant cathodic to a constant anodlc value can be Interpreted m terms of chargmg of the double layer capacitance C, where C = 22 ,uF/cm2 independent of the chargmg current density z, After reachmg the first plateau, the potential slowly rises, as shown by Fig 3 (which also shows the dlschargmg curve), until shortly before the change to the lmear increase of potential with time, typical for oxide-film growth, a sudden rise occurs. There 1s little doubt that 7P 1s the passlvatlon time, as the mflexlon at B corresponds to the expected potential behavlour for a change of current conduction through pores of a surface layer to conductlon by lomc transport across a non-porous oxide. lo If the property 7P 1s measured as a function of the charging current density lc, it 1s found (Fig 4) that log 7P 1s a linear function of log l,, with d log rp/d log I, equal to - 1 6 f 0 1. This relation 1s expected to hold m the present case, since other metal/electrolyte-solution systems, as Ag/H,SO, or Cu/HCl, where scarcely soluble salt layers are formed, generally also show a constancy of the product 1,~~~‘~lo l4 As shown elsewhere, lo a theoretlcal explanation other than the quahtatlve
FIG 4. Paswatlon
time Q, as a function of anodlc current density I,, m 1 M NaBCOB, 60°C
that the thickness of the oxide nuclei decreases with mcreasmg current density due to mcreasmg supersaturatlon, IS not possible at present In accordance with ths relation between the passlvatmg current density and the passlvatlon tnne It can be shown that the oxide IS formed at an early stage of the chargmg curve. As seen from Fig. 5, reducible matter 1s present on the electrode shortly after the begmmng of anodlc polanzatlon, and m fact the first traces of such reducible matter can be detected after only 2 ms of anodlc chargmg with 10m A/cm2 At a shghtly later stage, the cathodic reduction curve exhlblts a well-defined plateau, allowmg the determination For a comparison with thermodynamic data, slmllar of the reduction potential measurements were carried out at 2O”C, where the pH of the Na,CO, solution JS 12, and the potential of the reduction plateau (Fig 6) was found to be -1.15 f 0 05 V. As the eqmhbrmm potential of the Zn/ZnO electrode (standard potential -0 42 V)ll IS -l$B V, it IS seen that the reducible matter can be assumed to be zmc oxide In view of the low solublhty of the oxide (ca 1O-s mol/l at 2O”C), and transport of the zmcate ion into the bulk of the solution other than by dlffuslon bemg suppressed due to high salt concentration, an adherent solutlon layer of a tbckness of around 103 cm should indeed be supersaturated after chargmg with several ,&/cm2. Therefore, the behavlour of the electrode m Na,CO, solution departs from the behavlour m more alkaline solutions, where 7p is governed by diffunon-controlled supersaturation of the adherent layer of solutlon.6*7 As shown by Fig 7, temperature mfluences the shape of the anodlc charging curve Wtth increasing temperature, the mflexlon C attributed to oxygen evolution IS dlsplaced to less noble potentials, mdicatmg a decrease of oxygen over voltage. Also, the assumption
Passivity of zmc m Na,CO,.Q and NaHCOasp
387
Time,s
FIG 7 Anodlc chargmg curve of zmc m 1 M Na,CO,, at 20°C and 80°C
z, = 10 mA/cm2 Letters as used m Fig 1
temperature,
“C
FIG 8 Passivatlon time 7p as a function of temperature of 1M Na,COI.
I, = 10 mA/cma gradlent of the hnear portion of the chargmg curve between B and C decreases As this linear portion IS characterlstlc for the growth of non-porous oxide of constant specific lomc conductlvlty u, with transport of ions through the oxide layer, the decrease of the gradient can be interpreted m terms of an increase of cr with increasing temperature. With z, = 10 mA/cm2, d&/dt 1s 4 0 V/s at 20 “C, and 1.7 V/s at 80°C
The numerical value of (T, dependent on the roughness factor and on the density of the oxide, cannot be dzscussed at present Fmally, the passlvatlon time 7P decreases with decreasmg temperature, as shown m Fig. 8 It 1s seen that 7P approaches a lower hmltmg value of 0 1 s, and the product z x 7p therefore approaches a lower value of 1 mC/cm2. This corresponds to an approximately monomolecular oxide layer provided that the current efficiency with respect to oxide formation 1s 100 per cent The current efficency was checked at 60°C by comparing the amount of electriczty consumed durmg charging, ze the product zotc, were tc 1s the time of anodzc charging, with the amount of electnclty regained by oxide reduction, ze the product iDrE, where zD 1s the current density during discharge, and TV the time of oxide reduction (see
388
H. KAESCHE
Figs. 3, 5, 6). The ratio z,tc/zDTRwas found to be
FIG 9 The ratio QR/Qo = tDrB/Q, as a function of the anodlc charge z,to measured m 1 M Na,CO,, 6O”C, /z,l = /z,\ = 10 mA/cme
After passing through the mflexron at C during anodrc chargmg, the cathodic reduction curve becomes more complicated, as shown by Fig 10, mdicatmg the presence of reducrble matter not identical wrth ZnO, possrbly mdicatmg the reduction of absorbed molecular oxygen. The further rise of the potential beyond the second mflexlon at pomt D results m the appearance of a third distmct part of the reductron curve, as shown by Fig 11, and this possrbly Indicates zmc peroxide, the presence of which has been postulated. l3 It should be noted that after prolonged passrvatron a thermodynamic evaluation of potentials measured durmg cathodic reductron is not rmmedlately possible, since reduction of thrck oxrde layers may mvolve an appreciable overvoltage If anodic charging 1s further contmued, then, as shown by Fig. 12, the potential eventually rises to very high values until the external crrcurt can no longer furmsh the constant current Obvrously, the oxrde layer continues to thicken. It has been shown elsewhere1 that by such prolonged polarization oxrde films are obtamed that can be mvestigated by optical and X-ray analysis, ZnO being the only specres found Therefore the assumed presence of ZnO, IS still open to drscussron. As mentroned above, all measurements so far reported have been carried out with electrodes etched before each smgle experrment. If after passrvatron and subsequent reactivation the electrode is again anodrcally charged without mtermittent etchmg, the shape of the charging curve changes This may be seen from the sequence of charging curves A, B and C m Fig 13 A was measured with an etched electrode, B after anodic charging durmg 6 s and subsequent oxide reduction, wrthout etchmg, and C slmrlarly after 36 s of anodrc chargmg It IS seen that the major effect of pre-passlvatlon of the electrode IS a slowing down of the potential rrse, especially after onset of
Passivity of zmc m NaSCOI,a and NaHCO,,
389
FIG 13. Anodtc chargmg curve durmg prolonged polarzatlon m 1 M Na,CO,, 60°C zc = 30 mA/cmsI, with mterrmttent reactivation, without mterrmttent etchmg
20
I
‘!’ I-10
”
-08
”
-06
Potenhal, ” -04
”
V -02
”
0
” +O?
FIG 14 Potentlostatlc quasi-stationary anodlc current/voltage curve of zmc m 0 01 M NaHCO, solutron at 75°C (mcludmg change of curve urlth time of polanzatlon), and ..bZDfi
oxygen evolution. Thts change must be due to the influence of dtspersed metallic zinc In view of the observed on the electrode surface remammg after oxide reduction influence of excess metalhc zinc on the conductivrty of the passive oxide1 and m vrew of the similar mfluence of excess metallic coppers it may be assumed that the shape of curves B and C indicate an increase of electronic conductivrty of the oxide due to mcorporation of excess zinc, resultmg m an increase of the current density of oxygen evolution. However, as there also should be an increase of the roughness factor of the electrode surface, further experimental evidence with respect to the influence of the defect propertres of the oxide 1s necessary The quasi-stationary current/voltage curve of zmc m 1 M Na,C!Os solutron at 75 and 25°C measured with a potentiostatrc circuit, is shown m Fig 14. For measurmg the sequence of points shown at constant potential, at each single potentral the 5
390
H. KAFSCHE
electrode was etched and the cell filled with freshly prepared solution. It 1s seen that a statronary state is reached only very slowly, mdicatmg a considerable thickening of the oxide layer at each potential beyond the potential of passivatron. The latter potential corresponds to the first plateau (between pomts A and B, Fig. 1) of the galvanostatlc anodlc chargmg curve. Behavrour of zmc in O-1 M NaHCO, solutlon
The anodrc chargmg curve of zmc m deaerated 0.1 M NaHCOa solutron (75”C), measured as described above, is shown m Fig. 15 for different anodrc current den&es
-150
0
I
05
10
15 T/me,
mm
20
,?5
30
FIN 15 Galvanostatic anodlc chargmg curves of zmc m 0 1 M NaHCO* solution, 75”C, and cathodic dxhargmg curves Chargmg current as mdlcated, dshargmg current 10 mA/cma Anod~c chargmg mterrupted at $2 25 V
It IS seen that passlvatlon m this case proceeds very slowly. It is also seen that imtrally the potential passes through a distmct peak, returmng afterwards to a lower value and remarmng approximately constant for a time of the order of mmutes untrl the final rise due to passlvatlon. The discharging curves measured at 10 mA/cm2 are also shown m Fig. 15. It is seen that the current efficiency with respect to oxrde formatron is low. There is, therefore, an appreciable current loss due to zmc dlssolutron prior to the onset of passlvation From an mspectlon of the 10 mA/cm2 curve it is evident that approximately 4/5 of the current was not used for oxide budd-up, and the correspondmg amount of anodlcally dissolved zmc IS sufficient to saturate the solution with Zn(OH),. The appearance of flakes of loose white maternal is Indeed observed, and durmg experiments mvolvmg long polarization times, as for mstance at 40°C and I, = 10 mA/cm2, such flakes also adhere to the electrode, but this substance IS not identical with the passlvatmg oxide, and cannot be reduced durmg cathodrc discharge. Also, if chargmg curves are measured m a solution already used for previous passivatlon measurements, the passrvatlon time is the same as measured m a solution mrtlally free from dissolved zmc Smce the length of the potential plateau of the dlschargmg curves represented m Fig. 15 is approximately independent of the current density of the precedmg anodlc chargmg (m each case contmued untrl the potentral reached +2 25 V), the amount of oxide present is independent of the chargmg current density. Compared to the
Passlvltyof zmc m NaaCOs,Qand NaHCOIaa
394
behavlour m NazCOa solution the amount of oxide, correspondmg to an amount of electnclty of cu 300 mC/cma, IS very high, mdlcatmg a comparatively thick passlvatmg oxide layer By measurmg cathodic dlschargmg curves prior to the onset of passlvatlon, that IS, from potentials between the mltlal peak and the final nse of the potential m the region of the extended plateau of the chargmg curve, it IS found that m this case agam there IS reducible matter present on the electrode surface shortly after the start of anodlc charging. Durmg measurements of the passlvatlon at 75°C with zc = 10 mA/cm2 an mtlexlon of the discharging curve correspondmg to oxide reduction was found as early as 0 5 mm after the beginning of charging, where passlvatlon did not occur until 2 mm later After 0 5 mm of anodlc charging the amount of oxide already present corresponded to cu 80 mC/cm 2. Cathodic reduction curves measured at 60°C (pasavatlon time with z, = +lO mA/cm2, 7 mm) 5 mm after startmg the anodlc current are shown m Fig 16 as a function of the discharging current denstty zD It -1-00
-m? -Y-40
T/me FIG 16 Cathodic
swty,
reduction curves mdxatmg presence of oxide pnor to onset of pasmeasured at 6O”C, rc = 10 mA (chargmg curves not shown), cathodic current density as mdwated
IS seen that the plateau of the reduction curve markedly depends on zD, mdlcatmg a correspondmg overvoltage of oxide reduction The dependence of the oxide reduction potential on the dlschargmg current density ZDIS shown m Fig 17, and the unpolarized reduction potential is seen to be -0 9 V. Since the hydroxyl Ion concentration con_ at 60°C m O-1 M NaHCO, IS estimated to be lo4 mol/l, the equlhbrtum potential of the Zn/ZnO electrode should be -0.95 V, and therefore the oxide present on the electrode can be assumed to be ZnO. From the above-described observations It IS to be concluded that m this case again the eventually passlvatmg oxide IS present much earlier than passlvatlon occurs
392
-1.8_
FIG 17 Potential of oxide reduction m 0 1 M NaHCO*, 6O”C, as a fimctlon of dlschargmg current density z6
PIG 18 Chargmg and dlschargmg curves of zmc m 0 1 M NaHCOs as a fun&on the temperature I, = 10 mA/cmB, z = 10 mA/cms.
of
There IS no prmary passrvatmg layer drfferent from zmc oxrde. In contrast to the behavrour m Na,CO, solutrons m thus case the oxrde nuclei are relattvely thtck particles Such particles cannot, however, be observed dn-ectly, smce microscoprc mvestrgatron reveals only the presence of loosely adherent, powdery wlute material which may be either hydroxide or basrc carbonate,l and whtch cannot be Identified with the passrvatmg oxide As shown m Fig. 18, the passrvatron time decreases wrth mcreasmg temperature, m contrast to the observatrons made durmg experiments carrred out m Na&O, solutron Accordmg to the measurements recorded m Frg 18, the passrvatron time decreases from approxrmately 22 mm at 40°C to approximately 1 mm at 9O”C, with z, = 10 mA/cma. As the drschargmg current den&y was always 10 mA/cn?, and the plateau of oxrde reductron shortens considerably with mcreasing temperature, rt IS
393
Passlwty of zmc m NaBCOca and NaHCOJap
seen that the reason for the shortening of the passlvatlon trme 1s a marked decrease of the amount of oxrde necessary for passrvatron. From Fig. 18 the amount of oxide present at +25 V ISestrmated to correspond to 2800 mC/cm2 at 4O”C, and 150 mC/cm2 at 90°C Consrdermg that the presence of dlstmct oxrde particles prror to the onset of passrvrty has been shown already, the observed dependence of the amount of oxrde on the temperature can probably be best explamed by an Increase of the number of oxide nuclei with increasing temperature. Clearly, If the rate of growth of each oxide nucleus
.:. . !L 5_ .. EK 1OW
.
/
- 1:. ’ temperature
0
/
*:
high
temperature
:
*.
*.
I S0lUtfCh-l
.
passwe active FIG 19 Model of growth of passlvatmg oxide, assummg a small constant number N of oxide nucla at low temperature, and a high constant N at hgh temperature
IS approxrmately constant, and the rate of nucleation increases with temberature, then the oxrde layer at the moment of passrvatlon will be thmner the higher the temperature A simple model of this behavrour IS sketched m Frg. 19, assuming the number of oxide nuclei to be constant during the growth of the oxrde layer This model quahtatrvely also explains the observed increase of passrvatlon time with decreasing number of oxide nuclei, E with decreasing temperature, and appears acceptable as a first approxlmatlon. The assumption of a constant number of oxrde nucler would be quantitatively Justified, rf the mrtral peak of the charging curve can be identified with a nucleation overvoltage. Smce anodlc peaks are frequently observed m other metal/electrolyte-solution systems such a detarled explanation needs, however, further experimental evidence. The quasi-stationary potentiostatlc anodlc current/voltage curve of zmc m the more dilute but otherwrse slmllar 0 01 M NaHCO, solution IS shown in Fig 20 The indicated potential values have been measured as described above for the case of 1 M Na,CO,. At 25”C, the electrode is not passive even after polanzatlon for 30 mm At 7X!, passive behavrour IS observed, with a passrvatron potentral of -0.95 V. Due to the low passlvatmg current density, thrs IS close to the unpolarized oxidereduction potential described above It 1s seen that at more positive potentrals the drop of current density with time IS very slow, mdlcatmg a considerable thrckenmg of the oxide layer during attainment of the stationary state. Also rt is seen that after passmg through the passrvatron potential the current becomes cathodrc between -0 8 and -0 9 V. At thrs stage the cathodic partial current of hydrogen deposltron therefore exceeds the anodrc partial current of oxide formatron. Consequently, there
K KAESCHE
394
1
-10 -08
-0.6
-04
-02
0
+02
FIG 20 Quasi-stationary potentlostatlc anodlc current/voltage curve of zmc m 0 01 M NaHCOI at 75°C (mcludmg change of curve with time of polamtion), and 25°C
IS a stable resting potential where the electrode 1s passive w&out externally applied current. It may be noted that a similar behavlour was observed m 0 01 M NaOH
solution. Acknowledgements-The author 1spleased to acknowledge the assistanceof H Gehrke, W Hopfner, and Mrs J Schneiderm the experlmentalwork He ISmdebted to Deutsche Forschungsgememschaft for financial support REFERENCES 1 K HUBER, Helv Chrm Acta 27, 1443 (1944), 26, 1037 (1943) 2. R LANDSBERG and H BARTELT, Z Elektrochem 61.1162 (1957) 3 1 SANGHI and T P RADHAICI~HNAN, S~J~OSJU~. on Ek&OhepOsJtJO~ and Metal Fwshmng, 1, 133 (19571 4 ‘s E s Ei WAKICAD,A M SHAMSEL DIN and H KOTL, J Electrochem Sot 105, 47 (1958) 5 K SCHWABE,Z phys Chem 205, 304 (1956), Electrochrm Acta 3,47 (1960) 6 T J POPOVA, V S BAGOTSKII, and B N KABANOV, Dokl Akad Nauk SSSR 132,639 (1960), Zh FJZ. Chum 36, 1432, 1439 (1962) 7 B N. KABANOV, Electrochlm Acta 6,253 (1962) 8 H KAFSCHE, Herzung, Lueftung, Haustechnrk 13, 332 (1962) 9 P T GILBERT, Pzttsburgh International Conference on Surface Reactrons, p 127 Electrochermcal Society, New York (1948) 10 U F ~ANCK, Werkst Korroston 14, 367 (1963) 11 W M LATIMER, Oxzdntron Potent&s Prentice-Hall, Englewood Chffs (1959) 12 J OS~RWALD,Z Elektrochem 66,492 (1962) 13 H FISCHER and N BUDILOFF, Z Metallk 32,100 (1940)