EEECTR1:CSL DOUBLE LAYER CAPACITY OF P9SSIVE IRON AND STAINLESS STEEL ELECTRODES BYPRANJIVAN V. POPAT AND NORMAN HACKERMAN Department of Chemistry, The University of Texas, Austin, Texas Received January 19, 1961
These measurements were made in sulfate solutions as a function of p H . The method of charging curves, utilizing square wave imput signals was used, and the metals were polarized over the range between oxygen and hydrogen evolution potentials. At potentials slightly less anodic than that for oxygen evolution the e.d.1. capacity for all three metals in neutral solution was about the same, viz., 18 pf./cm.z. For all three metals this is a minimum value which increased with increased acidity. The capacity-potential behavior of stainless steel was similar to that of platinum over the whole range. If there is no bulk oxide on the platinum under these conditions it can be postulated that the passivity of stainless steel is not caused by bulk oxide; certainly the behavior differs from that of aluminum and tantalum. There is a hump in the capacity value for both with increasing cathodic potential. With increasing pH the position of the maximum is more cathodic and the height is greater. With iron, it is not possible to distinguish between the metal dissolution region and the hydrogen evolution region by capacity values alone. However, there is a region where the iron potential changes suddenly from active to passive and this is paralleled hv a capacity decrease. The non-oxide argument for stainless steel (above) is equally valid here. Even while recognizing the problems imposed by polycrystalline, solid metals in interpreting these measurements in terms of differential capacity, it is suggested that comparison of the three mc*talsin these terms is valid.
Two principal schools of thought on the cause of electrochemical passivity of metals suggest (i) formation of a stoichiometric three dimensional oxide film on the metal surface, or (ii) some suitable sorbed oxygen-containing species. There is evidence for each of these views. A postulate reconciling both views has been proposed.2 I t suggests a sequence of events, the most important oi which IS the rapid sorption of up to a monolayer of some oxygen-containing species. This accourits for the rapid potential changes aiid the periodic phenomena associated with the onset and loss of passivity of iron. Passivity, then, is caused by a thin, insoluble, disordered film of oxygen-containing ions and metal ions. It is suggested that the film is formed by migration of the metal ions into the array displayed by the sorbed bpecies. The controversy ccrtuinly has not been resolved. Xleasurenieiit ol‘ the electrical double layer (e.d.1.) capacity, a t the metal-solution interface before, during, and after electrochemical passivation appears to us to be pertinent. The measured capacity of a metal electrode with a continuous oxide film should be lower in a giveii electrolyte than for the same metal without such a film. This is illustrated by the experimental results of Ershler, et uZ.,3 for a nickel electrode in 1 N NaOH before and after chemical oxidation, ie., 22 to 27 and 7 pf./cni.,2 respectively. KOfully satisfactory analog circuit is a;ailable for such an oxide-bearing electrode. However, lower capacity in the presence of an oxide film can be explained by assuming that its presence should lower the surface charge density, increase the thickness of the double layer, and thereby decrease the capacity. Alternatively, the oxide film may be considered as forming a capacitor in series with the e.d.1. capacity. Then since the capacity of the oxide film (lower dielectric constant) is small (1) Presented a t the symposium in commemoration of David C. C,rahame, sponsored b:; the Divsions of Colloid Chemistry and Physical Chemistry, a t the 1381h meeting of the American Chemical Soc.. New Tork, Sept 13, 1960. (2) N. Hackerman, Z. Cleklrochem., 62, 6.32 (1958). (3) A. Rakov, T. Elorisova and R Crshler, Zhur. Paz Kham , 22, 1390 (1948).
compared to that of the e.d.1. the nicasured capacity should be low. Work is in progress in this Laboratory on the e.d.1. capacity of several solid metals. Some results for iron and 18-8 stainless steel electrodes under passive and active conditions in solutions of various pH are reported here. The results are compared with published values for other metals. Experimental The method of charging curves based on the application of a square-wave signal was used for the double layer capacity measurement^.^ This method is suitable for solid metal electrodes having geometrical areas as large as 2 to 3 cm.2. The frequency of the square-wave signal in all the experiments reported here was 500 C.P.S. unless otherwise stated. The standard resistance through which the input squarewave signal passed before entering the cell was 15,000ohms. The Pyrex cell used in these experiments was similar to that already described.4a A large area, platinized platinum, cvlindrical wire gauze was used as a non-polarizable auxiliary electrode. All potential measurements were made against a saturated calomel electrode (SCE). Triple-distilled conductivity water was used for preparing all solutions. Analytical reagent grade chemicals were further purified by recrystallization from conductivity water. In some cases, the solutions were first filtered through activated charcoal to remove traces of organic impurities. Contamination by grease or other organic materials was carefully avoided. Bureau of Mines Grade A helium, reO2 ported as 99.997% pure and containing less than was bubbled through the experimental cell throughout the measurements. The helium wap pre-saturated by passing it through conductivitv water a t the temDerature of the exDerimentd solution. The gas passed out ihrough a similar ‘trap to Drevent back diffusion of air. Reagent grade iron wire (O.OOSff dia.) supplied by Baker Chemical Co. (Phillipsburg, K.J.) and reported as better than 99.8% pure (0.0275 C ) was used. Stainless steel electrodes were made from AIS1 302 wire (dia. 0.015”) supplied by Alloy Metal Wire Division inloore Station-Prospect Park, Pa.). Test pieces of the metal wire under investigation were sealed in 6 mm. soft glass tubing. The length of the test electrodes was such that a projected area of 1.0 cm.2 mas exposed to the electrolyte in each case. The glass tube holding the electrode was sealed to a ground glass holder which fitted the experimental cell. Electrical contact between the test electrode and the rest of the circuit was made by a column of mercury. Each electrode was polished with 4/0 emery paper, rinsed with acetone, washed throughly with distilled water, treated with 2 N H2S04 for about a minute (till HZevolution was (4) (a) R. J. Brodd and N. IIackerman. J . Electrochem. Soe., 104, 704 (1957); (b) J. J. McMullen and N. Hackerinan, $bid.. 106, 341 (1959).
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v. POPilT AND Y O I t M A X HACKERMAS
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Fig. 1.---Differential capacity-potential curves for stainless steel. Anodic (oxidizing) potential t o the left. a0
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Fig. 2.-As for Fig. 1 with 0.1 N HnSO,, and showing the absence of hysteresis. visible in the case ot iron), and washed again with conducivity water. Finally, the electrode was rinsed with the test ohtion before introducing it into the cell containing the test electrolyte. Each electrode was used only once. For the oxygen evolution and passivation of the stainless steel elertrode, a polarizing potentiometer with an upper limit of 1.5 v. was sufficient. For iron, however, i t was necessary to use a, high voltage (or high current) d.c. source to cause oxygen evolution. The current required for the oxygen evolution was generally determined by measuring the potential drop across a standard resistor of 50 ohms in series with the polarizing circuit. Oxygen evolution on an :ron electrode in a sulfate solution a t a pH of about 3 required no more than about 10 ma./cm.*. Once started, it was still visible a t 0.1 ma./cm.Z. For neutral and alkaline solutions, the anodic current density required for oxygen evolution and passivation for iron electrodes was considerably less. In some eases the electrode was first cathodically polarized to hydrogen evolution, and the potential then was gradually changed to more anodic valuee. All experiments were carried out a t room temperature. Before taking data a t each potential of the test electrode, sufficient time was allowed for the capacity values to become constant. This normally required 10 minutes or less.
of Fig. I , except that for 0.1 AVHZSO4, is an averag. of three independent experiments made with frehh solutions and new electrodes in each c a w The agreement between individual runs for each pH was better than 5%. The data for 0.1 AVHH,SOA is an ayerage of two rmis made with the same electrode and the same solution in the following way. Initially, this electrode was made sufficiently cathodic for hydrogen to evolve. The potential as then gradually changed to more anodic d u e s until the 0, evolution potential was reached. 111 the other run, the rlirection of the applied potential was reversed so that the electrode potential gradually changed from that for O2evolution to that for H2evolution. The agreement between the two runs 11 ab very good, as shown by the data of Fig. 2. The curves for 1.8 N HZSO, and 0.1 5 H,SO, are similar in shape to one reported by Kolotyrkin for 1.0 N H2SOA5 and are in fair agrecmolit (juantitaExperimental Results and Discussion tively. Stainless Steel Electrodes.-Figure 1 represents The steep braiich at thc extwnic left for each the differential e.d.1. capacity of 18-8 stainless steel curve of Fig. 1 comes u t potential. probably sufelectrodes as :t function of electrode potential in sulfate solutions. The data o€ each of the curves ( 5 ) 'I' 11 Iiolot\rkin Z I'lektrorhom , 62, (361 ' l q j 8 )
July, 1961
CA4PACITY O F PASSIVE I R O N AND S T A I N L E S S S T E E L
ELECTRODES
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ficient for oxygen evolution with little or no overA clean dropping mercury electrode near the voltage. The more or less shallow minimum in ZPC is not likely to have an oxide film on it. Howeach curve a t potentials slightly less anodic than ever, there is still some question whether oxygen this represents the passive region. The value of the evolution on platinum occurs on bulk oxide or on minimum capacity in this region is about 30 pf./ some oxygen-containing chemisorbed species. Unfor 0.1 and 1.8 N H2SO4, about 20 pf.jcm.2 fortunately, most published results can be explained for 1 iV S a O H and 12 to 13 uf. for 0.1 N Ka2- on either basis. Giner’s worklo on the behavior SO4. Bockris and Potter6 observed a similar effect of platinum electrodes in 1 N H2S04 indicates that of pH on Xi electrodes. They reported 28 pf./cm.2 the behavior is determined by chemisorbed oxygen for 0.01 1V HC1, dues in the anodic region (left is immeasurably slow a t these potentials. They of peak), is notable. Also, the C us. E behavior of postulated the formation of a polar ‘(oxygen evolustainless steel in the anodic region was similar to tion intermediate which is absorbed on the electrode that for platinum. In neither case was there a surface” as the first step in the oxygen evolution discontinuity in the curve on changing the potential reaction. gradually from oxygcn to hydrogen evolution. Kolotyrkin5 considers the absence of oxide on Moreover, within experimental error, no hysteresis platinum electrodes in the potential region ordiwas observed in either case in going from anodic to narily encountered, (including oxygen evolution), cathodic potentiads and reverse, within the poten- as a well-established fact. TrapnellI2 indicated tial range used. that oxygen chemisorption takes place with the The general similarity of the C us. E curves for formation of oxide ions a t the surface following platinum and mercury in the corresponding po- electron donation from the s and p bands of the tential range, ie.. no electrochcmical reaction tak- metals. ing place, has been established.* The e.d.1. Capacity-potential data for platinum generally capacity for clean dropping mercury in neutral support the proposition that bulk oxide is not influoride solutiong is about 20 pf./cm.2 on the anodic volved in the processes occurring prior to and durside of the potential of zero charge (ZPC) for mer- ing oxygen evolution. cury. Thiz is close to the values reported here for The minimum capacity on the anodic side before stainless steel and for platinum* under similar oxygen evolution on platinum in KazS04 solution is conditions. h rough estimate of the thickness of the same as for Clz (or Brz) evolution on platinum the compact double layer at anodically polarized from C1- or Br-.* Thus, there does not appear to mercury can be obtained if it is assumed that the be any fundamental difference in the nature of double layer structure is analogous to a parallel the metallic surface of the electrodes in the two plate capacitor v-ith the distance between the two cases. It is unlikely that C12 evolution takes place plates replaced by the thickness of the double layer. on a chloride covered surface, because of soluThen the capacity per unit area of the electrode, bility. C = 4 4 n d , where d is the thickness of the double The close similarity of the C us. E behavior for layer and e I S the dielectric constant of the medium stainless steel and platinum on the anodic side sugwithin it. Holverrer, a difficulty arises in selecting gests that bulk oxide is not involved during electrothe latter value Presumably it lies between unity chemical passivation and oxygen evolution on (corresponding to L: vacuum) and E for the bulk stainless steel. This view is further supported by solvent (about 80 for water at room temperature). the absence of hysteresis with stainless steel (Fig. Aqsuming a value 10, the thicknesq of the compact 2 ) . It is unlikely that hydrogen evolves on oxide layer having a czpacity of 20 pf./cm is 4.5 A. covered stainless steel. S o t e that C us. E for Using the same value of E with platinum and stain- platinum and for stainless steel were essentially less steel, the thickness of the compact double independent of the frequency of the square-wave layer a t the minimvm capacity on the anodic side signal, within experimental error. EnkeL3 reis of the order of 5 A. Because of the uncertainty ported similar behavior for platinum in HC104 in e it is perhaps more meaningful to think in solution. terms of ’d rather than of d alone. The behavior just discussed should be contrasted 6) J O’RI. Ilockriq a n d E C Potter, J Chorn Phys 20, 614 (10) J. Giner, Z . Elektrochem., 63, 386 (1959). (1952) (7) P V P o p ~ tand \i Haekerman, iinpubhshed results T Pop.tt ‘tnd N €farherman J . P h y s Chern, 62, 1198
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