The Electrochemical Behavior of Iron in Acetate Solutions Containing Perchlorate

Estudou-se a dissolução do ferro em meio de acetato de sódio, pH = 5, na presença de perclorato na faixa de concentração entre 10M e 0.10 M, utilizando a técnica da voltametria ciclica com eletrodo rotatório de disco e de disco e anel. Observou-se que a velocidade de dissolução do ferro aumenta com a concentração de perclorato, sendo influenciada pelas condições hidrodinâmicas. O acetato inibe os efeitos deletérios da presença do perclorato no sistema.


Introduction
The influence of anions on the dissolution of metais is very important in corrosion and in electrochemical machining, as well as in the attempts to understand some of the fundamental aspects of electrode kinetics.
The corrosion and passivation of iron in aqueous acid solutions, as well as the active to passive transition, depend on the solution composition 1 • 2 • 3 • Previous work on the electrodissolution of iron electrodes shows the direct participation of anions in this process 4 · 5 . The exchange currents forthe Fe/Fe + 2 system 5 proved to decrease in the series Cl04-, Cl-, CH3COO-, N03-. Foroulis 5 also observed the large variation of the exchange currents for iron with the nature of the acid anion. Bonhoeffer and Heusler 6 compared the exchange currents ofthe Fe/Fe+ 2 system in perchloric acid and in a mixture with formic acid at the sarne pH. The decrease in the exchange current when a readily adsorbed formic acid is introduced into the solution was explained as being due to the displacement of water molecules from the iron surface and a resulting decrease in the surface concentration ofthe oHions. Florianovich et al. 2 points out that the decrease in the iron dissolution rate could result from the displacement of CI04ions. ln fact, however, the anion actions are more complex 7 . Investigations of the dissolution of iron in acetate media were carried out by Bech-Nielsen 8 • 9 who proposed a reaction mechanism involving two coupled, parallel reactions. Later, this author studíed the influence of chloride, perchlorate and iodate 10 in this media and observed changes in the kinetic parameters of the dissolution reaction. Takahashi et al. 11 and Azambuja et al. 12 studied iron dissolution in acetate in the absence of other anions and concluded that the acetate takes part in the reaction mechanism by forming ferrous complexes with this anion.
The aim ofthis paper is to study the behavior ofiron in acetate solutions containing perchlorate in a concentration ranging from 0.001 M to 0.1 O M and to evaluate the kinetic parameters by using rotating disk and rotating ring disk electrodes.

Experimental
The working electrode consisted of highly pure iron (Specpure, Johnson Mattey Chemicals Ltd.) in the form of a rotating disk (0.07 cm 2 approximate arca) embedded in epoxy resin. Before each test the electrode was mechanically polished with silicon carbide paper up to 1200 grit, and then degreased in alcohol and rinsed with twice distilled water. The electrode was cathodically polarized at -1.2 V SCE for I O min in order to achieve a reproducible electroreduced iron surface just before the electrochemical measurement was perforrned. Potentials were measured against a SCE provided with a Luggin capillary tip. The potentials in the text are referred to the SCE scale. ln order to detect the forrnation of an iron soluble species during the voltammetric sweep of the working electrode the rotating ring disk electrode (RRDE) was employed. An iron working disk platinum detector ring was made for this purpose. The geometrical characteristics of the ring-disk electrode are the following: disk radius, rt 0.50 cm; ring interior radius, n = 0.66 cm; and ring exterior radius, r3= 0.72 cm. The calculated value ofthe collection coefficient according to Albery and Bruckenstein 13 is N 0.26. Solutions from analytical grade reagents and twice distilled water were prepared. The electrochemical tests were perforrned with two kinds of solutions with constant and varíable ionic strength. The first one consisted of a mixture of xMCH3COONa + yMNaCI04 (I < x < 0.90; 0.001 < y < 0.1 0), buffered with acetic acid to pH 5. The solutions with variable ionic strength were prcpared with 1 M acetate solutions, anda perchlorate concentration in the range ofO.OOl to 0.1 M.
The rotation speed ofthe working electrode (ro) varied in the range of 0-3000 rpm. The experiments were carried out under purified N2 gas saturation at 25 °C. Cyclic voltammetry was perforrned with a Pine Instrument Bipotentiostat model RDE3.

Results and Disçussion
The voltammogram of iron run in 1 M A c + 0.1 O M NaClÓ4 from -1.0 V to 0.3 V at v= 0.1 V s· 1 exhibits two anodic peaks at ca. -0.59 V (peak I) and -0.29 V (peak II), and the reverse scan presents two reactivation peaks corresponding to an intense eletrooxidation process at ca. -0.30 V (peak III), and a slight one at -0.63 V (peak IV) which appears only in perchlorate solutions. A comparison 14 with an iron voltammogram in l M acetate solutíon, at the sarne pH, shows that the height of the anodic peaks increases considerably in the presence of perchlorate, and the peak potentials are shifted to more anodic values.
According to literature data 15 • 16 the ano di c behavior of iron in weak acid solutions which are free of oxygen, shows two anodic peaks in thc potential range of the activepassive transition. El Miligy et a!. 15 poínt out that these results are not dependent on the measurement technique and can be confirmed by galvanostatic and potentiostatic pulse experiments. The voltammograms (Fig. l) indicate that the presence of perchlorate tends to stimulate the dissolution process ín the active-passive transition.
The contribution of the reactivation process observed during the reverse scan depends strongly on v (Fig. 2) and ro (Fig. 3), increasing as ro increases and v dccreases. These results suggest that the kinetics ofthe eletrooxidation proc-ess become mass transport controlled, particularly in the potential range of peak II.
ln the entire range of the solution composition tested, the height of peak I and II increases linearly with the square root of v (Fig. 2). The anodic to cathodic voltammetric chargc ratio is always greater than 1 and incrcases with v. This behavíor is similar to that found in acetate solution without perchlorate 14 • The rotation of the working electrode produces a con-    literature data on acetate solutions 12 shows that with an increaseofro from O to 600 rpm the heightofpeaki changes from ca. 1 to 2 mA cm· 2 , but an increase of ro from 600 to 2500 rpm does not alter the current peak. For peak II the effect of ro is similar 12 to that observed in this paper. This means that the influence of ro on the charge and heíght of peak I ean be related to the presence of perchlorate. This anion tends to stimulate the dissolution process in the early stages of oxidation. The charge Q corresponding to peaks I and II, at constant ro increases linearly with v·' (Fig. 4). The value of Q extrapolated to v~ oo is 0.4 mC/cm 2 . For peak II a charge of 1 O mC/cm 2 was found. These results suggest that the anodic reactions involve at Ieast two stages. The first one, associated with peak I, implies a dissolution reaction influ-  enced by a mass transport control which is dependent on perchlorate concentration. The small charge in this potential region can be assigned to the formation of a monolayer of Fe(OH)2 species as has been suggested for iron in strongly alkaline media 17 and in slight acid solutions containing sulphate ion 18 • 19 • The second stage, which is related to the peak II, implies a mass transport control which is coupled to the electrochemical reaction and is mainly influenced by acetate concentration 10 • 12 • Previous work on acetate solution 9 • 14 has also found a high charge value for this peak. Bech-Nielsen 9 suggests for this peak a structure equivalent to some 30-50 atomic layers of iron. Several works 9 • 10 • 20 have postulated the participation of acetate in the iron dissolution reaction by forming acetate complexes at this potential range. The pseudo-reaction orders 9 • 11 • 21 with respect to perchlorate are determined (Table I)   .
The results for 1 M Ac + 0.10 M NaCI04 are in Table L The Tafel constant for iron in I M acetate soiution, at the sarne pH is 90 mV/decadc and 160 mV/decade 14 , respectively. For the solution of I M Ac + O.OOI M NaCI04, 60 mV/decade and 80 mV/decade were obtained, respectively. For the second Tafel region the decrease observed is Iess pronounced. Both Tafel slopes decrease with increasing perchlorate concentration. The RRDE data (Fig. 5) provide information about the nature and concentration of soluble Fe(II) electroactive species. When the ring potential (Er) was set at 0.60 V only oxidation occurred on the ring, and for Er = -0.60 V only reduction occurred on the ring.
For Er 0.60 V (Fig. 5b) the soluble species relative to disk peaks I and II appear on the ring voltammogram, and Fe(II) soluble species are detected as soon as Fe(II) is formed. For Er -0.60V (Fig. 5c) only the reductíon of Table 2. Effect of perchlorate concentration on the Fe(Il) soluble species concentration, calculated from the ring current at Er = 0.6 V, relative to peaks I (C* I) and II (C*u).

NaCI04(M)
w-3 w·l C*1 (M) w·1 w·6 peaks II and III can be observed on the ring. No soluble species have been detected at a more reducing potential (peak I).
From the ring current it is possible to evaluate the concentration of electroactive Fe(II) soluble species coming from the disk 23 for Ed = E1 and En . Defining ldr as the fraction of the disk current that is detected by the ring, we have the following relation: (1) where ild is the number of electrons involved in the disk reaction and nr the number of electrons in the ring reactions.
Regarding the disk, one may assume: Fe =Fe(Il)aq + 2e· with nd = 2 The ring reaction at O. 60 V is: Fe(II)aq Fe(III) + 1 e· with nr= 1 By using the Levich equation 24 one may determine the concentration ofthe soluble species on the disk surface in quasi-stationary conditions (v= 0.0 l VIs): (2) where v is the kinematic viscosity (0.01 cm 2 s· 1 ), D the diffusion coefficient of Fe+ 2 2.10· 5 cm 2 /s), r the disk radius, and w is the rotation speed. The calculation was made for Er= 0.60 V. Table 2 shows that the concentration (C*) of soluble Fe(Il) species increases according to perchlorate concentration. ln the potential zone of peak I, according to RRDE experiments and confirmed by thermo-dynamic25 data, only Fe(II) species are expected as reactíon products. ln the potential region of peak II, Fe(II) species can be formed 25 and were detected by the ring. Fe(III) species are also observed by using RRDE at this potential (Fig. 5c ). The concentratíon of the ferrous ion at the disk surface is determined by the solubility product of the ferrous hydroxide 25 (Kps 8.10- 16 ). Comparing the concentration of Fe(II) species values on the disk (Table 2) with that predicted from the solubility product, it turns out that it is always lower than necessary to precipitate ferrous hydroxide.
On the other hand, the concentration of these soluble species in perchlorate concentration solution is greater than those obtained in acetate solution in the absence of per-chlorate 14 • These facts point out that perch1orate influences the nature ofthe products forrned in the disk reaction.
The results obtained suggest that acetate inhibits the aggressive action of perchlorate. The effect of the acetate may be compared with those observed by Boenhoeffer and Heusler 6 with forrnic acid in the presence of perchlorate. According to these authors the presence of a strongly absorbing anion inhibits the increase in the dissolution rate caused by the presence of perchlorate. Bech-Nielsen 8 points out that anions like sulphate and perchlorate, nonspecifically adsorbed, are able to influence the electrode reactions, except in the case of sulphate the change in the dissolution rate is higher than in the case of perchlorate 2 • According to MacDougall and Bardwell 23 the passivation of iron in sulfate and perchlorate solution would be inefficient because these anions, unlike those derived from borate, do not have inhibitive abilities, nor do their solutions have buffering capacity. Previous works 9 • 11 • 14 have suggested that the beneficial action of acetate in F e passivation is due to its strongly interacting anions which give it an inhibiting capability.

Conclusions
Based on the data obtained, it appears that perchlorate acts in the iron dissolution process by increasing the dissolution rate in the early stages of oxidation, at a more negative potential. This is confirmed by kinetic data on the dependence ofthe dissolution rate on perchlorate concentration. This can again be related to an increase in the Fe(Il) soluble species concentration in the presence of perchlorate. The acetate anion has an inhibiting ability for iron and the passivity can be achieved at more positive potentials.