Acta Chim. Slov. 2003, 50, 43-55. 43 IMPEDANCE MEASUREMENTS OF STAINLESS DIFFERENT HEAT TREATED STEELS IN THE ACTIVE - PASSIVE REGION Mojca Slemnik, Valter Doleček, and Miran Gaberšček? Faculty of Chemistry and Chemical Engineering, Smetanova 17, 2000 Maribor, Slovenia ? National Institute of Chemistry, Hajdrihova 19, 61115 Ljubljana, Slovenia Received 25-09-2002 Abstract The behaviour of differently heat-treated X20Crl3 steels in 0.1 M H2S04 was studied using the classical potentiodynamic method and electrochemical impedance spectroscopy. On the base of the potentiodynamic curve the constant potentials have been chosen at which the impedance spectra were recorded: at the active peak, the potential next to the active peak and in the active - passive region. Ali impedance spectra show typical shapes: the high frequency portions have typical semicircular shapes of complex plane plots, whereas at low frequencies the plots deviate into semicircles ranning in opposite direction and yielding the so-called negative resistance as the frequency approaches 0 Hz. These spectra are interpreted in terms of a model by R.D. Armstrong describing, in a general way, the electrochemical reaction at interfaces with adsorbed intermediates.10 Applying the model to the measured impedance spectra, we have been able to distinguish between the cases in which the observed charge transfer resistance is soleh/ determined by nature of metal surface and the cases in which, beyond this inherent metal property, the charge transfer is determined by the degree of surface coverage with adsorbed intermediates. For example, while the oil-quenched X20Crl3 steel shows the lowest inherent charge transfer resistance of bare surface, the air-quenched sample exhibits the largest resistance due to surface passivation by intermediates. AH results of impedance analysis are well-correlated to the measured potentiodynamic curves. Introduction Potentiodynamic polarization techniques permit the measurement of polarization behaviour by continuously scanning the potential while monitoring the current response. In the čase of passivating systems, these techniques give useful information about the potential regions which correspond to the active and passive state and, in particular, about the active-passive transition region. However, this information is rather qualitative. It is not possible to discern betvveen various reactions steps, such as adsorption and charge transfer reaction at the passivating interface. Hence, the corresponding quantitative parameters, e.g. charge transfer resistance, adsorption rate constant or adsorption capacity etc, are also not obtainable by these techniques. By contrast, such information can be gained using the electrochemical impedance spectroscopy (EIS). M. Slemnik, V. Doleček, M. Gaberšček: Impedance measurements of stainless different heat treated… 44 Acta Chim. Slov. 2003, 50, 43-55. EIS is a well-established technique for investigating electrochemical and corrosion systems.1"6 An ac signal is applied to an electrode, here a corroding material, and the response is measured. Usually a small voltage signal is applied and the resulting current measured. The measuring equipment processes the current - tirne and the voltage - tirne measurements to provide the impedance at different frequencies, the impedance spectrum.7 In our former work, stainless steel X20Crl3 was exposed to various heat treatments: quenched on air, quenched in oil and quenched in oil and tempered. After the passivation, the measurements of electrochemical noise 8 and the impedance spectra 9 on the passive layers have been made with the purpose to determine the influence of the heat treatments on corrosion resistance of steels at the passivation potential. The values of passive layers resistance, Rph and interfacial capacitance, Q, were determined.9 However; the information was limited to the high and intermediate frequency regimes, while the low-frequency part of EIS spectra was considered only qualitatively. In the present work we broaden the previous work by interpreting the EIS spectra measured in the active-passive transition in terms of a quantitative model developed by R.D. Armstrong in 1972.10 In short, the model assumes that the active -passive transition occurs due to adsorbed species on a metal surface which block the charge transfer process. The adsorption of species is potential-dependent and is increased as the potential increases. This results in a pronounced increase of charge transfer resistance because charge transfer can only occur at spots on the metal surface where no species have been adsorbed. During the measurement of impedance spectra, two types of processes occur (Fig.l): a) at high frequencies the adsorption process is essentially »frozen in«, and mainly the charge transfer is detected (the high-frequency are A is observed in the complex plane); b) The low-frequency response is dominated by the adsorption/passivation process (are B is observed). Note, however, that the parameters pertaining to the charge transfer reaction and adsorption/passivation processes are not clearly separated, i.e., they cannot be mapped directly into the two ares observed. For example, although are A in Fig.l is dominated by the charge transfer reaction, the size of the are is not directly related to the charge transfer resistance - in fact, as we will see M. Slemnik, V. Doleček, M. Gaberšček: Impedance measurements of stainless different heat treated... Acta Chim. Slov. 2003, 50, 43-55. 45 later, charge transfer resistance splits to several contributions in this model. We emphasize this issue because there is a general tendency in the field of EIS that the authors ascribe individual processes to individual arcs. -Z’’ R ctsum O Z’ Figure 1. Typical shape of impedance response resulting from a model by R.D. Armstrong 10 presented in complex plane. A: high-frequency are, B: low-frequency are. Rctsum is defined by Eq. (1), eo: angular frequency of excitation signal. The aim of the present work was to determine the change of charge transfer resistance when moving from aetive to passive region for three types of heat treatment on stainless steels. It is reasonable to assume that the larger this change the more effective is the passivation process. In this way, we should be able to get a quantitative measure which would teli us which of the selected heat treatment of steel is passivated most effectively. Theoretical predietions of the adsorption-charge transfer reaction model The model of R.D. Armstrong 10 contains 5 parameters, i.e., charge transfer resistance at »frozen in« adsorption, Rct00, charge transfer resistance appearing at low frequencies, Rct0, rate constant for adsorption, k, and high- and low-frequency capacity, C0 and Coo. Analytic expressions for ali these parameters are given in ref 10 and are not repeated here for the sake of conciseness. M. Slemnik, V. Doleček, M. Gaberšček: Impedance measurements of stainless different heat treated… 46 Acta Chim. Slov. 2003, 50, 43-55. Ti me/a. u. Ti me/a. u. Figure 2. Schematic presentation of current response (below) to a potential step (above) within the active-passive transition region. For detailed explanation see text. Let us briefly comment only on the meaning of both charge transfer resistances, because these will be the main object of present analysis. Let us consider what happens when the potential within the active-passive transition region slightly increases in a stepwise manner (Fig. 2). At first instant, the current will also increase, in accordance to the charge transfer reaction kinetics which is usually well described by the Butler-Volmer reaction and which, at low amplitudes, simplifies to Ohm's law. In terms of the present model, this initial increase in current (AL) is determined by charge transfer resistance at »frozen in« adsorption, R^. At a later stage, the current starts to decrease because at higher potential an additional portion of metal surface is gradually covered by adsorbing species/passive film. Finally, a new steady-state is reached with a current lower than before the excitation. The decrease of current in this second stage (AI2) is determined by Rct0. Clearly, because the current decreases while the potential is increased, the value of Rct0 is always negative. Also, as can be deduced from Fig.2, the absolute value of Rct0 is always lower than that of Rctx. M. Slemnik, V. Doleček, M. Gaberšček: Impedance measurements of stainless different heat treated… Acta Chim. Slov. 2003, 50, 43-55. 47 Although the meaning of both Rct values is quite straightfonvard, they, unfortunately, cannot be extracted from the impedance spectra, such as that in Fig.l, in a simple way. However, the sum of their inverse values: is quite easy to determine from impedance spectra: namely, i?ctsum is exactly equal to the value of impedance at frequency 0 Hz. Obviously, like i?ct0, i?ctsum < 0 (see Fig.l). Furthermore, Rctsum is also equal to the inverse of the slope in I-U steady state curve at the point at which the impedance spectrum is recorded. Hence, Rctsum is a parameter which connects potentiodynamic and EIS measurements in a straightfonvard way. One of the specific aims of the present work was to determine the values of Rct? and Rcto at various potentials within the active-passive transition region. For this purpose, we decided to make simulations of the measured impedance spectra using the model presented above. These simulations, however, do not give a unique set of model parameters because, as mentioned, the model contains 5 parameters while the spectra containing 2 simple arcs (more precisely, semicircles) are uniquely determined already by 4 parameters. We have decided that for the purpose of the present paper, we set one of the parameters to an arbitrary and fixed value, i.e., k = 1 s"1. In other words, we assumed that the adsorption rate constant has a typical value as observed in similar systems by other techniques10 and that it does not change within the potential range of interest. In this way, we could then uniquely reproduce ali measured spectra by choosing appropriate values of the other 4 model parameters. Experimental method The material tested was a 13 - chromium stainless steel labelled as X20Crl3 made in Steel Works Ravne, Slovenia. The samples with 10 mm diameter and 3 mm thickness were treated under the following conditions: - annealed 20 minutes at 1000 °C, quenched on air - annealed 20 minutes at 1000 °C, quenched in oil M. Slemnik, V. Doleček, M. Gaberšček: Impedance measurements of stainless different heat treated… 48 Acta Chim. Slov. 2003, 50, 43-55. - annealed 20 minutes at 1000 °C, quenched in oil and tempered 1 h at 250 °C. The test samples were mechanically polished with 400, 800 and 1000 abrasive papers and fine polished with diamond pastes almost to a mirror quality. The degreasing was performed in acetone. The measurements were made in a standard Tacussel glass celi with the specimen as a working electrode, the platinum auxiliary electrode and a saturated calomel electrode (SCE) as reference electrode. The celi was filled with 300 ml of 0.1 M H2S04 solution. The measurements were performed at room temperature. Potentiodynamic polarization curves and impedance measurements were made with a Solarton 1287 Electrochemical Interface and a 1250 Frequency response analyzer. The data were collected using CorrWare and Zplot and interpreted with CorrView and ZView softwares. Ali softwares were developed by Scribner Associates, Inc. Potentiodynamic measurements were made from -1.0 V to 1.0 V vs. SCE at the scan rate lmV s"1. Impedance measurements were performed in a frequency range from 60 kHz down to 0.001 Hz. The amplitude of excitation voltage was 30 mV. EIS measurements were carried out after a 10-min polarization at a given potential. Measured potentials were chosen on the base of the classical polarization curve for X20Crl3 (Fig. 3). Results and discussion Annealed X20Crl3 steel contains alloying elements, which reduce the carbon diffusion and increase corrosion resistance. Quenching of steel prevents carbon diffusion, increases hardness, but leads to brittleness. Therefore, the steel has to be tempered before use. Tempering reduces the tension and increases the toughness, but on the other hand, causes the carbon to precipitate, leading to pitting corrosion.11 Anodic polarization curves for differently heat treated steels are shown in Fig.3. Anodic currents differed with heat treating particularly in the active and active - passive transition region. At the active peak, the current for oil - quenched steel was the highest. At potentials below this peak, the anodic currents are associated with the anodic oxidation M. Slemnik, V. Doleček, M. Gaberšček: Impedance measurements of stainless different heat treated… Acta Chim. Slov. 2003, 50, 43-55. 49 of steel. Higher peak currents for tempered and oil – quenched steel demonstrate that these heat treatments cause the material to corrode faster than air – quenched steel. But on the other hand, the anodic current drops more rapidly for the oil – quenched steel than for the tempered sample. To get a clearer picture about how all these phenomena are related to passivation (adsorption) and charge transfer processes, EIS measurements were performed. I ( A / cm2 ) 0,0030 0,0025 0,0020 0,0015 0,0010 0,0005 ¦air quenched oil - quenched tempered -0,7 -0,6/ / -0,5 -0,4 / -0,3 -0,2 E ( V ) -0,1 00 -0,0005 -0,0010 -0,0015 0,1 0,2 0,3 Figure 3. A potentiodynamic curve for X20Crl3 in 0.1 M H2S04. Typical measured complex - plane impedance spectra for the materials studied are presented in Fig.4. Ali spectra have the shapes as predicted by the model of Armstrong described above and displayed schematically in Fig.l. Of course, the model of Armstrong is only one of the many which can yield such a shape of impedance spectra. One could also use a simple equivalent circuit consisting of two RC terms, whereby the high frequency are would represent charge transfer resistance and the low-frequency are could correspond to adsorption. Analysis of measurements in Fig.4 using such a simple circuit, however, leads to quite unconvincing results. For example, when moving from the aetive dissolution peak to -0.33 V/SCE (the beginning of passive state), the size of the high frequency are (i.e., in this circuit corresponding directly to the charge transfer resistance) inereases merely by a factor of 4. This, of course, cannot explain the much M. Slemnik, V. Doleček, M. Gaberšček: Impedance measurements of stainless different heat treated… 50 Acta Chim. Slov. 2003, 50, 43-55. larger drop in current in Fig.3 (by at least a factor of 20). Even larger discrepancy occurs, if one wants to correlate the changes in impedance and potentiodynamic curves when moving from the active peak potential to -0.4 V/SCE. -300 -200 -300 Z" (nem2 ) •¦¦ active peak f>--0.40 V / SCE -0.33 V / SCE -100 100 200 300 Z (nem2) Figure 4. Impedance spectra for air - quenched X20Crl3 in 0.1 M H2S04 at different potentials. Much more consistent results are obtained if one analyses the measured spectra using the model of Armstrong. Fig.5 shows simulated spectra using this model. The simulated spectra reproduce quite well the measured spectra in Fig.4. The model parameters for each spectrum are displayed in Table 1. Interestingly, the development of the spectra with potential affects only two model parameters, Rcta0 and Rct0 - at least at given accuracy of parameter determination. Rcta0 changes from 60 Qcm"2 to more than 1300 Qcm"2, which seems a reasonable result if the corresponding change of current in Fig.3 is considered. Note, however, that the aetual change in Rctx might be even larger. Namely, many simulations have shown that once the ratio Rctx/\Rct0\ exceeds 10 - 15, the spectra become quite insensitive to the value of Rctoo. In other words, the spectrum at -0.33 V/SCE could be reproduced well by any value of i?ctoo larger than about 1300. o M. Slemnik, V. Doleček, M. Gaberšček: Impedance measurements of stainless different heat treated… Acta Chim. Slov. 2003, 50, 43-55. 51 aetive peak -0,40 V/SCE -0,33 V/SCE 0 50 Z' (nem2 ) Figure 5. Simulated impedance spectra using the model of Armstrong.10 The simulated spectra coincide well with the measured spectra in Fig.4. Some difference is observed in the spectra for the aetive peak. This is probabh/ mainly due to surface inhomogeneities, especially when it gets passivated. Note, however, that this deviation of shape into extended semicircle (see Figure 4) does not affect significanth/ the average values of model parameters. The model parameters for the simulated graphs are shown in Table 1. Table 1. Model parameter values at 3 different potentials for air-quenched steel. The value of k was selected arbitrarily, Rctsum was calculated from Eq. (1). The other parameters were set to sueh values that the tvpical features of model spectra (peak frequencies, are dimensions, frequency of high-frequency/low-frequency are transition etc.) coincided with the measured parameters with an accuracy of F (1± 0.1) where F is the value of feature under consideration. Potential Rctoo/ Qcm2 Rcto/ Qcm2 Aetive peak - 0.40 V/SCE - 0.33 V/SCE 60 120 >1300 -34 -44 <-130 Rctsum/ Qcm2 -80 -70 -150 C0/ HFcm-2 8300 8300 8300 Coo/ nFcm-2 190 190 190 k/s"1 1.0 1.0 1.0 It is interesting that the absolute value of i?ct0 also inereases during the passivation process. This means that the current drop A/2 (see Fig.2) becomes smaller when the passivation proceeds. On the other hand, the absolute value of Rctsum , which corresponds to the inverse of the slope of I-U curve, first decreases and then inereases again. This variation correlates well to the fact that the I-U curve in this region exhibits an inflection. -150 -100 -50 00 150 M. Slemnik, V. Doleček, M. Gaberšček: Impedance measurements of stainless different heat treated… 52 Acta Chim. Slov. 2003, 50, 43-55. The finding that the capacities do not change with potential in this rather small potential window is not very surprising; it is common that in interfacial electrochemistry those capacities are much less sensitive to potential than resistances. active peak -100 ! Z1 ( ?em2 ) -100 -80 -60 -40 -20 0 20 Z ( ?em2 ) 40 air quenched oil quenched tempered 60 80 100 Figure 6. Impedance spectra for X20Cr13 in 0.1 M H2SO4 at the potential of the active peak. Figures 6-8 compare the impedance spectra of different samples at the potential of active peak, at -0.40 V/SCE and at -0.33 V/SCE, respectively. Ali graphs have the same shapes which indicate that the model of Armstrong should be applicable in ali cases. The values of model parameters related to charge transfer resistances are presented in Table 2. As regards Rct?, some interesting results can be observed. At the active peak, the oil-quenched sample shows the lowest value while the air-quenched sample exhibits the highest Rct?. This correlates well with potentiodynamic polarization data in Fig.3, i.e. the current peak is the highest for the oil-quenched and the lowest for the air-quenched sample. At -0.33 V/SCE, the Rct? for the M. Slemnik, V. Doleček, M. Gaberšček: Impedance measurements of stainless different heat treated… Acta Chim. Slov. 2003, 50, 43-55. 53 -0.40 V / SCE -80 ¦¦¦*¦¦ air - quenc Z'' (?cm2 ) ..... 0.075 .....•.. --o--oil - quencr —*tempered 0.03 .--•'' 0.047 '"'^ ,• ^so,- A*N •' / L]*" 0.023 -B "* -50 - \ / V / • \ \ • . ' / / / / / V / 1 1 0.03 -40 - 0.095 \ '"i \ '-l • / J / 1 -30 - \ • \ \ \ * 0.047 g? 0.0075 \ B 0.0075 -20 --10 -755C HzJ 18.9 *** &P^ 18.9^»» 1 * J ^§ U 23« 1.5 1.5 -80 -60 -40 -20 0 20 Z ( ?em2 ) 40 60 80 Figure 7. Impedance spectra for X20Crl3 in 0.1 M H2S04 at -0.4 V / SCE. -0.33 V / SCE -300 -Z" ( ?em2 ) 0.037" * 4 - - • - - air - quenched -a- oil quenched A tempered 0.4*6 ••• Z' ( ?em2 ) Figure 8. Impedance spectra for X20Cr13 in 0.1 M H2SO4 at -0.33 V / SCE. 0.03 0.0075 4.766 0.003 -300 -200 -100 0 100 200 300 M. Slemnik, V. Doleček, M. Gaberšček: Impedance measurements of stainless different heat treated… 54 Acta Chim. Slov. 2003, 50, 43-55. Table 2. Summary of ali model-related Rct values for ali samples at ali potentials measured. Air-qu. = air-quenched, Oil-qu.= Oil-quenched, temp. = tempered X20Crl3. The meaning of model parameters is explained in text. Potential Sample RctJQcm2 7WQcm2 i?ctsum /Qcm2 Active peak Air-qu. 60 -34 -80 Oil-qu. 30 -23 -95 Temp. 53 -33 -90 - 0.40 V/SCE Air-qu. 120 -44 -70 Oil-qu. 120 -35 -50 Temp. 60 -32 -70 - 0.33 V/SCE Air-qu. >1300 <-130 -150 Oil-qu. 300 -80 -110 Temp. 300 -80 -110 air-quenched sample is more than 4 times larger than for the other two samples indicating the much better passivation of the former (presumably, a larger percentage of surface is covered with passive film in air-quenched sample). The model values for tempered sample at -0.40 V/SCE can be used to check the model assumption that Rctsum is correlated with the slope of the corresponding steady state I-U curve (we chose this sample because at -0.4 V/SCE the slope of the potentiodynamic polarization curve is best-defined). The inverse of this slope is about 100 m V/l.5 mAcm"2 which vields a resistance of 66 Qcm2. As seen from Table 2, the corresponding Rctsum equals 70 Qcm2, which is not a bad agreement, especially if we take into account the fact that potentiodynamic curves are not equal to true steady state curves. In any čase, it holds quite well for ali measurements that the smaller the slope in the I-U curves, the higher the absolute value of i?ctsum. To summarize, the air-quenched sample exhibits the largest Rctx, at ali conditions. At the active peak potential, when the surface is stili almost free of adsorbates, these means that the inherent charge transfer resistance of this sample is higher than that of the other two samples. By contrast, at this potential the oil-quenched sample has much lower Rctx, which means that it dissolves most easily among the samples. At the passive potential the air-quenched sample shows at least 4 times higher Rctx values than the other two samples. In fact, the difference has much increased, which indicates that passivation was more successful in the čase of air-quenched sample (e.g. coverage with M. Slemnik, V. Doleček, M. Gaberšček: Impedance measurements of stainless different heat treated... Acta Chim. Slov. 2003, 50, 43-55. 55 intermediates and products was more efficient; a smaller portion of the initial surface remained free). As regards the oil-quenched sample, it shows the lowest Rct? at active peak. This means that the bare surface of this material dissolves at a higher rate than in the case of the other two samples. However, at -0.33 V/SCE Rct? for this sample becomes equal to i?ct? of tempered sample. This means that this sample was protected more successfully with a passive film than the tempered one. References 1. F. Mansfield, W. J. Lorenz, J. Wiley 1991, 581–647. 2. J.L. Dawson, M.G.S. Ferreira, Corrosion Science 1986, 26, 1009–1026. 3. M. Bovine, G. Fabricius, T. Laitinen, T. Saario, Electrochimica Acta 1999, 44, 4331–4343. 4. Y.J. Tan, S.Bailey, B. Kinsela, Corrosion Science 1996, 38, 1545–1561. 5. I.Epelboin, M. Keddam, J. Electrochem. Soc. 1970, 117, 1052–1056. 6. C. Gabrielli, F.Huet, M. Keddam, Electrochimica Acta 1986, 31, 1025–1039. 7. R. Cottis, S. Turgoose, Electrochemical impedance and noise 1999, NACE international, Huston, USA. 8. M. Slemnik, A. Petek, V. Doleček, Werkst. Korros. 1995, 46, 13–17. 9. M. Slemnik, V. Doleček, M. Gaberšček, Acta Chim. Slov. 2002, 49, 613–621. 10. R.D.Armstrong, M.Henderson, J. Electroanal. Chem. 1972, 39, 81–90. 11. G. Okamoto, Corrosion Science 1973, 13, 471–489. Povzetek Merili smo lastnosti različno toplotno obdelanih jekel X20Cr13 v 0.1 M H2SO4 s klasično potenciodinamsko metodo in impedanco. Na osnovi potenciodinamske krivulje smo izbrali konstantne napetosti, pri katerih smo izmerili impedanco: v točki aktivnega vrha, pri napetosti tik pod aktivnim vrhom in v aktivno – pasivnem področju. Pri vseh impedančnih krivuljah opazimo podobno obliko: pri visokih frekvencah dobimo lepo izražene polkroge v kompleksni ravnini, medtem ko se z zniževanjem frekvence le-ti obrnejo v nasprotno smer, katerega rezultat je t.i. negativna upornost, medtem ko se frekvenca približuje 0 Hz. Za razlago impedančnih spektrov smo uporabili model, ki ga je razvil R. D. Armstrong za elektrokemijsko reakcijo na površinah z adsorbiranim intermediatom.10 Z uporabo modela na eksperimentalno dobljenih impedančnih spektrih smo lahko razločili med primeri, pri katerih je opazovana upornost prenosa naboja posledica izključno narave površine kovine in primeri, kjer je poleg tega prenos naboja določen še s stopnjo adsorbiranih intermediatov, ki dodatno pokrijejo površino kovine. Na primer, medtem ko v olju kaljeno jeklo pokaže najnižjo pripadajočo upornost prenosa naboja nepokrite površine kovine, pa na zraku kaljen vzorec pokaže največjo vrednost upornosti zaradi površine, ki jo dodatno pasivirajo intermediati. Vsi rezultati impedančne analize se dobro skladajo z izmerjenimi potenciodinamskimi krivuljami. M. Slemnik, V. Doleček, M. Gaberšček: Impedance measurements of stainless different heat treated…