Local electrochemical impedance measurements on

Electrochimica Acta 54(2009)6070–6074Contents lists available at ScienceDirectElectrochimicaActaj o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /e l e c t a c taLocal electrochemical impedance measurements on inclusion-containing stainless steels using microcapillary-based techniquesH.Krawiec a ,∗,V.Vignal b ,J.Banas aa AGH,University of Science and Technology,Department of Foundry,ul.Reymonta 23,30-059Cracow,Poland bICB,UMR 5209CNRS-Universitéde Bourgogne,BP 47870,21078Dijon Cedex,Francea r t i c l e i n f o Article history:Received 31October 2008Received in revised form 29November 2008Accepted 14December 2008Available online 24December 2008Keywords:Local impedance spectroscopy Inclusions Stainless steel Equivalent circuita b s t r a c tMnS inclusions are good precursor sites for pitting corrosion of stainless steel.The objective of this paper was to quantify the passive properties of resulfurized stainless steel after immersion in chloride media.This was done by combining microcapillary techniques with electrochemical impedance spectroscopy and numerical analysis (specific equivalent circuit).It was shown that sulfur species produced in the electrolyte during the dissolution of inclusions react with the native passive film to CrS and FeSO 4.Local electrochemical impedance spectroscopy measurements provided data describing the behaviour of the affected matrix at the microscale.For example,the value of the charge transfer and migration of point defects resistance decreases from 51,700 cm 2,in sites free of any metallurgical heterogeneity down to 12,200 cm 2,in sites containing a high density of inclusions.It was also shown that the integrity of the microcapillary can be altered by the presence of high quantity of sulfur in the electrolyte.Local impedance data allowed the detection of such problems.©2008Elsevier Ltd.All rights reserved.1.IntroductionMnS inclusions are good precursor sites for pitting corrosion of stainless steel [1–3].The first step leading to pit initiation is the dis-solution of MnS inclusions and the adsorption of the sulfur species,released into the solution on the surrounding passive film.Data collected from microcapillary techniques enabled to evaluate some key parameters,such as the potential for MnS dissolution [4–8].On the other hand,sulfur species that appear in the solution were detected using the scanning electrochemical microscope (SECM)with I −/I 3−as a redox mediator [9].Experiments performed,using UV spectrophotometry,confirmed that S 2O 32−ions are formed dur-ing electrochemical dissolution [10,11].Changes in the passive film composition induced by the presence of sulfur in the electrolyte species have then been quantified [12].On the other hand,only a few research groups have attempted to combine microcapillary-based techniques with other techniques.These include scanning vibrating electrode technique [12],elec-tron backscatter diffraction measurements [13]and impedance spectroscopy [14].By combining different techniques it is possible to describe more accurately processes taking place on the speci-men surface.Regarding MnS inclusions,small wires of W,Ag and Pt were placed as second microprobes within an electrochemical∗Corresponding author.Tel.:+48126172763;fax:+48126336348.E-mail address:krawiec@.pl (H.Krawiec).microcell to detect changes in the pH value,sulfide and thiosul-fate concentrations during voltammograms [4].The object of this paper was to combine microcapillary techniques with electrochem-ical impedance spectroscopy to quantify the passive properties of resulfurized stainless steel after immersion in chloride media.2.ExperimentalMeasurements were performed on resulfurized 304L stain-less steel (chemical composition Cr:18.3wt.%,Ni:8.75,Mn:1.7,S:0.17,Si:0.5,N:0.055,C:0.05,P:<0.035and Al:0.02).Spec-imens were mechanically sanded with SiC emery papers down to 4000grit and polished with diamond pastes down to 1␮m.Between each polishing stage samples were ultrasonically rinsed in ethanol.Local electrochemical impedance data were obtained using the electrochemical microcell technique [15].Glass capillaries with a diameter in the range of 50–250␮m were used.The microcap-illary tip was covered with a layer of silicon rubber.When the microcapillary was in contact with the specimen surface an elastic deformation of the silicon rubber was ing this method,it was possible to do measurements with extremely small microcapil-laries (diameter less than 10␮m)[16].The microcell was mounted on a microscope for precise positioning of the microcapillary on the surface.In addition,the wetted area on the specimen was accurately calculated using a specific image analysis procedure.Cal-culation of the current density could be validated by performing0013-4686/$–see front matter ©2008Elsevier Ltd.All rights reserved.doi:10.1016/j.electacta.2008.12.022H.Krawiec et al./Electrochimica Acta 54(2009)6070–60746071measurements with different microcapillary diameters and numer-ical simulation [17].Experiments were carried out using a PGSTAT30AUTOLAB potentiostat/frequency response analyzer.Impedance spectra were determined using ac signals of amplitude 20mV peak to peak within the frequency range of 10kHz to 10mHz.All the potentials were measured vs.Ag/AgCl reference electrode (saturated solution)and a platinum wire counter electrode.Measurements were per-formed after immersion of specimens in 1M NaCl,pH 3(adjusted with HCl),at +150mV vs.Ag/AgCl for 10min.3.Results and discussion3.1.Potentiodynamic and potentiostatic measurementsLocal polarisation curves (1mV s −1)were first obtained on sites of the resulfurized 304L stainless steel containing the pure matrix and the matrix with a MnS inclusion (Fig.1(a)).These experi-ments were carried out using 50␮m in diameter microcapillaries.MnS inclusion started to dissolve from an applied potential of +150mV vs.Ag/AgCl,leading to the current increase observed in Fig.1(a).Stable pitting was systematically observed during this pro-cess (above 250mV vs.Ag/AgCl).By contrast,a large current plateau was observed in the pure matrix and no pitting potential could be identified.As it was explained in Section 1,dissolution of MnS inclu-sions and pit initiation and propagation have widely been studied in theliterature.Fig.1.(a)Local polarisation curves (1mV s −1)obtained on sites containing the purematrix and the matrix with a single MnS inclusion of resulfurized 304L stainless steel in 1M NaCl,pH 3,at 25◦C.(b)Evolution of the current density vs.time in various sites of pure 304L and resulfurized 304L stainless steels in 1M NaCl,pH 3,at +150mV vs.Ag/AgCl.Fig.2(a)shows that sulfur was only detected in inclusions after polishing.Sulfur species released in the electrolyte,react with the native passive film formed on the surrounding grains.Sulfur species were then detected in the matrix using secondary ion mass spec-trometry (SIMS),as shown in Fig.2(b).Previous works based on XPS analysis [12]have shown that this yields the formation of CrS and FeSO 4on the specimen surface.Such changes affect the elec-trochemical behaviour of stainless steel (both cathodic and anodic reactions).The evolution of the current density vs.time was then monitored at +150mV vs.Ag/AgCl using 250␮m in diameter microcapillaries.The applied potential corresponds to the onset of MnS dissolution.On the pure 304L sample (no inclusion),the current at the steady state was very low—about 0.25␮A cm −2.This indicates that the passive film was protective.In the case of sites with a few inclu-sions (resulfurized stainless steel),a small quantity of sulfur species was released in the electrolyte.Fig.2(c)shows an example of such sites.The passive film was not significantly affected and the cur-rent density at the steady state was then found to increase up to values around 0.9␮A cm −2.When the quantity of sulfur species in the solution was high (sites with numerous inclusions,such as in Fig.2(c)),the chemical composition of the passive film was signifi-cantly changed.High current values were then reached at the steady state (1.5␮A cm −2,Fig.1(b)).3.2.Simulation fitting procedure for impedance dataThe fitting procedure was performed using the equivalent cir-cuit insert in Fig.3.This circuit takes into account the response of both the material and the microcapillary.The resistance R 1and the capacitance C 1were associated to an additional time constant related to ohmic drop effects induced by the use of the microcap-illary [18].On the other hand,R 2and CPE2were added to model semi-infinite linear diffusion of species in the microcapillary where only the linear distance from the electrode was taken into account.Previous publications,using a finite element approach [17,19],have proved that the diffusive flux of species,depends on the cell geom-etry.The diffusive flux is much higher in the microcapillary than in the classical macrocell.These results justify the use of R 2and CPE2in the low frequency range.Impedance data were fitted to the appropriate equivalent circuit,using the Z-view software package.The first fitting was performed without any constraints.Parameters with a relative error less than 2%were then fixed,and the fitting procedure was re-started.This method was applied until all the relative errors were smaller than 2%.3.3.Impedance measurements on the pure 304L stainless steel at the microscaleImpedance measurements were first carried out on the pure 304L stainless steel,after immersion at +150mV vs.Ag/AgCl for 10min,as shown in Fig.3.Experimental (black line)and numerical (black symbols)diagrams show very good correlation.Numerical values used for each component are reported in Table 1.The power index of CPE2(CPE2-␸)was systematically around 0.5,confirming that CPE2can be assimilated to the Warburg impedance given by Z W =ω1/2(1−i )=1√2CPE2−T ω1/2(1−i )(1)where is the Warburg coefficient ( s −1/2).Once the fitting procedure was performed,it was possible to plot the response of the material (red diamonds corresponding to R HF −R 3/CPE3).The two numerical diagrams (black circles corre-sponding to the full equivalent circuit and red diamonds related to the material response)were then compared.As the used electrolyte6072H.Krawiec et al./Electrochimica Acta 54(2009)6070–6074Fig.2.Repartition of sulfur species determined from SIMS experiments on various sites (resulfurized stainless steel).(a)After mechanical polishing and (b)after immersion in 1M NaCl,pH 3,for 10min.(c)Optical micrograph of the resulfurized stainless steel surface.is highly conductive,the use of the microcapillary has nearly no influence on impedance measurements in the high frequency range (above 100Hz).In this case,the two diagrams almost overlap and impedance measurements roughly describe the sample behaviour.By contrast,diffusion processes are strongly affected by the pres-ence of the microcapillary below 0.5Hz.Under these conditions,the equivalent circuit has to be used to extract the material response and to avoid misinterpretation of impedance data.3.4.Impedance measurements on the resulfurized 304L stainless steel at the microscaleImpedance measurements were then carried out on sites of the resulfurized stainless steel containing a few inclusions (Fig.4).As it was explained previously,the chemical composition of the passive film was affected (formation of CrS and FeSO 4).The same values of R 1,C 1,R 2and CPE2,as in the absence of inclusions,were obtained (Table 1).This confirms that these parameters only depend on the characteristics of the microcapillary.Note that similar results have been obtained on various passive systems.They include stainless steel [18]and pure aluminum [20]in chloride media.The resistance R 3decreases significantly (16,600 cm 2instead of 51,700 cm 2in the absence of inclusions),as reported in Table 1.It is considered that this parameter includes the contributions of the charge-transfer resistance and the migration of point defectsmediating the growth of the passive film and dissolution of metal through it.Observed decrease in the value of R 3indicates that the protective properties of the passive film decrease with the forma-tion of CrS and FeSO 4.In sites containing numerous inclusions (resulfurized stainless steel),the chemical composition of the passive film was strongly modified.Fig.5shows impedance diagrams obtained in such sites.The resistance R 3again decreases to values of about 12,200 cm 2(Table 1).This value is four times lower than that obtained on sites free of any metallurgical heterogeneity.The values of R HF ,R 1and C 1(related to ohmic drop effects)were unchanged,whereas R 2decreases significantly,as reported in Table 1.In some sites,fitting could not converge because of large errors in the value of R 2.Convergence conditions were fulfilled by removing this element from the equivalent circuit.This indicates that the micro-capillary characteristics evolve during the corrosion test.When the concentration of sulfur is high,one may assume that it is absorbed on the silicon rubber.This process perturbs diffusion mechanisms in the close vicinity of the sample surface and affects the values of R 2and CPE2.Analysis of the chemical composition of the silicon rubber has been performed after corrosion tests to confirm this assumption.In addition,it appears that impedance measurements are a good method to detect any alterations of the microcapillary during corrosion tests that could generate arti-facts.H.Krawiec et al./Electrochimica Acta 54(2009)6070–60746073Table 1Comparison of numerical fitting obtained on various sites in 1M NaCl,pH 3,at +150mV vs.Ag/AgCl for 10min.R HF ( cm 2)R 1( cm 2)C 1(×10−5F cm 2)R 2( cm 2)CPE2-T(×10−5F cm 2),CPE2-␸R 3( cm 2)CPE3-T(×10−5F cm 2),CPE3-␸Pure 304L,pure matrix2.550.87 1.7184,4609.21,0.651,6878.71,0.93Resulfurized 304L,matrix +few inclusions2.770.821.93184,500 4.55,0.416,623 5.43,0.90Resulfurized 304L,matrix +numerous inclusions 2.16 1.070.71342,77010.2,0.5915,3838.62,0.90Resulfurized 304L,matrix +numerous inclusions2.21.170.7292.5,0.6612,2137.9,0.93Fig.3.Bode diagrams determined on pure 304L stainless steel (no inclusion)after immersion in 1M NaCl at +150mV for 10min.Both experimental data (line)and numerical plots obtained from equivalent circuits (symbols)are presented.Fig.4.Bode diagrams determined on sites containing the matrix and a few inclu-sions (resulfurized 304L stainless steel)after immersion in 1M NaCl at +150mV for 10min.Both experimental data (line)and numerical plots obtained from equivalent circuits (symbols)are presented.Fig.5.Bode diagrams determined on two different sites containing numerous inclu-sions (resulfurized stainless steel),after immersion in 1M NaCl at +150mV for 10min.Both experimental data (line)and numerical plots obtained from equivalent circuits (symbols).4.ConclusionsThis study shows that local electrochemical impedance spec-troscopy measurements,based on the use of microcapillaries,combined with the proposed numerical analysis,provide data describing the behaviour of metallic alloys at the microscale.As these data do not depend on the microcapillary characteristics,no misinterpretation of the results can be done.This method was applied in case of stainless steel contain-ing MnS inclusions which was considered as a model system.SIMS analysis demonstrated that sulfur species,released in the electrolyte,react with the native passive film formed on the sur-rounding grains.CrS and FeSO 4are then formed,modifying the passive properties of the passive film.This was quantified from impedance measurements.It has been shown that the value of R 3decreases from 51,700 cm 2,in sites free of any metallurgical het-erogeneity to 12,200 cm 2,in sites containing a high density of inclusions.It has also been shown that the integrity of the micro-capillary can be altered by the presence of a high quantity of sulfur in the electrolyte.Although impedance measurements 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