Anodic Dissolution of 304 Stainless Steel Using

Anodic Dissolution of 304 Stainless Steel Using Atomic Emission Spectroelectrochemistry K. Ogle and S. Weber J. Electrochem. Soc. 2000, Volume 147, Issue 5, Pages 1770-1780. doi: 10.1149/1.1393433 Email alerting service

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Journal of The Electrochemical Society, 147 (5) 1770-1780 (2000) S0013-4651(99)10-067-3 CCC: $7.00 © The Electrochemical Society, Inc.

Anodic Dissolution of 304 Stainless Steel Using Atomic Emission Spectroelectrochemistry K. Ogle*,z and S. Weber Irsid, Usinor Research, 57283 Maizières-lès-Metz, France In this work a new spectroelectrochemical method based on an inductively coupled plasma atomic emission spectrometry has been developed and used to measure the elementary dissolution rates of Fe, Cr, Ni, Mn, Mo, and Cu simultaneously during linear scan voltammetry of a 304 stainless steel in the active region. Simultaneous dissolution was observed for all elements with the exception of copper, which appeared in solution at a potential approximately 100 mV more positive. The Tafel slopes for Fe, Cr, Ni, and Mn partial dissolution rates were measured around the corrosion potential and found to be identical within experimental error, between 59 and 68 mV/decade. The anodic dissolution of copper in acidic chloride and sulfate solutions was used to establish the quantitative relationship between the concentration transients and the dissolution rate. The residence time distribution of the electrochemical flow cell was determined using galvanostatic pulses of copper or stainless steel dissolution. The experimental residence time distribution could be approximated to a high degree of accuracy at both long and short times by a log-normal distribution. The effect of the residence time distribution on the shape of partial elemental current transients during linear scan voltammetry was investigated by numerical simulation. © 2000 The Electrochemical Society. S0013-4651(99)10-067-3. All rights reserved. Manuscript submitted October 18, 1999; revised manuscript received January 12, 2000.

Even though electrochemical corrosion occurs through the coupling of the anodic and cathodic half reactions, it is the anodic reaction which plays the essential role in corrosion research. The variation of the anodic dissolution rate with potential is often considered sufficient information to predict the corrosion behavior of the material in a wide range of conditions.1,2 A variety of electrochemical methods are often used for this measurement and linear scan voltammetry can be considered as a “work horse” method for the characterization of metals and alloys, particularly for materials exhibiting passivation phenomena.2 Nevertheless, there are two major difficulties which limit the usefulness of electrochemical methodology. First, the electrical current is proportional to the difference between the anodic and cathodic half reaction rates. Therefore, the half reactions must be decoupled, either by working under conditions in which the cathodic partial reaction is negligible over the potential range of interest (as in a conventional Tafel slope measurement) or by using in situ or ex situ measurements to independently measure one of the half reactions. This difficulty applies to polarization resistance (Rp) measurements as well: although Rp can be readily measured around the open-circuit potential, quantitative interpretation in terms of a corrosion rate requires knowledge of the anodic and cathodic Tafel slopes, which must themselves be measured under different conditions in which the two reactions are decoupled.3 The second problem concerns multicomponent materials, for which the anodic partial reaction is a sum of elementary reactions involving different elements and different phases of the material. It is often necessary to know how different elements contribute to the overall electrochemical behavior, as, for example, to distinguish between simultaneous and selective dissolution. Therefore, there is a real need for a method of measuring partial anodic currents which is widely applicable to a variety of materials and electrolytes. Ex situ gravimetric methods are commonly used to decouple anodic and cathodic reactions near the open-circuit potential, and such experiments were important in the establishment of the mixed potential theory for corrosion processes.4 Recently, in situ gravimetric methods using the quartz crystal microbalance (QCM) have proven useful for decoupling the anodic dissolution rate from the cathodic reaction for pure metals. For example, Seo et al.5 have measured the anodic and cathodic partial currents as a function of potential near the open-circuit potential for iron thin films deposited on the QCM. Nevertheless the technique is limited to materials * Electrochemical Society Active Member. z E-mail: [email protected]

which can be deposited on the quartz surface, and does not in any case solve the problem for multicomponent materials. Downstream electrochemical detection has been extensively developed in the form of channel flow cells, since the pioneering work of Cahan and Haynes6 and Bockris et al.7 in which downstream polarographic methods were used to determine the partial dissolution currents of copper and nickel from copper-nickel alloys. More recently, Tsuru et al.8 have investigated the dissolution of pure iron in a channel flow cell with a double detection electrode. Tafel behavior was observed around the active region of iron dissolution, and was measured to about 220 mV below the corrosion potential. Similar measurements were performed by Vuillemin and Oltra9 for pure Cr. The rotating ring disk electrode has been used to study binary alloy systems such as Fe-Ni,10-12 and to measure partial elementary currents of Fe-Cr alloys during linear scan voltammetry experiments in the passive region13 and during pitting transients.14,15 The more difficult problem of measuring several partial anodic currents simultaneously and in situ has not been dealt with extensively in the literature. A channel flow cell with a triple detection electrode has been used16 to measure Fe and Mo partial dissolution currents of an Fe-Mo alloy. Tafel behavior was observed for both elements around the active potential region extending to about 245 mV below the open-circuit potential. A split ring disk electrode17 has also been used to measure the partial currents of Fe and Cr from binary Fe-30% Cr alloys. It was found that, just below the passivation potential, the Cr partial current exhibits two Tafel regions while the iron partial dissolution curve showed only one Tafel region. A fundamental difficulty of downstream electrochemical detection is that only a single species can be detected at a given time at the detector electrode, and the species must be electroactive in a potential domain where there are no interfering reactions. This greatly limits the applicability of the method as concerns the dissolution products which can be detected and the composition of the electrolyte. The latter is particularly troublesome for the study of surface treatment processes, as the electrolytes often contain other electroactive species which could interfere. As mentioned above multielectrode systems have been developed, however the design of the experimental cell is notably more complex, and the absolute number of elements which may be detected simultaneously is limited. Therefore, although these techniques have proven extremely useful in some cases, they cannot be considered as generally applicable to a wide range of materials. Although the use of flow cells for optical spectroelectrochemistry is well established,18,19 it is surprising that multielement techniques, such as atomic emission or absorption


Journal of The Electrochemical Society, 147 (5) 1770-1780 (2000)

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spectroscopy have not been previously used to measure partial dissolution currents on-line. Off-line measurements of steady-state partial currents have been performed by Kirchheim,20 and the overall stoichiometery of passivation has been determined by inductively coupled plasma-mass spectrometry (ICP-MS).21 As concerns on-line spectroscopic measurements of metal dissolution, several very simple electrochemical flow cells coupled with spectroscopic techniques such as atomic absorption spectroscopy and ICP-AES have been proposed for the flow-injection analysis of bulk alloys.22-25 However the objective of these studies was to develop a method of on-line dissolution of a solid matrix for the purposes of elemental analysis of the bulk material. The cell design described in these references would not be practical for the study of dissolution mechanisms because of the lack of potential control and the proximity of the counter and working electrodes. To address this problem, we have developed a spectroelectrochemical technique using an ICP atomic emission spectrometer (AES) to directly measure the elemental concentration transients downstream from an electrochemical flow cell. In this way we can measure the dissolution rate of each component of an alloy system independently and simultaneously, and when combined with total current measurements, we can independently determine the anodic and cathodic half reactions. ICP emission spectrometry is perhaps an ideal choice for this experiment as it is sensitive to most elements, has a linear dynamic range of around five to six orders of magnitude, and is extremely sensitive, detecting trace quantities of elements at the parts per billion level, and is relatively insensitive to the matrix effects. Further, quantification is relatively simple as it only involves calibration with standard electrolyte solutions. Compared with other techniques often used for this purpose (rotating ring-disk electrode, jet-flow cell, quartz crystal microbalance, UV-visible spectroscopy), this technique has a much greater elemental sensitivity and is more easily adapted to a variety of metal samples and electrolytes. The temporal resolution and hydrodynamic range are however, more limited due to the aspiration-nebulization system of the spectrometer. The prototype for this cell was developed by de Gelis et al.26 Their system consisted of a small volume flow cell, open to the atmosphere, in which a metallic sample was exposed to an aggressive electrolyte. This original system was applied primarily to the analysis of galvanized steel products by dissolving the metallic coating in an acidic electrolyte at open-circuit and measuring the ICP emission intensities as a function of time. In many cases, each different layer of the coating would give rise to transitory signals in the ICP emission intensitytime profiles permitting a layer by layer analysis of the zinc/steel interface. The electrochemical cell described here was originally proposed as an extension of this earlier method, in which the selectivity of the dissolution was improved by the addition of potentiostatic control.27 More recently we have been interested in the ability of this system for spectroelectrochemical studies of surface reactivity. In this paper, the multielement capability of the system is demonstrated by investigating the anodic dissolution of a 304 stainless steel in the active region. The flow cell was characterized by analysis of transients resulting from the galvanostatic dissolution of pure copper. In other work, we have studied the anodic dissolution of Al-Zn galvanic steel coatings in alkaline solutions,28 and we have coupled the system to a quartz crystal microbalance to follow reaction kinetics during the exposure of zinc to alkaline solutions29 and the chromating of zinc.30 Experimental Spectrometer system.—A block diagram of the atomic emission spectroelectrochemistry system is shown in Fig. 1. The ICP spectrometer was used to measure the composition of the electrolyte downstream from the dissolution cell permitting a qualitative identification of the soluble reaction products and a quantitative measure of the simultaneous dissolution rates of the products. A commercial ICP atomic emission spectrometer from Jobin Yvon, Inc., (JY 74) was used in this work. The plasma source consisted of a 40 MHz, 1 kW inductively coupled Ar plasma into which the electrolyte sample was continuously aspirated. The spectrometer consisted of a polychroma-

tor for the simultaneous detection of 17 predetermined elements, and a monochormator for the detection of an additional element of choice. The polychromator was based on the Paschen-Runge configuration and was equipped with a holographic grating of 3600 groves/mm and 22 photomultiplier tubes. The number of photomultipliers is larger than the number of elements because several elements are measured at more than one wavelength to avoid interelement interference. The theoretical resolution of the polychromator was 0.028 nm covering a range from 165 to 425 nm. The monochromator was based on a Czerny-Turner configuration with a focal length of 64 cm and a holographic grating of 2400 groves/mm, yielding a practical resolution of 0.010 nm covering a spectral range of 165-800 nm. Both the polychromator and the monochromator were nitrogen purged. With the system as configured, the concentration of 18 different elements could be measured simultaneously. This number could be increased by adding additional photomultipliers, limited by the spatial requirements in the spectrometer focal plane. The commercial spectrometer was modified for the purposes of this work by the addition of a rapid data acquisition system, which monitors the entire array of photomultipliers and displays the signals in real time, and permits software control of the photomultiplier voltage. With this system, the photomultiplier voltages are sampled at a rate of 0.1 kHz with a 12 bit A/D converter, and the results are averaged over an arbitrary integration period, which in this work, was normally 0.5 or 1 s. Because of the averaging period, the dynamic range of the system was significantly greater than that of the 12 bit A/D converter. The spectrometer data acquisition electronics and software were further modified so that the potential and current signals from the analog output of the EG&G PAR M273A potentiostat/galvanostat could be recorded simultaneously with the spectroscopic emission data using the same A/D converter and integration/collection algorithm. This ensures that all simultaneous data are on exactly the same time base. A cyclonic aspiration chamber, chosen for its rapid response time, and a concentric glass nebulizer which has good performance when using electrolytes with high salt concentration, were used in this work. For the pulsed stainless steel dissolution experiment, an ultrasonic nebulizer was used, which has a somewhat better response time than the cyclonic nebulizer, and in general increases the detection limits of the system. However, because of a decrease in longterm stability with electrolytes of high salt concentration, the ultrasonic nebulizer was not routinely used. For the work described here the following analytical wavelengths were used: Fe, 259.94 nm; Cr, 205.56 nm; Ni, 216.55 nm; Mn, 257.61 nm; Mo, 202.03 nm; and Cu, 324.75 nm. The emission intensity vs. electrolyte concentration was calibrated using standard solutions (Titrosol™) in the electrolyte of interest. Elemental interefer-

Figure 1. Functional block diagram of the ICP atomic emission spectroelectrochemistry system.


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ences were checked for and found negligible for these wavelengths and elements under the conditions of these experiments.

Electrochemical flow cell.—A two-compartment, three-electrode flow cell was constructed from Teflon as illustrated in Fig. 2. The surface of the working electrode (0.52 cm2) is brought in contact with the flowing electrolyte (2-12 mL/min) in a small volume compartment ( 5 MV cm. NaCl, HCl, and H2SO4 were all reagent grade. Spectropure™ copper (Johnson Mathhey, 99.9995%) and a commercial 430 stainless steel were used in galvanostatic pulse experiments. A commercial 304 stainless steel sample was in the polarization experiments. The stainless steel samples were used as received following a degreasing with ethanol in an ultrasonic bath. The elemental composition of the stainless steels is described in the Results section. Results Steady-state dissolution rate and emission intensity.—ICP emission spectroscopy is a well-established method of elemental trace analysis for liquid samples and a large number of commercial systems are available. The methodology is described in numerous texts and review articles, and the treatise of Bowmans31 can be considered as a standard reference. For the work described here it is important to understand that the spectrometer system can be divided into three modules: (i) the nebulization/aspiration system into which the liquid sample is transformed into a fog of very small droplets, a fraction of which are aspirated into the (ii) inductively coupled plasma. The high temperature of the plasma ( I 8l, the noise level is determined by fluctuations in the nebulization system and is a constant percentage of the analytical signal. Under steady-state conditions, the elemental dissolution rate of component M of the sample working electrode within the flow cell, [RM(t) expressed in micrograms per second], is equal to its concentration downstream from the cell multiplied by the flow rate of the electrolyte ( f expressed in cm3/s) RM(t) 5 f CM(t) 5 kf [Il(t) 2 I8l]

[4]

where n is assumed to be 1 for copper dissolution in 1.2 M HCl. Again, note that the units are chosen to give the concentration in the conventional analytical units of micrograms per centimeter cubed. Following the imposition of a constant current, there is a lag period, t8, of approximately 10 s, during which no copper emission is detected. This time is associated with the transport of the ions to the spectrometer and depends on the flow rate and the length of tubing between the cell and the nebulizer. This is followed by a signal increase to a steady-state value. Approximately 43 s are required to reach 95% of the steady-state value. Quantitatively, the steady-state value is 10% higher than that predicted from the imposed current using Eq. 3. After 5 min, the programmed current returns to zero. Following the period t 8, the measured concentration of copper drops to 5% of the steady-state value after approximately 50 s. Figure 4 illustrates how this technique may be used to directly measure the number of electrons, n, transferred during the anodic dissolution of pure metals. The rate of dissolution of copper was measured as a function of applied current in 1.2 M HCl and 0.60 M H2SO4. Values of n 5 0.86 in 1.2 M HCl and n 5 1.94 in 0.6 M H2SO4 were determined, in good agreement with the well-known Cu(I) and Cu(II) species formed, respectively, in these two electrolytes .32 The right side of Fig. 3 shows an equivalent concentration transient induced by passing a 5 ppm Cu standard solution (1.2 M HCl) while bypassing the electrochemical cell as described in the experimental section. This experiment was performed immediately after the electrochemical experiment on the right so as to insure that all hydraulic and optical conditions were identical. Some differences are observed in the respective signals. For the concentration transient on the right, the noise level is reduced from 2.5 to 1.6%, which is probably due to the elimination of a low frequency noise visible in the cop-

[2]

In order to compare spectroscopic dissolution rates with the faradaic current measured by the potentiostat, it is convenient to express the elemental dissolution rate in terms of a partial elemental current, in microamperes iM(t) 5 nFRM(t)/MA

[3]

where MA is the atomic mass of element M. In this manner, the external current iex (defined as the current measured by the electrometer of the potentiostat, thus an external circuit, in contrast to the partial elemental current which is calculated from the spectroscopic transient) would be identical to iM if the only reaction were M r M1n 1 ne2 with 100% faradaic efficiency. In general however, this cannot be assured because either the n value will be unknown or metal ion release may be due to nonelectrochemical dissolution of oxides or other corrosion products. The expression of the metal ion release in terms of an equivalent current should be considered only as a convenience to facilitate comparison of two fundamentally different data sets (spectroscopic and electrochemical) and does not

Figure 3. Typical Cu atomic emission intensity transients produced by step events. Left: Anodic dissolution of copper in 1.2 M HCl resulting from a galvanostatic step from 0 to 300 mA assuming n 5 2. (dotted line 5 applied current expressed as concentration; solid line 5 measured concentration transient) Right: concentration step using a 5 mg cm23 Cu solution in 1.2 M HCl.


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Figure 4. Steady-state copper dissolution rate, measured from galvanic step experiments such as that shown in Fig. 3, as a function of the applied current in 1.2 M HCl and 0.6 M H2SO4. The slope of the curves yields n5 0.86 and 1.94 in HCl and H2SO4, respectively.

per dissolution transient and having a period of approximately 14 s. Furthermore, the response time of the electrochemical experiment is markedly increased due to diffusion and mixing in the flow cell. Electrochemical experiments are often associated with significant gas formation which could perturb the nebulization process and thus the spectroscopic measurement. Therefore, the effect of gas production on the emission intensity-concentration relationship was investigated. A concentration transient experiment was obtained, such as is shown in Fig. 3b, by aspirating 5 mL of a standard electrolyte containing Zn, Fe, and Al, into plasma passing by the electrochemical flow cell. During the aspiration, hydrogen gas was injected into the flowing electrolyte by imposing a cathodic current to a Pt foil used as a working electrode in the flow cell. Cathodic currents up to 2500 mA were applied without a measurable change in the signal intensity, the area of the resulting transient, or the noise level. The important point in the design of the system is that the electrochemical flow cell be placed between the pump and the nebulization system, thereby maintaining a constant volume of electrolyte per unit time into the nebulization chamber. With the pump between the cell and the spectrometer, the volume of hydrogen produced at the Pt cathode directly replaces the electrolyte and thus lowers the electrolyte flow rate leading to significant changes in the intensity levels. Therefore, we conclude that the quantification of metal ion release is correct even in the presence of significant gas production. Nonsteady-state instrument response.—Figures 3 and 4 demonstrate the quantitative nature of steady-state measurements. However, a significant response time was observed associated with diffusion and mixing in the flow cell and, to a lesser degree, mixing in the nebulization chamber. The limited time resolution is a problem for many applications, and therefore, it is of interest to treat the problem in more detail. The measured concentration transient, CM(t), is determined by the convolution of the dissolution rate, RM(t) with the distribution of residence times, H(t).33 Because of the complexity of the system, H(t) must be determined experimentally. This was done by measuring the concentration-time response to a “delta” function of dissolution as follows: A copper electrode was placed in contact with the flowing 1.2 M HCl for 5 min at the rest potential, followed by a 0.5 s galvanostatic pulse, and then a return to the rest potential. The transitory emission intensities were measured with a time resolution of 0.5 s. A typical result is shown in Fig. 5, with an applied current

of 10 mA and at a flow rate of 2.3 mL/min. Three different experiments are superimposed to demonstrate the reproducibility. The 0.5 s galvanostatic pulse is considered to be sufficiently short, on the time scale of these experiments, that it approximates a delta function. The pulsed anodic dissolution of copper gives rise to an asymmetric peak, which increases quickly to a maximum and then returns slowly to the background signal, I8l. Although, copper dissolution is known to proceed via the formation of a CuCl film followed by chemical dissolution of the film,34-36 the conditions of these experiments were chosen so that the initial film would be formed very rapidly, and its dissolution would be practically instantaneous by reaction with Cl2 in the electrolyte. This is also one of the advantages of working with a pulse experiment rather than a steady-state experiment as in Fig. 3. It is of interest to define several time parameters in Fig. 5. The previously mentioned lag time, t 8, is the time between the anodic pulse and the first point of data which rises above background. The inset to Fig. 5 shows the signal variation around t 8, and demonstrates that this point is readily recognizable to within the error of the data acquisition rate (0.5 s). This time is associated with the time necessary for the copper ions to travel between the between the dissolution cell and the nebulization system. Considerable progress has been made in modeling transients obtained by flow-injection analysis from first principles,37 and it is tempting to try a similar approach here. Unfortunately, a number of complex physical processes contribute to the broadening of H(t). These processes include diffusion from the surface into the flowing electrolyte stream, mixing in the channel flow cell, spreading out during the laminar flow in the capillaries between the cell and the spectrometer, and the complicated nebulization system itself. Further, the dissolution cell is not ideal and the diffusion distances probably differ from the edge to the center of the sample and from top to bottom of the channel flow cell. Finally, there is probably a significant dead volume near the O-ring. The situation could be simplified by using a small electrode mounted in a nonconducter and centered in the flow cell, however, this approach would lower the surface area and the sensitivity accordingly, and would further complicate sample preparation. Nevertheless the use of geometrically limited electrodes represents a particularly promising method of improving the time resolution of the system for certain applications. Despite the complexity of the system, the curve of Fig. 6 can be simulated by an empirical function in the form of a log-normal distribution

Figure 5. Residence time distribution of the flow cell. A 10 mA pulse was applied to pure copper in 1.2 M HCl and the Cu emission intensity was measured in response. Three measures are superimposed to show the reproducibility. t8 is defined as the time between the initial pulse and the first point at which the signal rises above the background (I 8l). A blowup of the onset near t8 is shown in the inset. t is defined as the time between t8 and the peak maximum.



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H (t ) 5

I 2 Io 5 Q

b b ln 2 ( tt* ) e pt 2

[5]

where b and t* are characteristic time constants for the log normal distribution and t 5 t9 (experimental time scale) 2 t8. Q is the integral of the transient in arbitrary units. The advantage of the log-normal distribution is that it predicts the parabolic form of the rising part of the residence time distribution. For the treatment of experimental data, it is convenient to define a simplified version of the lognormal distribution in which the pre-exponential factor is constant H (t ) 5

I 2 Io 5 Q

b 2 41b b ln 2 ( tt ) e e pt 2

[6]

Figure 6 displays the data of Fig. 5 as a function of t on a logarithmic scale. A symmetrical peak is observed with a maximum at 14.8 s. The solid curves show nonlinear least square curve fits to Eq. 5 and 6 superimposed. Obviously, the two models cannot be distinguished from one another. For Eq. 6, t is located at the peak maximum, in this case 14.8 s, and for Eq. 5, t* is located to the right of the peak maximum, at 25.7 s. By taking the derivative of Eq. 5, it can be shown that t 5 t*e21/(2b) and that b is the same for the two models. In the remainder of the discussion, Eq. 6 is used as an approximation of the experimental residence time distribution. The linearity of H(t) can be verified by performing the anodic dissolution with stainless steel so that a large number of elements are released into the electrolyte at very different concentrations. Figure 7 gives the time constant distribution for seven elements obtained after a 1.0 A, 0.5 s pulse applied to a 430 stainless steel. An ultrasonic nebulizer was used in this work to enhance the detection limits, as well as more rapid flow rate of 12 mL/min, making quantitative comparison with the copper experiments impossible. Nevertheless, it is seen that the log-normal distribution function accurately simulates the experimental transfer function for all the elements. Table I recapitulates the curve fit data for this experiment and compares them with the elemental analysis provided by the steel manufacturer. Note that, although the concentration of the elements varies over three orders of magnitude, the curves are essentially identical within experimental error, showing little trend with concentration. The only significant deviation is for copper which does appear at shorter times, perhaps due to the facility of forming the Cu(I) species in the chloride-containing electrolyte.

cal experiment, we have measured the release of metal ions during the polarization of a 304 stainless steel in an aggressive electrolyte (2 M H2SO4 and 0.1 M NaCl). Figure 8 shows the variation of the total “external” current density, Jex, and the sum of the elemental current densities SJM at variable temperature between 24 and 408C. Figures 9 and 10 give the individual curves decomposed into elemental currents. (Note that all currents are expressed in terms of current density unless otherwise noted: J 5 i/A, where A 5 0.52 cm2.) The elemental data are expressed in units of current in order to facilitate their comparison with the current measured by the electrometer (Jex). These conversions were made using Faraday’s law, assuming n 5 2 for each element in accordance with Eq. 3. Initially (t < 0) the sample is at the open-circuit potential for approximately 5 min. The potential program began at t 5 0, with a reduction at 2700 mV for 60 s, followed by a linear potential sweep at n 5 0.75 mV s21. Detectable metal ion release occurred at two different periods of the experiment: (i) during the active period where metal ion dissolution correlated closely with the external current and (ii) during the initial application of 2700 mV as shown in the inset to Fig. 8 and decomposed into partial currents in Fig. 10. During this period, the reactions leading to the release of metal ions were negligible compared to the cathodic current associated with hydrogen formation ( 2200 mV. Furthermore, these results indicate that secondary reactions such as the oxidation of adsorbed hydrogen atoms could only make a minor contribution to the total current. These points are discussed in more detail in the Discussion section of this paper. The cathodic transients are decomposed into elemental dissolution rates in Fig. 10. Note that, in order to facilitate comparison with the results of Fig. 9, the dissolution rates are presented with the same multiplicative factors. The rates are expressed as equivalent currents assuming n 5 2, but in this case the choice of n is arbitrary since the stoichiometry of the dissolution is unknown. Although the origin of these peaks could not be confirmed in this work, it is likely that they are due to the reduction of the passive film which is initially present on the surface and which evolves during the initial exposure to the electrolyte at open circuit and is reduced during the cathodic polarization. This hypothesis is supported by the fact that the rate of Cr dissolution is significantly higher here than in the passivation peak consistent with the well-accepted hypothesis of Cr enrichment in the passive layer. Likewise the Mn/Fe ratio is slightly increased. Integration of the three cathodic elemental transients at 408C yields 450 6 6 ng Fe, 390 6 20 ng Cr, and 8.5 6 1 ng Mn. The errors are calculated from the standard deviation of the background signal, I8l, measured immediately after the transient. This quantity of Cr would correspond to 570 ng of Cr2O3. In turn, this would correspond to a passive film of 1.1 nm assuming 520 ng/nm cm2. 38 Note that no significant rise of the Ni and Mo signals above the background noise was detected in this experiment. The quantitative nature of this experiment permits a kinetic analysis of the anodic dissolution reaction near and below the corrosion potential. This is shown in Fig. 12 where the total current and the elementary currents are shown on a logarithmic scale, as a function of potential. The potential of zero external current is indicated by an arrow. Linear behavior is observed over two orders of magnitude for the elementary reactions. Tafel slopes from these curves were nearly identical and varied between 59 mV for Fe to 68 mV for Ni as indicated in the figure. The error for the latter was quite large



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Figure 9. Partial elemental currents for the polarization curves of Fig. 8 for six elements. The bold face curve gives the total external current, Jex. The dotted curve gives the sum of the elemental currents, SIM. Note that the curve for the partial elemental current of copper has been offset for clarity. (n 5 0.75 mV s21, flow rate 5 3 mL min21). (a) JFe; (b) JMn 3 40; (c) JCr 3 2; (d) JNi 3 3; (e) JMo 3 100. Downloaded on 2012-11-15 to IP 87.91.69.175 address. Redistribution subject to ECS license or copyright; see www.esltbd.org

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due to the high noise level for Ni at low potentials. The noise level for Mo was too high for this type of analysis. These values are within the expected range (25-100 mV) for the dissolution of steel in an acid solution.39,40 The measurements obtained at lower temperatures indicate a slight decrease in the Tafel slope with temperature, from 60 mV at 408C to 42 mV at 258C. Within the detection limits of this experiment, we have not detected any selective dissolution between the elements other than copper. This is consistent with the literature where simultaneous dissolution has been consistently observed for ferritic Fe-Cr alloys in the active region.14,20,41,42 Selective dissolution of iron is believed to occur for high alloyed austenitic steels.43 The cathodic step experi-

Figure 11. Total quantity of metal dissolved, expressed in mC cm22, as determined by atomic emission spectroscopy assuming n 5 2 (y axis) and by integration of the current transient (x axis). By this calculation the faradaic yield is 91.5%. The dotted line gives the expected results for a Faradaic yield of 100%, n 5 2.

ments of Fig. 10 indicate that the native oxide layer contain a marked enrichment in Cr which suggest that selective Fe dissolution occurs in the passive state and at the open-circuit potential.

Figure 10. Partial elemental currents before and during the initial cathodic polarization. The potential-time program applied to the sample began at t 5 0 as indicated in the upper curve. For t < 0, E 5 open-circuit. For 0 < t < 60 s, E 5 2700 mV. The open-circuit potentials just before the potential step were 257 mV (248C), 285 mV (308C), 310 mV (358C), and 375 mV (408C).

Figure 12. Determination of the Tafel slope at the corrosion potential for four major elements during the early stages of the dissolution peak. Note that the onset of the linear region is observed below the corrosion potential, and extends over approximately two orders of magnitude. The Mo data set is not shown as the noise level was too high for this type of analysis.



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Discussion Mechanism of stainless steel dissolution.—The presence of double peaks in sulfate medium has been previously observed for 304 stainless steel and a number of possible explanations have been proposed including the dissolution of a copper-enriched surface film,44 or the oxidation of hydrogen atoms adsorbed during the cathodic polarization, facilitated by the presence of copper enrichments.45,46 The polarization curves shown in Fig. 8 and 9 demonstrate that, with the exception of copper (which makes a negligible contribution to the total current) the dissolution stoichiometry remains constant and proportional to the bulk alloy composition throughout both peaks. Therefore, as concerns the experiments reported here, mechanisms which attribute the second peak to the oxidation of foreign elements such as adsorbed hydrogen can be eliminated, although the oxidation of such species may contribute indirectly to the increased reactivity of the stainless steel alloy. It has been shown previously that copper enrichment at the surface of 304 stainless steel leads to a suppression of anodic dissolution and gives rise to a second anodic peak in the sulfate electrolyte.44 However, in that work the second anodic peak was shifted 150 mV more positive than what is observed here and was about an order of magnitude less intense than the first peak. The differences can be attributed to the presence of Cl2 ions in the electrolyte used in this work which, as demonstrated in Fig. 4, stabilizes the Cu(I) species by forming the CuCl21 2 complex, which shifts the corrosion potential of the copper into the active region of the stainless steel. Thus the dissolution of the copper film leads to a reactivation of the stainless steel, followed by passivation only at higher potentials. In the sulfate electrolyte, the dissolution of the copper film occurs in the passivation region, and thus does not lead to a reactivation of the surface. Due to the limited number of experiments presented here, it is difficult to generalize these results to other conditions such as those in Ref. 44-46. A more detailed study of these phenomena under a wider variety of conditions is called for.

effect of the log-normal time constant on the form of the measured polarization curve is clearly seen in the upper curves. These effects include a dampening of the signal intensity and a shift of the passivation peak to higher potentials. This distortion of the polarization curve makes it difficult to compare quantitatively the instantaneous electrical current and the measured partial currents without some form of correction. Deconvolution methods have been proposed for a similar situations encountered with the rotating ring-disk electrode,47 and with the use of flow cells with optical detection.48,49 A residence time distribution model for a flowing liquid with longitudinal diffusion has been proposed by Levenspiel,50 and the resulting distribution is very similar to a log-normal distribution. The lower curves in Fig. 12 give the same data in a log current vs. time (potential) format. Despite the distortions of the overall peak, the log normal residence time distribution does not affect the exponential slope of the leading edge of the passivation peak even at very large values (120 s). The only practical problem is that the linear region is shortened, limited at low signal levels by the spectral background noise and at high signal levels by the decreasing amplitude of the peak. Therefore the kinetic analysis such as that of Fig. 11 is justified, although the alignment of the elementary currents with potential may have an error estimated on the order of 5 mV (