ISSN 2070-2051, Protection of Metals and Physical Chemistry of Surfaces, 2011, Vol. 47, No. 3, pp. 402–409. © Pleiades Publishing, Ltd., 2011. Original Russian Text © I.A. Safonov, Yu.Ya. Andreev, E.A. Matvienko, 2011, published in Fizikokhimiya Poverkhnosti i Zashchita Materialov, 2011, Vol. 47, No. 3, pp. 322–329. INVESTIGATION METHODS FOR PHYSICOCHEMICAL SYSTEMS Methods of Measuring Activation and Passivation Potentials of Fe–Cr Alloys I. A. Safonov, Yu. Ya. Andreev, and E. A. Matvienko MISIS National University of Science and Technology, Leninskii pr. 4, Moscow, 119991 Russia e-mail: Aivan85@yandex.ru Received December 29, 2009 Abstract—A novel experimental method of estimating the activation potentials of passive layers on metallic materials is developed taking into account the advantages and drawbacks of conventional methods (switching off the anodic polarization and reverse potentiodynamic activation–passivation). Theoretical principles of the proposed method of successive approximations are based on the thermodynamic possibility of forming a passive layer on the alloy surface. The method involves potentiostatic exposures in the potential range of presumable activation. The change in the current direction with time at a selected potential is considered to be an indicator of the surface state. The error of measurements is related solely to the accuracy of potentiostat and is independent of a particular mechanism of the passive layer formation. The method is supported by the example of Fe–Cr and Ni–Cr alloys, as well as 4541 steel X6CrNiTi18-10, AISI 321. The experimental data obtained are compared to the results of theoretical calculations of Flade potentials. DOI: 10.1134/S2070205111030142 INTRODUCTION EXPERIMENTAL The development of methods of measuring activation potentials of metals and alloys is very important for solving the problem of improving the corrosion resistance of metallic materials and constructing theoretical models that can describe the passivity phenomenon [1–7]. The use of modern techniques presents new possibilities in studying materials and processes that take place on the surfaces [8–12]. One can now observe effects that were unnoticed or previously neglected because of the low sensitivity of measuring devices. A standard three-electrode cell with the separated spaces filled with the working solution and a saturated potassium chloride solution respectively was taken for recording polarization curves. A standard silver-chloride electrode was used as the reference electrode, while a gold electrode served as the auxiliary electrode. 0.5 M H2SO4 solution was used as the electrolyte. Electrochemical measurements were carried out with an IPC 2000 Pro digital potentiostat with a modern base controlled by a built-in microprocessor connected to a personal computer. Fe–Cr alloy specimens containing 0–23% chromium were taken as the studied subjects. The working surface was nearly 0.5 cm2 and was different for each particular specimen, which was taken into account in the calculation of current density. Electrolytically pure chromium and carbonyl iron decarburized in hydrogen flow atmosphere (repeated 2-h annealing at 1100°С) were used when smelting the alloys. The alloys were produced in a Lay bold Heraues electric arc furnace by repeated smelting, followed by recrystallizing annealing at T = 830°С for 1 h and quenching in oil. The purity of alloys was controlled by X-ray fluorescent analysis and spark emission spectroscopy, which guarantee the purity of alloys in carbon (< 0.01) and other admixtures. Before experiments, the specimens were mechanically cleaned with a set of sandpaper starting with grade no. 80 (coarsest) up to no. 1000 (finest), degreased with acetone, and washed in distilled water. Then, they were purified in an ST-405 CTBRAND ultrasound bath in distilled water for 3 min. Upon An important theoretical parameter that characterizes the stability of the passive state of metals is Flade potential EF, which is the equilibrium potential of the passive-layer (PL) formation (E > EF) and dissolution (E < EF) [1, 2] corresponding to the following reaction in the film theory of metal passivity: mMe + nH 2O ←→ MemO n + 2nH + + 2ne −. (1) In this work, conventional techniques are used to determine the activation and passivation potentials of Fe–Cr alloys, i.e.,, switching off the anodic polarization, as well as forward and reverse potentiodynamic activation. Taking into account their advantages and drawbacks, we proposed a method of estimating the critical potential of the passive film stability on metallic materials that most adequately reflects the thermodynamic meaning of the Flade potential. The experimental results are compared to the theoretically calculated Flade potentials of the alloys. 402 METHODS OF MEASURING ACTIVATION immersing a specimen in the working solution, it was cathodically treated as follows: a 2-min exposure at a potential of –980 mV (S.H.E.) accompanied by a strong hydrogen evolution and a 2-min exposure at a potential of –380 mV (S.H.E.) accompanied by weak hydrogen evolution and the final removal of residual gas bubbles. The examination of the specimen surfaces at various stages of the experiment was carried out with an MBC-10 binocular microscope and a HITACHI TM-1000 scanning electron microscope. MEASURING TECHNIQUES Activation by Switching Off Anodic Polarization The switch-off method was proposed by Flade in 1911 [15] when he studied the electrochemical behavior of iron in sulfuric acid. According to this technique, upon the cathodic treatment at a Ec potential, the specimen is exposed at the Ep passive-state potential. Upon certain time ∆τ, the polarization is switched off and the rapid potential change in the negative direction is followed in time. The horizontal segment of a Е(τ) potential–time curve, i.e., the temporal delay before the potential drop, is called the Flade potential in the literature. Typical Е(τ) curves on Fe–Cr alloys are shown in Fig. 1. If we take into account that Flade used the method on individual iron, we can see that the data obtained with the method on multicomponent systems may be incorrect; sharply switching the potential from Ec to Ep eliminates the stage of active alloy dissolution, so that alloy components do not selectively dissolve in the active range and the surface does not become enriched in a component that is more stable under particular conditions. Therefore, in the case of multicomponent systems, it is reasonable to bring the alloy to the passive state by means of potentiodynamic scanning followed by exposure at the selected Ep value. Upon the passivation of a Fe–4 Cr alloy at Ep = 1222 mV for 30 min (Fig. 1a), the horizontal segment is shorter (cf. curves 1 and 2) when the specimen surface undergoes active dissolution before passivation. In this case, chromium predominantly dissolves because its standard potential is much more negative compared to that of iron. It is important that, in both cases, the potential of the end of the horizontal segment on Fe–4 Cr is the same and equal to Ea = 580 mV (S.H.E.). This means that iron oxides play a key role in the passivation of the alloy. Studying the effect of the duration and potential of passivation in the Flade method based on the example of the Fe–6 Cr alloy showed that a decrease in the passivation duration from 30 to 10 min does not result in the shortening of the horizontal segment in Е–τ curve, while an increase in Ep from 822 to 1022 mV leads to the increase in the time interval before activation by a factor of three. In all cases, Ea is about 470 mV, which indicates the similar nature of passive layers grown 403 Potential, mV (S.H.E.) (а) 1250 1050 850 650 450 1 2 250 50 –150 Axis 1 –350 3300 3350 3400 3400 3500 3550 3600 3650 1750 1800 1850 1900 1950 2000 2050 2100 2150 Axis 2 Time, s Potential, mV (S.H.E.) 1100 900 700 500 300 1 100 –100 –300 500 1000 (b) 2 3 1500 2000 2500 3000 Time, s Potential, mV (S.H.E.) (c) 700 1 600 2 500 400 300 200 100 3 0 –100 –200 –300 500 1000 1500 2000 2500 3000 3500 4000 Time, s Fig. 1. E–τ curves of activation by switching off anodic polarization of Fe–Cr alloys: (a) Fe–4 Cr: (1) active dissolution potential + 1222 mV, 30 min (axis 1), and (2) 1222 mV, 30 min (axis 2); (b) Fe–6 Cr: (1) 822 mV, 10 min, (2) 822 mV, 30 min, and (3) 1022 mV, 30 min; and (c) Fe–13 Cr: (1, 2, and 3) horizontal potential segments. under different conditions (Fig. 1b). The character of the horizontal segment remains nearly the same, regardless of the exposure at the passive-state potential, but substantially changes depending on the method of attaining the potential and its value. With an increase in the chromium content up to 13%, the shape of E–τ curves changes noticeably. Even at a short exposure (10 min), the Flade method reveals the multilayer structure of the passive film (Fig. 1c), which is the main advantage of the method. In the E–τ curve, there are three horizontal segments that correspond to the potential delays. Let us assume PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 47 No. 3 2011 404 SAFONOV et al. Current density, mA/cm2 Current density, mA/cm2 Current density, mA/cm2 (а) 190 180 170 160 cr 150 Ep 140 130 120 110 100 200 250 300 350 400 450 500 550 600 650 Potential, mV (S.H.E.) 350 (b) Еa cr 300 Ep 1 250 200 2 1 150 2 100 Еao 50 0 35 30 25 20 15 10 5 0 300 350 400 450 500 550 600 650 Potential, mV (S.H.E.) 700 (c) 1 2 3 1 2 3 –300 –250 –200 –150 –100 –50 0 Potential, mV (S.H.E.) 50 that the end of the segment only looks like an ordinary inflection point when the E–τ dependence is recorded with an inertial device. Oscillographic investigation shows that there is actually two, not one delay [16]. This means that data obtained with a potentiostat are approximate. A shift of the potential in the segment toward the more negative values is determined by the inconstancy of the passive layer composition in the depth. Close to the base metal, the layer is enriched in the electrochemically negative chromium. This fact agrees with numerous data [1–7, 11, 12]. When chromium content is further increased, the length and slope of the segment increase and, if the steady state of the alloy is passive in the particular environment (due to self-passivation), activation does not take place. Thus, we can conclude that the switch-off method is well applicable for qualitatively determining the number of layers on the metal surface and the potential ranges of their stable existence. In the case of multicomponent systems, it is reasonable to bring the alloy to the passive state by means of potentiodynamic scanning followed by an exposure at the selected potential. 100 Fig. 2. Polarization curves recorded at potentiodynamic scanning at a rate of 1 mV/s: (a) forward branch with oscillations on individual iron; (b) forward and reverse branches on low-chromium alloys with the critical potentials shown: (1) Fe–2 Cr, and (2) Fe–6 Cr; and (c) same on high-chromium alloys: (1) Fe–10 Cr, (2) Fe–13 Cr, and (3) Fe–23 Cr. Arrows show scanning direction. that each oxide phase (film or adsorption layer) has certain dissolution potential. Then, every time a new layer begins to dissolve on the passive metal surface, one should see a delay before the potential drop, i.e., a horizontal segment in the E–τ dependence. The length of the segment corresponds to the time interval in which a particular oxide phase is present on the surface. As soon as the layer disappears or becomes sufficiently porous, the potential drops until the next phase begins to dissolve or until the steady-state potential of the metal is reached. The drawbacks of the method are as follows. In an E–τ curve recorded with a potentiostat, the segment of the potential delay is never strictly horizontal. Sukhotin, who studied the activation of iron, found Potentiodynamic Activation–Passivation The method involves successively recording forward and reverse potentiodynamic curves. The beginning of the increase in the current density in the reverse curve is thought to be related to the Eao potential of the activation onset, while the maximum i value is thought to be related to the E acr, critical activation potential, which can be treated in a first approximation as the EF potential. An advantage of the method is cr its express character; one can obtain a set of E a values under particular conditions with no long exposures. The high potential scanning rate is an essential tool when studying cathodic loops on individual iron [17]. A drawback of the method is the dependence of the activation potential and critical passivation potentials on the scanning rate [12]. The effect of the chromium content on the shape of polarization curves is illustrated in Fig. 2. Spontaneous current oscillations typical of individual iron [4, 5] in a potential range of 390–490 mV (Fig. 2a) are caused by the formation of the adsorption layer of FeO oxides. In fact, the thermodynamic calculation of the Flade potential of individual iron based on the formation conditions of the adsorbed FeO oxide on the iron surface provides EF = 432 mV [13], and in Fig. 2a, this value falls in the first range of current oscillations. The second potential range of current oscillations in individual iron is close to the critical passivation potential E pcr (Fig. 2a). A similar phenomenon is typical of Fe– Cr alloys with low chromium contents (2 and 4%) as well, while at a chromium concentration of 6%, no current oscillation is observed during the passivation (Fig. 2b). Starting from the chromium content of 6%, PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 47 No. 3 2011 METHODS OF MEASURING ACTIVATION in accordance with the data of [8, 9]. The dependence on the chromium content in the alloy, which was obtained in those experiments, is symbate to the E pcr curve and shifted in the negative direction. In this cr case, the E a values are closest to the theoretically estimated EF potentials of the Fe–Cr alloys [14] (see below). With an increase in the chromium content (10– 23%), the shape of polarization curves noticeably changes (Fig. 2c). The passivation and activation curves are smoother and there is a substantial hysteresis between the critical passivation and activation potentials, similarly to the curves recorded on the lowchromium alloys. It is worth noting that activation does not take place on an alloy containing 23% Cr, and even lowering the potential scanning rate cannot solve the problem. Thus, the method of potentiodynamic passivation–activation provides a rapid estimation of cr E pcr and E a potentials of Fe–Cr alloys with a chromium content below 10% in non-oxidizing environments (0.5 M sulfuric acid, sodium sulfate, etc.). Method of Successive Approximations There is still no common concept about the nature of the passive layer. Many researchers [18–23] believe that the layer has an adsorption structure because the protective properties appear sometimes when the layer thickness is only part of the monolayer. Some others adhere to the phase nature of the passive layer [2, 4, 5]. The third point of view proposed by Schwabe [24], which is most popular among corrosionists [7], is as follows. During the metal passivation, oxygen-containing particles first adsorb on the metal to produce chemisorption oxide film. Then, with an increase in the anodic potential, the oxygen content in the layer increases, the layer becomes thicker, and transforms into the barrier film. Later, a phase layer can appear that is porous and cannot provide a protective effect. Based on various concepts, many variants of the definition of passivity were proposed, though, according to Tomashov [7], none of them reflects the phenomenon in full. Moreover, the interpretation of Flade potential varies from the assumption that it equals the end point of the second horizontal segment in E–τ curve [5] to the critical passivation potential [7]. The interpretation of the decrease in the current density during the exposure in a passive range is by no means more definite. The authors of [17] attribute it to the decrease in the passive layer thickness, which is in turn related to the decrease in defectiveness of the layer during its formation that causes the decrease in the ionic flow via the film along its structure defects and the deceleration of the layer formation. In classical works [1–4, 7], an assumption about the increase in the passive layer thickness at the concurrent decrease (а) 700 600 500 400 300 200 100 0 –100 –200 3 2 1 Potential, V (S.H.E.) cr Ea 0 Potential, V (S.H.E. E pcr shifts in the negative direction (Fig. 3a), which is 405 2 4 700 600 500 400 300 200 100 0 –100 –200 6 8 10 12 14 16 18 20 22 24 Chromium content, at %. (b) 3 2 1 0 2 4 6 8 10 12 14 16 18 20 22 24 Chromium content, at %. Fig. 3. Critical potentials of alloys depending on chromium content: (a) (1) Flade potential calculated according to [14], (2) activation potential, and (3) critical passivation potential; and (b) (1) Flade potential calculated according to [14], (2) activation onset potential determined by means of potentiodynamic scanning, and (3) activation onset potential determined by method of successive approximations. in its ionic conductivity was adopted. Still, only the existence of the passive layer upon reaching the passivation potential and the corresponding current density drop are the common facts of all of the theories. Taking into account that it is necessary to know the stability range of the passive layer when improving the corrosion resistance of metallic materials based on their passivability, we focused chiefly on determining the Eao activation-onset potential that would be closest to the Flade potential under the selected conditions. Therefore, we propose a method of estimating Eao of a passive layer that is independent of the passivation mechanism and based on the thermodynamic possibility of the passive layer formation on the alloy surface solely. Here, we assume that the EF Flade potential is a theoretical value that characterizes the equilibrium formation–dissolution of a passive layer. In this cr respect, E a is significantl closer to it than E pcr . The graphical estimation of the Flade potential proposed by Vetter [2] is illustrated by Fig. 4. In the energetic aspect, it can be treated as dynamic equilibrium (1) between the active surface and the passive layer as follows: PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 47 No. 3 2011 406 SAFONOV et al. The Eao indicator as the boundary between the active and passive ranges is assumed to be the change in the current direction at the selected potential. A scheme of the method is shown in Fig. 5. Passive layer Current density Current density Active surface Time E < EF At the first step, the Ep passivation potential corresponding to the smallest passivity current is determined (Fig. 5, curve 5). For this purpose, potentiodynamic polarization curve is recorded up to the repassivation potential range, and Ep value, which is typically close to the middle part of the passive range, is found. In Fig. 6, this potential of Fe–6 Cr alloy is denoted as E p1 and equal to 822 mV. Time EF E > EF Potential Fig. 4. Estimation of Flade potential according to Vetter [2]. Current densit (а) Potential Current densit 1 Eao1 < Eao < Eao2 (b) 2 Ep 3 4 5 Time Fig. 5. Scheme of determining activation onset potential by successive approximations in (a) current density– potential coordinates and (b) current density–time coordinates: (1, 2) activation curves; (3) plateau-activation curve; (4) plateau curve; and (5) current density in the passive state. Active Passive EF ← → surface layer      (2) [E < E F ⇒ ∆GPL > 0] [ E > E F ⇒ ∆ GPL < 0] , where ∆GPL is the Gibbs formation energy of the passive layer [14], EF is the Flade potential, and E is the electrode potential. At the second step, Eao potential is determined by successive approximations. Using the potentiodynamic scanning at a rate of 1 mV/s, the alloy surface is brought to the passive state below Ep Ⰷ EF. In this case, the Gibbs formation energy of the passive layer is always negative irrespectively of the layer composition: ∆GPL < 0 (Eq. (2)). The active–passive transition is clearly seen in the current–potential coordinates (Fig. 5a). The specimen is exposed at the Ep potential until the current becomes constant (about 30 min). This passive layer has the best protective properties in the particular alloy–environment system, which is indicated by the decrease in i to 10–20 µA/cm2. Then, the potential is abruptly changed to a certain Eao1 value from the active dissolution range. Now, the ∆GPL sign is determined by the Eao1 position with respect to EF. If Eao1 < EF, then ∆GPL > 0 (the film formation is thermodynamically unfavorable), and the current in I–T curve increases, which indicates the active anodic dissolution of the alloy surface (Fig. 4; Fig. 5b, curve 1). If Eao2 > EF, then ∆GPL < 0 and the current in I–T curve increases up to a certain value equal to the steady-state passivity current at the selected potential and then remains unchanged for an infinitely long time, which indicates the stabilization of the passive layer thickness at the selected potential (Fig. 5b, curve 4). A logical conclusion is that Eao is intermediate between Eao1 and Eao2 potentials. Therefore, in the next experiment, the alloy is exposed at Eao3 potential, which is the arithmetic mean of Eao1 and Eao2 values. Repeating this step, we make the Eao range progressively narrower by 50% each time at a corresponding increase in the total duration of the experiment. Here, the accuracy of determining the Eao potential depends on the sensitivity of potentiostat and the Eaoi boundary values (i = 1, 2, 3, etc.) of the range. It is worth noting that the i–τ dependence can also have the shape of curve 4 (Fig. 5b) in the case of the local alloy dissolution, which is determined by the heterogeneity of the metal surface and the passive layer. In the general case, the imperfect structure of the passive layer may be caused by distortions of the base metal structure, namely, grain boundaries; various (metallic or nonmetallic) inclusions; crystal faces with the orientation less favorable for passivation; or some finer PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 47 No. 3 2011 RESULTS AND DISCUSSION On Fe–Cr alloys containing 2–23% chromium, hysteresis is pronounced in the curves constructed using the passivation–activation method (Figs. 2b, 2c). On all 407 1.0 (а) 0.8 0.6 0.4 1 Ep 0.2 0 400 600 800 1000 1200 1400 1600 1800 2000 Potential, mV (S.H.E.) Current density, mA/cm2 220 (b) 180 140 Activation 100 60 20 –20 1 3 5 7 10 Current density, mA/cm2 defects, such as dislocations or inclusions of foreign atoms in the metal lattice [7]. At potentials close to Eao, the shape of i–τ curves changes (Fig. 5b, curves 2, 3). At first, a plateau at small current values is observed; then, upon a certain time, the alloy becomes active (Fig. 6b). Scaling up the curve in the i direction, one can notice that current increases with time, which indicates the gradual decrease in the protective properties of the passive layer (Fig. 6c). The Fe–6 Cr alloy was completely activated (i = 230 mA/cm2) at 502 mV (S.H.E.) upon 17-h exposure. Occasional current waves, which are especially strong at the 13th hour of exposure, are explained by the breakdown of the passive layer at sites where the surface microrelief is uneven. The passive layer is more strained at convexes where its internal energy is higher and, hence, the activation potential is shifted in the positive direction with respect to the mean surface value. Upon breakdown, the active dissolution of the site begins and leads to the general smoothening. When the height at the breakdown site levels off with respect to the neighboring surface, the potential at the site shifts in the positive direction under the effect of the neighborhood, the site again becomes passive and the passive layer is sealed. The closer the potential of exposure to the Eao value (on a condition that Eaoi < EF), the longer the complete dissolution of the passive layer before the active dissolution of the alloy itself and the more substantial the role played by grain boundaries in the surface activation. For example, at the exposure of Fe–15 Cr alloy at a potential of –58 mV, no visible activation was noticed and the current density was only 0.35 mA/cm2, even at 20 h. In this case, the i–τ curve was oscillating and did not reach a plateau (Fig. 7a). The examination of the specimen surface after the experiment with a scanning electron microscope showed that grain boundaries are strongly etched out compared to those upon the exposure in a passive range (Figs. 8b, 7c). Pits can also be seen at sites where dislocations exit to the surface. Comparing the data obtained by the method of successive approximations and the results of potentiodynamic activation reveals the similarity in the potential dependence on the chromium content (Fig. 3). The Eao values obtained by the former method are more positive, since in this case, the potential measurements are not affected by the scanning rate. The advantage of the method developed is as follows: it takes into account the thermodynamics of the active– passive transition and, hence, yields an Eao value close to ЕF. This method enabled us to estimate Eao on Fe– 23 Cr alloy, where all the residual techniques failed. Current density, mA/cm2 METHODS OF MEASURING ACTIVATION 9 11 Time, h 13 15 17 13 15 17 (c) 8 6 4 2 0 1 3 5 7 9 11 Time, h Fig. 6. Example of successive approximations on Fe–6 Cr alloy: (a) selecting passivation potential in passive range; (b) activation by exposure at potential of 502 mV; and (c) same scaled up in i direction. alloys, the critical passivation potential is shifted in the positive direction with respect to E ac r value. However, the character of the polarization curves on the lowchromium alloys (2–6% Cr) differs from that of the curves on the alloys containing more than 10% chromium; a current jump is observed both at the active– passive and passive–active transitions (Fig. 2b). Thermodynamic calculation of the passive layer composition on the low-chromium alloys provides a very low Cr2O3 content compared to the main FeO oxide layer directly adjacent to the Fe–Cr alloy surface [14]. For this reason, the E ac r potential is close (≤500 mV) to the Flade potential of individual iron. However, besides the inner FeO layer slightly enriched in Cr2O3, the outer layer composed of the higher iron oxides is probably formed on the alloy surfaces. This layer is also enriched in Cr2O3 oxide to a certain degree. The exist- PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 47 No. 3 2011 Urrent density, mA/cm2 408 SAFONOV et al. (а) 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 2 (b) 6 10 14 Time, h 18 22 100 µm determines the potential delay in curve shown in Fig. 1c, since there is no similar segment in the curves recorded on the alloys with lower chromium contents (Figs. 1a, 1b). Judging from the switch-off curve (Fig. 1c), we can suppose that the passive film still has a two-layer structure on this alloy. The convergence of the experimental curves of the passivation and activation onset potentials of the alloys (Fig. 3a, curves 2, 3), which correspond to the outer layer compositions, and theoretical curve 1 that corresponds to the Flade potential of the inner layer indicates the transition from a two-layer (or possibly three-layer) structure to a one-layer film. This is confirmed by the data of X-ray photoelectron spectroscopy of the passive layer formed on Fe–20 Cr alloy. The analysis revealed only one layer composed of 80% Cr2O3 and 20% FeO [25]. The enrichment of the inner passive film layer with Cr2O3 oxide strongly decelerates the potentiodynamic activation of the alloys with a high chromium content (10–23%). As is shown by Fig. 2c, this is reflected in the smooth shape of the polarization curves, as well as the decrease in the critical passivation and activation currents. The novel method of successive approximations alone enables one to activate the high-chromium alloy (23% Cr). Based on the condition of the steady passive state, Е > EF, we can say that the theoretical dependence on the chromium content in the alloy is the boundary that determines the passivity range, in particular above the curves in Fig. 3. Experimental values obtained using different methods are satisfactorily close to the theoretical dependence. CONCLUSIONS (c) 100 µm Fig. 7. Activation of Fe–15 Cr alloy along grain boundaries: (a) current density–time dependence and (b, c) microphotos of surface upon exposure at potential, mV: (b) –58 (presumable activation) and (c) 670 (passive range). ence of the layer is indicated by the potential delays in a range around Е ≈ 600 mV (Fig. 1a), which is typical of the passivation of individual iron. The absence of a horizontal segment corresponding to the thin inner layer is explained by the low Cr2O3 concentration, which results in its rapid dissolution upon switching off the potential. Thermodynamic analysis of the composition of the inner passive layer on Fe–13 Cr alloy provides an estimate of 70% Cr2O3. It is this enrichment that probably 1. Passivation and activation potentials of Fe–Cr alloys are studied using three methods, namely, switching off the anodic polarization, potentiodynamic passivation–activation, and successive approximations. Critical passivation and activation potentials obtained by the three methods are in satisfactory agreement with each other. 2. An experimental method is proposed for determining the activation potentials (the method of successive approximations to the equilibrium Flade potential) of passive layers formed in metallic materials, which enables one to estimate the activation onset potential of an alloy more accurately and correctly than the conventional methods. 3. 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