RU2662272C9 - Passive oxide film on iron charge threshold detecting method - Google Patents

Abstract

FIELD: physics.SUBSTANCE: invention relates to the solid state physics studying visual aids and its applications to the corrosion process. Electrode is placed into the electrolyte aqueous solution. Linearly changing the electrode potential in the cathode and anode directions. Setting the alternating current through the electrode. At the alternating current frequency, recording the electrode oscillations amplitude. Representing this amplitude as the estans, equal to the derivative of the electrode surface tension over the surface charge density. Recording the estans modulus dependence on the electrode potential. With the potential sweep cathode direction, finding the estans modulus change following stages: linear growth, sudden decline, exceeding linear growth in steepness, minimum and peak. Showing the estans modulus linear growth and the sudden decline reversibility. Reversible charging stage is completed with increase in the electrons consumption in the oxides reduction electrochemical reaction.EFFECT: method allows to observe the processes associated with the change in the electric field in the oxidized metal surface layer.24 cl, 7 dwg

Description

The invention relates to visual aids for studying solid state physics and its applications to the corrosion process.

Passivation of iron and a number of other metals is known due to the formation of a dense oxide film on them that slows down corrosion (A. Groysman "Corrosion for everybody" 2010, Springer). The natural coating that protects metals from destruction has been the subject of research for over 150 years. The level of science and technology achieved in this area is described in E. McCaffery's 2010 Introduction to Corrosion Science, Springer, New York Heidelberg. On page 224 of this book, a lack of information about the state of a passive oxide film covering iron is indicated:

).""The nature of the passive film on iron has been studied in great detail by many investigators who have used a variety of techniques. However, even with intense study and even with the remarkable capabilities of modern surface analytical techniques, the exact nature of the passive film on elemental iron remains elusive. This is due in part to the fact that the passive film on iron is very thin (of the order of 1.5-5.0 nm, i.e. 15-50

). "

).»“The nature of the passive film on iron has been studied in great detail by many researchers using a variety of techniques. However, despite the thoroughness of the research and the remarkable capabilities of modern equipment for surface analysis, the true nature of the passive film on pure iron remains unsolved. The reason for this, in particular, is the fact that the passive film on iron is very thin (of the order of 1.5-5.0 nm, i.e. 15-50

). ”

Similarly, the problem of a passive film on iron is described in a recently published review:

A. Veluchamy, D. Sherwood, B. Emmanuel, I.S. Cole "Critical review on the passive film formation and breakdown on iron electrode and the models for the mechanism underlying passivity" Journal of Electroanalytical Chemistry, Volume 785, January 15, 2017, pages 196-215:

"Though several theories, models and experimental works on passivity have been published in the literature, the mechanisms underlying the stability of the passive oxide over the metals remain still a mystery."

“Although a number of theories, models, and experimental studies have been published in the literature, the mechanisms underlying the stability of passive oxide on metals are still a mystery.”

According to the sources cited above, the passive film on iron is similar in structure to the spinel FeOFe ₂ O ₃ , includes excess iron ions and is an n-type semiconductor acting as an insulator.

Metal-insulator transitions are known, caused by mechanical deformation of a solid body or by a change in its temperature (NF Mott "Metal-insulator transitions", Moscow. Ed. "Science" 1979).

A known method proposed by the author for demonstrating a charge-induced metal-insulator transition (RF Patent 2601921 IPC G09B 23/18, 11/10/2016 Bull. No. 31). This method makes it possible to show that the outer monolayer of metal atoms free of oxide has the properties of an insulator.

There is a known method for recording variable surface tension of solids (estans method), first proposed by the author and described in detail (Auth. Certificate. USSR 178161 IPC G01N 13/00, 01/08/1966 Bull. No. 2; Aut. Certificate. USSR 277399 IPC G01N 13 / 02, 07/22/1970 Bull. No. 24; Electrochemistry 1966, volume 2, p. 1061; Reports of the USSR Academy of Sciences 1969 volume 187, p. 601; Electrochimica Acta 1970, vol. 15, p.219; book “Surface tension of solids and adsorption ", Moscow, Nauka, 1976, 400 s; Electrochemistry 1981, Volume 17, p. 780; Uspekhi Fizicheskikh Nauk 2000, Volume 170, p. 779; J. Solid State Electrochemistry 2013, vol. 17, p. 1743; RF Patent 2552116 IPC G01N 13/00, 06/10/2015 Bull. No. 16).

A method for detecting the charge threshold of a passive oxide film on iron is not known.

The objective of the present invention is to overcome, by the example of iron, the difficulties of standard methods in determining the mechanism of metal passivation, to apply the experimental method proposed by the author based on registration of variable surface tension of solids, to eliminate in this way the influence of electrochemical reactions involving porous corrosion products covering a passive oxide film , and select for observation the physical process of charging a passive oxide film, which changes its electronic state Without changing its chemical composition and prepares the subsequent electrochemical removal of oxides.

The problem is solved due to the fact that, according to the invention, an electrode is made of iron, the electrode is brought into contact with an aqueous electrolyte solution and the electrode potential is linearly changed in the cathode and anode directions, while alternating current is passed through the electrode, the oscillation amplitude is recorded at an alternating current frequency electrode, represent this amplitude as an estans equal to the derivative of the surface tension of the electrode with respect to the surface charge density, register the dependence of the estance modulus on the potential electrode, at the cathodic direction of potential sweep, the stages of change in the estance modulus are found, including linear growth, sudden decline, exceeding linear growth in steepness, minimum and peak, alternating directions of potential sweep show the reversibility of linear growth and sudden decline of the estance module, while the linear growth of the module estansa is characterized by increased electrostriction of the passive oxide film as an insulator with an increase in the negative charge of the oxidized electrode, and a sudden decrease in the modulus of estansa har it is activated by reversible charging of a passive oxide film, inducing a transition of the passive oxide film from the insulator state to the metal state, and the sudden onset of the stage of reversible charging is characterized by a threshold value of the electrode potential at which the bottom of the conduction band of the passive oxide film reaches the Fermi level of the electrode and the process of reversible charging is characterized by electrostatic accumulation of excess electrons that tunnel into the passive oxide film from the electrode and contribute to irradiation of valence electrons, after which the completion of the reversible charge stage is determined by the increase in electron consumption in the electrochemical reaction of oxide reduction due to the displacement of the bulk of the interphase potential jump from the volume of the passive oxide film to its boundary with the electrolyte solution.

The threshold value of the electrode potential is measured as the potential of the maximum modulus of estance between the reversible stages of linear growth and the sudden decline of the modulus of estance, this maximum is described as the threshold for charging a passive oxide film, expressing the suddenness of its transition from the state of the insulator to the state of the metal.

The combination of the Fermi level with the conduction band of the passive oxide film is defined as the condition for the ohmic contact to be established at the interface between the electrode and the passive oxide film, and the insulator-metal transition is characterized by a decrease in the ohmic resistance in the volume of the passive oxide film, which ensures a decrease in electrostriction, with a sudden drop in the module estans are used as a sign of an insulator-metal transition in the volume of a passive oxide film.

The accuracy of preserving linearity at the stage of linear growth of the estans modulus is estimated, a relative deviation from linearity is found that does not exceed 1% of the maximum modulus of estans for any potential value from an interval of length 0.8 V with a cathode boundary located at a distance of 0.05 V from the charging threshold, consider this stage as a characteristic of the non-trivial phenomenon of linear nano-electrostriction.

The suddenness of the transition of the passive oxide film from the state of the insulator to the state of the metal is characterized by the shortness of the stage of sudden decline of the estance modulus as compared to the linear growth stage and the asymmetry of the steepness of the dependence “estance modulus - potential” on both sides of the charge threshold, in the vicinity of which the maximum slope of this dependence is a sudden decline is five times the slope at the linear growth stage.

The reversibility of the indicated growth and decline of the estance modulus is considered a characteristic of preserving the chemical composition of a passive oxide film during its charging by electrons, while the stage of charging a passive oxide film precedes the stage of electrochemical reduction of oxides.

A linear change in the electrode potential is limited to the region of linear growth and sudden decline of the estance module, note the coincidence of the decay form of the estance module in the cathode direction with the form of the increase of the estance module in the anode direction, the length of the reversible charging stage of the passive oxide oxide is estimated from the potential interval occupied by the sudden decline of the estance module films larger than 0.2 V.

The dependence of the current through the electrode on the electrode potential is recorded, at the cathodic direction of the potential scan, the total oxide recovery current wave is found, characterized by its redistribution of the interphase potential jump as the passive oxide film is charged, as a result of which the decrease in the potential drop in the volume of the passive oxide film is compensated by an increase in the potential drop by the boundary of the passive oxide film with the electrolyte solution to values sufficient for the beginning and completion of electrochemical reactions cts of reduction of oxides.

The lag of the total wave of the oxide reduction current from the charge threshold of the passive oxide film is noted, while the lag is equal to the length of the stage of reversible charging of the passive oxide film along the potential axis.

The redistribution of the interphase potential jump in a passive oxide film is characterized by the simultaneous action of two factors, including an increase in the electrical conductivity of the oxide film and an increase in its dielectric constant when excess electrons flow into the oxide film, which enhance the screening of positive metal ions and promote the displacement of valence electrons in the electric field at the boundary of the oxidized electrode with an electrolyte solution.

The lag of the recovery current wave from the charge threshold of the passive oxide film is characterized as a sign of the leading role of the reversible charge of the passive oxide film in the process of iron activation by electrochemical removal of oxide.

A linear change in the electrode potential is used in the form of a zigzag scan of the potential, consisting of more than two alternating sections of the cathode and anode directions with a predominance of the duration of one of these directions.

A zigzag scan of the potential is used to identify the degree of local reversibility of the change in the state of the electrode surface in the vicinity of a certain potential value, the sign of local reversibility is considered to be the same direction or merging of the sections of the recorded dependence of the estance modulus on the potential obtained by changing the potential in opposite directions.

The local reversibility of the sudden decline in the estance modulus is found over a potential interval longer than 0.2 V in accordance with the length of the reversible charge stage, found by continuous scanning of the potential.

As an aqueous electrolyte solution, a solution of potassium hydroxide in a concentration of 0.1 to 0.5 mol / l is used, the electrode is made of sheet metal with a thickness of 0.4 to 1 mm with an iron content of 99.95% and higher, while the electrode is polished chemically or electrolytically, the range of potential changes the electrode is selected in the range from +1.3 V to -0.3 V relative to the equilibrium hydrogen electrode in the solution used at a temperature of 20 ° C ± 2 ° C.

The dependence of the estans module on the electrode potential is recorded at two alternating current frequencies.

The dependence of the estans modulus on the potential at the stage of linear growth is extrapolated to the zero value of the estans modulus; in this way, the value of the zero charge potential of the oxidized iron electrode is obtained.

For the zero-charge potential of the oxidized iron electrode in a 0.5 M KOH aqueous solution, a value of +2.0 V is found relative to the equilibrium hydrogen electrode in the same solution.

The dimensionless slope of the dependence of the estans modulus on the potential at the stage of linear growth is calculated and the scale of the estans modulus is determined on the oscillogram in the coordinates "estance modulus-potential", using the formula

∂ | ∂γ / ∂q | / ∂ϕ = - [1+ (∂С / ∂ϑ) С],

where ∂ | ∂γ / ∂q | / ∂ϕ is the dimensionless slope,

γ is the surface tension of the iron electrode,

q is the surface charge density,

ϕ is the electrode potential,

C = ∂q / ∂ϕ is the specific capacity of the oxidized electrode,

ϑ is the relative elastic deformation of the electrode area (the ratio of the increment of the electrode area with its elastic elongation to the original electrode area),

(∂С / ∂ϑ) / С is a constant coefficient characterizing the linear nano-electrostriction of a passive oxide film,

(∂С / ∂ϑ) / С = ν / (1-ν),

ν = 0.29 - Poisson's ratio for spinel as a substance of a passive oxide film.

The peak of the estance modulus is characterized by the desorption of a monomolecular layer of hydroxyl ions adsorbed on iron by the reaction

(FeOH) _ad + e ^- → Fe + OH ^-

when the electrode potential changes in the cathode direction.

The current peak corresponding to the peak of the estance modulus is found, and the true surface area of the iron as a dispersed system of crystallites is estimated from the amount of electricity consumed by the current peak.

An aqueous solution of 0.2 M K ₃ Fe (CN) ₆ + 0.2 M K ₄ Fe (CN) _{6 is} used as the aqueous electrolyte solution, the initial alternating current is passed through the boundary of this solution with the iron electrode, the signal caused by the alternating thermal tension of the iron electrode is recorded, use this signal as a reference for measuring the modulus of estance.

To account for roughness, multiply the measurement result with a standard by a factor

Λ = α ^{1 + ν (m)} ,

where α is the roughness factor of the electrode, equal to the ratio of the area of the rough surface to the projection area of this surface on the plane,

ν (m) is the Poisson's ratio for the electrode,

Λ - coefficient taking into account the dispersion of the orientation of the crystallite faces.

The potential and estans dependences are interpreted in accordance with the following successive changes in the composition of the oxide film on iron in the process of irreversible electrochemical reactions involving oxygen:

Fe → (FeOH) _ad → (FeO) _ad → (FeFe ₂ O ₄ ) _ad , γ-Fe ₂ O ₃ , γ-FeOOH when the electrode potential changes in the anode direction and

γ-FeOOH, γ-Fe ₂ O ₃ , (FeFe ₂ O ₄ ) _ad → (FeO) _ad → (FeOH) _ad → Fe

when the electrode potential changes in the cathode direction, where the adsorption bond of the oxide film with the iron electrode is indicated by _ad , the existing compounds are separated by commas, and the compound (FeFe ₂ O ₄ ) _ad stands for mixed oxide Fe ₃ O ₄ of the spinel type, which ensures passivity of the oxide film and containing Fe ²⁺ and Fe ³⁺ ions with the possibility of releasing a valence electron when exchanging it while maintaining equilibrium Fe ²⁺ ↔ Fe ³⁺ + е ^- .

The advantage of the proposed approach to the observation of processes in a passive oxide film on iron is due to the selective sensitivity of the surface tension of a solid to the state of the surface of a solid, regardless of the current density of redox reactions. The study of a passive oxide film is hindered by the external porous layer of corrosion products, which is much larger than the passive film by mass, overshadows it mechanically and, in addition, makes the main contribution to the overall cathode current wave of the reduction of iron oxides, in which the beginning of the reduction of the passive film is lost.

In contrast to its predominant influence on current, the external porous layer of corrosion products is weakly bonded to the iron electrode and therefore does not make a significant contribution to the recorded surface tension. In contrast to the external porous layer, the passive oxide film adheres tightly to the iron electrode and is the main factor determining the change in the surface tension of oxidized iron with a change in the potential of the iron electrode.

Under these conditions, the registration of an alternating surface tension of an iron electrode makes it possible to observe a previously unknown physical process of charging a passive oxide film, leading to a reversible change in its electronic state without the participation of electrochemical reactions. An important feature of the observed charging process is its sudden onset at a certain threshold value of the potential of the iron electrode.

The invention is illustrated by the following illustrations.

FIG. 1 - The waveform “estance modulus - potential”, taken in the cathode direction, | ∂γ / ∂q | -ϕ, obtained on a chemically polished iron electrode in an aqueous solution of 0.5 M KOH at an alternating current frequency of 4.60 kHz, potential variation rate of 0.1 V / s from +1.56 V to -0.31 V. The waveform relief outlines three stages of a change in the state of a passive oxide film: electrostriction (E), charging (C), reaction (R). The first two stages correspond to physical processes in a passive oxide film without a significant change in its chemical composition. At the third stage, a sequential electrochemical reduction of corrosion products occurs, including a porous layer of oxides and a passive oxide film. The potential is given on the scale of the equilibrium hydrogen electrode in the same solution (REE). The signs + and - restore the polarity of ∂γ / ∂q in the sections of the waveform closest to them. The arrow indicates the direction of potential sweep.

FIG. 2 - Oscillograms of "estans modulus - potential" obtained in the cathodic (from left to right) and anode (from right to left) directions in the conditions of FIG. one.

FIG. 3 - Two adjacent reversibility regions obtained by shortened potential sweep under the conditions of FIG. 1: linear growth of the passive film electrostriction and the insulator-metal transition in it.

FIG. 4 - Oscillograms of "estance module - potential" obtained at an alternating current frequency of 1.27 kHz with the remaining basic conditions of FIG. one.

FIG. 5 - Oscillograms "estans module - potential" obtained in the conditions of FIG. 4 using a zigzag potential sweep to detect the reversibility of recorded processes. A sign of reversibility is the fusion of the forward and reverse beam. The merger is expressed by the thickening of individual segments of the waveform. It is present in the regions of rise and fall of the estance module, where the electrode is covered with an oxide film, and also in the extreme cathode region of the waveform (right), where the electrode is free of oxide. Reversibility is characteristic of physical processes that proceed without changing the chemical composition of the oxide film — the increase in the electrostriction of the oxide film in the region of rise of the estance module, the charge-induced insulator-metal transition in the oxide film in the region of decay of the estance module, where the weakening of electrostriction is caused by an increase in the conductivity of the oxide film. Kinks mark those sections of the waveform where irreversible redox reactions occur with a change in the mass and composition of the oxide film.

FIG. 6 - Cathode and anode waveforms “current density potential” j - ϕ in the conditions of FIG. one;

FIG. 7 is a diagram of a device for detecting a charge threshold of a passive oxide film on iron.

In the device for detecting the charge threshold of a passive oxide film on iron, the iron electrode 1 is made curved, includes a vertical part 2, a horizontal part 3 and an inclined board 4. The lower surface 5 of the horizontal part of the electrode is brought into contact with the meniscus 6 of the electrolyte liquid solution 7. The solution is in the reservoir 8. An additional electrode 9 and a reference electrode 10 are introduced into the same solution. The electrode 1 is grounded.

An inductance 12 and a capacitance 13 are connected to the additional electrode via a load resistance 11. The inductance 12 is connected to the output of the differential amplifier 14, the inverse input of which is connected to the reference electrode 10, and the non-inverse input is connected to the potential shaper 15. The capacity 13 is connected to the output of the alternator 16 through a ballast 17. The key 18 is inserted into the circuit of the generator 16. The ends of the load resistance 11 are connected to the inputs of the differential amplifier 19.

The electrode 1 using the holder 20 is fastened to a piezoelectric element 21 which is fixed in the housing 22 and connected to the input of the selective amplifier 23. The alternator 16 and the selective amplifier 23 are configured to the same frequency that matches the resonant frequency of the mechanical system of the electrode - piezoelectric element. The spectrum of this mechanical system includes a number of resonant frequencies in the sound and ultrasonic ranges.

A rectifier 24 is installed at the output of the selective amplifier 23. The switch 25 is made on-off with the possibility of alternately connecting the outputs of the rectifier 24 (position E) and the differential amplifier 19 (position C) to the vertically deflecting system of the oscilloscope recorder 26. The horizontally deflecting system of the recorder 26 is connected to the comparison electrode 10 .

The differential amplifier 14, the driver 15, the inductance 12 and the additional electrode 9 provide the setting of the potential of the electrode 1 relative to the reference electrode 10 (that is, setting the potential difference between the electrode 1 and the reference electrode 10). The potential shaper 15 is configured to vary the law of variation of the potential of the electrode 1 with time. Inductance 12 plays the role of a low-pass filter and suppresses the potential component at the frequency of a given alternating current passing through the capacitance 13. The use of inductance broadens the range of potential setting by reducing the ohmic drop in the circuit of the additional electrode. The change in electrode potential over time corresponds to a horizontal scan on the screen of the oscilloscope recorder 26.

Ballast resistance 17 in combination with inductance 12 and capacitance 13 provides a mode of independence of the amplitude of the alternating current through the electrode 1 from the electrode potential.

Work on the described device is as follows. Set the switch 25 to position E, turn on the alternator 16 and set the frequency from the desired range, for example, 5 kHz. When this alternating current of a fixed amplitude passes through the capacitance 13, the load resistance 11, the additional electrode 9, the solution 7 and the surface 5 of the electrode 1.

The fluctuations in the charge of the surface of the sample caused by alternating current are accompanied by fluctuations in surface tension. The vibration of the electrode 1 is perceived by the piezoelectric element 21 and is fed to the input of the selective amplifier 23 as an electric signal. The amplified alternating signal is processed by the rectifier 24 and then fed to the vertical deviation input of the oscilloscope recorder 26 as a constant voltage. The signal received from the piezoelectric element is displayed on the screen of the recorder by the deviation electron beam from the initial zero level. By smoothly varying the frequency of the alternating current, the signal on the screen is increased to a maximum and in this way the nearest resonant frequency, for example, 4.60 kHz, is found, as in the conditions of FIG. 1. The selective amplifier 23 is tuned to the same frequency.

The recorded signal is proportional to the amplitude Δγ of an alternating surface tension γ, swinging the electrode-piezoelectric system. This amplitude depends on the electrode potential ϕ, the change of which corresponds to a horizontal scan on the screen of the recorder 26. Therefore, the dependence recorded on the screen is proportional to the dependence | Δγ | from ϕ.

The constancy of the amplitude of the alternating current from the potential of the electrode, provided by the inductance 12 and the ballast resistance 17, is equivalent to the constancy of the amplitude Δq of the surface density q of the charge of the polarizable electrode when the potential changes, Δq = const. Under these conditions, which are practically fulfilled in the demonstrated experiments, the dependence recorded on the screen is proportional to the dependence of | Δγ / Δq | on ϕ.

Preservation of the waveform relief upon a twofold change in the amplitude of the alternating current is an easily controlled criterion for the smallness of the amplitudes Δγ and Δq. It makes it possible to consider the equality | Δγ / Δq | ≈ | ∂γ / ∂q | and consider the recorded dependences as oscillograms in the coordinates | ∂γ / ∂q | -ϕ.

In contrast to capacitance recording methods, in the method of recording the alternating surface tension of solids (in the estans method) there are no restrictions on the frequency of the alternating current associated with ohmic losses. Practically available frequencies within 1 MHz. At sufficiently high frequencies, the estans ∂γ / ∂q is a complex number, which can be represented by the module | ∂γ / ∂q | and phase arg ∂γ / ∂q. At frequencies less than 10 kHz used in the demonstrated experiments, the estance is real, the module | ∂γ / ∂q | is equivalent to the absolute value of the number, and the phase takes two values 0 and π, equivalent to the plus and minus signs. These signs are affixed to the given oscillograms of the modulus (absolute value) of the estance and, if necessary, make it possible to distinguish the positive portions of the ∂γ / ∂q-ϕ dependence from the negative ones. The passage of estans through zero, transmitted by V-shaped fragments of the waveforms, is accompanied by a change of signs.

Shaper 15 potential set a linear scan of the potential of the electrode 1 in the cathode or anode direction. In this case, a unidirectional oscillogram of the estance in the coordinates "module of the estance-potential" is obtained on the screen of the recorder 24.

The potential shaper 15 is also used to specify zigzag scans of the potential of the electrode 1, in which the cathodic and anodic directions of the potential change alternate over the course of one sweep with the predominance of the duration of one of the directions. Obtained in this way zigzag oscillograms of estans in the coordinates "module estansa potential" is used to identify the degree of reversibility of the recorded processes.

Key 18 open the circuit of the alternator 16 and put the switch 25 in position C. Set the linear scan of the potential and get the current waveform in the coordinates of the "current-potential".

The electrode 1 is made of Armco grade iron sheet (99.95% Fe) 0.4 mm thick. A strip of 30 mm × 6 mm is cut from the sheet, bent to form a vertical part 2, a horizontal part 3 and an inclined bead 4. The active area of the electrode is determined by the length of the horizontal part 9 mm 0.6 × 0.9 = 0.54 cm ² to take into account the roughness. Demagnetize the electrode in an alternating magnetic field of the solenoid with a frequency of 50 Hz.

Subject the electrode to chemical polishing. An aqueous solution of oxalic acid with the addition of hydrogen peroxide is used for this (Tegart B. Electrochemical and chemical polishing of metals, Moscow, IL, 1957), then washed with water.

Solution 7 is prepared from potassium hydroxide qualification of analytical grade. (pure for analysis) and twice distilled water with a concentration of 0.5 mol per liter. Pour 150 ml of the solution into the reservoir 8. As a reference electrode 9, a nickel wire is used, the potential of which in the specified solution is measured relative to the equilibrium hydrogen electrode. As an auxiliary electrode 10, a glassy carbon plate is used. Traces of oxygen are removed from the solution by passing nitrogen.

Oxygen removal is not necessary when registering the estans module, since the surface tension of the solid electrode is not sensitive to side electrochemical reactions, the products of which remain in solution and do not change the state of the surface. In the absence of a glassy carbon plate, it can be replaced by a nickel plate.

Lower the electrode 1 until its horizontal part 3 comes in contact with solution 7, then raise it by 1-2 mm to form a meniscus 6 under the electrode. Set the switch 25 to position E, set the linear sweep of the potential of the electrode 1 in the range from +1.56 V to -0.31 V.

Close the key 18 and using the generator 16 pass through the boundary of the electrode 1 with the solution an alternating current with a frequency of 5 kHz and an amplitude of 5 mA. The potential is fixed at the sweep boundaries and the beam deviation is changed continuously by changing the frequency of the beam on the screen of the recorder 26. The closest resonance of the electrode-piezoelectric system at a frequency of 4.60 kHz is found from the maximum deviation.

At the resonance frequency, oscillograms are recorded in the coordinates "estance modulus - potential" with a linear change in potential in the cathode and anode directions at a speed of 0.1 V / s (Fig. 1, 2). The duration of the potential sweep does not exceed 30 s. At the same time, the duration of the anodic oxidation of the iron electrode is limited. Due to this, the thickness of the passive oxide film in the experiments presented is close to the minimum value of 2 nm.

On the oscillogram of the cathodic direction, the following stages of changing the modulus of estance are found: 1) linear growth, 2) sudden decline, 3) flattened minimum, 4) peak, 5) passage through zero with a change in sign. At first, the estance retains a positive value, ∂γ / ∂q> 0, and after the peak it changes sign sharply and becomes negative, ∂γ / ∂q <0 (Fig. 1). The stages of linear growth and sudden decline of the estance modulus are separated by the maximum of the estance modulus at an electrode potential value of +0.58 V relative to the equilibrium hydrogen electrode.

A linear change in the electrode potential is limited by the region of the linear growth stages and the sudden decline of the estans modulus in the potential range from +0.33 V to +1.46 V (Fig. 3). The shape of the oscillograms obtained in the cathodic and anodic directions along the entire length of this region is noted to coincide, which indicates the reversibility of both stages. From the potential interval occupied by the decrease in the estance modulus from +0.58 V to +0.33 V, the extent of the stage of the reversible charging of the passive oxide film is estimated to be 0.25 V.

At the stage of linear growth of the estans modulus, the passive oxide film remains a semiconductor with a significant band gap between the bottom of the partially filled conduction band and the ceiling of the completely filled valence band. In this case, the passive oxide film has the properties of an insulator, and the Fermi level of the iron electrode is located inside its forbidden zone. In the considered process of evolution of a passive oxide film, the initial potential is the zero charge potential of the oxidized electrode. Due to surface dipoles of adsorbed oxygen, it is strongly shifted in the anode direction. At this electrode potential, there is no potential difference in the passive oxide film, there are no electrostrictive stresses, and the estance modulus is zero. A shift of the electrode potential in the cathode direction leads to a linear increase in electrostriction and estance modulus.

At the same time, the cathodic shift of the potential lowers the electronic zones of the passive oxide film relative to the Fermi level of the iron electrode. The threshold value becomes the potential at which the bottom of the conduction band of the passive oxide film reaches the Fermi level of the iron electrode. This creates an ohmic contact at the boundary of the electrode with the passive film. Isoenergy tunneling transitions of electrons from the iron electrode to the free states of the conduction band of the passive oxide film become possible.

Excess electrons accumulate in a passive oxide film in the form of a space charge, which is embedded in a double electric layer at the interface of the electrode with the solution. The accumulation of charge is facilitated by the inhibition of its leakage through this boundary, which remains closed until intense electrochemical reactions begin.

The average charge density accumulated in a passive oxide film electrostatically can be estimated by the expression

ρ = CΔϕ _o / δe,

where C = ε _o ε / δ is the specific capacity of the passive oxide film,

ε is its dielectric constant,

δ is its thickness,

ε _o is the electric constant,

e is the electron charge,

Δϕ _o - the interval of the potential, measured from the potential of the zero charge of the oxidized electrode.

At δ = 2 nm, ε = 10, Δϕ _o = 1, this implies ρ = 1.38⋅10 ²⁰ cm ^-3 , which is three orders of magnitude less than in a normal metal.

The stage of linear growth of the estans modulus is explained by the increased electrostriction of the passive oxide film in the electric field of the interfacial layer with an increase in the negative charge of the oxidized electrode. The sudden decline in the estans modulus is characterized as the stage of reversible charging of a passive oxide film by injection of electrons from the electrode into it. The beginning of injection is associated with reaching a threshold value of the potential at which the Fermi level of the electrode enters the conduction band of the passive oxide film.

The threshold potential is determined on the basis of its coincidence with the potential of the maximum modulus of estance equal to +0.58 V in the experimental conditions of FIG. 1. Injection of electrons depends on the state of the boundary of the electrode with the passive oxide film and is not directly related to the resistance in the volume of the passive oxide film. As shown above, the electron-induced accumulation of electrons is commensurate with the concentration of excess current carriers in the space charge region of the double electric layer. This concentration is not enough for metallic conductivity. Therefore, electron injection alone cannot explain the observed almost complete decrease in electrostriction.

An additional factor is the interaction of injected excess electrons with valence electrons bound in the material of a passive oxide film. Excess electrons enhance the screening of positive ions, weaken their electric field and in this way contribute to the release of valence electrons. The increase in the number of current carriers induced by the mutual repulsion of electrons sharply increases the electrical conductivity in the volume of the passive oxide film, which is reflected in its transition from the state of the insulator to the state of the metal.

Experimental evidence of the insulator-metal transition in a passive oxide film is the almost complete decrease in electrostriction after reaching the charge threshold. The recession depth is characterized by the ratio of the extreme values of the estans modulus before and after the recession. Under the conditions of FIG. 1, this ratio exceeds 10. The corresponding decrease in the resistance of a passive oxide film occurs while maintaining the chemical composition of this film, without the accompanying electrochemical reactions of oxide reduction. Thus, the observed decrease in the resistance of the passive oxide film is a purely physical phenomenon.

Before the insulator-metal transition, the interphase potential drop is concentrated in the volume of the passive oxide film. During the growth of its electrical conductivity, redistribution of the interphase potential drop occurs. Most of it is displaced from the volume of the passive oxide film to its boundary with the electrolyte solution. This initiates the electrochemical reduction reactions of the oxides of the passive oxide film and its coating, consisting of porous corrosion products.

According to the described mechanism, the stage of charging a passive oxide film without changing its chemical composition precedes the stage of electrochemical reactions at its interface with the electrolyte solution.

The electrochemical reduction of a passive oxide film begins as a result of a redistribution of potential occurring during a sudden decline in the estance modulus. A reversible change in the electronic structure of the oxide film, which does not affect its chemical composition, prepares the beginning of irreversible electrochemical reactions that affect the chemical composition of the oxide film.

The accuracy of preserving linearity at the indicated stage of growth of the estans modulus is estimated, the relative deviation from linearity does not exceed 1% of the maximum of the estans modulus for any potential value from an interval of length 0.8 V with a cathode boundary located at a distance of 0.05 V from the charging threshold.

The suddenness of the insulator-metal transition in a passive oxide film is characterized by the shortness of the stage of sudden decline in the estance modulus as compared to the stage of linear growth and the asymmetry of the steepness of the dependence “estance modulus – potential” on both sides of the charge threshold, where the maximum slope of this dependence at the stage of sudden decline is five times more slope at the linear growth stage.

The reversibility of the linear rise and sudden decline of the estans modulus along their entire length is illustrated by the oscillograms of FIG. 3, where the dependences "estans modulus - potential", obtained in opposite directions, repeat each other with a slight bias.

Oscillograms are taken at an alternating current frequency of 1.27 kHz (Fig. 4) and compared with the result obtained at a frequency of 4.60 kHz (Fig. 1, 2). They find that the form of the dependence of the estance potential is preserved when the frequency changes in the sound range. The reversibility of a sudden decline is used as evidence of the conservation of the chemical composition of a passive oxide film in the process of changing its electronic structure.

As an illustration of the difference between the physical and chemical transformations of an oxide film on a metal, the oscillograms of FIG. 5 obtained by zigzag scans of potential. Relatively smooth sections of reversible linear rise and sudden decline of the estance modulus stand out against broken lines associated with irreversible electrochemical oxidation and reduction reactions. The irreversibility of these reactions is associated with the participation of oxygen in them.

For comparison, the oscillograms obtained at the same iron electrode are shown in the coordinates “current density - potential”, FIG. 6. The current is spent mainly on electrochemical reactions with the participation of corrosion products covering the passive oxide film.

The participation of the passive film under study in the current is obscured by the prevailing background, whereas on the oscillograms of the estance module, the studied passive film is presented in isolated form.

The stage of linear growth of the estans modulus makes it possible to reveal the electrostriction of a passive oxide film and to find the zero charge potential of the oxidized iron electrode, which is an essential parameter of the phase boundary. Under the conditions of FIG. 1 extrapolation of the linear portion of the waveform to the zero line gives a value of +2.0 V for the potential of the zero charge of the oxidized iron electrode.

For the first time, a change in the sign of the charge of a metal during oxidation of its surface was detected by the deviation of a thin platinum electrode in an external electric field and explained by the formation of a dipole layer with the participation of adsorbed oxygen (Balashova N.A., Frumkin A.N. Electrokinetic behavior of platinum at various potentials, Doklady AN USSR, 1938, volume 20, p. 293; Frumkin A.N. Zero-charge potentials, Moscow, Nauka, 1979, p. 88).

The linear dependence of the estans modulus on the potential of the electrode coated with an oxide film allows theoretical justification based on the equations of electrocapillarity of solids. These equations obtained by the author earlier have the form:

∂γ / ∂ϕ = -q-∂q / ∂ϑ,

∂γ / ∂q = ∂ϕ / ∂ϑ,

where γ is the surface tension of a solid serving as an electrode,

ϕ is the electrode potential,

q is the surface charge density of the electrode,

ϑ is the relative elastic deformation of the electrode area (the ratio of the increment of the electrode area with its elastic elongation to the original electrode area),

The derivative ∂ϕ / ∂ϑ is measured at a constant full charge of the electrode, which corresponds to the open circuit mode and can be implemented in a circuit with a sufficiently high resistance. The value ∂γ / ∂q = ∂ϕ / ∂ϑ is called “estance” and is calculated in units of potential (in volts).

Since both coordinates of the “modulus estans - potential” waveform have the same dimension, the slope ∂ (∂γ / ∂q) / ∂ϕ of the waveform is a dimensionless criterion.

At the stage of linear growth of the estans modulus, a passive oxide film acts as an insulating strip between a negatively charged metal electrode and a positively charged electrolyte solution, which are plates of an interfacial capacitor. The shift of the electrode potential in the cathode direction increases the charges of the plates and the electric field acting on the passive oxide film. The electrostriction caused by the electric field of the passive oxide film is expressed in an additional tangential stress, which is part of the surface tension of the solid electrode.

The contribution of electrostriction to the surface tension of a solid electrode coated with a dielectric layer was calculated in the author’s book cited above using the equations of electrocapillarity of solids (Surface tension of solids and adsorption, chapter 6, p. 131, equation 6.5.3). In relation to the estans of an electrode coated with a passive oxide film, the equation derived in the book is the equation of nano-electrostriction:

∂γ / ∂q = - [1+ (∂С / ∂ϑ) / С] (ϕ-ϕ _o ) + ∂ϕ _o / ∂ϑ,

where C = ∂q / ∂ϕ is the specific capacity of the oxidized electrode,

(∂C / ∂ϑ) / C = (∂ε / ∂ϑ) / ε- (∂δ / ∂ϑ) / δ,

δ is the thickness of the passive oxide film (δ ≈ 2 nm),

ε is the dielectric constant of the passive oxide film (ε ≈ 10).

ϕ _o - the potential of the zero charge of the oxidized electrode,

The experiments of FIG. 1, 2 demonstrate a linear change in ∂γ / ∂q with a potential ϕ in the range of the order of 1 V. This implies the linearity of nano-electrostriction, which is not trivial from the physical point of view due to the relative small thickness of the passive oxide film.

Under the assumption that the oxide film is isotropic, the term (∂δ / ∂ϑ) / δ can be estimated by the equality

(∂δ / ∂ϑ) / δ = -ν / (1-ν),

where ν is the Poisson's ratio. From the above equation, a linear dependence of the estance ∂γ / ∂q on the potential ϕ follows, provided that the values (∂δ / ∂ϑ) / δ and (∂ε / ∂ϑ) / ε are constant.

The reversibility of the stages of linear growth and a sudden decline in the modulus of estans shows that the thickness δ and permittivity ε remain constant along these stages.

When the thickness of the passive oxide film is 2 nm, a change in the electrode potential by 1 V corresponds to a change in the electric field strength between the electrode and the electrolyte solution by 5 × 10 ⁸ V / m. Thus, from the experiments it follows that the values of (∂δ / ∂ϑ) / δ and (∂ε / ∂ϑ) / ε are independent of the electric field strength up to 5⋅10 ⁸ V / m, which is essential information on the properties of the passive oxide film at her electrostriction.

Under conditions of a strong electric field, the constancy of the quantity (∂ε / ∂ϑ) / ε serves as an indication of the smallness of this quantity, (∂ε / ∂ϑ) / ε << 1, which simplifies the equation for estance at the stage of linear growth:

∂γ / ∂q = [- 1+ (∂δ / ∂ϑ) / δ] (ϕ-ϕ _o ) + ∂ϕ _o / ∂ϑ

and gives an expression for dimensionless tilt:

∂ (∂γ / ∂q) / ∂ϕ = -1 + (∂δ / ∂ϑ) / δ = - [1 + ν / (1-ν)].

Since the passive oxide film is close in structure to spinel, the value ν = 0.29, known for magnesia spinel, can be used as the Poisson coefficient (Balkevich V.L. Technical Ceramics, Moscow, Stroyizdat 1984, p. 209). Then

∂ (∂γ / ∂q) /∂ϕ=-1.41.

At the stage of linear growth, estans is positive and equal to the modulus,

∂γ / ∂q> 0, | ∂γ / ∂q | = ∂γ / ∂q.

Therefore, a value of -1.41 is also characterized by the slope of the oscillogram of the estans module

∂ | ∂γ / ∂q | / dϕ = ∂ (∂γ / ∂q) /∂ϕ=-1.41.

The minus sign corresponds to an increase in the modulus of estance when the potential ϕ shifts in the negative direction.

The well-known slope allows you to find the scale of the estance modulus on the waveform "estance modulus - potential". If the slope is calculated, the interval 1 In the horizontal axis of the potential corresponds to the interval 1.41 In the vertical axis of the estance module. In this way, the scale of the estance modulus on the oscillograms of FIG. 1-5.

To independently determine the scale of the estans modulus, a reference signal is used, created by the known variable thermal tension on the same working surface of the electrode, where in the main experiment the variable surface tension to be measured was applied.

An aqueous solution of a redox system of 0.2 M K ₃ Fe (CN) ₆ + 0.2 M K ₄ Fe (CN) _{6 is used} , which corresponds to the thermoelectric effect with a Peltier coefficient close to 0.5 V at 20 ° C. The initial alternating current is passed through the boundary of this solution with the iron electrode, leading to alternating heat generation on the electrode surface. An alternating thermal voltage in the surface layer of the electrode creates a reference signal, which is calculated taking into account the penetration of heat into the interior of the electrode (formulas for calculation are derived in the cited book of the author, p. 247).

In particular, when the thickness of the iron electrode is 0.4 mm and the frequency of the alternating current is 4.60 kHz (as in the conditions of Fig. 1), the reference estance signal is 0.33 V. The depth of penetration of temperature fluctuations into the iron electrode at a frequency of 4.60 kHz is 38 μm, which significantly exceeds the maximum height protrusions of the rough surface of the metal (1 μm). Therefore, the reference thermal signal does not feel protrusions and reproduces estans on a smooth surface.

When repeating potential scans that occur in the region of anodic metal oxidation and cathodic reduction of oxides, recrystallization occurs on the electrode surface with the formation of the preferred orientation of the metal crystallite faces (100 in the case of a cubic lattice). In this case, over several scans (of the order of 10), the roughness factor of the iron electrode may exceed 4.

After the first two scans, an increase in the roughness factor does not change the shape of the waveforms, but should be taken into account when determining the scale. The estance signal, measured on a rough surface by comparison with the thermal standard, is part 1 / Λ of the true value of the estance. For the roughness formed by multiple superposition of protrusions,

Λ = α ^{1 + ν (m)} ,

where α is the roughness factor (the ratio of the areas of the rough surface and its projection onto the plane),

ν (m) is the Poisson's ratio for the metal of the electrode (in the case of iron, ν (m) = 0.28),

Λ - coefficient taking into account the dispersion of the orientation of the crystallite faces.

At α = 4, the signal measured on iron by the thermal standard should be multiplied by the coefficient

Λ = 4 ^{1 + 0-28} = 5.90.

The roughness factor of the iron electrode can be estimated by the amount of electricity consumed by the current of the desorption peak (Fig. 6), which corresponds to the restoration of a monolayer coating of external iron atoms by adsorbed OH groups. For iron crystallites with a predominant orientation of the (100) faces, the surface density of iron atoms is 1.214 × 10 ¹⁵ 1 / cm ² .

In the single-electron OH ion desorption reaction ^, this corresponds to an electricity consumption of 1.94 × 10 ^-4 C / cm ² . In the experience of FIG. 6, the electricity consumption per desorption peak current is estimated at 7.37 × 10 ⁻⁴ C / cm ² , whence the value for the roughness factor is obtained

α = 7.37⋅10 ^-4 C / cm ² /1.94⋅10 ^-4 C / cm ² = 3.80,

which corresponds to Λ = 5.52.

An increase in the roughness factor can be observed by an increase in the height of the total current wave upon repeated potential sweeps with oxidation and reduction cycles immediately after chemical polishing of the iron electrode.

The conclusions of the described experiments are based on the form of the recorded dependence of the estans on the potential, which serves as the horizontal coordinate axis on the oscillograms. If it is necessary to simplify the experiments, these conclusions can be made without determining the vertical scale of the estance. In this case, the linearity of the growth of the estance modulus and the suddenness of the decline of the estance modulus after reaching the charge threshold remain obvious.

Claims (46)

1. A method for detecting the charge threshold of a passive oxide film on iron, characterized in that an electrode is made of iron, the electrode is brought into contact with an aqueous electrolyte solution and the electrode potential is linearly changed in the cathode and anode directions, while an alternating current is passed through the electrode at a frequency alternating current register the amplitude of the oscillations of the electrode, represent this amplitude as an estans equal to the derivative of the surface tension of the electrode with respect to the surface charge density, record the dependence the estance modulus of the electrode potential, at the cathodic direction of the potential sweep, find the stages of change in the estance modulus, including linear growth, sudden decline, exceeding linear growth in steepness, minimum and peak, alternating directions of potential development show the reversibility of linear growth and sudden decline of the estance module, while the linear growth of the estans modulus is characterized by increased electrostriction of the passive oxide film as an insulator with an increase in the negative charge of the oxidized electrode, and a sudden the decrease in the estance modulus is characterized by the reversible charging of the passive oxide film, inducing the transition of the passive oxide film from the insulator state to the metal state, and the sudden onset of the reversible charging stage is characterized by a threshold value of the electrode potential at which the bottom of the conduction band of the passive oxide film reaches the Fermi level of the electrode and the process of reversible charging is characterized by electrostatic accumulation of excess electrons that tunnel into a passive oxide film of elec genus and facilitate release valence electrons, whereupon the completion stage reversible charging is determined by the increase of consumption of electrons in an electrochemical reduction reaction of the oxides by the movement of the main part of the interfacial potential drop from the scope of the passive oxide film on its boundary with the electrolyte solution. 2. The method according to p. 1, characterized in that the threshold value of the electrode potential is measured as the potential maximum of the estance modulus between the reversible stages of linear growth and the sudden decline of the estance modulus, this maximum is characterized as the charge threshold of a passive oxide film expressing the suddenness of its transition from the state of the insulator in a state of metal. 3. The method according to p. 1, characterized in that the combination of the Fermi level with the conduction band of the passive oxide film is defined as the condition for establishing ohmic contact at the interface between the electrode and the passive oxide film, and the insulator-metal transition is characterized by a decrease in the ohmic resistance in the volume of the passive oxide films, which provides a decrease in electrostriction, while a sudden drop in the estance module is used as a sign of an insulator-metal transition in the volume of a passive oxide film. 4. The method according to p. 1, characterized in that they evaluate the accuracy of maintaining linearity at the stage of linear growth of the estance modulus, find a relative deviation from linearity that does not exceed 1% of the maximum of the estance modulus for any potential value from an interval of 0.8 V s the cathode boundary located at a distance of 0.05 V from the charge threshold, this stage is considered as a characteristic of a non-trivial phenomenon of linear nanoelectrics. 5. The method according to p. 1, characterized in that the sudden transition of the passive oxide film from the state of the insulator to the state of the metal is characterized by the brevity of the stage of sudden decline of the estance modulus compared to the linear growth stage and the asymmetry of the steepness of the dependence “estance modulus - potential” on both sides of charging threshold, in the vicinity of which the maximum slope of this dependence at the stage of sudden decline is five times greater than the slope at the stage of linear growth. 6. The method according to p. 1, characterized in that the reversibility of the indicated growth and decay of the estance module is considered to be a characteristic of preserving the chemical composition of the passive oxide film during its charging by electrons, while the stage of charging the passive oxide film precedes the stage of electrochemical reduction of oxides. 7. The method according to p. 1, characterized in that the linear change in the electrode potential is limited to the region of linear growth and sudden decline of the estance module, note the coincidence shape of the decline of the estance module in the cathode direction with the shape of the rise of the estance module in the anode direction, along the potential interval occupied by a sudden decline in the estance modulus, the extent of the stage of reversible charging of the passive oxide film is estimated to be greater than 0.2 V. 8. The method according to p. 1, characterized in that the dependence of the current through the electrode on the electrode potential is recorded, at the cathode direction of the potential scan, the total wave of the oxide reduction current is found, characterized by its redistribution of the interphase potential jump as the passive oxide film is charged, resulting in a decrease the potential difference in the volume of the passive oxide film is compensated by the growth of the potential difference at the boundary of the passive oxide film with the electrolyte solution to values sufficient for the beginnings and the completion of electrochemical reactions for the reduction of oxides. 9. The method according to p. 8, characterized in that there is a lag in the total wave of the oxide reduction current from the charge threshold of the passive oxide film, the lag being equal to the length of the stage of reversible charging of the passive oxide film along the potential axis. 10. The method according to p. 8, characterized in that the redistribution of the interphase potential jump in the passive oxide film is characterized by the simultaneous action of two factors, including an increase in the electrical conductivity of the oxide film and an increase in its dielectric constant when excess electrons flow into the oxide film, which enhance the screening of positive metal ions and contribute to the displacement of valence electrons in an electric field at the interface of the oxidized electrode with the electrolyte solution. 11. The method according to p. 9, characterized in that the lag of the recovery current wave from the charge threshold of the passive oxide film is characterized as a sign of the leading role of the reversible charge of the passive oxide film in the process of iron activation by electrochemical removal of oxide. 12. The method according to p. 1, characterized in that a linear change in the electrode potential is used in the form of a zigzag scan of the potential, consisting of more than two alternating sections of the cathode and anode directions with a predominance of the duration of one of these directions. 13. The method according to p. 12, characterized in that the zigzag scan of the potential is used to determine the degree of local reversibility of the change in the state of the electrode surface in the vicinity of a certain potential value, the sign of local reversibility is considered to be the same direction or merging of the sections of the recorded dependence of the estans modulus on the potential obtained by changing potential in opposite directions. 14. The method according to p. 13, characterized in that the local reversibility of the sudden decline of the estance module is found in the potential interval longer than 0.2 V in accordance with the length of the stage of reversible charging, found by a continuous scan of the potential. 15. The method according to p. 1, characterized in that as an aqueous solution of an electrolyte using a solution of potassium hydroxide in a concentration of from 0.1 to 0.5 mol / l, the electrode is made of sheet metal with a thickness of from 0.4 to 1 mm with a content iron 99.95% and higher, while polishing the electrode with a chemical or electrolytic method, the range of variation of the electrode potential is selected in the range from +1.3 V to -0.3 V relative to the equilibrium hydrogen electrode in the solution used at a temperature of 20 ± 2 ° C . 16. The method according to p. 1, characterized in that the registration of the dependence of the estans module on the potential of the electrode is carried out at two frequencies of alternating current. 17. The method according to p. 1, characterized in that the dependence of the estans modulus on the potential at the stage of linear growth is extrapolated to the zero value of the estans modulus, and in this way the value of the zero charge potential of the oxidized iron electrode is obtained. 18. The method according to p. 17, characterized in that for the potential of zero charge of the oxidized iron electrode in an aqueous solution of 0.5 M KOH, a value of +2.0 V relative to the equilibrium hydrogen electrode in the same solution is found. 19. The method according to p. 1, characterized in that the dimensionless slope of the dependence of the modulus of estans on the potential at the stage of linear growth is calculated and the scale of the module of estans on the oscillogram in the coordinates "modulus of estans - potential" is determined using the formula ∂ | ∂γ / ∂q | / ∂ϕ = - [1+ (∂С / ∂ϑ) / С], where ∂ | ∂γ / ∂q | / ∂ϕ is the dimensionless slope, γ is the surface tension of the iron electrode, q is the surface charge density, ϕ is the electrode potential, C = ∂q / ∂ϕ is the specific capacity of the oxidized electrode, ϑ is the relative elastic deformation of the electrode area (the ratio of the increment of the electrode area with its elastic elongation to the original electrode area), (∂С / ∂ϑ) / С is a constant coefficient characterizing the linear nanoelectrics of a passive oxide film, (∂С / ∂ϑ) / С = ν / (1-ν), ν = 0.29 - Poisson's ratio for spinel as a substance of a passive oxide film. 20. The method according to p. 1, characterized in that the peak modulus of estans is characterized by the desorption of a monomolecular layer of hydroxyl ions adsorbed on iron by the reaction (FeOH) _ad + e ^- → Fe + OH ^- when the electrode potential changes in the cathode direction. 21. The method according to p. 1 or 20, characterized in that the peak of the current corresponding to the peak of the modulus of estance is found, and the true surface area of the iron as a dispersed system of crystallites is estimated by the amount of electricity consumed by the peak of the current. 22. The method according to p. 1, characterized in that as an aqueous solution of an electrolyte using an aqueous solution 0.2 M K ₃ Fe (CN) ₆ + 0.2 M K ₄ Fe (CN) ₆ , the initial alternating current is passed through the boundary of this solution with the iron electrode, the signal caused by the variable thermal tension of the iron electrode is recorded, this signal is used as a reference for measuring the estance modulus. 23. The method according to p. 22, characterized in that to account for the roughness, multiply the measurement result by the standard by a coefficient Λ = α ^{1 + ν (m)} , where α is the roughness factor of the electrode, equal to the ratio of the area of the rough surface to the projection area of this surface on the plane, ν (m) is the Poisson's ratio for the electrode, Λ - coefficient taking into account the dispersion of the orientation of the crystallite faces. 24. The method according to p. 1, characterized in that the dependences of current and estans on potential are interpreted in accordance with the following successive changes in the composition of the oxide film on iron in the process of irreversible electrochemical reactions involving oxygen: Fe → (FeOH) _ad → (FeO) _ad → (FeFe ₂ O ₄ ) _ad , γ-Fe ₂ O ₃ , γ-FeOOH when the electrode potential changes in the anode direction and γ-FeOOH, γ-Fe ₂ O ₃ , (FeFe ₂ O ₄ ) _ad → (FeO) _ad → (FeOH) _ad → Fe when the electrode potential changes in the cathode direction, where the adsorption bond of the oxide film with the iron electrode is indicated by _ad , the existing compounds are separated by commas, and the compound (FeFe ₂ O ₄ ) _ad stands for mixed oxide Fe ₃ O ₄ of the spinel type, which ensures passivity of the oxide film and containing ions of Fe ²⁺ and Fe ³⁺ with the possibility of releasing a valence electron during its exchange in the process of maintaining equilibrium Fe ²⁺ ↔ Fe ³⁺ + е ^- .

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