Method for predicting the corrosion rate of a metal material in soil and model thereof

Abstract

The application provides a metal material soil corrosion rate prediction method and model, and belongs to the technical field of metal life evaluation and environmental geotechnical disaster prevention and reduction; a mathematical model determined based on the basic reaction process of an electrode system response is used for predicting the corrosion rate of a metal in a soil environment; a dissolution reaction current I 1 , a passivation reaction current I 2 , and a corrosion product layer coverage θ are calculated to represent the strength of metal corrosion; the method explicitly includes the key characteristics of the surface electrochemical reaction of the metal under the action of different soil environments, and makes up for the deficiency of existing data analysis; the application can more reliably predict the charge transfer mechanism between the metal surface and the surrounding soil, and solves the uncertainty of traditional analysis methods in interpreting impedance data.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of metal life evaluation and environmental geotechnical disaster prevention and reduction, and relates to a method for predicting the corrosion rate of metal materials in soil and a model thereof. BACKGROUND

[0002] Underground pipelines for transporting oil and natural gas are mainly built by a large number of low-carbon steels. However, due to the aging and local damage of the coating, the outer wall of the pipeline is exposed to the soil, causing the pipeline to be subjected to the harm of electrochemical corrosion. Long-term corrosion damage can cause the pipeline to collapse under the action of soil and surface load, and then ground subsidence occurs. In addition, fuel leakage caused by pipeline corrosion can also lead to serious ecological environmental problems and have a negative impact on nearby residences and industries. Therefore, the corrosion damage of buried metal pipelines is an important phenomenon that cannot be ignored in the transportation pipeline system.

[0003] Electrochemical impedance spectroscopy (EIS) can characterize the dynamics of bound or mobile charges in the bulk or interfacial region of any solid or liquid material, as well as the interaction of electrons with conductive electrodes. However, given the various processes of impedance response of conductive systems, the total impedance can be associated with various complex variables, such as medium transfer, chemical reaction rate, corrosion, dielectric properties, microstructure, and the influence of chemical composition on the conductivity of the system. Therefore, it is challenging and uncertain to examine and evaluate the relevant corrosion processes in a specific frequency domain.

[0004] Based on the defects existing in the analysis of metal corrosion based on existing electrochemical impedance spectroscopy, it is necessary to develop new and suitable mathematical methods in order to more reliably predict the charge transfer mechanism between the metal surface and the surrounding soil and solve the uncertainty of traditional analysis methods in interpreting impedance data. SUMMARY

[0005] The present application overcomes the deficiencies of the prior art and proposes a method for predicting the corrosion rate of metal materials in soil and a model thereof. Based on the basic reaction process and kinetic theory of electrode system response, the present application analyzes the key characteristics of the surface electrochemical reaction of metals under the action of different soil environments to make up for the deficiencies of traditional equivalent circuit methods in interpreting impedance data.

[0006] In order to achieve the above purpose, the present application is realized by the following technical solutions.

[0007] A method for predicting the corrosion rate of metal materials in soil, comprising the following steps:

[0008] Step one: Based on anion-assisted dissolution reaction and metal passivation reaction, the reaction rate expressions are I1 and I2, as shown in equations (3) and (4):

[0009]

[0010]

[0011] where E represents the potential relative to the reference electrode: E = E ss + ΔE, E ss is the steady-state potential, and ΔE is the perturbation potential; C Cl- is the anion concentration, C Cl is combined into the parameter k1, k1, k2, r1, and r2 are all kinetic parameters; θ is the proportion of the surface area covered by corrosion products; the rate of change of θ is shown in equation (6):

[0012]

[0013] where K is the conversion coefficient.

[0014] Step two: The rate of change of surface coverage of corrosion products is a function of all system variables, and under the condition of potential perturbation, the rate of change According to Taylor series transformation, equations (12) and (13) are obtained:

[0015]

[0016]

[0017] Z F The expression is equivalent to a circuit composed of a resistor R and a complex element (RC) in series; therefore, Z F The equivalent circuit symbol of Z s can be given by R(RC), as shown in equation (14):

[0018]

[0019] An equivalent circuit model R dl (C t (R p (R p ))) corresponding to the complex impedance curve is established, where R s is the bulk resistance of the soil, C dl is the double-layer capacitance of the electrode surface, R t is the charge transfer resistance of the metal dissolution reaction, and R p and C p are the resistance behavior and capacitance behavior of the metal passivation reaction, respectively.

[0020] The evolution of the metal interface corrosion kinetics is explained by calculating the fractional surface coverage of the oxide film;

[0021] The calculation formulas of the kinetic parameters I1, I2 and the physical parameter θ are as follows:

[0022] I1= 1 / (R t +R p )(r1-r2) (29)

[0023] I2= R t +R p -r1R t -2r1R p / r2R t (R t +R p ) (30)

[0024]

[0025] The dissolution reaction current I1 represents the rate of the metal matrix loss, the passivation reaction current I2 represents the rate of the corrosion product generation, and the corrosion product layer coverage θ represents the coverage of the corrosion product layer on the metal matrix surface.

[0026] Further, the anion-assisted dissolution reaction in step one is shown in equation (1):

[0027]

[0028] The metal passivation reaction is shown in equation (2):

[0029] Fe + 3OH - →FeOOH + H2O + 3e - (2).

[0030] Further, the total current density I is composed of the Faraday current density I F and the non-Faraday current density I dl , as shown in equation (5):

[0031]

[0032] Where C dl is the double-layer capacitance; the state variable X affecting I F is the electrode potential E and the surface coverage of the corrosion product.

[0033] Further, the step two is based on partial differential equation to establish the relationship between the Faraday current expression and the equivalent circuit model; the impedance response process in the corrosion process is decomposed through mathematical modeling, so as to establish the mathematical model of the impedance response,

[0034] The Faraday current density is shown as formula (7):

[0035] I F = f (E, X, C) (7)

[0036] Where E is the electrode potential, X is the system variable on the electrode surface, and C is the ion concentration near the electrode surface.

[0037] Under the condition that sufficient oxygen can reach the metal surface and the environmental temperature of the soil is approximately constant, the change of dissolution and passivation with time tends to be constant; under these conditions, the concentration polarization is not considered; therefore, the Faraday current density I F is shown as formula (8):

[0038]

[0039] By expanding the multi-variable function I F into Taylor series to express as the function of ΔE and ΔX, the function I F is expressed as the function of ΔE and ΔX. F The change ΔI ss is shown as formula (9):

[0040]

[0041] Where ΔE = |ΔE|·exp(jωt) is a small-amplitude sinusoidal potential perturbation, X ss and E ss are the steady-state values, and ΔX is the change of the state variable after the potential perturbation;

[0042] Y F is derived from the Faraday definition, which is shown as formula (10):

[0043]

[0044] The Faraday impedance Z F = 1 / Y F is shown as formula (11):

[0045]

[0046] Further, the parameters R t , R p , and C p in formula (14) are solved by formula (15), formula (16), and formula (17) respectively:

[0047]

[0048]

[0049]

[0050] Further, the simultaneous equations (18) ~ (20) are obtained A, |B|;

[0051]

[0052]

[0053]

[0054] Again, the simultaneous equations (20) ~ (25) are solved to obtain the dissolution reaction rate I1 and the passivation reaction rate I2:

[0055]

[0056]

[0057]

[0058]

[0059]

[0060] Where F is the Faraday constant, 96485.33289 C / mol, R is the gas constant, 8,314 J / (mol·K), T is the absolute temperature, and α1 and α2 are transfer coefficients.

[0061] By the simultaneous Faraday's law of electrolysis equation (26), the corrosion product film thickness δ calculation equation (27) and equation (28), the surface coverage change rate - charge density conversion coefficient K is solved

[0062] K' = M / Fn (26)

[0063]

[0064] K = K' / ρδ (28)

[0065] Where M is the relative molecular mass of the corrosion product, n is the absolute value of the total number of positive and negative valence of the corrosion product, ε is the relative dielectric constant of the corrosion product film, ε0 is the vacuum dielectric constant, 8542 × 10 -14 F·cm -1 , C ∞is the frequency limit capacitance value in the test result complex capacitance graph, and p is the density of the corrosion product film; then the conversion coefficient K is brought into formula (21), to obtain the surface coverage rate θ of the corrosion product.

[0066] Based on the model of the prediction method of the soil corrosion rate of the metal material, the calculation formula of the kinetic parameters I1, I2 and the physical parameter θ is as follows:

[0067] I1=1 / (R t +R p )(r1-r2) (29)

[0068] I2=R t +R p -r1R t -2r1R p / r2R t (R t +R p ) (30)

[0069]

[0070] The greater I1 is, the higher the degree of metal loss is, and the stronger the corrosion of the metal is; the greater I2 is, the higher the degree of ionization of the metal matrix is, and the stronger the corrosion of the metal is; the greater the corrosion product layer coverage rate θ is, the higher the degree of coverage of the corrosion product on the surface of the metal matrix is, the stronger the protection of the corrosion product layer to the metal is, and the weaker the corrosion of the metal is.

[0071] The beneficial effects of the present application relative to the prior art are:

[0072] The mathematical model determined based on the basic reaction process of the electrode system response is used for predicting the corrosion rate of the metal in the soil environment. The method clearly includes the key characteristics of the surface electrochemical reaction of the metal under the action of different soil environments, and makes up for the shortcomings of the existing data analysis. The present application can more reliably predict the charge transfer mechanism between the metal surface and the surrounding soil, and solves the uncertainty of the traditional analysis method in interpreting impedance data. BRIEF DESCRIPTION OF DRAWINGS

[0073] Figure 1 It is a schematic diagram of an equivalent circuit model corresponding to a complex impedance curve.

[0074] Figure 2 It is a complex impedance curve schematic diagram of electrochemical measurement under the condition of 20 to -20 DEG C and 0% salt content in the embodiment.

[0075] Figure 3 It is a complex impedance curve schematic diagram of electrochemical measurement under the condition of 20 to -20 DEG C and 0.3% salt content in the embodiment.

[0076] Figure 4 Figure 2 is a schematic diagram of the complex impedance curve measured electrochemically for the Example at 20 to -20 °C with a salt content of 1.0%.

[0077] Figure 5 Figure 3 is a schematic diagram of the complex impedance curve measured electrochemically for the Example at 20 to -20 °C with a salt content of 2.0%.

[0078] Figure 6 Figure 4 is a schematic diagram of the dissolution reaction current density I1 for the Example.

[0079] Figure 7 Figure 5 is a schematic diagram of the passivation reaction current density I2 for the Example.

[0080] Figure 8 Figure 6 is a schematic diagram of the corrosion product surface coverage Θ for the Example.

[0081] Figure 9 Figure 7 is a schematic diagram of the area ratio distribution of the O element in the EDS analysis results for the Example. DETAILED DESCRIPTION

[0082] In order to make the technical problems to be solved by the present application, technical solutions and beneficial effects more clearly understood, the present application will be further described in detail in conjunction with the embodiments and drawings. It should be understood that the specific embodiments described herein are only used to explain the present application and do not limit the present application. The technical solutions of the present application will be described in detail below in conjunction with the embodiments and drawings, but the protection scope is not limited thereto.

[0083] The mathematical model for analyzing the metal corrosion in soil includes the following steps:

[0084] Step one

[0085] The impedance of the metal in the corrosion process is controlled by the transient response of the metal-electrolyte interface and the corrosion product layer in series. Based on the assumption of anion-assisted dissolution reaction and metal passivation reaction, a physical model for the corrosion of the pipeline in the soil environment is proposed, as shown in equations (1) and (2).

[0086] Anion-assisted dissolution reaction:

[0087]

[0088] A s- represents the acceptor anion.

[0089] Passivation reaction:

[0090] Fe + 3OH-→ FeOOH + H2O + 3e - (2)

[0091] The reaction rate expressions based on equations (1) and (2) are I1and I2, respectively, as shown in equations (3) and (4):

[0092]

[0093] where E represents the potential relative to a reference electrode: E = E ss + ΔΕ, E ss is the potential at steady state, and ΔΕ is the perturbation potential. C Cl- is the anion concentration, C Cl- are combined into the parameter k1, k1, k2, r1, and r2 are kinetic parameters, F is the Faraday constant, 96485.33289 C / mol, R is the gas constant, 8,314 J / (mol·K), T is the absolute temperature, and α1and α2are transfer coefficients. Given that corrosion products present on the metal surface can prevent the occurrence of dissolution and passivation reactions, the rates represented by equations (1) and (2) are directly proportional to the ratio of the surface reactive area (1-θ), where θ is the proportion of the surface area covered by corrosion products.

[0094] The total current density I is composed of the Faradaic current density I F and the non-Faradaic current density I dl , as shown in equation (5):

[0095]

[0096] where C dl is the double-layer capacitance.

[0097] In this model, the state variables X that affect I F are the electrode potential E and the surface coverage of corrosion products.

[0098] The corrosion products formed during the metal passivation reaction cause θ to increase, and the rate of change of θ is given by equation (6):

[0099]

[0100] where K is the conversion factor.

[0101] Step two

[0102] Based on the partial differential equation, the relationship between the Faradaic current expression and the equivalent circuit model is established. By mathematical modeling, the impedance response process in the corrosion process is decomposed, thereby establishing a mathematical model of the impedance response:

[0103] The Faradaic current density I F is given by equation (7):

[0104] I​F = f(E, X, C) (7)

[0105] where E is the electrode potential, X is the system variable on the electrode surface, and C is the ion concentration near the electrode surface;

[0106] Under conditions where sufficient oxygen can reach the metal surface and the environmental temperature of the soil remains substantially constant, the changes in dissolution and passivation with time tend to be constant. Under these conditions, the concentration polarization is not considered. Therefore, the Faradaic current density I F As shown in equation (8):

[0107]

[0108] By expanding the multivariate function I F as a Taylor series to express as a function of ΔE and ΔX, for the function I F The resulting change ΔI F As shown in equation (9):

[0109]

[0110] where ΔE = |ΔE|·exp(jωt) is a small-amplitude sinusoidal potential perturbation, X ss and E ss are steady-state values, and ΔX is the change in the state variable after the potential perturbation.

[0111] Y F is derived from the Faradaic definition, as shown in equation (10):

[0112]

[0113] The Faradaic impedance Z F = 1 / Y F is shown in equation (11):

[0114]

[0115] The rate of change of the surface coverage of the corrosion product is a function of all the system variables, including the electrode potential E, and under the condition of a potential perturbation, the rate of change According to the Taylor series transformation, equations (12) and (13) are obtained:

[0116]

[0117]

[0118] Z F The expression is equivalent to a circuit composed of a resistor R and a complex element (RC) in series. Therefore, ZF The equivalent circuit symbol of can be given by R(RC), as shown in formula (14):

[0119]

[0120] The parameter R t , R p , C p Solving formula (15), formula (16) and formula (17) respectively, we can get:

[0121]

[0122]

[0123]

[0124] Based on the above derivation process, an equivalent circuit model R corresponding to the complex impedance curve is established. s (C dl (R t (R p C p ))), as attached Figure 1 As shown, where R s is the volume resistivity of the soil, C dl is the double layer capacitance of the electrode surface, R t is the charge transfer resistance of the metal dissolution reaction, R p and C p They are the resistance behavior and capacitance behavior of metal passivation reaction respectively.

[0125] Based on steps 1 and 2, the theoretical calculation process of the kinetic parameters I1, I2 and the physical parameter θ is as follows:

[0126] Combining formulas (18) to (20), we can obtain A, |B|.

[0127]

[0128]

[0129]

[0130] Combining equations (20) to (25) again, we can obtain the dissolution reaction rate I1 and the passivation reaction rate I2:

[0131]

[0132]

[0133]

[0134]

[0135]

[0136] By simultaneous Faraday's electrochemical law equation (26), corrosion product film thickness δ calculation equation (27) and equation (28), the surface coverage change rate-charge density conversion coefficient K is obtained

[0137] K' = M / Fn (26)

[0138]

[0139] K = K' / pδ (28)

[0140] Wherein K' represents the chemical equivalent, M is the relative molecular mass of the corrosion product, n is the absolute value of the total number of positive and negative valence of the corrosion product, ε is the relative dielectric constant of the corrosion product film, ε0 is the vacuum dielectric constant, 8542 x 10 -14 F·cm -1 , C ∞ is the frequency limit capacitance value in the test result complex capacitance diagram, and p is the density of the corrosion product film.

[0141] Then the conversion coefficient K is brought into equation (21) to obtain the surface coverage of the corrosion product θ. The evolution law of influencing the metal interface corrosion kinetics is explained by calculating the change of the fractional surface coverage of the oxide film.

[0142] The calculation equations of the kinetic parameters I1, I2 and the physical parameter θ are as follows:

[0143] I1 = 1 / (R t +R p )(r1-r2) (29)

[0144] I2 = R t +R p -r1R t -2r1R p / r2R t (R t +R p ) (30)

[0145]

[0146] The dissolution reaction current I1represents the rate of loss of the metal matrix, and the greater I1is, the higher the degree of metal loss and the stronger the corrosion of the metal. The passivation reaction current I2represents the generation rate of the corrosion product, and the greater I2is, the higher the degree of ionization of the metal matrix, and to some extent, the stronger the corrosion of the metal. The corrosion product layer coverage θ represents the coverage of the corrosion product layer on the surface of the metal matrix, and the greater θ is, the higher the degree of coverage of the corrosion product on the surface of the metal matrix, and to some extent, the stronger the protection of the corrosion product layer on the metal and the weaker the corrosion of the metal.

[0147] The following takes the corrosion test of X80 steel in a certain soil sample as an example, and the corrosion rate and corrosion product surface coverage of the X80 steel are calculated according to the mathematical model proposed above, and the specific implementation is as follows:

[0148] (1) Experimental process:

[0149] The electrochemical measurement is performed using a three-electrode system: the working electrode is a certain X80 steel sheet sample, the reference electrode is a saturated calomel electrode, and the counter electrode is a platinum electrode. Before testing, the electrodes and soil are buried in the mold together, and the soil is a mixture of different concentrations of NaCl solution and silt. This test tests the electrochemical impedance spectroscopy under the conditions of 20℃-20℃. The test frequency range is 100 kHz-0.01 Hz, and the linear sinusoidal potential perturbation amplitude is 10 mV. The test results are shown in Figure 2-Figure 5 .

[0150] (2) Mathematical analysis:

[0151] Due to the non-uniformity of the medium, C dl and C p are replaced by equivalent constant phase angle elements Q dl and Q P . The equivalent circuit model R s (Q dl (R t (R p Q p ))) is used to fit the EIS data by the fitting software Zsimdemo. Appendix Table 1 shows the values of various electrochemical parameters of each sample fitted by the equivalent circuit (R s C s )(Q dl (R t (R p Q p ))). The parameter values R t , R p , C pSubstitute formula (18) and formula (19) to solve A and |B|, as shown in Table 2. By solving formula (20)-(25), approximately considering that α1≈1.5, α2≈0.5, the current density I1, I2 of the dissolution reaction and the passivation reaction are calculated, wherein the kinetic parameters r1, r2 required for calculation are shown in Table 3, and the current density I1, I2 of the dissolution reaction and the passivation reaction calculated are shown in Figure 6 and Figure 7 .

[0152] I1 and I2 can be used to evaluate the relationship between the corrosion rate of X80 steel in silt and the salt content and temperature of NaCl.

[0153] The conversion coefficient K is calculated according to formula (26)-(28). According to the Faraday electrolysis law K'=M / Fn, wherein K' is the electrochemical equivalent of FeOOH, M is the relative molecular mass of FeOOH, and n is the absolute value of the total number of positive and negative valences of FeOOH. The thickness δ of the corrosion product layer is calculated according to formula (27), wherein the dielectric constant value of FeOOH is 14.1, and the density ρ of FeOOH is 4.26 g / cm 3 . By calculating the thickness of the corrosion product of all test results, I2 solved by the equation is substituted into θ=A-KI2 / A to obtain the coverage of the oxide film θ. The relationship between the temperature, salt content and surface coverage θ in silt is established as shown in Figure 8 and Figure 9 .

[0154] (3) Verification result:

[0155] Combined with the results of Energy Dispersive Spectroscopy (EDS) plane scanning O element and ImagePro Plus (IPP) software analysis, the distribution area of O element is calculated by using the black and white binary method. The calculation results are shown in Figure 8 and Figure 9 . The distribution law of the area ratio of O element is roughly similar to the calculation results, which confirms that the model can effectively analyze the surface coverage of FeOOH on the metal.

[0156] The analysis results of the examples show that the calculation method and prediction model of the soil corrosion rate of the metal material proposed in the application can realize the analysis of the metal corrosion rate and the interface distribution under any condition in the soil environment, in view of the uncertainty of the traditional equivalent circuit method in impedance data interpretation.

[0157] (4) Prediction result

[0158] Based on the relationship between temperature, salt content and kinetic parameters I1 and I2 in the silt system above. Through regression analysis, the prediction model of current density of dissolution and passivation reaction of metal in silt and the coverage rate θ of oxide film when the temperature and salt content change can be determined, as shown in equations (32) to (34).

[0159] I1=10- 6 ×(1.391+0.3952·T+4.452·c+0.01959·T 2 +0.1216·T·c-0.9592·c 2 ) (32)

[0160]

[0161] θ=1-10- 6 ×(-0.07896·T-6.474·c-0.004264·T 2 -0.1205·T·c+2.036·c 2 )(34)

[0162] Where T represents the soil temperature, ℃; c represents the NaCl salt content in the soil, %.

[0163] Table 1

[0164]

[0165] Table 2

[0166]

[0167] Table 3

[0168]

[0169] Table 4

[0170]

[0171] The above is a further detailed description of the present application in combination with specific preferred embodiments, which cannot be considered as limiting the specific embodiments of the present application to only this. For ordinary skilled persons in the technical field to which the present application belongs, a number of simple deductions or substitutions can be made without departing from the present application, which should be considered as belonging to the scope of patent protection determined by the claims submitted.

Claims

1. A method for predicting the soil corrosion rate of metal materials, characterized in that: The following steps are involved: Step 1: Based on the anion-assisted dissolution reaction and the metal passivation reaction, the reaction rate expressions are established as I1 and I2, respectively, as shown in formula (3) and formula (4): Where E represents the potential relative to the reference electrode: E = E ss +ΔE,E ss is the potential in steady state, ΔE is the perturbation potential; C Cl- is the anion concentration, C Cl- is incorporated into the parameter k1, k1, k2, r1 and r2 are all kinetic parameters; θ is the coverage of the corrosion product layer on the metal substrate surface; F is the Faraday constant, 96485.33289 C / mol; α1 and α2 are transfer coefficients; the rate of change of θ is shown in formula (6): Where K is the conversion factor; Step 2: Surface coverage change rate of corrosion products is a function of all system variables. Under the condition of potential disturbance, the rate of change is According to Taylor series transformation, we get formula (12) and formula (13): Z F The expression is equivalent to a circuit consisting of a resistor R and a composite element (RC) in series; therefore, Z F The equivalent circuit symbol of can be given by R(RC), as shown in formula (14): Establish an equivalent circuit model R corresponding to the complex impedance curve s (C dl (R t (R p C p ))), where R s is the volume resistivity of the soil, C dl is the double layer capacitance of the electrode surface, R t is the charge transfer resistance of the metal dissolution reaction, R p and C p They are the resistive and capacitive behaviors of the metal passivation reaction; The evolution of corrosion dynamics at metal interfaces is explained by calculating the changes in the fractional surface coverage of the oxide film. The calculation formulas for the kinetic parameters I1, I2 and the physical parameter θ are as follows: I1=1 / (R t +R p )(r1-r2) (29) I2=R t +R p -r1R t -2r1R p / r2R t (R t +R p ) (30) The dissolution reaction current I1 represents the rate of metal matrix loss, the passivation reaction current I2 represents the rate of corrosion product generation, the corrosion product layer coverage θ represents the coverage of the corrosion product layer on the metal matrix surface, and K is the conversion coefficient.

2. The method for predicting the soil corrosion rate of metal materials according to claim 1, characterized in that: The anion-assisted dissolution reaction described in step 1 is shown in equation (1): The metal passivation reaction is shown in equation (2): Fe + 3OH - →FeOOH + H2O + 3e - (2)。 3. The method for predicting the soil corrosion rate of metal materials according to claim 1, characterized in that: The total current density I is calculated from the Faraday current density I F and the non-Faraday current density I dl Composition, as shown in formula (5): Among them C dl is the double layer capacitance; affecting I F The state variables X are the electrode potential E and the surface coverage of corrosion products.

4. The method for predicting soil corrosion rate of metal materials according to claim 1, characterized in that: The second step is to establish the relationship between the Faraday current expression and the equivalent circuit model based on the partial differential equation; decompose the impedance response process in the corrosion process through mathematical modeling, thereby establishing a mathematical model of the impedance response. The Faraday current density is shown in formula (7): I F =f(E,X,C) (7) Where E is the electrode potential, X is the system variable on the electrode surface, and C is the ion concentration near the electrode surface; Under the premise that sufficient oxygen can reach the metal surface and the ambient temperature of the soil remains roughly constant, the changes of dissolution and passivation with time tend to be constant; under these conditions, concentration polarization is not considered; so the Faraday current density I F As shown in formula (8): By transforming the multivariate function I F Expanded to a Taylor series to express as a function of ΔE and ΔX, used to express the function I F The resulting change ΔI F , as shown in formula (9): Where ΔE=|ΔE|·exp(jωt) is a small amplitude sinusoidal potential disturbance, X ss and E ss is the steady-state value, and ΔX is the change in the state variable after the potential disturbance; Derivation of Y by Faraday's definition F , as shown in formula (10): Faraday impedance Z F =1 / Y F , as shown in formula (11):

5. The method for predicting soil corrosion rate of metal materials according to claim 1, characterized in that: The parameter R in the formula (14) t , R p , C p Solving formula (15), formula (16) and formula (17) respectively, we can get:

6. The method for predicting soil corrosion rate of metal materials according to claim 1, characterized in that: Combining formulas (18) to (20), we get A, |B|; Combining equations (20) to (25) again, we can obtain the dissolution reaction rate I1 and the passivation reaction rate I2: where F is the Faraday constant, 96485.33289 C / mol, R is the gas constant, 8,314 J / (mol·K), T is the absolute temperature, and α1 and α2 are the transfer coefficients; By combining Faraday's law of electrolysis (26), the corrosion product film thickness δ calculation formula (27) and formula (28), the surface coverage change rate - charge density conversion coefficient K is obtained K′=M / Fn (26) K=K′ / ρδ (28) Where M is the relative molecular mass of the corrosion product, n is the absolute value of the total number of positive and negative valences of the corrosion product, ε is the relative dielectric constant of the corrosion product film, ε0 is the vacuum dielectric constant, 8542×10 -14 F·cm -1 , C ∞ is the frequency limit capacitance value in the complex capacitance diagram of the test results, ρ is the density of the corrosion product film; then the conversion coefficient K is substituted into formula (21) to obtain the surface coverage of the corrosion product θ.

7. A model based on a method for predicting soil corrosion rate of a metal material according to any one of claims 1 to 6, characterized in that: The calculation formulas for the kinetic parameters I1, I2 and the physical parameter θ are as follows: I1=1 / (R t +R p )(r1-r2) (29) I2=R t +R p -r1R t -2r1R p / r2R t (R t +R p ) (30) The larger I1 is, the higher the degree of metal loss is and the more corrosive the metal is; the larger I2 is, the higher the degree of ionization of the metal matrix is ​​and the more corrosive the metal is; the larger the corrosion product layer coverage θ is, the higher the degree of corrosion product coverage on the metal matrix surface is, the stronger the protection of the corrosion product layer to the metal is, and the weaker the corrosiveness of the metal is.

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