CN113553784B - Organic coating life assessment method in seawater full immersion environment - Google Patents
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Abstract
The invention provides a method for evaluating the service life of an organic coating in a seawater full-immersion environment by considering the influence of temperature and chloride ions, which comprises the following implementation steps: and (3) a step of: establishing an organic anti-corrosion coating aging model; and II: establishing an environmental profile of the coating structure during service; thirdly,: fitting a distribution function of a reaction constant theta; fourth, the method comprises the following steps: determining a failure threshold of the electrochemical parameters of the coating according to the natural environment data; fifth step: performing reliability evaluation on the organic coating by using a stress-intensity interference theory; through the steps, the qualitative logic relation and quantitative model method for evaluating the reliability of the organic coating by integrally establishing a coating accelerated aging test and an environmental factor correlation model; the definition of the service life and the reliability of the organic coating is realized, so that the failure criterion of the organic coating system is more timely, accurate and reasonable; the invention has the advantages of simple testing method, convenient operation and no damage to the structure of the coating.
Description
Technical Field
The invention provides a method for evaluating the service life of an organic coating in a seawater full-immersion environment, namely a method for evaluating the service life of the organic coating in the seawater full-immersion environment by considering the influence of temperature and chloride ions, which relates to a method for evaluating the time-varying service life of an organic anti-corrosion coating taking polyurethane as a base material in the seawater full-immersion environment, and is based on electrochemical test, stress-intensity interference theory and organic matter
The organic anticorrosive coating life evaluation method aims at the accelerated ageing test data of a coating in an organic anticorrosive coating system, the corrosion current change data of a base metal under the coating and corrosion environmental factors, and integrally establishes an acceleration test, a correlation model of coating degradation and the environmental factors for describing qualitative logic relation and quantitative model methods of the organic coating degradation process. The aging state of the organic anti-corrosion coating is mapped to the impedance characteristic of the organic anti-corrosion coating, so that the coating aging state can be conveniently identified, and the reliability evaluation can be further carried out. The method is suitable for the field of evaluating the service life of the coating structure taking polyurethane as a main coating material in a seawater full immersion environment.
Background
Organic coatings have been successfully applied in a variety of metal structures as an effective corrosion protection, but so far no extensive research has been done on coating failure. Statistical analysis shows that the failure or accident of the equipment structure caused by corrosion damage is mainly caused by the failure of the corrosion prevention function of the coating. The organic coating inevitably has some defects during the coating process through which smaller sized water molecules, ions and oxygen can pass to reach the metal substrate. Since the defects in the coating are not uniformly distributed, there is a difference in the order and extent of damage at the metal interface of the coating, which can lead to the formation of corrosion microcells. When electrochemical corrosion of the coating metal occurs, the cathodic reaction or the products of the cathodic reaction affect the bonding of the coating to the base metal, causing the coating to separate from the base metal. In addition, the weak part of the bond site of the high molecular chemical structure in the coating can be broken into free radicals under the action of different external factors, so that the free radical reaction is initiated, the damage of the coating molecules is caused, the original protective performance is gradually lost, and the matrix metal is seriously corroded to fail. The method has the key effects of researching a protective coating test under the corrosion condition, determining the service life and reliability of the coating, making a reasonable maintenance or repainting period of the coating and ensuring that the substrate structure reaches the specified calendar service life.
The existing evaluation test for the aging state of the organic anti-corrosion coating generally only comprises the light loss rate, the color change degree, the differentiation cracking degree, the bubble density and the peeling area, the anti-corrosion performance of the organic anti-corrosion coating cannot be evaluated timely and accurately, and the accuracy is low. It is found that the degradation of the corrosion resistance of the organic corrosion-resistant coating is caused by the local or overall degradation of the electrochemical properties and the corrosion environment isolation capability of the coating due to the change of the permeation of water molecules and ions in the organic corrosion-resistant coating, so that the corrosion resistance of the coating is reduced. Therefore, environmental factor data and a degradation model of the electrochemical performance of the coating under different environments are utilized to analyze and obtain the distribution of the electrochemical performance parameters of the coating, and then the reliability calculation method of the organic anti-corrosion coating is obtained according to the electrochemical performance threshold value of the coating with normal anti-corrosion performance under the corrosion environment.
Based on the method, the invention combines the accelerated aging test data of the organic anti-corrosion coating and the environmental factor data, and provides a method for evaluating the service life of the organic coating in the seawater full immersion environment by considering the influence of temperature and chloride ions, so as to realize the definition of the reliability and the service life evaluation of the organic coating.
Disclosure of Invention
(1) The purpose of the invention is that:
the invention aims to provide an organic coating life assessment method under a seawater full immersion environment, namely an organic coating life assessment method under the seawater full immersion environment considering the influence of temperature and chloride ions, which aims at the problems that an organic coating structure is seriously degraded in a severe service environment and no accurate and reasonable detection and reliability assessment method exists, and provides an organic coating life assessment method which is an organic coating failure judgment and reliability assessment method comprising electrochemical performance aging rule analysis, environmental factor analysis and coating electrochemical performance and corrosion resistance relation analysis; and establishing a model of electrochemical parameters and environmental factors by measuring degradation curves of electrochemical parameters of electrochemical impedance modulus of the coating under different environments, and determining the reliability and service life of the organic coating under different seawater full-immersion environments and service time according to the corresponding relation between the electrochemical parameters and the corrosion resistance of the organic coating.
(2) The technical scheme is as follows:
the invention needs to establish the following basic settings:
setting 1 a low-frequency impedance module value of the organic anti-corrosion coating, wherein the low-frequency impedance module value is an electrochemical performance parameter affecting the anti-corrosion performance of the organic anti-corrosion coating, and the organic anti-corrosion coating is invalid because water molecules, ions and oxygen corrosion mediums contact a metal matrix through the coating, so that the impedance of the coating is reduced and the anti-corrosion effect is reduced;
setting 2 the low-frequency impedance of the organic coating is an ageing characteristic parameter of the organic coating, and the quantitative relation between the low-frequency impedance and the ageing time obeys an ageing dynamics formula:
wherein: t is the accelerated aging test time (h) |Z| t For the ageing period t, the coating impedance modulus value (omega) at a frequency of 0.01Hz, |Z| 0 Is the coating impedance modulus value (omega) with ageing time of 0 and frequency of 0.01Hz m Is the impedance modulus (Ω) of the metal substrate at a frequency of 0.01Hz, θ is the reaction constant, and its magnitude is related to the coating characteristics and the severity of the aging environment;
setting 3: obeying the Arrhenius model between θ and thermodynamic temperature T:
wherein: z is a constant greater than 0, K is a Boltzmann constant, T is absolute temperature (K), E a Is an activation energy (J);
setting 4: θ and chloride ion concentration c obey Fick's second law, and are obtained according to the relation between diffusion speed and chloride ion concentration c:
θ＝α·c β (3)
wherein: alpha is a constant greater than 0 and beta is the reaction index.
To sum up, a model of the relationship between θ and thermodynamic temperature T and chloride concentration c is obtained:
θ＝a·c b ·e -d/T (4)
wherein: a. b and d are undetermined constants;
setting 5: before the low-frequency impedance of the organic anti-corrosion coating is reduced to a certain threshold value, the excellent anti-corrosion performance of the organic anti-corrosion coating perfectly isolates the metal matrix from the corrosion medium, and under the condition, the metal matrix cannot generate electrochemical corrosion, and the organic anti-corrosion coating is regarded as normal in anti-corrosion function.
The method mainly comprises the steps of analyzing electrochemical performance aging rules of the organic anti-corrosion coating, establishing an electrochemical parameter and environmental factor model of the organic anti-corrosion coating, determining a coating failure threshold value, establishing coating degradation parameter distribution based on external natural environment data, and estimating reliability according to a stress-intensity interference model;
based on the basic setting, the method for evaluating the service life of the organic coating in the seawater full-immersion environment, which is provided by the invention, is characterized by considering the influence of temperature and chloride ions: the method is realized by the following steps:
step one: establishing an aging model of the organic anti-corrosion coating
Firstly, estimating unknown parameters in an aging dynamics model (1) based on accelerated aging test data of an organic anti-corrosion coating, and interpolating to obtain a change rule of electrochemical parameters of the coating along with time in an external natural environment; secondly, establishing a reaction constant and environmental factor data pair in the aging dynamics model, and fitting a reaction constant model (4) according to the reaction constant and environmental factor data pair; further, obtaining a quantitative corresponding relation between electrochemical parameters of the organic anti-corrosion coating and aging time and environmental factors according to the reaction constant model;
the method comprises the following specific steps:
I. estimating aging dynamics model parameters
According to formula (1), the following equation is established:
wherein: t is t ij For the jth test at the ith ambient stress levelTime (h), |Z| tij At t ij Impedance modulus (Ω), |z|of the organic coating at a frequency of 0.01Hz measured at the moment 0i Is the coating impedance modulus value (omega) at the ith environmental stress level and aging time of 0 and frequency of 0.01Hz, |Z| mi Is the impedance modulus (omega) and theta of the metal substrate at the frequency of 0.01Hz at the ith environmental stress level i Is the reaction constant at the ith ambient stress level; for |Z| mi From 10 5 The starting interval 1 is assigned with value; for each given |Z| m Calculating the sum of squares of errors:
wherein: y is Y ij ＝ln(|Z| tij -|Z| m )，
Wherein->
Respectively->
Least squares estimation of (c); when S is 2 When the minimum value is taken, the |Z| is obtained m Optimal estimate of +.>
Further obtain |Z| 0 ,θ i Estimate of (2)
II, estimating parameters of a reaction constant model
According to equation (4), the following equation is established:
in the middle of：θ ij C is the reaction constant at the ith temperature stress and the jth chloride ion concentration level i Is the i-th chloride concentration level value (%), T j Is the jth temperature stress level value (°c); according to the aging rate estimated value obtained in the last step
Performing least square fitting according to the formula to obtain the relation between the reaction constant and the temperature stress T:
wherein:
a least square estimated value for the constant a, b, d to be determined;
III, determining the relation between the low-frequency impedance of the coating and environmental factors
Based on the aging kinetic model and the reaction constant model of the organic coating, the low-frequency impedance |Z|of the organic coating at the ambient temperature T and the chloride ion concentration c is obtained by combining the formulas (1), (5) and (8) t Correspondence with aging time t:
step two: establishing environmental profile during service of coating structure
Corresponding natural environment factor data, time data and longitude and latitude, carrying out Kriging interpolation calculation and natural environment modeling, removing trend terms, calculating autocorrelation coefficients, checking the stability of trend term data, solving Kriging equation sets, interpolating the natural environment data, adding trend terms, carrying out error analysis and space-time variation function model selection, and obtaining a space-time distribution model of environmental stress; there are various methods for realizing the kriging space-time interpolation method, for example, python, java, matlab, R language and ArcGIS; taking Matlab as an example, calling a dacafit function in a dace toolbox, and directly performing Kriging interpolation calculation on the environmental data according to the selected variation function; interpolation error analysis and space-time variation function model selection are key steps of a Kriging interpolation method, and the space variation function model used for fitting at present comprises a Gaussian model, a linear model, a spherical model, an exponential model and a circular model; using the half variance as a measure for measuring the space-time correlation degree between each two points, finding out a variation function model which is best fitted with the half variance, wherein the interpolation error brought by the model with the better fitting degree is lower;
after the environmental stress space-time distribution model is obtained, the actual use task surface information of the coating-containing structure is combined to obtain an environmental profile under the actual use condition, for example, the environmental profile experienced by the ship in the sailing process is obtained through statistics according to the route and the stay time of the ship in a certain sea area and according to the natural environmental data space-time interpolation model in the sea area;
the 'Python and java' refers to a computer programming language which is applied to scientific calculation and statistics; the Matlab refers to commercial mathematical software used for data analysis and matrix operation; the R language refers to a language and an operation environment for statistical analysis and drawing; the ArcGIS refers to a technical system for collecting, storing, managing, calculating, analyzing, displaying and describing related geographic distribution data in the whole or part of the earth surface (including the atmosphere) space under the support of a computer hard and software system;
the "dacafit function in dace toolbox" refers to a Matlab software operation program for establishing a kriging model according to the existing test data points;
step three: fitting the distribution function of the reaction constant θ
According to the natural environment space-time distribution data obtained in the second step, calculating to obtain a reaction constant theta of each time node in the area by combining the formula (8); for the calculated reaction constant data θ 1 ,θ 2 ,…,θ n Find its maximum value L a And minimum value S m Grouping the data using formulas (10) and (11) and calculating the group number k andgroup distance Δt:
k＝1+3.3lgn (10)
Δt＝(L a -S m )/k (11)
counting the frequency falling into each group, and taking the reaction constant as an abscissa and the frequency of each group as an ordinate to make a frequency histogram; connecting the straight points into a curve, regarding as an approximation of the distribution density curve, primarily judging what kind of distribution the obtained reaction constant data overall belongs to, and writing out the distribution density function f (theta, eta) of the overall 1 ,η 2 ,…,η n ) Wherein eta 1 ,η 2 ,…,η n For the parameters to be estimated, according to the reaction constant data theta 1 ,θ 2 ,…,θ n Write out eta 1 ,η 2 ,…,η n Likelihood function of (2):
solving the following likelihood equation to obtain an estimated value
Based on the estimated value
Deriving an estimated distribution density function->
Adopting a hypothesis testing method to carry out fitting goodness test and judging whether the selected distribution is reasonable or not;
step four: determining failure threshold of electrochemical parameters of coating according to natural environment data
The corrosion of the metal matrix has a relatively large relationship with the environmental temperature, the concentration of the corrosive medium and the corrosion resistance factor of the organic coating, and in general,the greater the conductivity of the corrosive medium, the higher the ambient temperature, and the more easily the metal substrate is corroded; the method comprises the steps of measuring corrosion current of a metal matrix and electrochemical parameters of a coating in different environments, namely establishing a relation among environmental factors, the corrosion current of the metal matrix and the electrochemical parameters of the coating, and constructing a relation model among the three; deducing failure threshold value data of electrochemical parameters of the coating in the area range by combining space-time distribution data of environmental stress in the second step with a relation model of environmental factors, metal matrix corrosion current and electrochemical parameters of the coating; in general, the threshold value data of the low-frequency impedance failure of the coating is extremely small, and the maximum value of the threshold value data of the low-frequency impedance realization of the coating is taken as the electrochemical parameter threshold value |Z||when the organic coating fails th ；
Step five: evaluation of reliability of organic coating using stress-intensity interference theory
The electrochemical performance aging dynamic model and the reaction constant model of the organic coating obtained in the first step are combined with the reaction constant distribution in the third step to obtain the electrochemical performance distribution of the organic coating and the relation between the electrochemical performance distribution and the aging time of the organic coating, and the electrochemical performance distribution and the reaction constant distribution are used as stress distribution of the corrosion resistance of the organic coating; the electrochemical threshold value of the coating failure obtained in the fourth step is used as the strength requirement of the corrosion resistance of the organic coating; describing the reliability of the organic coating under different ageing times through a stress-intensity interference model, and obtaining a calculation formula of the reliability R of the organic coating by defining stress and intensity:
R＝P(|Z| t ＞|Z| th ) (14)
after obtaining an organic coating reliability R curve, calculating a reliable life parameter of the organic coating;
through the steps, the qualitative logic relation and quantitative model method for evaluating the reliability of the organic coating by integrally establishing a coating accelerated aging test and an environmental factor correlation model; the definition of the service life and the reliability of the organic coating is realized, so that the failure criterion of the organic coating system is more timely, accurate and reasonable; the invention has the advantages of simple testing method, convenient operation and no damage to the structure of the coating.
(3) Advantages and efficacy: the invention relates to a method for evaluating the service life of an organic coating in a seawater full-immersion environment by considering the influence of temperature and chloride ions, which has the advantages that:
(1) the invention integrally establishes a qualitative logic relation and a quantitative model method of a coating accelerated aging test and an environmental factor association model for evaluating the reliability of the organic coating;
(2) the invention realizes the definition of the service life and the reliability of the organic coating, so that the failure criterion of the organic coating system is more timely, accurate and reasonable;
(3) the method has the advantages that the measurement parameters of the coating structure are electrochemical parameters, the testing method is simple, the operation is convenient, and the structure is not destructive;
(4) the evaluation method is scientific, good in manufacturability and wide in popularization and application value.
Drawings
Figure 1 is a flow chart of the method of the present invention.
Response constant data profile in the case of figure 2.
The reliability of each coating in the case of fig. 3 is plotted.
Detailed Description
The invention will be described in further detail with reference to examples.
The organic coating structure takes polyurethane series coating as finishing coat, and the whole thickness of the coating system is about 80 mu m; test the polyurethane coating system was subjected to a soak test in 8 test environments; specific test conditions and low frequency impedance data records during the aging process of the coating are shown in table 1;
table 1 data of the soaking test of the polyurethane coating system
The invention relates to a method for evaluating the service life of an organic coating in a seawater full-immersion environment, namely a method for evaluating the service life of the organic coating in the seawater immersion environment by considering the influence of temperature and chloride ions, which is shown in a figure 1, and is realized by the following steps:
step one: establishing an organic anti-corrosion coating aging model;
I. estimating aging dynamics model parameters
First, the undetermined parameter |Z| is determined m The method comprises the steps of carrying out a first treatment on the surface of the For |Z| m From 10 5 The start interval 1 assigns it, for each given |Z| m Calculating the sum of squares of errors; when different |Z| m Sum of squares of linear regression residual errors S under value 2 At minimum, the value of |Z| in the different experiments was obtained m A value; on the basis, the low-frequency impedance modulus value and the soaking time of the coating in the table 1 are fitted by a least squares method, and the result is shown in the table 2, R 2 Fitting goodness;
TABLE 2 aging kinetics model parameters and soaking time fitting results
II, estimating parameters of a reaction constant model
Obtained in Table 2
Value and corresponding T i And c i Performing least square fitting, and calculating to obtain an aging rate estimated value +.>
The relationship with the temperature T and the chloride ion concentration c is:
R 2 0.9813, the fitting result of the fitting curved surface to the observed value is better;
III, determining the relation of the low-frequency impedance of the coating and the environmental factors
According to the fitting equation obtained by the calculation method, extrapolating estimated values of low-frequency impedance modulus values of the coating corresponding to different soaking times, eliminating the intermediate reaction constant theta to obtain the temperatureThe low-frequency impedance modulus value of the coating is |Z| under the conditions of the degree T (DEG C) and the chloride ion concentration c (%) t The correspondence of (Ω) and the soak time t (h) is:
step two: establishing an environmental profile of the coating structure during service;
selecting natural seawater environment data of the south China sea area in the world ocean database in 2015-2020, corresponding the seawater temperature and salinity data and time data in the natural seawater environment to longitude and latitude, and carrying out Kriging interpolation calculation and natural environment modeling by taking 0.5-degree longitude and latitude as interpolation precision. Using Matlab software to call a dacefat function in a dace toolbox to perform Kriging interpolation calculation on environmental data according to the selected variation function, and selecting a 0-order index space-time variation function model through error analysis to obtain space-time distribution data of environmental stress; due to the annual cycle repetitive nature of natural environment data, natural seawater environment data are integrated into 1 year. Taking three-to-cissand route as an example, the route is approximately nine-point connection of (18 °,108 °), (18 °,109 °), (18 °,110 °), (18 °,111 °), (18 °,112 °), (18 °,113 °), (17 °,113 °), (16 °,113 °), (15 °,113 °; according to the geographical position of the route, 3285 data in nine points and one year on the route are selected as natural seawater environment data backgrounds;
step three: fitting a reaction constant theta distribution function;
finding out the maximum value L according to the reaction constant data obtained by calculating the natural environment space-time distribution data obtained in the step two a =520 and minimum value S m =110, grouping the data using formulas (10) and (11) and calculating the group number k=13 group distance Δt=31.5;
counting the frequency falling into each group, and taking the reaction constant as an abscissa and the frequency of each group as an ordinate to make a frequency histogram;
TABLE 3 reaction constant data sheet
The obtained reaction constant data population is primarily judged to belong to the mixed Weibull-normal distribution according to the curve formed by connecting the straight points, and the distribution density function of the population is as follows:
wherein a, m, eta, gamma, mu and sigma are parameters to be estimated, likelihood functions of a, m, eta, gamma, mu and sigma are written according to the reaction constant data, and likelihood equations are solved to obtain estimated values
Table 4 list of parameter estimates
By pearson χ 2 The method for testing carries out fitting goodness test, and an original assumption is established:
then, the observed values of the probabilities that the variable θ falls into the ith interval are calculated respectively:
wherein: a, a i Is the i-th interval endpoint. Finally calculate statistics
Is:
wherein: n is n i The frequency number of the ith interval is the frequency number of the ith interval, and n is the sample data size;
taking the significance level alpha=0.01, and finding the critical value as
Due to
The original assumption is accepted that the reaction constant data is considered to belong generally to the mixed weibull-normal distribution;
table 5 calculation of goodness-of-fit test
Step four: determining a failure threshold of the electrochemical parameters of the coating according to the natural environment data;
selecting a self-etching current i CORR As characteristic quantity for representing corrosion of the metal matrix, according to an empirical relation model of self-corrosion current of the metal steel matrix and environmental factors:
ln1.08i CORR ＝8.37+0.618ln0.46c-3034/T-0.000105R c +2.32t -0.215 (21)
wherein i is CORR Is the self-corrosion current density (mu A/cm) of the steel matrix 2 ) C is chloride concentration (%), T is metal substrate surface temperature (K), R c Is the surface resistance (omega) of the metal matrix, t is the corrosion time (years);
the self-corrosion current density of the steel substrate in the initial stage of soaking in the electrolyte at the time of coating failure is set to be 1 mu A +.cm 2 Approximating the surface resistance of the metal matrix to the low-frequency impedance module value of the coating, and calculating to obtain a low-frequency impedance threshold value |Z|of the coating failure by combining the space-time distribution data of the natural environment obtained in the step two th Find its maximum value L a = 914722.31 and minimum value S m = 913034.92 due to |z| th The difference between the maximum value and the minimum value is smaller, so the maximum value 914722.31 is taken as the low-frequency impedance threshold value |Z| of the coating failure in the sea area th ；
Step five: performing reliability evaluation on the organic coating by using a stress-intensity interference theory;
for the polyurethane system coating and the metal steel matrix in the example, the reliability under different ageing times is described by a stress-intensity interference model, and the low-frequency impedance threshold value |Z| of the coating failure is determined according to the distribution of the reaction constant theta th And (3) obtaining a calculation formula of the reliability R of the polyurethane coating according to the definition of stress and strength:
according to the reliability calculation formula, calculating to obtain the reliability life t (R) of the organic coating;
the result shows that the method of the invention realizes the estimation of the reliability of the organic anti-corrosion coating by analyzing the change of the electrochemical parameters of the organic coating along with the time and the environmental factor data, thereby achieving the expected purpose.
In summary, the invention relates to a method for evaluating the service life of an organic coating in a seawater full-immersion environment, namely a method for evaluating the service life of an organic coating in a seawater full-immersion environment by considering temperature and chloride ion influence, which relates to a method for evaluating the service life of an organic coating in a seawater full-immersion environment by considering temperature and chloride ion influence; the method integrally establishes a qualitative logic relation and a quantitative model method of a coating accelerated aging test and an environmental factor association model for evaluating the reliability of the organic coating; the aging state of the corrosion resistance of the organic coating is mapped to the electrochemical parameters of the coating, so that modeling and calculation are facilitated; the method comprises the following specific steps: 1. establishing an organic anti-corrosion coating aging model; 2. establishing an environmental profile of the coating structure during service; 3. fitting a distribution function of a reaction constant theta; 4. determining a failure threshold of the electrochemical parameters of the coating according to the natural environment data; 5. performing reliability evaluation on the organic coating by using a stress-intensity interference theory; the invention is suitable for the field of evaluating the reliability of the coating structure serving in the seawater full immersion environment by using polyurethane or a main coating material, has the characteristics of simple testing method, convenient operation and no damage to the structure, and has wide popularization and application values.
Claims (1)
1. The life evaluation method of the organic coating in the seawater full immersion environment needs to establish the following settings:
setting 1: the low-frequency impedance modulus of the organic anti-corrosion coating is an electrochemical performance parameter affecting the anti-corrosion performance of the organic anti-corrosion coating, and the failure of the organic coating is caused by the fact that water molecules, ions and oxygen corrosion mediums contact a metal matrix through the coating, so that the coating impedance is reduced and the anti-corrosion effect is reduced;
setting 2: the low-frequency impedance of the organic coating is an ageing characteristic parameter of the organic coating, and the quantitative relation between the low-frequency impedance and the ageing time obeys an ageing dynamic formula:
wherein: t is the accelerated aging test time (h) |Z| t For the ageing period t, the coating impedance modulus value (omega) at a frequency of 0.01Hz, |Z| 0 Is the coating impedance modulus value (omega) with ageing time of 0 and frequency of 0.01Hz m The impedance modulus (omega) of the metal substrate at the frequency of 0.01Hz, and theta is a reaction constant, and the magnitude is related to the coating characteristics and the severity of the aging environment;
setting 3: obeying the Arrhenius model between θ and thermodynamic temperature T:
wherein: z is a constant greater than 0, K is a Boltzmann constant, T is absolute temperature (K), E a Is an activation energy (J);
setting 4: θ and chloride ion concentration c obey Fick's second law, and are obtained according to the relation between diffusion speed and chloride ion concentration c:
θ＝α·c β (3)
wherein: alpha is a constant greater than 0 and beta is a reaction index;
to sum up, a model of the relationship between θ and thermodynamic temperature T and chloride concentration c is obtained:
θ＝a·c b ·e -d/T (4)
wherein: a. b and d are undetermined constants;
setting 5: before the low-frequency impedance of the organic anti-corrosion coating is reduced to a certain threshold value, the anti-corrosion performance isolates the metal matrix from the corrosion medium, in this case, the metal matrix does not generate electrochemical corrosion, and the organic anti-corrosion coating is regarded as normal in anti-corrosion function
The method is characterized in that: the method is realized by the following steps:
step one: establishing an aging model of the organic anti-corrosion coating
Firstly, estimating unknown parameters in an aging dynamics model (1) based on accelerated aging test data of an organic anti-corrosion coating, and interpolating to obtain a change rule of electrochemical parameters of the coating along with time in an external natural environment; secondly, establishing a reaction constant and environmental factor data pair in the aging dynamics model, and fitting a reaction constant model (4) according to the reaction constant and environmental factor data pair; further, obtaining a quantitative corresponding relation between electrochemical parameters of the organic anti-corrosion coating and aging time and environmental factors according to the reaction constant model;
the method comprises the following specific steps:
I. estimating aging dynamics model parameters
According to formula (1), the following equation is established:
wherein: t is t ij Is the ith ringTime (h) of jth detection at the level of the environmental stress, |Z| tij At t ij Impedance modulus (Ω), |z|of the organic coating at a frequency of 0.01Hz measured at the moment 0i Is the coating impedance modulus value (omega) at the ith environmental stress level and aging time of 0 and frequency of 0.01Hz, |Z| mi Is the impedance modulus (omega) and theta of the metal substrate at the frequency of 0.01Hz at the ith environmental stress level i Is the reaction constant at the ith ambient stress level; for |Z| mi From 10 5 The starting interval 1 is assigned with value; for each given |Z| m Calculating the sum of squares of errors:
wherein: y is Y ij ＝ln(|Z| tij -|Z| m )，
Wherein->
Ln (|Z|) respectively 0 -|Z| m ),
Least squares estimation of (c); when S is 2 When the minimum value is taken, the |Z| is obtained m Optimal estimate of +.>
Further obtain |Z| 0 ,θ i Estimate of (2)
II, estimating parameters of a reaction constant model
According to equation (4), the following equation is established:
wherein: θ ij C is the reaction constant at the ith temperature stress and the jth chloride ion concentration level i Is the i-th chloride concentration level value (%), T j Is the jth temperature stress level value (°c); according to the aging rate estimated value obtained in the last step
Performing least square fitting according to the formula to obtain the relation between the reaction constant and the temperature stress T:
wherein:
a least square estimated value for the constant a, b, d to be determined;
III, determining the relation between the low-frequency impedance of the coating and environmental factors
Based on the aging kinetic model and the reaction constant model of the organic coating, the low-frequency impedance |Z|of the organic coating at the ambient temperature T and the chloride ion concentration c is obtained by combining the formulas (1), (5) and (8) t Correspondence with aging time t:
step two: establishing environmental profile during service of coating structure
Corresponding natural environment factor data, time data and longitude and latitude, carrying out Kriging interpolation calculation and natural environment modeling, removing trend terms, calculating autocorrelation coefficients, checking the stability of trend term data, solving Kriging equation sets, interpolating the natural environment data, adding trend terms, carrying out error analysis and space-time variation function model selection, and obtaining a space-time distribution model of environmental stress; calling a dacafit function in a dace toolbox by using Matlab, and directly performing Kriging interpolation calculation on the environmental data according to the selected variation function; interpolation error analysis and space-time variation function model selection are key steps of a Kriging interpolation method, and the current space variation function model used for fitting comprises a Gaussian model, a linear model, a spherical model, an exponential model and a circular model; using the half variance as a measure for measuring the space-time correlation degree between each two points, finding out a variation function model which is best fitted with the half variance, wherein the interpolation error brought by the model with the better fitting degree is lower;
after the environmental stress space-time distribution model is obtained, the environmental profile under the actual use condition is obtained by combining the actual use task surface information of the coating-containing structure;
step three: fitting the distribution function of the reaction constant θ
According to the natural environment space-time distribution data obtained in the second step, calculating to obtain a reaction constant theta of each time node in the region by combining the formula (8); for the calculated reaction constant data θ 1 ,θ 2 ,…,θ n Find its maximum value L a And minimum value S m Grouping the data and calculating the group number k and the group distance Δt using formulas (10) and (11):
k＝1+3.3lg n (10)
Δt＝(L a -S m )/k (11)
counting the frequency falling into each group, and taking the reaction constant as an abscissa and the frequency of each group as an ordinate to make a frequency histogram; connecting the straight points into a curve, regarding as an approximation of the distribution density curve, primarily judging what kind of distribution the obtained reaction constant data overall belongs to, and writing out the distribution density function f (theta, eta) of the overall 1 ,η 2 ,…,η n ) Wherein eta 1 ,η 2 ,…,η n For the parameters to be estimated, according to the reaction constant data theta 1 ,θ 2 ,…,θ n Write out eta 1 ,η 2 ,…,η n Likelihood function of (2):
solving the following likelihood equation to obtain an estimated value
Based on the estimated value
Deriving an estimated distribution density function->
Adopting a hypothesis testing method to carry out fitting goodness test and judging whether the selected distribution is reasonable or not;
step four: determining failure threshold of electrochemical parameters of coating according to natural environment data
The corrosion of the metal matrix is related to the environmental temperature, the concentration of the corrosion medium and the corrosion resistance factor of the organic coating, and the greater the conductivity of the corrosion medium is, the higher the environmental temperature is, and the more easily the metal matrix is corroded; the method comprises the steps of measuring corrosion current of a metal matrix and electrochemical parameters of a coating in different environments, namely establishing a relation among environmental factors, the corrosion current of the metal matrix and the electrochemical parameters of the coating, and constructing a relation model among the three; deducing failure threshold value data of electrochemical parameters of the coating in the area range by combining space-time distribution data of environmental stress in the second step with a relation model of environmental factors, metal matrix corrosion current and electrochemical parameters of the coating; the threshold value data of the low-frequency impedance failure of the coating is extremely small, and the maximum value of the threshold value data realized by the low-frequency impedance of the coating is taken as the electrochemical parameter threshold value |Z|| when the organic coating fails th ；
Step five: evaluation of reliability of organic coating using stress-intensity interference theory
The electrochemical performance aging dynamic model and the reaction constant model of the organic coating obtained in the first step are combined with the reaction constant distribution in the third step to obtain the electrochemical performance distribution of the organic coating and the relation between the electrochemical performance distribution and the aging time of the organic coating, and the electrochemical performance distribution and the reaction constant distribution are used as stress distribution of the corrosion resistance of the organic coating; the electrochemical threshold value of the coating failure obtained in the fourth step is used as the strength requirement of the corrosion resistance of the organic coating; describing the reliability of the organic coating under different ageing times through a stress-intensity interference model, and obtaining a calculation formula of the reliability R of the organic coating by defining stress and intensity:
R＝P(|Z| t ＞|Z| th ) (14)
after the reliability R curve of the organic coating is obtained, the reliability life parameter of the organic coating is calculated.
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