CN113553784B - Organic coating life assessment method in seawater full immersion environment - Google Patents

Abstract

The invention provides a method for evaluating the life of an organic coating in a seawater full-immersion environment considering the influence of temperature and chloride ions. The implementation steps are as follows: 1: Establish an aging model of an organic anti-corrosion coating; 2: Establish an environmental profile of the coating structure during service; Three: Fit the distribution function of the reaction constant θ; Four: Determine the failure threshold of coating electrochemical parameters according to the natural environment data; Five: Utilize the stress-intensity interference theory to evaluate the reliability of organic coatings; through the above steps, the integrity of the present invention The qualitative logic relationship and quantitative model method for the reliability evaluation of organic coatings are established by the accelerated aging test of coatings and the correlation model of environmental factors; the definition of life and reliability of organic coatings is realized, and the failure criterion of organic coating systems It is more timely, accurate and reasonable; the measurement parameters of the coating structure in the present invention are electrochemical parameters, the test method is simple, easy to operate, and has no destructive effect on the structure.

Description

A method for evaluating the life of organic coatings in a seawater immersion environment

Technical Field

The present invention provides a method for evaluating the life of an organic coating in a seawater immersion environment, that is, a method for evaluating the life of an organic coating in a seawater immersion environment taking into account the influence of temperature and chloride ions. The method relates to a method for evaluating the time-varying life of an organic anticorrosive coating based on polyurethane in a seawater immersion environment. The method is based on electrochemical testing, stress-intensity interference theory and organic

The life assessment method of organic anticorrosion coating based on the coating aging kinetics model is based on the accelerated aging test data of the organic anticorrosion coating system, the corrosion current change data of the base metal under the coating, and the corrosion environment factors. It comprehensively establishes the correlation model of accelerated test, coating degradation and environmental factors to describe the qualitative logical relationship and quantitative model method of the degradation process of organic coatings. It maps the aging state of the organic anticorrosion coating to its impedance characteristics, which is convenient for the identification of the coating aging state and further reliability assessment. It is suitable for the field of life assessment of coating structures with polyurethane as the main coating material serving in a full immersion environment in seawater.

Background Art

Organic coatings have been successfully applied in various metal structures as an effective corrosion protection measure, but so far, the research on coating failure has not been in-depth. Statistical analysis shows that the failure or accident caused by corrosion damage to equipment structures is mainly caused by the failure of the coating's anti-corrosion function. Organic coatings will inevitably have some defects during the coating process, and smaller water molecules, ions and oxygen will pass through these defects to reach the metal substrate. Since the defects in the coating are not evenly distributed, there are differences in the order and degree of damage at the coating-metal interface, and this difference will lead to the formation of corrosion micro-batteries. When the coated metal undergoes electrochemical corrosion, the cathode reaction or the cathode reaction product will affect the bonding between the coating and the base metal, causing the coating to separate from the base metal. In addition, the weak parts of the polymer chemical structure bonds in the coating will break into free radicals under the action of different external factors, triggering free radical reactions, leading to the destruction of the coating molecules and gradually losing the original protective performance, thereby causing the base metal to be severely corroded and fail. Studying protective coating tests under corrosive conditions, determining the service life and reliability of coatings, and formulating reasonable coating maintenance or re-spraying cycles are key to ensuring that the substrate structure reaches the specified calendar service life.

Existing evaluation tests for the aging status of organic anti-corrosion coatings usually only include gloss loss rate, discoloration degree, differentiation and cracking degree, bubble density, and peeling area. They are unable to timely and accurately evaluate the anti-corrosion performance of organic anti-corrosion coatings and have low accuracy. Studies have found that the decline in the anti-corrosion performance of organic anti-corrosion coatings is due to changes in the penetration of water molecules and ions inside them, which causes local or overall degradation of the electrochemical properties of the coatings and the ability to isolate the corrosive environment, resulting in a decline in the anti-corrosion performance of the coatings. Therefore, the degradation model of the coatings in different environments is obtained by analyzing the environmental factor data and the electrochemical performance degradation curve of the coatings, and the distribution of the electrochemical performance parameters of the coatings is obtained. Then, based on the electrochemical performance threshold of the coating with normal anti-corrosion performance under a corrosive environment, a reliability calculation method for organic anti-corrosion coatings is obtained.

Based on this, the present invention combines the accelerated aging test data of organic anti-corrosion coatings and environmental factor data, and proposes a method for evaluating the life of organic coatings in a seawater immersion environment that considers the influence of temperature and chloride ions, thereby realizing the reliability definition and life evaluation of organic coatings.

Summary of the invention

(1) Purpose of the invention:

The purpose of the invention is to provide a method for evaluating the life of an organic coating in a seawater immersion environment, that is, a method for evaluating the life of an organic coating in a seawater immersion environment taking into account the influence of temperature and chloride ions. It is aimed at the problem that the organic coating structure deteriorates seriously in a harsh service environment and there is no accurate and reasonable detection and reliability evaluation method. A method for evaluating the life of an organic coating is provided. It is an organic coating failure judgment and reliability evaluation method that includes analysis of electrochemical performance aging laws, environmental factors, and analysis of the relationship between the electrochemical performance of the coating and the anti-corrosion performance. A model of electrochemical parameters and environmental factors is established by measuring the degradation curve of the electrochemical impedance modulus of the coating under different environments, and then according to the corresponding relationship between the electrochemical parameters and the anti-corrosion performance of the organic coating, the reliability and service life of the organic coating under different seawater immersion environments and service times are determined.

(2) Technical solution:

The present invention needs to establish the following basic settings:

Setting 1 The low-frequency impedance modulus of the organic anti-corrosion coating is an electrochemical performance parameter that affects the anti-corrosion performance of the organic anti-corrosion coating. The failure of the organic coating is due to the contact of water molecules, ions and oxygen-type corrosive media with the metal substrate through the coating, resulting in a decrease in the coating impedance and a decrease in the anti-corrosion effect;

Setting 2 The low-frequency impedance of the organic coating is the aging characteristic parameter of the organic coating. Its quantitative relationship with the aging time obeys the aging kinetics formula:

Where: t is the accelerated aging test time (h), |Z| _t is the impedance modulus of the coating at the aging time t and frequency 0.01Hz (Ω), |Z| ₀ is the impedance modulus of the coating at the aging time 0 and frequency 0.01Hz (Ω), |Z| _m is the impedance modulus of the metal substrate at a frequency of 0.01Hz (Ω), θ is the reaction constant, and its size is related to the coating characteristics and the severity of the aging environment;

Setting 3: The relationship between θ and thermodynamic temperature T obeys the Arrhenius model:

Where: Z is a constant greater than 0, k is the Boltzmann constant, T is the absolute temperature (K), and E _a is the activation energy (J);

Setting 4: θ and chloride ion concentration c obey Fick's second law. According to the relationship between diffusion velocity and chloride ion concentration c, we get:

θ＝α·c ^β (3)

Where: α is a constant greater than 0, β is the reaction exponent.

In summary, the relationship model between θ, thermodynamic temperature T and chloride ion concentration c is obtained:

θ＝a·c ^b ·e ^-d/T (4)

In the formula: a, b, d are constants to be determined;

Setting 5: Before the low-frequency impedance of the organic anti-corrosion coating drops to a certain threshold, its excellent anti-corrosion performance perfectly isolates the metal substrate from the corrosive medium. In this case, the metal substrate will not produce electrochemical corrosion, and the organic anti-corrosion coating is considered to have normal anti-corrosion function.

The method proposed in the present invention mainly includes analyzing the aging law of the electrochemical performance of the organic anti-corrosion coating, establishing the electrochemical parameters and environmental factor model of the organic anti-corrosion coating, determining the failure threshold of the coating, establishing the distribution of coating degradation parameters based on the external natural environment data, and performing reliability estimation based on the stress-strength interference model;

Based on the above basic settings, the present invention proposes a method for evaluating the life of an organic coating in a seawater immersion environment, that is, a method for evaluating the life of an organic coating in a seawater immersion environment taking into account the influence of temperature and chloride ions, which is characterized by: it is achieved through the following steps:

Step 1: Establish an aging model for organic anticorrosive coatings

Firstly, the unknown parameters in the aging kinetics model (1) are estimated based on the accelerated aging test data of the organic anti-corrosion coating, and the variation patterns of the electrochemical parameters of the coating with time under the natural environment are interpolated; secondly, the data pairs of reaction constants and environmental factors in the aging kinetics model are established, and the reaction constant model (4) is fitted accordingly; further, the quantitative correspondence between the electrochemical parameters of the organic anti-corrosion coating and the aging time and environmental factors is obtained according to the reaction constant model;

The specific steps are:

I. Estimation of aging kinetic model parameters

According to formula (1), the following equation is established:

Wherein: t _ij is the time of the jth detection under the i-th environmental stress level (h), |Z| _tij is the impedance modulus of the organic coating measured at 0.01 Hz frequency at time t _ij (Ω), |Z| _0i is the impedance modulus of the coating when the aging time is 0 and the frequency is 0.01 Hz under the i-th environmental stress level (Ω), |Z| _mi is the impedance modulus of the metal substrate at the i-th environmental stress level at the frequency of 0.01 Hz (Ω), θ _i is the reaction constant under the i-th environmental stress level; for |Z| _mi , it is assigned a value at intervals of 1 starting from 10 ⁵ ; for each given |Z| _m , the sum of squared errors is calculated:

其中

分别为

的最小二乘估计值；当S²取最小值时，得到|Z|_m的最优估计值

进一步得到|Z|₀,θ_i的估计值

In the formula: Y _ij =ln(|Z| _tij -|Z| _m ),

in

They are

The least squares estimate of |Z| _m is obtained when S ² takes the minimum value.

We can further obtain the estimated value of |Z| ₀ ,θ _i

II. Estimation of Reaction Constant Model Parameters

According to formula (4), the following equation is established:

按上式进行最小二乘拟合，得到反应常数与温度应力T的关系：Where: θ _ij is the reaction constant under the i-th temperature stress and the j-th chloride ion concentration level, c _i is the i-th chloride ion concentration level value (%), T _j is the j-th temperature stress level value (℃); Based on the aging rate estimate obtained in the previous step

According to the above formula, the least squares fitting is performed to obtain the relationship between the reaction constant and the temperature stress T:

为待确定常数a，b，d的最小二乘估计值；Where:

is the least squares estimate of the constants a, b, d to be determined;

III. Determine the relationship between the low-frequency impedance of the coating and environmental factors

Based on the aging kinetics model and reaction constant model of organic coatings, combined with equations (1), (5), and (8), the corresponding relationship between the low-frequency impedance |Z| _t of the organic coating and the aging time t under ambient temperature T and chloride ion concentration c is obtained:

Step 2: Establish the environmental profile of the coating structure during service

Corresponding the natural environmental factor data, time data and longitude and latitude, Kriging interpolation calculation and natural environment modeling are carried out. By removing the trend term, calculating the autocorrelation coefficient, testing the stability of the detrended data, solving the Kriging equations, interpolating the natural environmental data and adding the trend term, error analysis and spatiotemporal variogram model selection steps are performed to obtain the spatiotemporal distribution model of environmental stress. Currently, there are many methods to implement the Kriging spatiotemporal interpolation method, such as using Python, Java, Matlab, R language, and ArcGIS. Taking Matlab as an example, the dacefit function in the dace toolbox is called to perform Kriging interpolation calculation on the environmental data directly according to the selected variogram. Interpolation error analysis and the selection of spatiotemporal variogram models are the key steps of the Kriging interpolation method. The spatial variogram models currently used for fitting include Gaussian model, linear model, spherical model, exponential model, and circular model. The semivariance is used as a measure of the spatiotemporal correlation between points to find the best fitting variogram model. The better the fitting degree of the model, the lower the interpolation error.

After the spatiotemporal distribution model of environmental stress is obtained, the environmental profile under actual use conditions is obtained by combining the actual use mission information of the coated structure. For example, according to the route and stay time of the ship in a certain sea area, the spatiotemporal interpolation model of the natural environment data in the sea area is used to statistically obtain the environmental profile experienced by the ship during navigation;

The above-mentioned “Python, Java” refers to a computer programming language used in scientific computing and statistics; the above-mentioned “Matlab” refers to a commercial mathematical software used for data analysis and matrix operations; the above-mentioned “R language” refers to a language and operating environment used for statistical analysis and drawing; the above-mentioned “ArcGIS” refers to a technical system that, with the support of computer hardware and software systems, collects, stores, manages, calculates, analyzes, displays and describes relevant geographical distribution data in the entire or part of the earth's surface (including the atmosphere) space;

The above-mentioned "dacefit function in the dace toolbox" refers to a Matlab software operation program for establishing a Kriging model based on existing experimental data points;

Step 3: Fitting the distribution function of the reaction constant θ

According to the spatiotemporal distribution data of the natural environment obtained in step 2, the reaction constant θ at each time node in the region is calculated in combination with formula (8); for the calculated reaction constant data θ ₁ ,θ ₂ ,…,θ _n , the maximum value _La and the minimum value S _m are found, and the data are grouped using formulas (10) and (11) to calculate the number of groups k and the group distance Δt:

k＝1+3.3lgn (10)

Δt＝(L _a −S _m )/k (11)

Count the frequencies falling into each group, and make a frequency histogram with the reaction constant as the horizontal axis and the frequency of each group as the vertical axis; connect the midpoints of each histogram into a curve, which is regarded as an approximation of the distribution density curve. Preliminarily judge what kind of distribution the reaction constant data belongs to, and write out the distribution density function of the population f(θ,η ₁ ,η ₂ ,…,η _n ), where η ₁ ,η ₂ ,…,η _n are the parameters to be estimated. According to the reaction constant data θ ₁ ,θ ₂ ,…,θ _n, write out the likelihood function of η ₁ ,η ₂ ,…,η _n :

Solve the following likelihood equation to get the estimated value

得出估计的分布密度函数

采用假设检验的方法进行拟合优度检验，判断所选分布是否合理；According to the estimated value

Get the estimated distribution density function

Use hypothesis testing method to conduct goodness of fit test to determine whether the selected distribution is reasonable;

Step 4: Determine the failure threshold of coating electrochemical parameters based on natural environment data

The corrosion of the metal substrate is closely related to the environmental temperature, the concentration of the corrosive medium and the anti-corrosion performance of the organic coating. In general, the greater the conductivity of the corrosive medium and the higher the environmental temperature, the easier it is for the metal substrate to be corroded. By measuring the corrosion current of the metal substrate and the electrochemical parameters of the coating under different environments, the relationship between the environmental factors, the corrosion current of the metal substrate and the electrochemical parameters of the coating is established, and a relationship model between the three is constructed. Combined with the spatiotemporal distribution data of the environmental stress in step 2 and the relationship model between the environmental factors, the corrosion current of the metal substrate and the electrochemical parameters of the coating, the failure threshold data of the electrochemical parameters of the coating within the region are derived. In general, the range of the failure threshold data of the low-frequency impedance of the coating is relatively small, and the maximum value of the threshold data of the low-frequency impedance of the coating is taken as the electrochemical parameter threshold |Z| _th when the organic coating fails.

Step 5: Reliability evaluation of organic coatings using stress-strength interference theory

The electrochemical performance aging kinetics model and reaction constant model of the organic coating obtained in step one are used in combination with the reaction constant distribution in step three to obtain the electrochemical performance distribution of the organic coating and its relationship with the aging time as the stress distribution of the organic coating's anti-corrosion performance; the coating failure electrochemical threshold obtained in step four is used as the strength requirement of the organic coating's anti-corrosion performance; the stress-strength interference model is used to describe the reliability of the organic coating at different aging times, and the organic coating reliability R calculation formula is obtained from the definition of stress and strength:

R＝P(|Z| _t ＞|Z| _th ) (14)

After obtaining the reliability R curve of the organic coating, the reliable life parameters of the organic coating are calculated;

Through the above steps, the present invention holistically establishes the qualitative logical relationship and quantitative model method of the accelerated aging test of the coating and the correlation model of environmental factors for the reliability evaluation of the organic coating; realizes the definition of the life and reliability of the organic coating, and makes the failure criterion of the organic coating system more timely, accurate and reasonable; the present invention measures the coating structure parameters as electrochemical parameters, and the test method is simple, easy to operate, and non-destructive to the structure.

(3) Advantages and effects: The present invention is a method for evaluating the life of an organic coating in a seawater immersion environment taking into account the influence of temperature and chloride ions. The advantages are:

① The present invention establishes the qualitative logical relationship and quantitative model method of coating accelerated aging test and environmental factor correlation model for organic coating reliability evaluation;

② The present invention realizes the definition of the life and reliability of organic coatings, making the failure criterion of organic coating systems more timely, accurate and reasonable;

③ The present invention measures the coating structure using electrochemical parameters, and the test method is simple, easy to operate, and non-destructive to the structure;

④This evaluation method is scientific, has good processability, and has broad promotion and application value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a flow chart of the method of the present invention.

Figure 2. Distribution of reaction constant data in the case.

Figure 3 shows the reliability curves of each coating in the case.

DETAILED DESCRIPTION

The present invention will be further described in detail below with reference to examples.

A certain organic coating structure uses a polyurethane series coating as the topcoat, and the overall thickness of the coating system is about 80μm. The polyurethane coating system was immersed in 8 test environments. The specific test conditions and low-frequency impedance data records during the coating aging process are shown in Table 1.

Table 1 Immersion test data of polyurethane coating system

The present invention provides a method for evaluating the life of an organic coating in a seawater immersion environment, that is, a method for evaluating the life of an organic coating in a seawater immersion environment taking into account the influence of temperature and chloride ions, as shown in FIG1 , and is implemented by the following steps:

Step 1: Establish an aging model for organic anticorrosive coatings;

I. Estimation of aging kinetic model parameters

First, determine the unknown parameter |Z| _m ; for |Z| _m , assign values to it at intervals of 1 starting from 10 ⁵ , and for each given |Z| _m , calculate the sum of squared errors; when the value of the sum of squared linear regression residuals S ² is the smallest under different |Z| _m values, the |Z| _m values in different tests are obtained; on this basis, the least squares method is used to fit the low-frequency impedance modulus value of the coating and the immersion time in Table 1, and the results are shown in Table 2, and R ² is the goodness of fit;

Table 2 Aging kinetic model parameters and immersion time fitting results

II. Estimation of Reaction Constant Model Parameters

值与对应的T_i和c_i进行最小二乘拟合，计算得到老化速率估计值

与温度T及氯离子浓度c的关系为：The results in Table 2

The aging rate estimate is calculated by performing the least squares fitting with the corresponding _Ti and _ci .

The relationship with temperature T and chloride ion concentration c is:

R ² is 0.9813, which means that the fitting surface has a good fitting result for the observed value;

III. Determine the relationship between the low-frequency impedance of the coating and environmental factors

According to the fitting equation obtained by the above calculation method, the estimated values of the low-frequency impedance modulus of the coating corresponding to different immersion times are extrapolated, and the intermediate reaction constant θ is eliminated. The corresponding relationship between the low-frequency impedance modulus of the coating |Z| _t (Ω) and the immersion time t (h) under the conditions of temperature T (℃) and chloride ion concentration c (%) is obtained as follows:

Step 2: Establish the environmental profile of the coating structure during service;

The natural seawater environment data of the South my country Sea from 2015 to 2020 were selected from the World Ocean Database. The seawater temperature and salinity data, time data and longitude and latitude in the natural seawater environment were matched. The longitude and latitude of 0.5° were used as the interpolation accuracy to carry out Kriging interpolation calculation and natural environment modeling. Matlab software was used to call the dacefit function in the dace toolbox to perform Kriging interpolation calculation on the environmental data according to the selected variogram. Through error analysis, the 0-order exponential spatiotemporal variogram model was selected to obtain the spatiotemporal distribution data of environmental stress. Due to the annual repetitive nature of natural environmental data, the natural seawater environmental data was integrated into 1 year. Taking the route from Sanya to Xisha as an example, the route is roughly connected by nine points (18°, 108°), (18°, 109°), (18°, 110°), (18°, 111°), (18°, 112°), (18°, 113°), (17°, 113°), (16°, 113°), and (15°, 113°); according to the geographical location of this route, a total of 3285 data points in one year at nine points on this route are selected as the natural seawater environment data background;

Step 3: Fitting the reaction constant θ distribution function;

According to the reaction constant data calculated from the natural environment spatiotemporal distribution data obtained in step 2, find out its maximum value _La = 520 and minimum value _Sm = 110, use formulas (10) and (11) to group the data and calculate the number of groups k = 13 and the group interval Δt = 31.5;

Count the frequencies that fall into each group, and make a frequency histogram with the reaction constant as the horizontal axis and the frequency of each group as the vertical axis;

Table 3 Reaction constant data table

According to the curve formed by connecting the midpoints of each histogram, it is preliminarily judged that the overall reaction constant data belongs to the mixed Weibull-normal distribution, and its overall distribution density function is:

Among them, a, m, η, γ, μ, σ are the parameters to be estimated. According to the reaction constant data, write the likelihood function of a, m, η, γ, μ, σ, solve the likelihood equation, and get the estimated value

Table 4 Parameter estimation list

The Pearson ^χ2 test method was used to conduct a goodness of fit test and establish the null hypothesis:

Then calculate the observed values of the probability that the variable θ falls into the i-th interval respectively:

的观测值为：Where: a _i is the endpoint of the i-th interval. Finally, the statistic is calculated

The observed values are:

Where: _ni is the frequency of the ith interval, n is the sample data size;

由于

所以接受原假设，即认为反应常数数据总体属于混合威布尔-正态分布；Taking the significance level α = 0.01, the critical value is

because

Therefore, the null hypothesis is accepted, that is, the reaction constant data are considered to belong to the mixed Weibull-normal distribution;

Table 5 Calculation of goodness of fit test

Step 4: Determine the failure threshold of the coating electrochemical parameters based on natural environment data;

The self-corrosion current i _CORR is selected as the characteristic quantity to characterize the corrosion of the metal matrix. According to the empirical relationship model between the 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)

Where i _CORR is the self-corrosion current density of the steel substrate (μA/cm ² ), c is the chloride ion concentration (%), T is the surface temperature of the metal substrate (K), R _c is the surface resistance of the metal substrate (Ω), and t is the corrosion time (years);

The self-corrosion current density of the steel substrate in the initial stage of immersion in the electrolyte when the coating fails is set to 1μA/ ^cm2 , and the surface resistance of the metal substrate is approximated as the low-frequency impedance modulus of the coating. The low-frequency impedance threshold value |Z| _th of the coating failure is calculated by combining the spatiotemporal distribution data of the natural environment obtained in step 2, and its maximum value _La = 914722.31 and minimum value _Sm = 913034.92 are found. Since the difference between the maximum value and the minimum value of |Z| _th is small, the maximum value of 914722.31 is taken as the low-frequency impedance threshold value |Z| _th of the coating failure in the sea area;

Step 5: Use stress-strength interference theory to evaluate the reliability of organic coatings;

For the polyurethane system coating and metal steel substrate in this example, the reliability under different aging times is described by the stress-strength interference model. According to the distribution of the reaction constant θ and the determined low-frequency impedance threshold of coating failure |Z| _th , the calculation formula of the polyurethane coating reliability R is obtained from the definition of stress and strength:

According to this reliability calculation formula, the reliability life t(R) of the organic coating is calculated;

The results show that the method of the present invention can estimate the reliability of the organic anti-corrosion coating by analyzing the changes in the electrochemical parameters of the organic coating with time and environmental factors, thereby achieving the desired purpose.

In summary, the present invention provides a method for evaluating the life of an organic coating in a seawater full immersion environment, that is, a method for evaluating the life of an organic coating in a seawater full immersion environment considering the influence of temperature and chloride ions, which relates to a method for evaluating the life of an organic coating in a seawater full immersion environment considering the influence of temperature and chloride ions; it integrally establishes a qualitative logical relationship and a quantitative model method for evaluating the reliability of an organic coating between a coating accelerated aging test and an environmental factor correlation model; the aging state of the organic coating's anti-corrosion performance is mapped to the coating's electrochemical parameters, which is convenient for modeling and calculation; the specific steps of the method are: 1. Establishing an aging model for an organic anti-corrosion coating; 2. Establishing an environmental profile of the coating structure during service; 3. Fitting the distribution function of the reaction constant θ; 4. Determining the failure threshold of the coating's electrochemical parameters based on natural environmental data; and 5. Using stress-strength interference theory to evaluate the reliability of an organic coating; the present invention is suitable for the field of reliability evaluation of coating structures with polyurethane or as the main coating material serving in a seawater full immersion environment, has the characteristics of a simple testing method, easy operation, and no destructiveness to the structure, and has broad value for promotion and application.

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.

Images