Investigating the serve life of common earthed materials based on evaluations of the soil corrosion in offshore areas of East China

The serve life of common earthed materials in power systems is essentially affected by the soil environment, especially the potential corrosion problem of sodium chloride contaminated soil in offshore areas. This study measured the corrosion rates of three common earthed metal materials, Q235 carbon steel, pure Al, and H62 brass, in typical chloride saline soil using accelerated and field corrosion tests. The acceleration ratio and kinetic correlation coefficient under different acceleration conditions were calculated, and the laboratory and field corrosion time correlation models of common earthed metal materials were obtained. The results show a good correlation between soil accelerated and field corrosion tests. Under the same temperature and salinity, the correlation degree of Q235 carbon steel is larger when the water content is 30%. In comparison, the correlation degree of pure Al and H62 brass is larger when the water content is 20%. The laboratory and field corrosion time correlation models of Q235 carbon steel, pure Al, and H62 brass in chloride type saline soil environment are T1=18.32T0, T1=95.56T0, T1=15.78T0, which can effectively evaluate the serve life of common earthed metal materials in typical chloride type saline soil in the offshore area of East China.


Introduction
The grounding device in the power system plays a key role in ensuring the safe operation and maintenance of power equipment [1].In recent years, the power system has continued to develop in the direction of higher voltage and larger capacity, and the requirements for the performance of grounding devices have gradually increased [2,3].Based on satisfying the electrical performance, the anti-soil corrosion capacity of the grounding materials directly determines the operating life of the grounding device [4,5].The soil in the eastern coastal areas of China can be classified as chloride-type coastal saline soil, rich in corrosive components such as chloride, sulfate, and magnesium salts [6][7][8].This soil environment makes grounding metal materials more susceptible to corrosion damage, and the degree and speed of corrosion are usually significantly higher than those in inland areas [9,10].Therefore, studying the service life of grounding metal materials in a chloride-type saline soil corrosion environment is significant.
In the field corrosion test, standard specimens are buried in a typical soil environment, and the corrosion mass loss of the specimens is tested according to a certain buried duration.This basic method can truly reflect the corrosion rate and performance evolution of metal materials in soil.However, the test data are precious due to the long time and high maintenance cost [11].Compared with the field corrosion test, the accelerated corrosion test is an experimental method that can control the test conditions and accelerate the corrosion process through manual intervention.Its main purpose is to evaluate the corrosion trend of materials in specific media in a relatively short period [12].The corrosion acceleration test is based on the premise that the results of the field corrosion test have a good correlation, and the acceleration test with a good correlation has more practical significance [13].
In this study, the corrosion rates of three grounding materials, Q235 carbon steel, pure Al, and H62 brass, in typical chloride-type saline soil in coastal areas of East China were measured based on the soil accelerated corrosion test and field corrosion test results.The laboratory and field corrosion time correlation models of three grounding materials in a chloride-type saline soil corrosion environment were obtained, which provided a theoretical basis for optimizing the depreciation strategy of metal equipment in electric power enterprises.

Materials
The test materials are Q235 carbon steel, pure Al, and H62 brass, and the sample size is 50 mm×25 mm ×2 mm, as shown in Figure 1.After the sample preparation is completed, the sample is ground, de-oiled, cleaned, and dried, weighed with an electronic balance with an accuracy of 0.1 mg, and placed in a dryer for use.  1, and the soil sample can be classified as a low-liquid limit clay (CL).The configured test soil information is shown in Table 2, and the test soil is in Figure 2. The undisturbed soil samples were air-dried and ground in the natural environment and passed through a 0.83 mm sieve.They were placed in an oven at 110℃ for 12 hours and then set aside.To simulate the corrosion of chloride-type saline soil in the coastal areas of East China, according to the 'Code for geotechnical engineering investigation', the NaCl analysis pure and deionized water were configured into 1.5%NaCl contaminated liquid for backup.The dry soil and NaCl contaminated solution were artificially mixed to prepare sodium chloride contaminated soil with 20% and 30% water content, respectively.In Table 2, the oxygen content in 1 # soil is higher than in 2 # soil.
Table 2. Test soil information.

Methods
Figure 3 shows the test process.The accelerated soil corrosion test was conducted in a programmable constant temperature and humidity test chamber.To accelerate metal corrosion in sodium chloride contaminated soil, the relative humidity of the test environment was controlled at 80%, and the temperature was set at 50°C.The metal samples were placed in parallel in the configured sodium chloride contaminated soil.At the same time, the distance from the soil in each direction of the test box was controlled to be 50 mm, and the sample spacing was set to be 25 mm.Three parallel samples were set respectively.Each test period was one month, and samples were taken every three days after the start of the test for weight loss measurement.During the test, the soil moisture content was measured and recorded every 24 hours, and deionized water was added regularly to supplement the water evaporated in the soil during the test.After the corrosion test, the brush was used to remove the number of soil particles attached to the surface of the metal sample.Then, the metal sample was placed in the rust removal cleaning solution prepared according to the specifications.After the rust removal, the surface of the sample was cleaned with deionized water, dried, and weighed.The formula for the rust removal cleaning solution, cleaning time, and cleaning temperature of each metal material are shown in Table 3.
Table 3. Cleaning liquid formula and cleaning parameters.

Field corrosion test
The field corrosion mass loss results of Q235 carbon steel, pure Al, and H62 brass in typical chloridetype saline soil are from the Dagang Material Corrosion Experimental Station of the National Material Environmental Corrosion Network.The specific corrosion weight loss values of the three earthed materials are shown in Table 4.  4 shows that the corrosion rate of the three grounding materials in chloride-type saline soil is Q235 carbon steel > H62 brass > pure Al.In chloride-type saline soil, pure Al is relatively corrosion resistant, and the passivation film on the surface of Al is partially penetrated under the action of Cl-, forming pitting corrosion.H62 brass has obvious dezincification corrosion, and the corrosion rate is large.The corrosion rate of Q235 carbon steel is very high, above the national soil corrosion website standard (V) [14].The poor corrosion resistance of Q235 carbon steel is mainly due to its main components, iron and carbon, and the lack of alloy elements to resist corrosion.Although Q235 carbon steel has high strength and low cost, it is susceptible to corrosion, especially in the corrosive environment of chloride-type saline soil with wet and high salt content.
The power function fitting of the corrosion mass loss of the three grounding materials in the field of 0-3a was carried out by using Equation (1).The fitting results are shown in Figure 4.
Where M is the corrosion mass loss per unit area corrosion (g• dm -2 ).t is the corrosion time ().D and n represent constants.The fitted D values of Q235 carbon steel, pure Al, and H62 brass are 0.384, 0.002 and 0.005, and the fitted n values of three grounding materials are 0.333, 0.793 and 0.894, respectively.

Accelerated corrosion test in the laboratory
Figure 5 shows the corrosion mass loss kinetic curves of Q235 carbon steel, pure Al, and H62 brass under different accelerated conditions for one month.At the same temperature and salinity, with the decrease in water content, the corrosion mass loss of Q235 carbon steel and H62 brass increases by 3.0 and 8.1 times, respectively.In contrast, the corrosion mass loss of pure Al decreases by 1.6 times.The soil water content greatly affects the corrosion rate of Q235 carbon steel and H62 brass.
Soil is a multiphase system composed of solid, liquid, and gas.The physical and chemical properties of soil, such as Cl-content, water content, oxygen content, and temperature, affect soil corrosion.Under the same Cl -content and temperature, the oxygen content of soil with 20% water content is higher than that of soil with 30% water content.Iron in Q235 carbon steel reacts with oxygen and water to form iron oxide, which is common in rust.The corrosion rate of Q235 carbon steel is greatly affected by oxygen content.H62 brass is mainly composed of copper and zinc.The addition of zinc helps to form an oxide film, which can form a protective layer on the surface and prevent further oxidation reactions.Although H62 brass has good corrosion resistance, it is still affected by oxygen content.Especially in the harsh chloride-type saline soil environment, the presence of oxygen and water will promote the oxidation reaction of zinc and copper to form an oxide layer, which affects the appearance and properties of brass to a certain extent.The surface of pure Al will quickly form a dense and uniform oxide film due to the reaction of alumina with oxygen to form alumina. Alumina is a very stable compound with good adhesion and corrosion resistance.This layer of oxide film is formed quickly in the air.Even if it is damaged, it can be regenerated quickly.This oxide layer prevents further oxidation reaction, thereby reducing the oxidation rate of pure Al, and the corrosion rate of pure Al in 20% moisture content soil with higher oxygen content is lower.

Acceleration ratio of accelerated corrosion test in laboratory
The purpose of an accelerated corrosion test is to reduce the time required for the experiment to shorten the test cycle under the same corrosion degree.To measure the effect of the accelerated corrosion test directly, this paper uses Equation ( 2) to calculate the acceleration ratio [15].

𝑎 = 𝑡 0 (𝑀) 𝑡 1 (𝑀)
Where a is the acceleration ratio when the corrosion mass loss reaches M, t0(M) is the test period when the corrosion mass loss reaches M in the field corrosion test, and t1(M) is the test period when the corrosion mass loss reaches M in the accelerated corrosion test.This formula uses the ratio of the nonaccelerated test to the accelerated test cycle as the acceleration ratio of the accelerated corrosion test when the accelerated test and the non-accelerated test are at the same degree of corrosion mass loss.
The acceleration ratio is suitable not only for uniform corrosion but also for non-uniform corrosion and local corrosion.If the relationship between mass loss per unit area and time during the corrosion process satisfies the exponential law of Equation (1), namely: Field corrosion test: Accelerated corrosion test in laboratory: We calculate the acceleration ratio with Equation ( 5): Figure 5 shows that for the accelerated experiment with a soil water content of 20, the corrosion kinetics of Q235 carbon steel and H62 brass changed greatly in one month, and the corrosion kinetics of pure Al changed greatly in the accelerated experiment with a soil water content of 30%.Therefore, the corrosion mass loss was fitted in two time periods of 0-15 d and 0-30 d, respectively, and the two cases of 0-15 d and 0-30 d were calculated when the acceleration ratio was calculated.Through  3)-( 5), the corrosion mass loss of three grounding metal materials under different acceleration conditions was fitted, and the acceleration ratio was calculated under different acceleration conditions.The calculation results in Table 5 show that consistent with the change of accelerated corrosion mass loss, at the same temperature and salinity, Q235 carbon steel and H62 brass accelerated more in 20% water-bearing soil.In comparison, pure Al accelerated more in soil with 30% water content.

Grey correlation method and life prediction
The correlation of corrosion mass loss between the accelerated corrosion test in the laboratory and the field corrosion test is calculated by the grey correlation analysis method.Grey correlation analysis is based on the proximity of the curve shape of each factor sequence to analyse the development trend.It provides a quantitative tool for the development and change trend of the system, which is very suitable for the analysis of dynamic processes [16].The calculation process of the correlation coefficient between the accelerated corrosion test in the laboratory and the field corrosion test results is as follows: (1) The relationship model between corrosion mass loss and time is obtained by fitting the results of accelerated corrosion test in laboratory and field corrosion test, and the acceleration ratio is calculated by Equation ( 5).
(2) The time series t1 of each accelerated corrosion test in the laboratory relative to the field corrosion test is calculated by Equation (6).
Where t1i is the i th data point in the accelerated corrosion test time series.t0i is the i th data point in the time series of field corrosion test.a is the acceleration ratio.
(3) The accelerated corrosion test in laboratory time series t1 is substituted into the M-t model to calculate the corresponding corrosion mass loss sequence of the accelerated corrosion test.
(4) The correlation coefficient between the comparison sequence xi and the reference sequence x0 at each moment can be calculated by Equation (7).
Where  is the resolution coefficient, which takes 0. (5) The correlation coefficient   is calculated by Equation ( 8) as a quantitative representation of the degree of correlation between the comparison sequence and the reference sequence.
ICAMIM-2023 Journal of Physics: Conference Series 2720 (2024) 012007 Based on the field corrosion test time of 365 d, 730 d, and 1095 d, the time series of the corresponding accelerated corrosion test in the laboratory is calculated by Equation ( 6), and the calculation results are shown in Table 6.6 and the fitting results of their corrosion mass loss kinetics, the corrosion mass loss is calculated, and the corresponding comparison sequence is obtained.The data are normalized, and the correlation coefficients of accelerated tests of three kinds of grounding metal materials under different conditions are calculated by Equations ( 7) and (8).Table 7 shows that under the same temperature and salinity, the correlation coefficient of Q235 carbon steel in the accelerated corrosion test of higher water content soil is higher, reaching 0.940.The kinetic correlation coefficients of pure Al and H62 brass in the accelerated corrosion test of soil with low water content were higher, reaching 0.830 and 0.934, respectively. 1 = 0.649 According to the correlation analysis, the accelerated corrosion test in the laboratory can effectively simulate the corrosion of Q235 carbon steel, pure Al, and H62 brass in a chloride-type saline soil environment.By fitting the corrosion data with a high correlation coefficient, the corrosion weight loss function of Q235 carbon steel is M = 0.973T 0.358 , the corrosion weight loss function of pure Al is M = 0.009T 0.670 , and the corrosion weight loss function of H62 brass is M = 0.047T 1.112 .The corrosion weight loss of the accelerated soil corrosion test is equal to the corrosion weight loss of the field corrosion test.The laboratory and field corrosion time-related time models of Q235 carbon steel, pure Al, and H62 brass in chloride type saline soil environment are T1 = 18.32T0,T1 = 95.56T0, and T1 = 15.78T0(T0 is the field soil corrosion time, a; T1 is the accelerated soil corrosion test in laboratory time, d).
The laboratory and field corrosion time correlation model can be used to predict the natural corrosion of Q235 carbon steel, pure Al, and H62 brass in a chloride-type saline soil environment.For example, the accelerated corrosion test of Q235 carbon steel in a chloride-type saline soil environment for 18.32 IOP Publishing doi:10.1088/1742-6596/2720/1/0120079 days can predict the corrosion weight loss kinetic characteristics of Q235 carbon steel in chloride-type saline soil environment in coastal areas of East China after 1 year, which has certain reference value for power enterprises.

Conclusion
(1) In this study, NaCl is added to the soil to simulate the corrosion environment of chloride-type saline soil in the coastal areas of East China.Controlling temperature, relative humidity, and salinity to accelerate the corrosion of metals in the soil can better simulate the corrosion weight loss characteristics of metal materials in saline soil environments.
(2) The water content of soil takes an essential role in the soil corrosion environment.In the correlation analysis of the accelerated corrosion test in the laboratory, under the same temperature and salinity, the kinetic correlation coefficient of Q235 carbon steel is larger when the water content is 30%.In comparison, the kinetic correlation coefficients of pure Al and H62 brass are larger when the water content is 20%.
(3) Through the field corrosion weight loss data and the accelerated soil corrosion test in laboratory weight loss fitting function under high correlation experimental conditions, the laboratory and field corrosion time correlation models of three commonly used earthed metal materials of Q235 carbon steel, pure Al and H62 brass in the typical chloride type saline soil environment in the coastal area of East China are obtained.It is divided into T1=18.32T0,T1=95.56T0, and T1=15.78T0,which provides a theoretical basis for the depreciation strategy of metal equipment in power enterprises.

Figure 1 .
Figure 1.Metal samples.The test soil was sampled from a site in Baoshan District, Shanghai.No gravel or other interference factors met the standard requirements of the China Soil Corrosion Test Network.The basic physical properties are shown in Table1, and the soil sample can be classified as a low-liquid limit clay (CL).Table1.Basic physical properties of test soil.

Figure 4 .
Figure 4. Mass loss curve of field corrosion test of grounding materials.

Figure 5 .
Figure 5.The mass loss curve of accelerated corrosion test of grounding materials (a) Q235 carbon steel; (b) Pure Al; (c) H62 brass.
) −   ()| is the minimum value of the absolute difference between the comparison sequence and the reference sequence.    | 0 () −   ()| is the maximum value of absolute difference.

Table 1 .
Basic physical properties of test soil.

Table 4 .
Mass loss of earthed materials in the field corrosion test.

Table 5 .
D1, n1, M value of Q235 carbon steel, pure Al, and H62 brass in an accelerated corrosion test.

Table 6 .
Different experimental conditions correspond to the time series of field corrosion tests.

Table 7 .
Dynamic correlation coefficients under different experimental conditions.