Influence of Xinjiang Saline Soil on Corrosion of X80 Pipeline Steel

A large-scale gas transmission pipeline project will pass through typical saline soil areas in Xinjiang, China, to find out the influence of the saline soil on the long-distance pipeline in this area, this paper investigates the corrosive behavior of X80 pipeline steel embedded in saline soil with multiple corrosion intensities. Firstly, NaCl, Na2SO4, NaHCO3, and deionized water were used to simulate the saline soil with different corrosion intensities, and then the corrosion test of X80 steel was carried out. A scanning electron microscope was used to observe the X80 steel before and after corrosion. Finally, the weight loss method was used to calculate the corrosion rate, and the corrosive behavior of X80 pipeline steel in Xinjiang saline soil environment was analyzed. The results show that: (1) X80 steel is susceptible to pitting corrosion in the saline soil environment; (2) the rate and level of corrosion were mainly related to the content of Cl− and SO4 2−. The greater the content, the stronger the pipeline is corroded; (3) the rate and level of corrosion were positively correlated with the corrosion intensities of saline soils.


Introduction
Soils containing more than 0.3% soluble salts and causing engineering problems such as collapsibility, salt expansion, and corrosion are called saline soils, which are categorized as special soils for corroding metal constructions [1].Corrosion caused by saline soils is mainly due to the presence of sulfates and chlorides.High levels of salts in saline soils will result in chemical corrosion of gas transmission pipelines, especially the chloride ions in the soil which produce strong corrosion on the metal.Many experiments have shown that the stress corrosion cracking of pipeline steel is often seen in the saline soil environment, which will produce serious damage to the oil and gas pipelines, or even oil and gas leakage, resulting in large economic losses.The alkaline soil in Xinjiang, Gansu, and other parts of China, containing high levels of salts and causing serious corrosion to metal materials, is where pipeline steel corrosion is most likely to occur [2].
This paper intends to test the corrosive behavior of X80 steel in the simulated solution of saline soil and obtain the surface morphologies of the X80 specimen using FEI-Quanta TM250 scanning electron microscope after removing corrosion products, to provide data and theoretical basis for the subsequent construction of pipeline in Xinjiang's saline soil areas.

Research Status of Saline Soil Corrosion on Pipeline Steel
Pitting is a form of corrosion that the "rust holes" on the surface of steel that develop rapidly and form pits. Before the pitting corrosion occurs, there are usually metastable pits, a few microns in diameter, appearing on the oxidation film which protects the surface of the stainless steel [3][4].Pitting is a dangerous localized corrosion that may penetrate the mass of the steel, and generally, the degree of pitting corrosion can not be evaluated by the amount of weight loss.
The corrosive behavior of pipeline steel in a saline soil environment has been studied by domestic and foreign scholars.Wang et al. [5] statistically analyzed the properties of saline soils along the West-East Natural Gas Transmission Project and the results showed that saline soils display undesirable properties such as melt sinking, salt swelling, and corrosive effects.Liu et al. [6] reviewed studies on the stress corrosion cracking of pipeline steels at home and abroad in the corresponding soil environments.Wang et al. [7] used the orthogonal test method to study and derive the corrosive behavior law of X80 steel under environments of different temperatures, pressures, and flow rates; Wang et al. [8] investigated the corrosive behavior of X80 steel in a simulated solution of soils in Shanshan, Xinjiang, by adopting the weight loss method and electrochemical testing; Xie et al. [9] conducted orthogonal tests to analyze the effects of environmental factors such as temperature, the pH value of solutions, and the content of dissolved oxygen (DO) on the electrochemical behavior; using SEM and EDS methods, Zhang and Wang [10][11] investigated the corrosion behavior of X80 steel in simulated solution of Shanshan soil; Zhang and Xie et al. [12][13] investigated the effect of changes in the concentrations of Cl -and SO4 2-on the corrosion behavior of X80 steel and its welds in simulated solutions of soil in Korla, Xinjiang; Li[14] investigated the corrosive behavior of X100 steel after 6, 10, 30, and 50 days of immersion in simulated solution of soil in Korla by conducting scanning electron microscopy observation and energy spectrum analysis; Tan et al. [15] investigated the effect of localized corrosion pits on the localized corrosion process by using scour corrosion experimental setup with high shear stress; Zhu and Jia [16] conducted slow strain rate test (SSRT) to study the effect of additional potential on the stress corrosion cracking (SCC) behavior of X80 steel and its welds in simulated solution of Lunnan soil; Liu [17] analyzed the corrosive electrochemical behavior of X70 steel in soil with different water contents and temperatures; Chong et al. [18] analyzed the corrosive behavior of pipeline steels in saline soils by focusing on factors such as electrical resistivity, salinity, and chloride ion content; Gu [19] studied the influence of genesis environment and salt types on the properties of saline soil; Tang et al. [20] studied the effect of sulfate ions in simulated concrete void solution on the corrosion of carbon steel.
In general, the current research on X80 steel at home and abroad is mostly focused on the high pH-SCC environment simulation solution and laboratory standard test solution NS4 (near-neutral pH-SCC test commonly used simulation solution), while the corrosive behavior of high-strength pipeline steel in the actual soil environment in China has not been systematically studied.

Experiment Procedure
X80 steel, which is widely applied in the building of oil and gas transmission pipelines, was selected as the object of the experiment.X80 steel contains elements such as C, Mn, Si, P, S, Cr, Nb, Ni, V, Ti, Cu, B, and Al; it has mechanical properties under room temperature as follows: tensile strength of 703 MPa, yield strength of 664 MPa, flexural strength ratio of 0.94, elongation of 26%.The specimens were processed into 40 mm×20 mm×20 mm size for weight loss test and corrosion morphology observation.
Before being immersed in simulated soil solution, the X80 steel block was polished with increasing grade of SiC sandpaper, then degreased in an acetone solution and anhydrous ethanol, cleaned with deionized water, and dried [21].Then the specimens were subjected to an immersion test, and the parallel specimens were taken out at 10d, 20d, 30d, 40d, 50d, and 60d, respectively.The weight loss method was adopted to calculate the corrosion rate.At the same time, the FEI-Quanta TM250 scanning electron microscope was used to observe and record the surface morphology of the specimens after removing the corrosion product layer after 30d and 60d of immersion.
According to the soil sample testing and analysis in the geotechnical engineering survey report along the pipeline, the primary salts in the soil are NaCl, and Na2SO4, followed by NaHCO3, and Na2CO3.Based on the Technical Code for Building in Saline Soil Regions (GB/T50942), the corrosion intensity of the soils was divided into four types, extra strong, strong, medium, and weak, representing four levels of corrosion in this orthogonal experimental design.The contents of soluble salts in the simulated solution, as shown in table 1, were determined according to the physicochemical data of the saline soils [10,21,22].In this experiment, the saline soil solution was used to simulate the saline soil environment, and the effects of humidity, electrical conductivity, and other soil properties on the pipeline were not considered.The polished and cleaned X80 steel specimens were put into the simulated solution, with their immersion periods, temperatures, and corrosion intensity as variables of the orthogonal experiment.The immersion periods were 30d and 60d respectively and the temperatures were controlled at 20℃ and 30℃ according to the environment where the pipelines were constructed.There are four groups of environment samples with 14 specimens in each group.The corrosive behavior of X80 steel was analyzed according to the results of the test (as shown in table 2).
a. Specimen temperature control.

Experiment Phenomenon and Corrosion Morphologies
After 10d of immersion, it is clearly observed that a layer of loose, porous, and brown-yellow corrosion products was generated on the surface of the X80 steel [23] (as shown in figure 2) and part of the layer has been peeled off, and the simulated solution exhibits the color of yellow-brown (as shown in figure 3).The same procedure was performed on specimens of 60d corrosion, and the results are shown in figure 5.
The photos taken by scanning electron microscope (SEM) were analyzed.After 30 days of immersion in the simulated saline soil solution, the surface of the X80 steel has been completely corroded and large pits can be seen on part of the specimen.The corrosion products showed a dense layer structure and the inner layer, uniform and dense, was closely combined with the steel block.The outer layer of corrosion products presented clusters and the corrosion was more obvious.There were irregular cracks on part of the outer layer.According to the results of XDR experiments [24], the corrosion products of the steel were mainly composed of α-FeOOH, β-FeOOH, and γ-FeOOH.FeOOH is soft and can hardly protect the pipeline.The inner layer was mainly composed of Fe3O4 which has a more dense texture and can protect the pipeline.According to figure 5, the longer the corrosion proceeds, the more serious the damage.The specimen that had been corroded for 60 days exhibited a large number of corrosion pits with larger depths and showed the structure of a honeycomb.

Analysis of corrosion rate.
The corrosion rate was calculated using the weight loss method [9] for Group A, Group B (20℃), and Group C, Group D (30℃) of parallel specimens taken out of the simulated solution.The corrosion rate was plotted as a line graph (figure 6) (vertical coordinate is the corrosion rate in mm/a) shown.a. Corrosion rate of 30d.b.Corrosion rate of 60d.

Figure 6. Trend of corrosion rate
As can be seen from figure 6, the corrosion rates of specimens with different immersion periods reduced with the increase of time.In addition, the corrosion rate of Group C is greater than that of Group A, and the corrosion rate of Group D is greater than that of Group B.
The results of the corrosive behavior experiment of X80 steel under a simulated saline soil environment with different salt contents show that: ① The four groups of specimens reached their maximum corrosion rate in the initial period of corrosion.The corrosion rate will gradually decline with the extension of corrosion time; ② The corrosion rate of X80 steel is positively correlated with the temperature.③ The maximum corrosion rate of X80 in the simulated solution of Xinjiang soil is less than 0.06mm/a.

Analysis of factors affecting the corrosion.
The corrosion rates of X80 steel under four corrosion intensities (specimens immersed for 60d in extra strong, strong, medium, and weak simulated saline soil solutions) were calculated using the weight loss method.As can be seen from table 3, the average corrosion rates of X80 steel were different in simulated solutions with different corrosion intensities, in which the corrosion rate of X80 steel in the extra-strong solution was the largest and the corrosion rate of X80 steel in the weak solution is the smallest.The average corrosion rate of X80 steel reached its highest in the initial period of the experiment and then it gradually reduced.

Analysis of Corrosion Mechanism
Ions contained in the simulated soil solution, which include Cl -, SO4 2-, CO3 2-, and HCO 3-, will all change the chemical composition of the surface of the X80 steel.With continuous chemical reactions, the surface will eventually exhibit obvious evidence of corrosion [25][26], of which Cl -and SO4 2-ions have a greater impact.Because the ion Cl -has a relatively small radius, it is possible for the ion to penetrate the corrosion layer and damage the passivation film, so that the corrosion rate is accelerated, resulting in localized corrosion and pits.A large amount of SO4 2-ions will make the solution acidic, and they can chemically react with Fe 2+ , accelerating the corrosion.That's why the greater the contents of Cl -and SO4 2-ions, the faster the corrosion rate of X80 steel.
The equation for the cathodic reaction of X80 steel in the simulated saline soil solution is 2H2O+O2+4e→4OH -.This is the main reason for the thickening of the rust and the deepening of its color.The equations for the anodic reaction are as follows: Fe-2e→Fe 2+  (1) 4Fe(OH)2+O2+2H2O→4Fe(OH)3 Fe(OH)3→FeOOH+H2O (4) 8FeOOH+Fe 2+ +2e→3Fe3O4+4H2O (5) 3Fe(OH)2+ During the corrosion process, on the one hand, Fe(OH)2 will continue to be slowly oxidized to Fe3O4, which is more stable; on the other hand, FeOOH can combine with the Fe 2+ which is on the surface of X80 steel to form Fe3O4, which is the reason for the higher content of Fe3O4 in the inner layer [27].
Pitting corrosion occurs when the metastable micropores grow rapidly and eventually develop into stable pits.Before the formation of pits, there are usually metastable pits, a few microns in diameter, appearing on the oxidation film protecting the surface of the stainless steel.Although many experts and scholars have conducted studies on the formation of these pits, there is no accurate explanation for the timing of their occurrence.Pitting corrosion, mostly occurring in the environment containing chlorine, bromine, iodine, etc., is a form of localized corrosion that is very dangerous and leads to the creation of small holes in the steel or even the penetration of the steel.

Conclusion
In this work, the corrosion behavior of X80 under the environment of Xinjiang saline soil was investigated, by using a simulated solution of the typical saline soil from a pipeline project in Xinjiang.An orthogonal experiment was conducted to analyze the effect of immersion period, temperature, and the saline soil intensities on the corrosion of X80 steel.The results show that, at the early stage of immersion, the main corrosion type of the steel in the saline soil environment is the selective corrosion of iron, and the ionic migration leads to the formation of Fe(OH)2 on the surface of the steel and Fe3O4 with certain protective ability on the deep inner layer.The average corrosion rate of X80 steel in the simulated solution of chlorine saline soil is faster than that in the simulated solution of sulfate saline soil; the corrosion trends continue with the extension of corrosion time and the instantaneous corrosion rate decreases at first and then increases; the corrosion rate is positively correlated to the corrosion intensity of the saline soil, and the corrosion rate of X80 steel in the extra-strong simulated saline soil solutions is the largest and it is the smallest in the weak solutions; pitting corrosion happens when the X80 steel contacts with saline soils and it may result in the damage of the pipelines.
It is intended to use X80 steel in most sections of the proposed pipeline project, and the steel will directly in contact with the soil.In areas with special and complicated geological conditions, the buried depth should not be less than 2 m, and the buried depth of steel in this project is about 1.5 m.Thus, after the excavation of the pipeline trenches, the project should be constructed in time, to avoid water seepage into the trenches and to prevent the pipeline foundations from being submerged or soaked by precipitation and floods; meanwhile, to protect the pipelines from direct corrosion, coating layer should be adopted to isolate the corrosive medium; use the magnesium alloy or aluminum alloy as sacrificial anode to form a closed circuit with the pipelines and protect them.
There are some neglected factors and shortcomings in this experiment.Future studies can focus on the effect of immersion period, temperature changes, and other factors.According to research results, the hazardous levels of pipeline project areas in saline soils can be classified, and regular monitoring of areas with high levels of hazard should be conducted to protect the pipelines.

Figure 2 .
Figure 2. The macro-morphology of X80 steel after 10d of immersion

Figure 3 .
Figure 3.Comparison of specimen corrosion in different solutions after 10d of immersion.

Table 1 .
Major soluble salts and ions in simulated solution.

Table 2 .
Orthogonal experiment scheme of environment samples.

Table 3 .
Average corrosion rate (mm/a) under four corrosion intensities.