The corrosion behavior and cracking susceptibility of the disbonded coating in the X80 steel pipeline by theoretical simulations and experimental measurements

2.1 Materials preparation

X80 steel pipelines were used as a steel substrate, and its chemical composition is as follows (wt%): C 0.036, Si 0.197, Mn 1.771, P 0.012, S 0.002, Cr 0.223, Ni 0.278, Cu 0.220, Al 0.021, Ti 0.019, Mo 0.184, V 0.001, Nb 0.110, and Fe balance. The simulated solution with a 1:1 mass ratio made from the actual soil was used as an experimental solution. This solution was prepared using the following compound of the denoted concentrations: 0.0475 g/L NaCl, 0.0167 g/L CaCl₂, 0.0391 g/L KNO₃, 0.0151 g/L NaHCO₃, 0.0266 g/L Na₂SO₄, 0.0170 g/L MgCl₂ 6H₂O, and adjusted pH value to 4.5 with a 5% aqueous acetic acid solution (Ma et al. 2020). The ionic composition is shown in Table 1.

2.2 Simulation method

A simulation device for the disbonded area was built for the experiment, Figure 1, to simulate the physicochemical properties of the trapped solution in the disbonded coating. A detailed description of the test equipment and procedures was reported in the literature (Ma et al. 2020). Ring-shaped plastic gaskets were sandwiched between these plates to adjust the gap between them to 0.24 mm. A reference electrode was designed to measure the trapped solution layer to address the incompatibility of the conventional reference electrode with the simulated disbonded system used in this study. The schematic diagram is shown in Figure 1c. The needles were immersed in the test liquid layer during testing.

Figure 1:

Schematic diagram of the device designed for the simulation of the disbonded area: (a) side view of the simulation device, (b) top view of the simulation device, and (c) the reference electrode.

2.3 Characterization

3.1 Compositional changes

The oxygen content in the gap is closely related to the electrochemical process on the steel surface, and it mainly depends on the oxygen diffusion and redox processes. Generally, the diffusion process is the primary control condition for the oxygen content in the gap (Liu et al. 2009a,b). Figure 2 shows the temporal evolution of dissolved oxygen content in the trapped solution within the disbonded area. There is no distinct difference in the dissolved oxygen content in the trapped solution at various positions, which exhibits a rapid decline with time dropping from 8.3 to 1.0 mg/L on Day 1 before reaching a stable level. The immediate oxygen consumption induces the formation of an anaerobic environment where the effect of oxygen becomes negligible.

Figure 2:

The evolution of the dissolved oxygen in the trapped solution within the disbonded area with time.

Figure 3 shows a relatively complex change in the pH value of the trapped solution in the disbonded area over time. The pH value of the trapped solution at each position firstly increases and then decreases. A maximal value is reached on Day 1, and it decreases with the progress of the corrosion reaction. However, the magnitude of the change varies with its distance from the holiday. It should be pointed out that the pH value change of the bottom of the disbonded area has the largest amplitude. The pH value is already lower than that of the bulk solution from Day 7 to 9. Besides, its pH value drops to about 4.0 on Day 25. The trend of the pH value change of the solution under the disbonded coating is consistent with the results reported in the literature (Ma et al. 2020; Wu et al. 2019; Yan et al. 2008), indicating that an acidic solution environment will form at the bottom of the disbonded coating (Qian et al. 2016).

Figure 3:

The evolution of the pH value in the trapped solution within the disbonded area.

Figure 4 shows the temporal change in the anion concentration of the trapped solution in the disbonded area. The Cl⁻ concentration exhibits the highest increase compared to other anions, following a monotonous increasing trend at each position of the disbonded area. The closer the position to the bottom of the disbonded area, the more significant the concentration increase. On Day 25, the Cl⁻ concentration at the bottom of the disbonded area (i.e., a distance of 25 cm) reaches 746.65 mg/L, increasing by 108% compared to the bottom of the disbonded area. The increase at the position near the holiday is smaller, being about 45% on Day 25. This difference in the Cl⁻ concentration increase is caused by the small diameter of Cl⁻, making it prone to electromigration under the influence of the electric field (Hołyst and Poniewierski 2012). Both NO₃⁻ and HCO₃⁻ contents show a declining trend. Compared with NO₃⁻, the HCO₃⁻ content declines with a greater amplitude. On Day 7, the concentration at each position in the disbonded area is close to its minimum, without apparent difference among the positions. Specifically, the concentration at each location is about 10 mg/L, which is nearly 90% lower than the initial concentration. The decrease in the NO₃⁻ concentration is relatively small, reaching its minimal value on Day 15 at each position. Specifically, the change in the NO₃⁻ concentration at the bottom of the disbonded area is more evident than at the position near the holiday. The maximal decrease rate is about 34%. Similar to the Cl⁻ concentration changes, the NO₃⁻ ions diffuse from a high concentration to a low-concentration position (i.e., 250.0 mm) in the late stage of corrosion. However, the amplitude of concentration diffusion is relatively low. The SO₄²⁻ concentration does not show a specific pattern, without a distinct concentration difference between the original bulk solution and the simulated solution collected on Day 25.

Figure 4:

The temporal evolution of the anion content in the trapped solution within the disbonded area: (a) Cl⁻, (b) HCO³⁻, (c) NO³⁻, (d) SO₄²⁻.

Figure 5 shows the concentration of Na⁺, K⁺, Mg²⁺, and Ca²⁺ cations in the trapped solution in the disbonded area over time. Among these cations, the Na⁺ concentration undergoes the most remarkable change. Besides, the changing pattern varies with the distance from the holiday. The Na⁺ concentration does not substantially change at the position near the holiday in the early stage of the corrosion reaction, reaching the maximum on Day 7, and then starting to decrease. There is no distinct change in the Na⁺ concentration at the bottom of the disbonded area during the early stage of the corrosion reaction. A monotonous decrease of Na⁺ concentration occurs after Day 7. The change in the Na⁺ concentration at the position located in the center of the disbonded area shows an intermediate level. Since Na⁺ ions do not participate in the corrosion reaction, the change in its concentration may be related to the formation of the electric field gradient and the concentration diffusion. More significant changes take place at the bottom of the disbonded area (i.e., Position 5). The Na⁺ concentration drops to about 77% of its original concentration on Day 25. In contrast, the K⁺ concentration drops to about 83% of its original at the same time point. The increase rate of Mg²⁺ concentration on Day 7 is about 35%. The decrease in the Ca²⁺ concentration is more significant than that of the wide slit, being only about 34% of its concentration in the bulk solution.

Figure 5:

The temporal evolution of the cation content in the trapped solution within the disbonded area: (a) Na⁺, (b) K⁺, (c) Mg²⁺, and (d) Ca²⁺.

3.2 In-situ corrosive electrochemical reaction variation

Figure 6 shows the change in the open-circuit potential at each position of the disbonded area over time. The initial potential of each position is at the same level of about −640 mV. After 2–3 days of corrosion, the open-circuit potential rapidly drops and reaches a relatively stable level. The open-circuit potential at the bottom of the disbonded coating (the holiday at 25 cm) retains a negative trend, indicating that the corrosion environment at the bottom of the disbonded coating is continuously changing. The open-circuit potentials at the holiday and the bottom of the disbonded area on Day 25 are −664 and −748 mV, respectively. The negative shifts from the initial open-circuit potential are about −20 and 108 mV, respectively, which indicates that with the progress of the corrosion reaction in the disbonded area, a large potential difference between the holiday of the disbonded area and the bottom of the disbonded area appears. With the corrosion development in the disbonded area, the holiday acts as a cathode. The bottom of the disbonded area acts as an anode, consistent with the conventional crevice corrosion phenomenon (Chen et al. 2015; Xu et al. 2016).

Figure 6:

The change in the open-circuit potential at each position of the disbonded area over time.

Figure 7 shows the EIS of X80 steel at each position of the disbonded area over time. The electrode impedance spectra at each position of the disbonded area are basically the same. They are composed of a high-frequency capacitance arc and a low-frequency inductive reactance arc, indicating a strong adsorption effect on the electrode surface. The radius of the arcs is almost the same, indicating the same reactions and reaction degrees (Zhao et al. 2021). The fitted equivalent circuit is presented in Figure 8a. With the progress of the corrosion reaction, the radius of the capacitance arc of each position increases on Day 1, while the inductive reactance arc disappears. This behavior may be related to a layer of protective corrosion products formed on the surface of X80 steel, which inhibits the corrosion reaction. The equivalent circuit is shown in Figure 8b. With the further progress of the corrosion reaction at each point in the disbonded area, the radius of the capacitive reactance arc at positions 1–3, i.e., closer to the holiday, increases monotonously with time. The largest increase in the radius of the capacitive reactance arc is observed in the impedance spectra of Position 3, indicating the highest resistance to the corrosion reaction, i.e., the corrosion products provide the best protection for the substrate at this position. The radius of the capacitive reactance arc at positions 4 and 5 firstly increases and then decreases. The maximal radius of the capacitive reactance arc at position 4 appears on Day 18, while at Position 5, it occurs on Day 3. Meanwhile, Warburg impedance appears in the impedance spectra, and the nature of the electrode reaction gradually shifts from the activation-controlled to the diffusion-controlled reaction (Zhao et al. 2018). The corresponding fitted equivalent circuit is presented in Figure 8c. The electrochemical impedance spectra of the disbonded area have the following characteristics: first, the radius of capacitive resistance arc at the pre-corrosion position, especially in the center of the disbonded area, exhibits a relatively large increase; second, during the post-corrosion stage, the radius of capacitive resistance arc at the bottom of the disbonded area undergoes a distinct decline. This behavior is characteristic of a crevice effect in the disbonded area, which can cause sealing of the disbonded area. This crevice effect inhibits corrosion in the center of the disbonded area but enhances it at the bottom of the disbonded area.

Figure 7:

The change in the EIS at each position of the disbonded area over time: (a) point1, (b) point 2, (c) point 3, (d) point 4, and (e) point 5.

Figure 8:

The equivalent circuit of EIS, shown in Figure 7, with different features: (a) capacitive loop with inductance, (b) single capacitive loop, (c) capacitive loop.

R _ct is the charge transfer resistance (Ω cm²), R_s is the solution resistance (Ω cm²), R_L is the inductance resistance (Ω cm²), L is inductance (H/cm²), Z_w is Warburg impedance (Ω cm²), Q is a constant phase element.

The constant phase angle element Q in the equivalent circuit in Figure 8 can be calculated using Eq. (1):

where j is the dimensionless square root of −1, ω (rad s⁻¹) is the angular frequency of the sinusoidal AC wave signal; Y₀ (Ω⁻¹ cm⁻² s⁻¹) always has a positive value; n is the diffusion coefficient. It is a dimensionless constant whose value ranges from 0 to 1. Among all these parameters, Y₀ and n are two parameters of Q. When n = 0, ZQ equals 1/Y₀. Under this circumstance, Q is resistive, and R is impedance; when n = 1, ZQ equals C. Under this circumstance, Q is capacitive, and the impedance equals 1/(jωC). It is generally perceived that n can be expressed as the electrode surface roughness caused by the dispersion effect [23].

By considering the charge transfer resistance R_ct, the relation between the two parameters (n and R_ct) and two variables (time and disbonding depth) can be illustrated by the curves presented in Figure 9.

Figure 9:

The changes in n and R_ct in the disbonded area with time and disbonding depth: (a) 3D map of n, (b) heat map of n, (c) 3D map of R_ct, and (d) heat map of R_ct.

Figure 9 shows the changes in n and R_ct in the disbonded area with time and disbonding depth. Both n and R_ct change in a regular manner. The X80 steel sample has a relatively rough surface. The value of n at the holiday is lower than that at the bottom of the disbonded area; in the center of the disbonded area, n is relatively large, and the sample has a relatively smooth surface. The charge transfer resistance R_ct is larger in the center of the disbonded area and increases faster. R_ct at the holiday is low, and the increase is slower. At the bottom of the disbonded area, R_ct firstly increases and then decreases, and a minimum appears in the later stage of the corrosion reaction.

3.3 SCC susceptibility

According to the test results presented in Section 2.1, the holiday (original solution), near the holiday (i.e., Position 1, at 5 cm from the holiday), the center of the disbonded area (i.e., Position 3, at 15 cm from the holiday), and the bottom of the disbonded area (i.e., Position 5, at 25 cm from the holiday) were chosen as four sampling points in the following experiment. Trapped solutions collected from the four sampling points on Day 25 were subjected to a SSRT test to study SCC susceptibility in different trapped solution environments. The composition of the trapped solution at four sampling points is shown in Table 2. Meanwhile, as the dissolved oxygen content in the solution trapped in the disbonded area is low, and there is a little difference in the distribution of dissolved oxygen, the treatment of deoxidation by bubbling N₂ into the solution was adopted before the stress corrosion test experiments. The specific deoxidation step is to pre-deoxygenate the simulated solution by bubbling high-purity nitrogen through the solution for 6 h before the experiment. The nitrogen continuously flew through the simulated solution until the end of the experiment.

Figure 10 shows the SSRT curves of X80 steel at different positions in the disbonded area. Compared with the air-exposed X80 steel, the fracture strength and elongation of the X80 steel soaked in the solution are reduced to a certain extent. This decline mainly originates from the fact that the simulated solution of the Yingtan district soil is a strong corrosive solution. Its trapped solution system is also corrosive. During fracture failure, the effects of corrosion factors and mechanical factors are coupled, which accelerates the loss of mechanical properties of steel.

Figure 10:

The SSRT curves of X80 steel at different positions in the disbonded area.

According to the test results, the breaking strength of the steel at the four positions can be ranked as YT2 > YT1 > YT0 > YT3. In contrast, the corresponding elongation at the break of the steel at the four positions can be sorted in the following order: YT2 > YT1 > YT3 > YT0. From the perspective of fracture strength and fracture elongation, the steel has relatively weak mechanical properties in YT0 and YT3. To illustrate further the relationship between the tensile strength and the SCC susceptibility of the material, we selected the shrinkage (ψ, %) and section elongation (δ, %) as parameters to evaluate the SCC susceptibility. The calculation equations are as follows:

where I₀ (mm) is the original gauge length of the tensile specimen; I_A (mm) is the gauge length of the tensile specimen after elongation; S₀ (mm²) is the original cross-sectional area of the tensile specimen at I₀; S_A (mm²) is the original cross-sectional area of the fracture site.

The smaller the values of the section shrinkage ψ and the elongation δ, the greater the SCC susceptibility of the material. Figure 11 shows that the shifting of the SCC susceptibility of X80 steel under the disbonded gap reflected by ψ is relatively consistent with that reflected by δ. From the holiday to the bottom of the disbonded area, the SCC susceptibility of the material firstly decreases and then increases. Among the four investigated points, the X80 steel at the holiday has a strong SCC susceptibility. The strongest and weakest SCC susceptibility values are recorded at the bottom of the disbonded area and in the center of the disbonded area, respectively. This finding is consistent with the loss of the material’s mechanical properties, indicating that the two environments at the holiday and the bottom of the disbonded area have a relatively strong SCC susceptibility to X80 steel. The elongation and section shrinkage of X80 steel at the bottom of the disbonded area reaches minimum values, indicating that the sample undergoes sudden fracture during the elongation process. The high SCC susceptibility at the bottom of the disbonded area is mainly attributed to the high concentration of hydrogen atoms at the bottom of the disbonded area (Liu et al. 2008). Some hydrogen atoms penetrate the steel and form Cottrell atmosphere pinning dislocations, which affects the material’s resistance to stress corrosion (Liu et al. 2011; Song et al. 2014).

Figure 11:

The SCC susceptibility of X80 steel at different positions in the disbonded area: (a) shrinkage (ψ) and (b) section elongation (δ).

The fracture area was sliced from the fractured tensile specimen. The morphology of the fracture was observed on macro- and micro-scales using SEM. Figure 12 shows macroscopic necking and fibrous zones in the primary fracture of the air-fractured X80 steel. Distinctive dimple structures can be observed from a microscopic scale. The walls of the local dimples feature the characteristics of serpentine slip, typical for ductile fracture. This indicates that X80 steel undergoes ductile fracture during a slow-fracture-rate process and possesses good ductility in the atmosphere environment (Liu et al. 2016). Under the four investigated conditions for the simulated disbonded area, relatively flat macroscopic fractures of the samples are observed. There is a certain tendency to brittle fracture, which belongs to the fracture characteristic of the combined ductile and brittle fracture pattern. YT0 and YT3 exhibit a higher flatness among all the points, indicating a more distinct brittle fracture tendency at the holiday and the bottom of the disbonded area. This feature is consistent with the curves obtained at a slow elongation rate. The microscopic morphology shows that X80 steel does not only undergo dimple fracture in the four different solution environments but also that a certain number of micropores appear in the fracture zones of all four samples. Among them, fewer micropores appear in the simulated environments at YT1 and YT2, located in the center of the disbonded area. Besides, the average diameter of the micropores is relatively smaller than that observed at other points. The fracture zones of the two samples tend to feature the characteristics of ductile fracture. This indicates that X80 steel does not have strong SCC susceptibility in the center of the disbonded area, showing a clear tendency to ductile fracture. The presence of micropores is apparent in the two simulated environments of the holiday (YT0) and the bottom of the disbonded area (YT3). The fracture zones are flatter and feature specific characteristics of brittle fracture. In the YT3 environment located at the bottom of the disbonded area, small cracks appear in the primary fracture of X80 steel. The crack size is larger than the micropores that appear in the simulated YT0 solution. In the YT3 environment, there are obvious cracks around the micropores on the X80 steel surface, indicating higher SCC susceptibility (Ma et al. 2020).

Figure 12:

The macro- and micro-scale morphology of the fracture of X80 steel at each positions: (a) in air, (b) in air, (c) YT0, (d) YT0, (e) YT1, (f) YT1, (g) YT2, (h) YT2, (i) YT3, and (j) YT3.

Figure 13 represents the lateral morphology of the fracture sample in Figure 12. Secondary microcracks do not appear on the lateral surface of the fracture of the control sample, which features the characteristics of plastic deformation (Liu et al. 2016). In the simulated solution environment, the lateral fracture morphology of YT1 and YT2 in the disbonded area exhibits relatively regular plastic deformation. It can be perceived that the material in the center of the disbonded area has a relatively low SCC susceptibility. However, there are some secondary microcracks on the lateral surfaces of the X80 steel fractures at the holiday (YT0) and the bottom (YT3) of simulated environments of the disbonded area. Specifically, the secondary cracks that appear on the steel surface under the YT3 environment are larger and more numerous than under the YT0 environment, which proves that the bottom of the disbonded area has higher SCC susceptibility. These secondary cracks exhibit straight shapes and apparent characteristics of trans-crystalline stress corrosion. It is worth noting that besides the obvious secondary microcracks, YT3 shows more pronounced corrosion morphology, which may be related to acidification of the solution at the bottom of the disbonded area (Mohtadi-Bonab et al. 2015). The trapped solution at the bottom of the disbonded gap is acidic and forms an anaerobic environment. The pH value of the trapped solution is close to 4, so the concentration of hydrogen atoms that can penetrate the base metal and promote the SCC process is significant, which supports the brittle fracture of the material (Wu et al. 2017).

Figure 13:

The lateral morphology of the fracture sample in Figure 12: (a) in air, (b) YT0, (c) YT1, (d) YT3, and (e) YT5.

4.1 Evolution of solution in the disbonded area

The initiation and development of stress corrosion of pipeline steel under the disbonded anticorrosion layer involve various interfacial chemical/electrochemical reactions between steel and solution. The generation of corrosion products induces the formation of an airtight zone in the disbonded area. Mass transfer and mass diffusion in the disbonded area are affected by the concentration gradient and the electric field (Yan et al. 2015). Newman et al. (1966) proposed that in the calculation or simulation of the flux of a substance in a solution, three factors should be taken into consideration: migration under the influence of the electric field, diffusion under the concentration gradient, and the convection in liquid with a certain velocity. Since the trapped solution is practically completely static, the effect of the convection factor on the mass diffusion is negligible. As shown in Eq. (4), the factors affecting the composition of the solution trapped in the disbonded area include only the mass transfer under electric field and concentration diffusion.

where N_i is the mass flux (mol/cm² s), U_i is the ionic mobility (m/V), F is Faraday constant (which is 96,500 C/mol), Z_i is electric charges (C), C_i is the molar concentration (mol/L), ϕ is potential (V), and D_i is the diffusion coefficient (cm²/s).

In this study, the difference in the compositions of the trapped solution is caused mainly by corrosion reactions. In aqueous solutions, the anodic reactions in the corrosion process of X80 steel mainly include the Fe dissolution reaction (Eq. (5)).

According to different conditions, such as the pH value of the solution, cathodic reactions including hydrogen evolution, Eq. (6), or oxygen absorption reaction, Eq. (7), may occur. In the disbonded area, due to a long and narrow gap, the mass transfer process is blocked. H⁺ and O₂ in the disbonded area are quickly consumed, and OH⁻ is generated, yielding a rapid increase in the pH value of the solution trapped during the early stage of the reaction.

Meanwhile, due to the formation of the airtight environment in the disbonded area, X80 steel undergoes locally unsteady chemical-electrochemical reactions (Liu et al. 2019), yielding a significant difference in the ion composition between the trapped solution and the external bulk solution. The ionization equilibria of H⁺, OH⁻, H₂CO₃, HCO₃⁻, and CO₃²⁻ in the trapped solution are shown in Eqs. (8) and (10) (Egbewande et al. 2014; Eslami et al. 2010). In the early stage of the reactions, the H⁺ ions in the trapped solution are quickly consumed, and the pH value of the trapped solution gradually increases, shifting the equilibria of the reactions presented in Eqs. (8)–(10) to the right. The HCO₃⁻ ions in the solution are converted into CO₃²⁻.

In the trapped solution, Ca²⁺, Mg²⁺, and Fe²⁺ ions are generated in the corrosion reaction from CaCO₃, MgCO₃, and FeCO₃ through chemical reactions, yielding a decrease of the ionic content in the trapped solution. As shown in Eqs. (11)–(13), besides the reduction of the total HCO₃⁻ concentration in the trapped solution, these reactions also cause a decrease in the cation concentration, e.g., such as Ca²⁺ and Mg²⁺.

The progress of the corrosion reaction induces the compositional difference of the solution at different positions in the disbonded area because of their different distances from the holiday. Specifically, closer to the holiday (Position 1), H⁺ and O₂ can be partially supplemented by the holiday via the concentration diffusion. As a result, reductive oxygen reactions and hydrogen evolution reactions occur continuously, yielding a high pH value of the trapped solution. Therefore, the electrode potential rises, and this spot becomes the cathode area of the electrochemical reactions. A long and narrow gap prevents the transport of H⁺ and O₂ ions penetrated in the trapped solution through the holiday to the bottom of the disbonded area. Therefore, the cathode reactants are quickly consumed and fail to promote the cathodic reactions. As a result, only anodic reactions occur in this area, and it then becomes the anodic zone of the corrosion reaction. The anodic reaction only involves the Fe dissolution reaction, Eq. (5). Meanwhile, under the anaerobic environment at the bottom of the disbonded area, Fe²⁺ ions undergo a hydrolysis reaction, and a large amount of H⁺ ions is generated (Eq. (14)). In the late stage of the corrosion reaction, the trapped solution in the bottom of the disbonded area is acidified due to the hydrolysis reaction, which can be verified by the results of the pH value measurement presented in Figure 4.

Overall, the above analysis shows that the continuous increase of the cation concentration at the bottom of the disbonded area disrupts the electroneutrality equilibrium in this area, so the electric field forms in the trapped solution from the bottom of the disbonded area to the holiday. To maintain electroneutrality, the anions outside of the disbonded area migrate inward. The cations inside of the disbonded area migrate outward (Wan et al. 2019). Na⁺, K⁺, and Cl⁻ ions exhibit relatively high ion mobility and strong migrating ability, so they mainly undergo long-range migration. In contrast, Mg²⁺, Ca²⁺, NO₃⁻, HCO₃⁻, and SO₄²⁻ mostly undergo short-range migration. Meanwhile, the acidification of the trapped solution at the bottom of the disbonded area causes a partial dissolution of the corrosion products. Fe²⁺ ions also migrate to the holiday. The electrode reaction in the disbonded area is gradually dominated by diffusion control, and Warburg impedance appears in the EIS. As the vicinity of the holiday is relatively rich in oxygen, Fe²⁺ ions that migrate to this area are converted into Fe-containing oxides. These oxides accumulate around the holiday, which further deteriorates the sealing of the disbonded area and aggravates the progress of the unique corrosion reaction in the disbonded area.

The above analysis shows that the electric field and the ion concentration difference in the trapped solution cause the local distribution of ions in the disbonded area. This yields the compositional difference of the trapped solution among different positions, resulting in local differences in the SCC of X80 steel among the positions in the disbonded area. The diagram in Figure 14 shows the formation of the local compositional difference in the trapped solutions. However, a comprehensive analysis of the corrosion in the solution trapped in the disbonded area and the SCC mechanism under applied stress should be performed to illustrate the stress corrosion behavior and the SCC susceptibility of X80 steel.

Figure 14:

Schematic diagram of the corrosion reaction in the disbonded area.

4.2 SCC susceptibility

In acidic soil solutions, pipeline steel in the disbonded area solution is prone to both anode dissolution and hydrogen embrittlement (HE) (Liu et al. 2016; Mohtadi-Bonab et al. 2015). Figure 12 indicates that the SCC susceptibility at the holiday and the bottom of the bonded area is relatively higher than at the other positions. The potential difference in the disbonded area cannot provide full-scale cathodic protection of the holiday because of the low corrosion resistance of X80 steel. Therefore, the anode corrosion reaction caused by the Fe dissolution also occurs during the holiday (Liu et al. 2008, 2009a,b; Wu et al. 2019). However, the solution at the holiday can be regarded as the bulk solution of the soil. As the pH of this bulk solution is only 4.5, the solution does not exhibit a distinctive acidity. Besides, since the solution is relatively pure, the anode is unlikely to undergo a chemical poisoning reaction. Therefore, hydrogen ions do not play a dominant role during the reaction, indicating that the decrease in the mechanical properties of the material may be caused mainly by the anodic dissolution (AD) resulting from the corrosion reaction.

The bottom of the disbonded area represents the anode section, where electrochemical corrosion, mainly the significant electron donation of iron, occurs. Meanwhile, the pH value in this area drops below 4.0, and the acidity of the trapped solution increases. Besides, as the trapped solution at this position is rich in Cl⁻, the side effect of chemical corrosion may also occur. The hydrogen precipitation increases because of the increase in acidity. Hydrogen ions are very likely to enrich steel defects, such as grain boundaries, inclusions, and dislocations, promoting HE. The coupling of the unsteady-state electrochemical behavior and stress corrosion reaction may result in a physical poisoning reaction, which, together with the increase in the degree of HE, may cause the shifting of the fracture failure mechanism to the mixed anode dissolution mechanism. This behavior and HE are critically important (AD + HE) (Ma et al. 2020; Zhao et al. 2021). The micromorphology, Figures 12e and 13e, can also confirm that the anode reaction at the bottom of the disbonded area has been aggravated.