The Effect of Crevice Structure on Corrosion Behavior of P110 Carbon Steel in a Carbonated Simulated Concrete Environment

Abstract

This study systematically investigated the corrosion behavior of P110 pipeline steel in simulated carbonated concrete environments through a combination of electrochemical testing and multiphysics simulation, with particular focus on revealing the evolution mechanisms of corrosion product deposition and ion concentration distribution under half crevice structures, providing new insights into localized corrosion in concealed areas. Experimental results showed that no significant corrosion occurred on the P110 steel surface in uncarbonated simulated pore solution. Conversely, the half crevice structure significantly promoted the development of localized corrosion in carbonated simulated pore solution, with the most severe corrosion and substantial accumulation of corrosion products observed at the crevice mouth region. COMSOL Multiphysics simulations demonstrated that this phenomenon was primarily attributed to local enrichment of Cl⁻ and H⁺ ions, leading to peak corrosion current density, and directional migration of Fe²⁺ ions toward the crevice mouth, causing preferential deposition of corrosion products at this location. This “electrochemical acceleration-corrosion product deposition” multiphysics coupling analysis of corrosion product deposition patterns within crevices represents a new perspective not captured by traditional crevice corrosion models. The established ion migration-corrosion product deposition model provides new theoretical foundations for understanding crevice corrosion mechanisms and predicting the service life of buried concrete pipelines.

1. Introduction

Downhole casing strings in oil and gas wells are secured and stabilized through cement sheaths, providing reliable structural integrity amidst the harsh downhole environment [1,2]. Localized corrosion of oil and gas well casing in CO₂ erosive environments is one of the main causes of wellbore integrity failure, and serious wellbore integrity failure can lead to perforation and cracking of the casing steel, resulting in serious environmental pollution and economic losses [3,4,5,6]. P110 carbon steel, as a medium to high strength metal material, is widely utilized in oil and gas well casing and reinforced concrete structures, where its durability directly determines the safety of Carbon Capture, Utilization, and Storage (CCUS) projects [7,8,9]. However, in the complex service environment of downhole, carbon steel is prone to localized corrosion, especially the hidden crevice corrosion, which rapidly expands once it occurs, and there is a great risk of structural failure.

Concrete forms a porous structure during the hydration process, in which the pores are filled with a highly alkaline solution [7,10,11]. Exposure of pipeline steel to highly alkaline cement pore solutions enables the development of a moderately protective passive film, significantly improving corrosion resistance through suppression of anodic dissolution [12,13,14,15]. However, in long-term geological storage scenarios, degradation of the cement sheath compromises the alkaline buffering reservoir, accelerating carbonation within the pore solution [16,17,18]. Wellbore fluids penetrating the wellbore cement barrier contact the well casing material, creating crevice corrosion conditions at the cement-casing boundary. This metallic–nonmetallic boundary promotes autocatalytic electrochemical dissolution, leading to localized corrosion failure of the pipeline steel and subsequent compromise of wellbore structural integrity [19,20,21]. Extensive research has established that chloride ions (Cl⁻) in formation water induce depassivation of pipeline steel by disrupting the protective films formed in highly alkaline cement pore solutions [8]. Concurrently, CO₂-mediated carbonation of concrete structures reduces the pH of pore solutions, elevating corrosion susceptibility of embedded steel components. Current research primarily focuses on elucidating the impact of Cl⁻ ions on the corrosion mechanisms of casing steels in simulated concrete electrolyte solution [22,23,24,25] and developing corrosion inhibitors effective in carbonated concrete pore environments [26,27]. Peng’s study on the destruction process of passivation film of P110 steel at different Cl⁻ ion concentrations found that pitting corrosion initiates under the condition of high concentration of Cl⁻ ions [28]. Gong et al. studied the depassivation mechanisms and electrochemical metrics for passive films formed on Q355B steel within chloride-containing simulated concrete solution environments [29]. The results demonstrate that in alkaline simulated pore solutions, passive films exhibit thicknesses of 3–5 nm, while defects localized at the interface accumulate with increasing Cl⁻ levels [29]. Furthermore, carbonation of concrete structures constitutes a critical factor in carbon steel corrosion. By reducing the pH of cement pore solutions, carbonation destabilizes passive films and fundamentally alters the electrolyte chemistry within the cement matrix. Li et al. fabricated a partially pre-corroded wire beam electrode (WBE) for assessing corrosion layer effects on rebar degradation within carbonated simulated concrete pore solutions [30]. Data indicated that passive films generated on pre-oxidized zones exhibited inferior protectiveness compared to those over polished steel surfaces under carbonation conditions. Furthermore, Cl⁻ ions preferentially triggered pitting corrosion at polished steel regions [30]. Yu et al. employed three-dimensional optical microscopy to characterize the degradation response of steel within simulated carbonated concrete pore electrolyte systems [31]. Quantitative assessment of defects under varying NaCl concentrations and inhibitor influences revealed a linear correlation between mean corrosion depth and NaCl content [31]. Existing research predominantly centers on carbon steel degradation within simulated concrete pore electrolytes under open systems, particularly addressing factors such as Cl⁻ ions or carbonation effects. However, limited attention has been paid on the degradation mechanisms of P110 carbon steel within simulated concrete pore environments in the tiny crevice structure, as well as the influence of the difference between the environment inside and outside of the crevice and the mass transport processes, and the influencing factors, corrosion mechanisms, and protective measures have not yet been clarified. Consequently, to enable accurate service life prediction of downhole pipeline steel encased in cement sheaths, dedicated research on concrete carbonation-induced corrosion mechanisms is imperative.

This study aims to elucidate the coupling mechanisms between HCO₃⁻ concentration in simulated concrete testing solutions and the corrosion kinetics of P110 steel with/without half crevice structure. Through electrochemical characterization and immersion testing, the degradation mechanisms of P110 steel were investigated, with subsequent morphological and compositional profiling of corrosion products. Numerical simulations using COMSOL Multiphysics elucidated the evolution of corrosion product deposition and ionic concentration gradients within crevices. Numerical simulations using COMSOL Multiphysics elucidated the evolution of corrosion product deposition and ionic concentration gradients within crevices and assessed the influence of concrete carbonation degree and crevice structure on tubular steel corrosion. The theoretical contribution of this work lies in establishing mechanistic linkages between cement carbonation progression and crevice geometry on casing corrosion, providing critical references for long-term wellbore integrity assessment in geological storage environments, and providing theoretical and data support for corrosion mitigation strategies for CCUS engineering applications.

2. Experiment

2.1. Materials and Methods

P110 casing steel conforming to API 5CT specifications was utilized as the test material and the main chemical composition is shown in

Table 1. Chemical element composition of API 5CT P110 casing steel.

Element	C	Si	Mn	Ni	Cr	Mo	Ti	Fe
Content(wt.%)	0.27	0.23	1.7	0.02	0.05	0.01	0.01	Balance

. The simulated concrete solution used in the experiment was a sulfur-resistant concrete filtrate containing 1 wt.% NaCl (Chengdu Jinshan Chemical Reagent Co., Ltd., AR, Chengdu, China.) with different degrees of carbonation, and the mineral composition of this concrete was mainly 56.5% C₃S, 18.33% C₄AF, 2.51% C₃A, 3.58% MgO, 0.6% Na₂O(K₂O), 2.04% SO₃, and so on (ZhuCheng 97 Building Materials Co., Ltd., industrial products, Zhucheng, China.). Before experimentation, sulfur-resistant concrete and deionized water were blended in a 1:10 mass proportion. The resultant suspension was intensively stirred for 30 min, followed by 24-h equilibration under room temperature conditions, after which the supernatant was filtered to yield the extracted concrete pore solution. Varying concentrations of NaHCO₃ (Chengdu Jinshan Chemical Reagent Co., Ltd., AR, Chengdu, China.) were introduced into the filtrate to emulate distinct carbonation levels, targeting concrete carbonation’s impact on pipeline steel degradation.

Electrochemical testing was performed employing a Gamry 1010E (Gangrui (Shanghai) Business Information Consulting Co., Ltd., Shanghai, China) instrument featuring a three-electrode cell assembly (GAMRY INSTRUMENTS, Gamry 1010E): Pt plate counter electrode (CE), saturated calomel reference electrode (SCE), and P110 steel working electrode (WE). Specimen dimensions were 10 mm × 10 mm × 5 mm for electrochemical tests and 15 mm × 10 mm × 5 mm for immersion experiments. Copper wire was welded to the backside of the P110 steel WE for establishing electrical contact. Non-working surfaces were encapsulated with epoxy resin to define a fixed exposed area, followed by drying and surface polishing. Before conducting electrochemical impedance spectroscopy (EIS) along with potentiodynamic polarization tests, the open-circuit potential (OCP) was tracked for 2 h to confirm system stability. EIS tests employed a 10 mV sinusoidal perturbation amplitude across a frequency spectrum of 10⁵ Hz to 10⁻² Hz. Potentiodynamic scans were conducted at 0.4 mV/s. All experiments were maintained at 30 ± 0.2 °C under atmospheric pressure to ensure electrolyte stability.

A P110 steel specimen measuring 15 mm × 10 mm × 5 mm was employed for the immersion experiment. Epoxy resin encapsulated half the WE’s exposed surface, with a polytetrafluoroethylene (PTFE) spacer inserted between the WE and resin creating a 400 μm wide crevice. Before the start of the immersion experiment, a solution was injected into the inside of the crevice to ensure that the crevice was filled with solution.

2.2. Analytical Test Methodology

A scanning electron microscope/energy spectrometer (SEM/EDS, S-3400N, HITACHI, Japan.) was employed to examine the corrosion layer morphology on P110 steel half crevice electrodes post 72-h immersion, with elemental species and content analyzed via EDS. X-ray diffraction (XRD, BRUKER D8 Discover, Cu Kα, BRUKER AXS GMBH, Karlsruhe, Germany.) was utilized to characterize the phase composition of the corrosion deposits. A 3D surface morphometry (VHX-7000, Keyence (China) Co., Ltd., Shanghai, China.) was used to analyze the corrosion product height profile and local corrosion depth near the crevice mouth of the half crevice electrodes post 72-h immersion.

2.3. Geometrical Model and Control Equations

To assess the deposition behavior of the products of corrosion on P110 steel in simulated concrete electrolyte solutions, a numerical model was developed using COMSOL Multiphysics 6.2 to simulate corrosion product precipitation and ionic mass transport within crevices. Based on experimental specimen geometry, an artificial crevice with an opening width w = 400 μm and depth L = 7.5 mm was constructed, as illustrated in Figure 1.

In the inside region of the crevice, neglecting intermolecular interactions, ionic transport within the electrolyte is governed by three mechanisms: electromigration, diffusion, and convection, conforming to the Nernst–Planck equation. Consequently, the flux density of chemical species i in dilute solutions can be expressed as [32,33]

$$N_i=-z_i{u}_{i}F{C}_i{\displaystyle \frac{\mathsf{\partial }\phi }{\mathsf{\partial }x}}-D_i{\displaystyle \frac{\mathsf{\partial }{C}_i}{\mathsf{\partial }x}}+v{C}_i$$

(1)

where N_i is the flux density of species i, mol·cm⁻²·s⁻¹; z_i denotes the charge number; u_i represents ionic mobility, m·V⁻¹; F denotes the Faraday constant, 96,500 C·mol⁻¹; C_i is the concentration of species i, mol·m⁻³; φ signifies the local electric potential within the crevice, V; D_i denotes the diffusion coefficient, m²·s⁻¹; and v signifies the convection velocity, m·s⁻¹. In the equation, the first item is the electromigration term, the second item is the diffusion term, and the third item is the convection term. In this experimental model, the electrolyte is static and the convective velocity is 0. Therefore, the flux density of species i within the crevice simplifies to

$$N_i=-z_i{u}_{i}F{C}_i{\displaystyle \frac{\mathsf{\partial }\phi }{\mathsf{\partial }x}}-D_i{\displaystyle \frac{\mathsf{\partial }{C}_i}{\mathsf{\partial }x}}$$

(2)

The ionic mobility u_i of chemical species i in dilute solutions can be determined from the Nernst–Einstein equation:

$$u_i={\displaystyle \frac{D_i}{RT}}$$

(3)

Therefore, the mass conservation of each chemical species along the crevice depth direction is governed by

$$R_i={\displaystyle \frac{\mathsf{\partial }{C}_i}{\mathsf{\partial }t}}+\nabla N_i$$

(4)

where R_i represents the reaction source term for the production or consumption of species i. Concurrently, electroneutrality is maintained in the solution, where the transport of charged ions via diffusion and electromigration generates an ionic current. On the basis of Faraday’s law, the current density in the electrolyte solution is expressed as [32,33]

$$i_l=F{\displaystyle \sum }_{i=1}^n{z}_i\left(-D_i\nabla C_i-z_i{u}_{i}F{C}_i\nabla {\phi }_l\right)$$

(5)

where i_l denotes the ionic current density within the electrolyte. By associating the above equations, the equation for the concentration of each chemical i at different distances from the crevice mouth opening as a function of time can be obtained, expressed as [32,33,34]

$${\displaystyle \frac{\mathsf{\partial }{C}_i}{\mathsf{\partial }t}}=z_i{\displaystyle \frac{F}{RT}}{D}_i\left({\displaystyle \frac{\mathsf{\partial }{C}_i}{\mathsf{\partial }x}}{\displaystyle \frac{\mathsf{\partial }\phi }{\mathsf{\partial }x}}+C_i{\displaystyle \frac{{\mathsf{\partial }}^2\phi }{\mathsf{\partial }{x}^2}}\right)+D_i{\displaystyle \frac{{\mathsf{\partial }}^2{C}_i}{\mathsf{\partial }{x}^2}}+{\displaystyle \frac{i_l}{nF}}$$

(6)

In crevice solutions, the electroneutrality equation must be introduced for numerical resolution, with all chemical species i satisfying the electroneutrality condition:

$$\sum z_i{C}_i=0$$

(7)

3. Results

3.1. Electrochemical Experiments

Figure 2 depicts polarization curves for P110 steel immersed in simulated concrete pore electrolytes across varying carbonation degrees. It can be seen that when pH = 12.70, the current density of the anodic polarization curve before the breakdown potential is always less than 10⁻⁵ A·cm⁻², and the electrode is in a passivated state. As the degree of solution carbonation increased, the pH decreased, the self-corrosion potential shifted negatively, and the anodic polarization curve displaced rightward, which indicated that the solution carbonation accelerated the electrode corrosion. For the half crevice electrodes, the electrodes were similarly passivated when the simulated concrete solution was uncarbonated (pH = 12.70). Under carbonated conditions, the polarization behavior of half crevice electrodes resembles that without crevice configurations, exhibiting a negative shift in corrosion potential and rightward displacement of the anodic polarization curve.

Table 2. Electrochemical kinetic parameters.

	pH	Icorr/(A·cm−2)	Ecorr/V	Rp/(Ω·cm2)	Corrosion Rate/(mm·y−1)
Without crevice electrode	12.70	1.84 × 10−7	−0.351	2.78 × 105	0.001
	11.50	1.31 × 10−6	−0.546	9.36 × 103	0.032
	10.50	2.46 × 10−6	−0.531	2.02 × 103	0.150
Half crevice electrode	12.70	2.14 × 10−7	−0.344	3.38 × 105	0.001
	11.50	2.93 × 10−6	−0.735	4.81 × 103	0.063
	10.50	4.84 × 10−6	−0.781	1.23 × 103	0.245

presents the specified electrochemical parameters of P110 steel electrodes across varying pH environments. The data demonstrate that the self-corrosion current density increases with increasing carbonation of the simulated pore solution under conditions with or without crevice structure. Without crevice P110 steel electrode, the self-corrosion potential decreased from −0.351 V to −0.531 V in pH = 10.50 solution, with a negative shift of 180 mV. The P110 steel half crevice electrode decreased from −0.344 V to −0.781 V, with a negative shift amplitude of 437 mV, indicating that the tendency of the electrode to corrode increased in the presence of the crevice structure. At the same degree of carbonization, degradation rates for the half crevice electrode exceed those of the specimens without crevice. This indicates that the presence of crevice structure increases the corrosion problem of P110 carbon steel in carbonated concrete electrolyte solution.

Figure 3 presents the results of EIS measurements for P110 steel without crevice electrode and half crevice electrode in simulated solution under graded carbonation levels. Without crevice condition, the maximum arc resistance radius was observed at pH = 12.70, which indicates the best corrosion resistance of P110 steel in uncarbonated simulated concrete electrolyte solution. When carbonation occurs, the pH decreases and the arc resistance radius decreases, which may be due to the destruction of the passivation film in the carbonated simulated pore solution by chloride, culminating in elevated corrosion rates at the electrode interface. Compared to the environment without crevice, the arc-resistant radius of the half crevice electrode was further reduced when carbonization of the simulated electrolyte solution occurred, which indicated that the presence of the crevice increased the corrosion tendency of the P110 pipeline steel.

Comparing the electrochemical impedance spectra of the two electrodes under different carbonation degree conditions, the corresponding circuit models are depicted in Figure 4. In the circuit models, R_s denotes the solution resistance, R_f and Q_f are the product film resistance and double layer capacitance, respectively, and R_ct and Q_dl are the charge transfer resistance and double layer capacitance at the metal/solution interface, respectively. For the uncarbonized concrete environment the circuit model shown in Figure 4a was used, and for the carbonized solution the circuit model shown in Figure 4b was used. The equivalent circuits corresponding to Figure 4a,b were fitted using ZsimpWin to obtain the corresponding data as presented in

Table 3. Equivalent circuit fitting data table.

	pH	Rs/ (Ω·cm2)	Qf/ (Ω−1·cm−2·s−nf)	nf	Rf/ (Ω·cm2)	Qdl/ (Ω−1·cm−2·s−ndl)	ndl	Rct/ (Ω·cm2)
Without crevice electrode	12.70	16.0	2.10 × 10−5	0.954	2.99 × 105	-	-	-
	11.50	24.6	4.01 × 10−5	0.950	236.4	7.50 × 10−5	0.750	1.38 × 104
	10.50	22.0	1.57 × 10−4	0.908	68.1	1.61 × 10−4	0.952	1.31 × 103
Half crevice electrode	12.70	42.3	1.32 × 10−5	0.975	1.13 × 106	-	-	-
	11.50	41.7	1.40 × 10−4	0.806	429.3	1.32 × 10−4	0.954	8.17 × 103
	10.50	46.4	1.63 × 10−4	0.820	34.1	1.06 × 10−4	0.909	3.18 × 103

. The R_f values representing the protective film resistance in the uncarbonized simulated concrete electrolyte solution with and without the half crevice structure are much larger than those in the carbonized solution, which indicates that the simulated solution carbonation reduces the stability of the passive layer on P110 steel. Meanwhile, the charge transfer resistance R_ct, which represents the metal/solution interface, decreases with decreasing pH, which indicates that the resistance of the charge transfer process at the electrode surface decreases with increasing carbonization and the corrosion of P110 steel increases.

Figure 5 displays electrochemical impedance spectroscopy (EIS) measurement results for P110 carbon steel half crevice electrodes immersed for 72 h in solutions with different degrees of carbonation. In the uncarbonated electrolyte solution, the P110 steel half crevice electrode showed expansion of the capacitive arc radius during prolonged immersion. This behavior likely stems from the deposition of Ca²⁺-containing corrosion products forming a protective Fe_xCa_1−xCO₃ passive film [35,36], which improves corrosion performance of P110 steel. Conversely, in the carbonated electrolyte solutions, the capacitive arc radius initially expands but subsequently diminishes over extended immersion compared to the simulated concrete pore solution without carbonation. This results from progressive compromise of the passive layer following initial surface film formation on P110 pipeline steel, consequently diminishing corrosion resistance during immersion.

3.2. Surface Morphology and Corrosion Product Composition

Figure 6 presents the macroscopic and microscopic morphologies of P110 steel half crevice electrodes after 72-h immersion in solutions with different degrees of carbonation. At pH = 12.70, there was no obvious corrosion on the inside and outside surfaces of the crevice, and some granular products could be noted on the outside surface of the crevice, which could be the carbonation deposition products of calcium-containing substances in the simulated concrete electrolyte solution. As the simulated pore solution pH decreased to 11.50, the macroscopic morphology showed that the color of the inside and outside areas of the crevice changed significantly, indicating the occurrence of corrosion. The microscopic morphology showed substantial accumulation of flocculent products of corrosion deposited inside the crevices, and the outside of the crevices was covered with uniform corrosion products to show that uniform corrosion had occurred. Minimal corrosion deposits were observed at the location of the crevice mouth, which had an elemental Fe:O atomic ratio of approximately 1:2, as shown in Figure 7a. Under pH 10.50 conditions, corrosion formation intensified on crevice surfaces internally, while a loosely adherent, yellowish-brown deposit gathered at the crevice entrance, evident in Figure 7b. As carbonation advanced in the simulated concrete pore environment, corrosion products increased inside the crevice and near the crevice mouth, while outside the crevice the change was insignificant and dominated by uniform corrosion. Figure 8 presents the XRD spectra of the P110 steel half crevice electrode corroded by immersion for 72 h in a carbonation-simulated pore solution. There is no significant change in the product film types at different carbonation degrees, and primary constituent phases in the passive layer on the electrode surface are FeOOH, FeCO₃, and CaCO₃.

Figure 9 depicts the corrosion product height profiles near the crevice mouth and localized corrosion depth maps after removal of corrosion products for P110 steel half crevice electrodes following 72-h immersion in simulated pore solutions at pH = 11.50 and pH = 10.50. Results demonstrate that at pH = 11.50, corrosion product deposits near the crevice mouth exhibit a height of approximately 34.64 μm and a width of 180 μm. In contrast, when the P110 pipeline steel half crevice electrode was immersed in pH = 10.50 simulated concrete pore solution, deposit dimensions increased significantly to approximately 111.96 μm in height and 300 μm in width. This confirms that in carbonated simulated concrete pore solutions, corrosion products predominantly deposit in the region bordering the crevice entrance. Microstructural analysis of the substrate was performed within the crevice mouth region subsequent to the elimination of corrosion products, and Figure 9c,d shows the local corrosion depth in the region bordering the crevice entrance of the P110 steel electrode. Results indicate a corrosion depth of 7.72 μm at pH = 11.50, increasing to 13.24 μm at pH = 10.50. Under pH = 10.50 conditions, both the height profile of corrosion product accumulation at the crevice mouth and the localized corrosion depth measured via 3D surface topography significantly exceed those observed at pH = 11.50. This demonstrates that intensified carbonation of simulated pore solutions exacerbates corrosion damage on P110 steel in highly carbonated environments.

4. Discussion

Current results demonstrate that in carbonated simulated cement pore solutions, the presence of crevice structures significantly accelerates the degradation rate of P110 steel in the inside region of the crevice, with corrosion intensification scaling proportionally to carbonation extent. Additionally, extensive corrosion product deposition and maximum localized corrosion depth were observed at the crevice mouth. In order to clarify the effect of crevice structure on the behavior of P110 steel corrosion in the carbonated simulated cement pore solution, Cl⁻/H⁺ concentration transients inside the crevice were calculated using the transient algorithm in COMSOL Multiphysics software, and by adding the ‘level set’ physical field interface to track corrosion-induced morphological evolution, quantifying the deposit accumulation process within half crevice structure.

Figure 10a presents the results of the distribution of the concentration of different kinds of ions inside the crevice. Compared with the bulk solution environment, there is an obvious enrichment of Cl⁻ ions and H⁺ ions inside the crevice, and this phenomenon is attributable to the critical crevice corrosion theory [37,38]. The increased concentration of Cl⁻ ions inside the crevice and localized acidification within crevices destabilize and accelerate the dissolution of passive films on P110 steel, significantly increasing corrosion rates inside the crevice regions [39]. This mechanistic interpretation is corroborated by characteristic shifts in potentiodynamic polarization curves. With an increasing degree of carbonization, elevated H⁺ and CO₃²⁻ concentrations in the solution facilitate the nucleation and growth of FeCO₃ and CaCO₃ deposits on P110 steel surfaces. In particular, the products of corrosion inside the crevice increased significantly, but the product film structure is loose and unable to provide effective protection (Figure 6). The results of Rf values in Table 3 similarly indicate that the elevated carbonation levels compromise the protective function of the corrosion product film in simulated carbonated concrete pore solution. The results of the corrosion current density distribution of P110 steel inside the crevice at different carbonation degrees are presented in Figure 10b. Compared with the condition of pH = 11.5, the current density of corrosion of P110 steel inside the crevice is higher when the carbonation-simulated cement pore solution is pH = 10.5, which indicates that the P110 steel corrosion inside the crevice is more serious with the increase in carbonation degree. Furthermore, Figure 10b reveals an inverse correlation between crevice depth and current density magnitude, and there is a maximum corrosion current density of P110 steel at the crevice mouth, and the larger current density of corrosion indicates that the rate of corrosion relevant region is larger, which corresponds well with the maximum local corrosion depth of P110 steel at the crevice mouth.

Figure 11 presents the simulation results of the product of corrosion deposition on the P110 steel surface after 72 h in different carbonation-simulated cement pore solutions. Substantial deposition occurs adjacent to the crevice mouth, with deposition mass escalating with carbonation severity. These computational results demonstrate a strong correlation with experimental observations in Figure 6. On the one hand, the higher concentration of Fe²⁺ ions inside the crevice diffusively migrated outside the crevice; on the other hand, OH⁻ and CO₃²⁻ ions in the solution migrated inside the crevice. These two types of ions in the crevice mouth region form a concentration convergence zone, resulting in the crevice mouth region of the deposition reaction kinetics being significantly enhanced, thereby promoting the deposition of corrosion products.

This study demonstrates significant practical value in engineering applications. It innovatively proposes a novel corrosion prediction methodology that evaluates internal crevice corrosion severity by analyzing deposition characteristics (such as deposit thickness) at crevice openings, providing new insights for assessing concealed corrosion. The findings not only advance theoretical understanding of crevice corrosion mechanisms but also establish a scientific basis for practical corrosion protection strategies, including optimized selection of external coatings, prevention of uneven concrete coverage that may create crevices, and mitigation of concrete carbonation. These discoveries offer critical reference value for safety assessment and maintenance of pipeline systems.

5. Conclusions

This paper examines the corrosion mechanism of P110 pipeline steel in simulated concrete electrolyte solutions by investigating the corrosion behavior of P110 steel in the presence and absence of a half crevice structure under varying carbonation levels using electrochemical testing and offline characterization methods. COMSOL Multiphysics software simulated ionic concentration changes within the crevice, while corrosion product deposition near the crevice opening was quantitatively modeled. The results demonstrated that P110 pipeline steel exhibits negligible electrochemical corrosion in uncarbonated simulated pore solutions, but under simulated carbonation conditions, Cl⁻ and H⁺ ions cause the surface product film to break down, resulting in localized corrosion that intensifies with increasing carbonation levels. In carbonated simulated pore solutions, corrosion products predominantly deposit near the crevice mouth region. This area experiences the most severe corrosion, with COMSOL Multiphysics simulations confirming that the maximum current density of corrosion occurs at the crevice mouth in carbonated environments. The current density of corrosion attenuates with increasing crevice depth. Correspondingly, the maximum deposit thickness of corrosion products accumulates near the crevice mouth. Furthermore, the crevice structure significantly accelerates corrosion of P110 steel, as internal Cl⁻ accumulation and localized acidification destabilize and dissolve the passive film, causing dissolved Fe²⁺ to migrate towards the crevice mouth and combine with OH⁻ to form corrosion products that partially occlude the crevice channel. This deposition intensifies acidification beneath the product layer, establishing a peak in localized corrosion penetration depth.