Corrosion phenomena investigated in this study are representative of conditions encountered in upstream oil and gas production systems. Figure 1 illustrates a typical oil well configuration, including the wellhead, tubing, production casing, packer, and perforated zones. In such systems, the production tubing commonly manufactured from L80 steel is continuously exposed to aggressive environments containing CO₂, H₂S and saline formation waters [23][24]. These fluids flow from the reservoir through the perforations into the production casing and tubing, where pressure and temperature variations promote complex physicochemical interactions. The presence of CO₂ leads to the formation of carbonic acid, while H₂S contributes to sulfide formation, both of which significantly influence corrosion mechanisms. In addition, the composition of formation water, particularly chloride concentration and the nature of dissolved cations (Na⁺, Ca²⁺, K⁺) plays a critical role in determining corrosion severity and scale formation. Therefore, the experimental conditions selected in this work (controlled temperature, CO₂/H₂S environments, and different brine compositions) were designed to simulate realistic downhole conditions, especially those affecting the tubing and production casing materials.

The material investigated in this study is tubing: 3-1/2”9.20 ≠ L80 API-NUE carbon steel [25], commonly used in oil and gas well tubing due to its favorable mechanical properties and moderate resistance to corrosion. The chemical composition of the steel is presented in Table 1.

Figure 1. Schematic of a typical oil well.

Table 1. Chemical composition of L80 steel (wt.%).

Rectangular specimens were machined from L80 Steel and embedded in epoxy resin, leaving an exposed working surface of approximately 1 cm². Prior to each experiment, the samples were sequentially ground using silicon carbide abrasive papers with grit sizes ranging from 320 to 1200, followed by polishing with alumina suspension to obtain a mirror like surface finish. After polishing, the specimens were thoroughly rinsed with distilled water, degreased with ethanol, and dried in warm air. The prepared samples were immediately immersed in the test solution to minimize surface oxidation prior to measurements. Three brines representative of oilfield formation and completion fluids were prepared using analytical grade salts dissolved in deionized water. The chemical compositions of the tested brine are summarized in Table 2.

Table 2. Electrochemical Tafel and Stern-Geary parameters for the tested brines.

All electrolytes were prepared using analytical-grade reagents and deionized water. The brine formulations used in the experimental program were NaCl (0.5 mol/L), KCl brine (specific gravity, sg = 1.02), and a heavy CaCl₂ completion brine (sg = 1.60). Analytical characterization of the KCl brine gave approximately 34,500 ppm Cl⁻ and 19,000 ppm K⁺. The CaCl₂ brine contained approximately 37,000 ppm Cl⁻, 18,500 ppm Br⁻, and 22,000 ppm Ca²⁺. For the 0.5 M NaCl solution, the ideal ionic strength is I = 0.5 M. For the field-formulated KCl and CaCl₂ brines, ionic strength was evaluated from the measured ionic composition using I = 0.5 ∑ ( c i z i 2 ) ; because only the principal reported ions are available in the analytical summary, the listed ion concentrations are reported explicitly rather than inferring an unverified complete formulation. Initial target pH values ranged from 3.5 to 6.5 and were adjusted using 1 M NaOH. Gas-pressure note: the supplied field documentation reports reservoir pressures of 808 psi (Low Case), 897 psi (Base Case), and 997 psi (High Case) at 1495 mTVDSS. These are field scenario pressures and are not presented as the electrochemical-cell total pressure. A unique H₂S partial pressure was not provided. The main experimental conditions are summarized in Table 3.

Electrochemical measurements were performed using a conventional three-electrode cell configuration. The L80 steel specimen served as the working electrode (WE), a saturated Ag/AgCl electrode was used as the reference electrode (RE), and a platinum electrode acted as the counter electrode (CE). Before testing, the brines were actively deaerated for at least 6 h and then brought to the thermostatically controlled test temperature. The field-gas specification used for the corrosion scenario contained CH₄ (85 - 100 mol%), C₂H₆ (0 - 6 mol%), C₃H₈ (0 - 5 mol%), C₄H₁₀ (0 - 3 mol%), C₅H₁₂ (0 - 2 mol%), CO₂ (0 - 2 mol%), N₂ (0 - 3 mol%), and O₂ (0 - 1 mol%), with combined inert compounds limited to 5 mol%. The modeled equilibrium pCO₂ was 0.9 bar at 40˚C, 0.8 bar at 60˚C, and 0.5 - 0.6 bar at 80˚C. H₂S was present as variable trace species associated with reservoir souring, but a unique experimental pH₂S value is not available in the supplied protocol and is therefore not assigned here. For long-term OCP/LPR monitoring, the cell was hermetically sealed after preparation; equilibrium pCO₂ was established from the multiphase simulation used for the test scenario. The electrochemical cell configuration is illustrated in Figure 2.

Figure 2. Schematic representation of the electrochemical cell used for corrosion measurements.

The corrosion rate (CR, mm·year⁻¹) was determined using the following relationship:

Legend: where k = 3.27 × 10⁻³ (mm·g·μA⁻¹·cm⁻¹·year⁻¹), i_corr is the corrosion current density (μA·cm⁻²), EW is the equivalent weight of the alloy, and ρ is the density on of iron (7.85 g·cm⁻³). For L80 steel, EW= 27.92 g·equiv⁻¹. In addition, polarization resistance (R_p) was determined from the near OCP region of the polarization curves and used to estimate the corrosion current density through the Stern Geary equation:

Legend: where β_a and β_b are the anodic and cathodic Tafel constants, respectively, and B is the Stern Geary constant [26]. After each experiment, the specimens were cleaned using ethanol, rinsed with distilled water, and dried in a desiccator prior to further analysis.

Table 3. Experimental conditions.

The corrosion mechanism of L80 steel in CO₂/H₂S containing brine is illustrated schematically Figure 3. The process involves coupled anodic and cathodic reactions occurring at the metal surface [27]-[29].

Figure 3. Schematic of L80 corrosion in CO₂/H₂S brines.

The chemical reaction equations from the diagram: The anodic reaction corresponds to iron dissolution: Corrosion begins with the oxidation of metallic iron and is fundamental to the corrosion of L80 steel.

The cathodic reactions include hydrogen evolution and carbonic acid reduction: This reaction is dominant in the presence of dissolved CO₂ and H₂S which acidify the environment. These reactions increase the concentration of H⁺ and accelerate corrosion. Proton reduction (acidic environment CO₂/H₂S which)

Additionally, dissolved CO₂ reacts with water to form carbonic acid, according to following equilibrium reactions (mild corrosion) [29][30]:

Carbonic acid (H₂CO₃) subsequently dissociates into hydrogen and bicarbonate ions and then (H⁺ and HCO 3 − ) into hydrogen and carbonate ions ( CO 3 2 − )

When the solution becomes supersaturated, ferrous ions (Fe²⁺) react with carbonate ions ( CO 3 2 − ) to form iron carbonate (FeCO₃) also known as siderite:

This compound can form a partially protective layer on the surface of L80 steel under favorable conditions.

In the presence of hydrogen sulfide (H₂S) iron sulfide may also form. The dissociation of H₂S occurs as follows:

Subsequently, ferrous ion sulfide (FeS):

These corrosion products (FeCO₃ and FeS) can either protect the steel surface or promote localized corrosion depending on their morphology and stability. FeS is generally less protective than FeCO₃ and promotes localized corrosion and hydrogen embrittlement. Overall reaction of corrosion in a CO₂ environment [31]

Overall reaction in H₂S environment

A thermodynamic analysis was performed to evaluate the stability and formation conditions of corrosion products in the investigated environments. FeCO₃ precipitation was assessed using a saturation index based on ionic activities. Temperature, pH, and ionic strength were considered because concentrated brines exhibit non-ideal behavior. Pourbaix diagrams for Fe-CO₂-H₂O and Fe-S-H₂O systems [32][33] were used to discuss the thermodynamic stability domains of iron species and corrosion products. For a reproducible non-ideal treatment, the activity-coefficient model and its parameters must be stated explicitly, together with the temperature dependence of the FeCO₃ solubility product

where a Fe 2 + and a CO 3 2 − denote the activities of Fe²⁺ and CO 3 2 − , respectively, and K_sp(T) is the temperature-dependent solubility product of FeCO₃. The thermodynamic calculations followed the Sun-Nešić formulation, in which the ionic-strength correction is incorporated into the apparent FeCO₃ solubility relation. The correlation used in the MATLAB calculations was: logK_sp(FeCO₃) = −59.3498 − 0.041377T − (2.1963/T) + 24.5724 logT + 2.518I^0.5 − 0.657I, where T is the absolute temperature (K) and I is the global ionic strength (mol/L). SI > 0 indicates thermodynamic supersaturation with respect to FeCO₃.

The density of various brine solutions was investigated under different conditions, including the presence of calcium chloride (CaCl₂), potassium chloride (KCl), and sodium chloride (NaCl), in an environment where the temperature reached 80˚C after 72 hours. The results are presented in Figure 4, which illustrates the relationship between density and salt concentration for each brine solution. Figure 4 shows that CaCl₂ brine exhibits the highest density compared to other chloride-based brines, followed by KCl brine. This observation is consistent with existing literature and reinforces the widespread use of CaCl₂ and KCl as medium to high-density brines in well completion and workover operations [13][34]. Furthermore, the data reveals that at 28˚C, KCl brine reaches a higher maximum density than NaCl brine. This result indicates that KCl is more effective than NaCl in increasing the density of aqueous fluids. Overall, brine density and pH vary significantly with both temperature and ionic composition.

KCl is the most stable brine in terms of both density and pH, making it the most suitable electrolyte for minimizing corrosion of L80 steel. NaCl exhibits intermediate stability, making it suitable for moderately corrosive environments such as wells. In contrast, CaCl₂ is the least stable brine, showing strong acidification, significant density variation, and a catalytic effect on corrosion (as reflected by corrosion rate (CR) and corrosion current density (i_corr)). The decrease in pH and density is directly correlated with the increase in corrosion rate observed experimentally. Figure 4 shows that corrosion of L80 steel in CaCl₂ is governed by acid driven kinetics, whereas corrosion in KCl is minimal, confirming trends widely reported in the literature.

Figure 4. Graph of dissolved salt concentration in water.

Figure 5. Evolution of density and pH of brines with temperature.

KCl brine exhibits the highest stability in terms of density and pH over the temperature range 50˚C - 80˚C. Its density remains nearly constant, and its pH shows minimal variation, indicating limited acidification. This behavior reflects lower ionic reactivity and reduced interaction between aggressive species and the metal surface, as shown in Figure 5. NaCl brine shows moderate density variations, consistent with its intermediate ionic strength. In contrast, CaCl₂ brine exhibits the largest fluctuations in density and the most significant decrease in pH, indicating strong ion exchange reactions and accelerated acidification. The observed acidification in CaCl₂ brine is associated with hydrolysis reactions and the release of acidic species, which reduce solution alkalinity. This effect destabilizes protective corrosion films and enhances steel dissolution. The strong dependence of corrosion on pH is consistent with established mechanisms, where decreasing pH increases carbonic acid concentration and accelerates the cathodic hydrogen evolution reaction.

Electrochemical responses were evaluated using potentiodynamic polarization. Figure 6 shows the polarization curves obtained for L80 steel in NaCl, KCl, and CaCl₂ electrolytes at different temperatures.

Figure 6. Polarization curves of L80 steel at different temperatures and electrolytes.

Potentiodynamic polarization analysis indicates competition between charge-transfer kinetics, mass transport, and the stability of interfacial corrosion-product films. After the stated OCP stabilization period, E_corr values remained within a relatively narrow range across the investigated conditions, while temperature and electrolyte composition modified the polarization response. The interpretation of E_corr should therefore be considered together with i_corr, Tafel slopes, and the cathodic transport-limited region rather than as an isolated measure of corrosion severity.

The cathodic branch is characterized by a diffusion-limited plateau between −700 and −850 mV, the magnitude of which increases with temperature in accordance with the temperature-dependent laws of diffusion coefficients. Notably, the nature of the cations radically influences anodic dissolution: while KCl promotes the formation of a stable interfacial film, NaCl acts as an intermediate medium, and CaCl₂ ultimately exacerbates iron dissolution. In calcium-rich chloride media, this behavior is punctuated near −500 mV by the emergence of a singular current peak. This phenomenon likely reflects the mechanical rupture of a mixed iron and calcium carbonate layer FeCO₃ and CaCO₃, preferentially precipitated due to local interfacial alkalization. The instability of this protective film, accentuated by the high ionic strength of the medium, underscores the deleterious role of Ca²⁺ and Cl⁻ ions in destabilizing passivating phases at high anodic overpotentials.

At lower temperature, the Tafel regions indicate an important contribution from activation-controlled charge transfer. The cathodic polarization curves also display a transport-limited region, showing that the process cannot be described as purely activation-controlled over the full potential range. At higher temperature, changes in the anodic and cathodic slopes, together with enhanced transport and corrosion-product development, are consistent with mixed activation-diffusion control. The applicable regime therefore depends on potential, temperature, electrolyte composition, and the extent of surface-film formation.

A shift of E_corr toward more positive values is observed with increasing temperature and salinity, indicating an increase in corrosion activity. KCl exhibits the highest E_corr values, confirming its lower aggressiveness, while CaCl₂ shows more active corrosion behavior. This trend reflects the modification of electrochemical equilibrium and surface film stability, in agreement with previous electrochemical studies. However, temperature also affects corrosion product stability. The solubility of FeCO₃ decreases with increasing temperature, promoting its precipitation on the steel surface. These results are consistent with previous studies reporting the influence of temperature on FeCO₃ precipitation kinetics [33]. The evolution of the corrosion potential (E_corr) of L80 steel in the different electrolytes is presented in Figure 7.

Figure 7. Evolution of corrosion potential (E_corr) of L80 steel in different electrolytes.

The evolution of corrosion rates (CR) is presented in Figure 8.

Figure 8. Corrosion rate vs. temperature for L80 steel in different electrolytes.

Corrosion rates (CR, mm/year) and i_corr values extracted from polarization curves and LPR measurements are discussed together with the electrochemical results. Across the reported experimental conditions, corrosion rate generally increased with temperature. Electrolyte aggressiveness in the reported experimental dataset followed the order CaCl₂ > NaCl > KCl. CaCl₂ produced the highest corrosion rates, whereas KCl showed the lowest values. Time-dependent datasets labelled as prediction are treated as modeled/predicted results and are not used as experimental replicates. An SI > 0 denotes thermodynamic supersaturation.

The effect of brine composition is clearly observed in Figure 7 and Figure 8. NaCl and KCl systems exhibit lower corrosion rates and more stable electrochemical behavior compared to CaCl₂. This difference is attributed to the higher ionic strength of CaCl₂, which increases FeCO₃ solubility and delays its precipitation. The delayed formation of protective films in CaCl₂ brine explains the higher corrosion rates observed. This delayed precipitation can be attributed to increased ionic strength, which enhances FeCO₃ solubility [33].

The weight loss measurements presented in Figure 9 confirm the electrochemical results.

Figure 9. Weightloss variation of L80 steel as a function of temperature.

Corrosion rate increased significantly from 50˚C to 80˚C for all electrolytes. At 80 ˚C the enhancement of mass transport, faster Fe dissolution, and increased molecular diffusion amplified corrosion. However, in KCl and NaCl brines, a partial reduction of the corrosion rate was observed after long exposure (>10 h), indicating the onset of FeCO₃ precipitation and partial surface passivation. The weight loss measurements under different temperatures conditions are grouped in Figure 9.

Lower pH caused a strong exponential increase in CR and i_corr. At pH = 3.5, all systems remained in the active corrosion domain, with no evidence of FeCO₃ stabilization. At pH = 6.5, the conditions favored the transition toward the passivation zone predicted by Pourbaix diagrams, especially in KCl and NaCl environments. The effect of pH on the weight-loss response of L80 steel is illustrated in Figure 10.

Figure 10.Weightloss variation of L80 steel as a function of pH.

The results indicate that lower pH values significantly accelerate corrosion, with CaCl₂ producing the highest level of material degradation among the electrolytes investigated. This behavior is attributed to the increased availability of H⁺ ions under acidic conditions, which enhances cathodic reactions, particularly the hydrogen evolution reaction, thereby accelerating the overall corrosion process. In addition, the high ionic strength of CaCl₂ further contributes to destabilizing protective films, leading to sustained metal dissolution. These findings are consistent with established corrosion mechanisms in CO₂ containing environments, where carbonic acid plays a central role in promoting steel degradation and controlling electrochemical reactions at the metal solution interface.

Scale formation increases with salinity, particularly when Fe²⁺ and CO 3 2 − reach supersaturation. However, scale protectiveness varies significantly. In CaCl₂ and NaCl, the scales are porous, weakly adherent, and easily disrupted by chloride ions, resulting in sustained corrosion. In KCl, the scale is denser and more uniform, providing partial protection.

Higher salinity promotes scale formation, particularly under conditions favorable for FeCO₃ precipitation: Fe 2 + + CO 3 2 − → FeCO 3 ( s ) . However, chloride ions reduce the stability of the protective layer, resulting in non-uniform scale formation; Increased localized corrosion; This confirms that scale formation does not necessarily guarantee corrosion protection. Average corrosion rates followed the order: CaCl₂ > NaCl > KCl. The variation in corrosion-scale mass with electrolyte type and salinity is shown in Figure 11.

Figure 11. Evolution of corrosionscale mass with electrolyte type and salinity.

KCl (CR ≈ 1.8 mm/year) showed the lowest corrosion, due to stable density, limited acidification, and more coherent film formation. NaCl (CR ≈ 2.5 - 2.6 mm/year) presented moderate corrosion, consistent with its typical behavior in CO₂ systems.

CaCl₂ (CR > 3.5 mm/year in the reported high-corrosion conditions) produced the highest corrosion rates. These results were interpreted in relation to Ca²⁺ interactions, ionic strength, and corrosion-product-film stability. Multifactorial analyses were also used to examine the influence of pH, temperature, salinity, and dissolved oxygen. Predicted/model-derived datasets are distinguished from direct experimental observations and are not treated as experimental replicates.

The influence of chloride concentration in different electrolytes is summarized in Figure 12.

Figure 12. Effect of chloride concentration on corrosion behavior of L80 steel.

Chloride ions can intensify corrosion by increasing electrolyte conductivity and contributing to disruption of surface films. In the reported experimental dataset, CaCl₂ exhibited the greatest corrosion response, NaCl showed intermediate behavior, and KCl showed the lowest corrosion response. Because chloride concentration differs substantially among the tested brines, the observed differences cannot be attributed solely to the cation identity; chloride level, ionic strength, pH, temperature, and film stability must be considered jointly. The thermodynamic evolution of FeCO₃ solubility and saturation is presented subsequently in Figure 13.

The overall corrosion mechanism is summarized in Figure 3. At elevated temperature, electrochemical reaction rates and mass transport increase, while FeCO₃ precipitation can become thermodynamically more favorable. The resulting corrosion behavior reflects the balance between activation-controlled charge transfer, cathodic transport limitations, and transport through or breakdown of corrosion-product films. Consequently, the system may evolve from predominantly activation-controlled behavior near OCP toward mixed activation-diffusion control as temperature increases and surface films develop.

3.9.1. Solubility of FeCO₃ (K_sp) and Saturation Index (SI)

Figure 13 presents the variation of the solubility product (K_sp) of iron carbonate (FeCO₃) as a function of temperature in the range 290 - 360 K for L80 steel. This evolution highlights two distinct thermodynamic regimes that govern precipitation behavior and corrosion protection. In the temperature range of 290 - 340 K, a pronounced decrease in the solubility product is observed. This trend indicates that the solubility of FeCO₃ decreases significantly as temperature increases, thereby enhancing its thermodynamic stability in the solid state. From a thermodynamic standpoint, the reduction in K_sp reflects a decrease in the Gibbs free energy of dissolution, which promotes the precipitation of FeCO₃. Previous studies S. Nešić and W. Sun have reported similar behavior in CO₂ containing systems, where increasing temperature favors FeCO₃ formation and leads to the development of protective surface layers [6][9]. Under these conditions, the system rapidly reaches supersaturation, which promotes nucleation and growth of FeCO₃ crystals and contributes to a progressive reduction in corrosion rate. Beyond 340 K, the evolution of K_sp becomes relatively stable, with only minor variations observed up to 360 K. This behavior indicates that the system approaches a quasi-equilibrium state in which FeCO₃ becomes only slightly soluble and the influence of temperature on its solubility becomes secondary. In this high-temperature regime, corrosion behavior is no longer controlled primarily by thermodynamics but rather by kinetic processes such as precipitation rate, mass transport, and film restructuring. At these temperatures, FeCO₃ precipitation is rapid and leads to the formation of a dense, adherent, and protective layer on the steel surface. As reported by Nesic [35][36]. above approximately 70˚C - 80˚C, corrosion control shifts from a thermodynamic regime to a diffusion-limited regime governed by transport through the FeCO₃ film. The overall decrease in the solubility product between 290 and 340 K reflects the increasing stability of FeCO₃ and its strong tendency to precipitate on the L80 steel surface. Above 340 K, the stabilization of K_sp indicates that FeCO₃ solubility becomes negligible and that corrosion behavior is primarily governed by the formation, integrity, and protective properties of the corrosion product layer.

Figure 13. FeCO₃ solubility, Fe²⁺ saturation and SI evolution.

As shown in Figure 13(A), the solubility product (K_sp) decreases significantly with increasing temperature, confirming that FeCO₃ becomes less soluble at elevated temperatures. This reduction in solubility promotes precipitation and favors the formation of a protective layer under suitable chemical conditions. Furthermore, Figure 13(B) shows that the saturation index (SI) increases progressively with time. When SI exceeds zero, the solution becomes supersaturated and precipitation becomes thermodynamically favorable. At 80˚C and pH 6.5, KCl and NaCl systems reach supersaturation earlier and maintain it for a longer duration, allowing the formation of a dense and protective FeCO₃ film. In contrast, the CaCl₂ system exhibits a delayed increase in SI, which is attributed to its higher ionic strength. This elevated ionic strength enhances FeCO₃ solubility and delays the onset of supersaturation, thereby hindering the formation of a stable protective film. As a result, corrosion remains more active in CaCl₂ brines, which is consistent with the experimental observations. The saturation index stabilizes after approximately 24 hours, indicating that the system approaches thermodynamic equilibrium and that the rate of film growth decreases significantly. This stabilization behavior correlates with the lower corrosion rates observed in KCl and NaCl systems and highlights the strong relationship between precipitation kinetics and corrosion protection efficiency.

3.9.2. Practical Implications for Well Integrity

The results highlight important operational implications: KCl is the most suitable brine for completion operations requiring minimal corrosion risk. NaCl is acceptable for wells of moderate corrosion severity CaCl₂ Brines should be avoided, when possible, due to their strong acidification, destabilization of protective films, and short predicted service life (≈3.9 years). Maintaining pH near neutral and minimizing dissolved oxygen are fundamental for extending tubing lifespan. Real time monitoring of Fe²⁺, pH, and salinity is essential for detecting transitions between active corrosion and passivation domains.