Influence of Calcium Scaling on Corrosion Behavior of Steel and Aluminum Alloys.

Calcium scaling is a serious problem encountered in the oil and gas industry because it is common that brines produced alongside oil and gas exhibit high concentrations of calcium ions, among others, which is expensive to remedy. The precipitation of calcium salts on the internal wall of the pipelines may occur because of the physical and chemical changes as fluids are produced from downhole to surface facilities. Although different researchers have address scaling and corrosion in the oil and gas industry, there are few reports in the literature relating the corrosion and scaling phenomena simultaneously. Despite there being indications that scales may produce corrosion problems, affecting the mechanical integrity of the infrastructure, there is minimal research in the literature addressing such relations. Previous studies presented aluminum alloys as excellent and reliable materials for applications in the petroleum industry, such as drilling activities. In this work, we evaluate the corrosion behavior of steel and aluminum alloys under highly scaling environments using supersaturated brines. Our results show that the presence of calcium carbonate and calcium sulfate as a scaling environment increases the corrosion rates for aluminum alloys and carbon steel; however, the same environments do not affect the corrosion behavior of stainless steel.

Introduction

The oil industry demands a significant amount of tubular goods—drill pipe, casing, tubing, and risers—to meet its operational goals. The development of tools and equipment to drill deeper and faster wells to ensure the production and transportation of crude oil, in an energy-competitive and economical approach, is of great interest to technicians, researchers, and engineers.[1]

In the last two decades, different authors have pointed out to the oil industry community the advantages that aluminum alloys (AAs) may present for tubular manufacturing because, compared to steel, the length of the AA pipe would be two times longer than that of a steel one.[1−7] This feature has allowed the development of aluminum drill pipes (ADPs) with excellent potential in well-extended reach drilling of 15 km. The manufacturing of ADPs was extensively used in the former Soviet Union, starting in the second half of the 1950s. In the last decade, the use of aluminum drilling riser strings in deep waters of offshore Brazil and the Eagle Ford formation in Texas resulted in the increase of studies on the application of light metal alloys including casing and tubing, expandable tubular technology, hybrid riser configurations, fatigue analysis of drill pipes, steel pipe coatings, and geothermal drilling.[1,4−7]

Recently, studies on the application of light metal alloys for the oil and gas industry became especially relevant in deep offshore field exploration and development. Some of the advantages of aluminum (AA), compared to steel, involve its light weight, nonmagnetic properties, and reduced elasticity modulus. Besides, aluminum and its alloys have excellent resistance to corrosion, for instance, the AAs that resist corrosion in a water environment include series 1xxx, 3xxx, 5xxx, and 6xxx.[8] So far, the study and application of AAs in the oil industry include the 2xxx, 5xxx, 7xxx, and 7xxxCu series, showing high corrosion resistance and no hydrogen embrittlement;[1,4,5,9−11] however, it is of paramount importance to further evaluate their properties under practical conditions.

On the other hand, the connate water (brine) associated to the oil reservoirs, exhibit chemical and physical properties of great significance for the production assurance of hydrocarbons. Scaling, for example, is a phenomenon in which originally dissolved mineral salts at supersaturated solutions and specific conditions of temperature, pH, and pressure precipitate, causing blockage of fluid channels, pipelines condenser tubes, among others. The formation of scaling is frequent and expensive to remedy.[12]

Two of the most common scales found in oilfield production wells and surface facilities pipelines are calcium carbonate (calcite) and calcium sulfate (anhydrite and gypsum).[12−18] To manage a potential scaling problem, it is essential to know where and how much scales form during oil and water production. Hence, several authors have studied the scaling formation, resulting in the development of experiments and models, which aim to control and minimize the scaling.[12,15−40]

Although it is known that high pressure and temperature, as well as supersaturated brines, yield scaling environments, there are only a handful of studies available that address the influence of calcium scaling on the internal corrosion of pipelines for carbon dioxide storage[41,42] and external corrosion of buried pipelines.[43] For instance, Mansoori and co-workers recently studied the effect of calcium ions and CaCO3 scaling on CO2 corrosion of mild steel, highlighting the gap of research on internal corrosion of oil and gas pipelines.[44−46]

In this work, we study the relationship between the corrosion behavior and calcium scaling on aluminum and steel alloys using three different supersaturated brines; calcium carbonate (Brine 2), calcium sulfate (Brine 3), and calcium carbonate + calcium sulfate (Brine 4). The evaluation herein showed the influence of carbonate and sulfate scaling on the corrosion resistance of aluminum and steel alloys by employing high temperatures and supersaturated brines. Significantly, and to the best of our knowledge, there are no references available that address the simultaneous study of the scaling formation of CaCO3 or CaSO4 and the corrosion behavior for AAs or steel alloys. Besides, there are few references available reporting the influence of scaling and the corrosion phenomena simultaneously for steel alloys.[47] Therefore, the gap in the research related to the internal corrosion of oil and gas pipelines, especially for aluminum, is notorious.

Material and Methods

Samples

Coupons of stainless steel 304-2B (50 mm × 10 mm × 0.45 mm), aluminum 3003-H14 (50 mm × 10 mm × 0.43 mm), carbon steel 1045 (50 mm × 10 mm × 2.2 mm), and aluminum 2024-T3 (57 mm × 9 mm × 2.0 mm) were obtained from commercial vendors. Before the experiments, the surface of the samples was abraded with emery paper (800, 1000, and 4000) and polished with alumina mixture (particle size of 3 μm) until a mirror finish was obtained. Afterward, coupons were rinsed with distilled water, degreased with ethanol and dried under a hot air stream. For the immersion of the coupons into brine solutions, a brass wire was welded and covered with an insulating coating to avoid conduction current. The exposed surface area for all samples was between 6.08 and 11.17 cm2. Roughness data were obtained prior to and after samples were abraded and polished using a SJ-310 Series 178 portable surface roughness tester.

Surface and Electrochemical Tests

Surface, texture, and composition of the alloys before and after the immersion experiments were characterized using a JEOL JSM-5600LV scanning electron microscope and Thermo Fisher Scientific energy-dispersive X-ray data using an ultrahigh resolution Electroscan JSM-7800F model, Schottky field-emission JEOL. The results confirm the expected composition for the materials according to their writ, Figures S1–S4 and Tables S1–S4. The characterization of the carbonate and sulfate salts, formed on the coupons’ surface, was carried out by powder X-ray diffraction (PXRD) (Figures S5 and S6) on a Bruker D8 Advance with a Cu anode (kα = 1.5406 Å).

Electrochemical impedance spectroscopy (EIS) was employed to study the corrosion behavior of steel and AAs exposed to the brine solutions. The electrochemical setup consisted of a standard three-electrode cell configuration: (a) the working electrode, with an exposed surface area between 6.13 and 11.05 cm2, (b) a saturated calomel electrode (SCE), as the reference electrode, and (c) a graphite rod as a counter electrode (CE). For EIS, the sweep frequency was from 104 to 10–1 Hz with an amplitude of 10 mVrms at open circuit potential (OCP).

Evaluation of the alloy performances was carried out employing supersaturated brines with calcium carbonate (Brine 2), calcium sulfate (Brine 3), and a combination of calcium carbonate + calcium sulfate (Brine 4). Coupons were immersed for 3 h in the brine solution before the first electrochemical test was performed. Samples were permanently immersed in the brine solutions for 576 h; electrochemical tests were carried out every 48 h.

Brine Solution 1

All reagents were obtained from commercial vendors and used without further purification. Brine 1 simulates seawater composition. Note that seawater is commonly used in oil field operations for maintaining the pressure of the reservoir; also, it forms an immiscible flood front for pushing the oil toward the production wells. The addition of calcium carbonate and sulfate ions into Brines 2 to 4 relies on the fact that these ions are present in the produced brines. To prepare this brine, we used distilled water, NaCl (0.451 M), MgCl2·6H2O (0.052 M), Na2SO4 (0.032 M), CaCl2·2H2O (0.010 M), and NaHCO3 (0.002 M). Ion content: [Na+] = 11,885.73 mg/L, [Ca2+] = 400.78 mg/L, [Mg2+] = 1,215.25 mg/L, [Cl−] = 20,243.66 mg/L, [SO42−] = 3,074.00 mg/L, and [HCO3−] = 122.03 mg/L. Afterward, Brine 1 was used to prepare Brine 2, Brine 3, and Brine 4, as described as follows. Moreover, Brine 1 was employed to do a series of baseline experiments, or nonscaling conditions, for comparing the performance of the coupons in the different scaling environments studied in this work.

Calcium Carbonate: Brine 2

A volume of 700 mL of Brine 1 + CaCl2·2H2O (0.0536 mol) was mixed in a 1 L flask, with the aid of magnetic stirring and heated up to 90 °C for few minutes. Note that coupons were immersed in the solution before heating. Once the temperature was reached, the heating was stopped, and NaHCO3 (0.0677 mol) was added. Ion content of Brine 2: [[Na+] = 14,109.17 mg/L, [Ca2+] = 3,469.61 mg/L, [Mg2+] = 1,215.25 mg/L, [Cl−] = 25,673.04 mg/L, [SO42−] = 3,074.00 mg/L, and [HCO3−] = 6,023.23 mg/L. A white solid was formed almost immediately. Then, the solution was allowed to cool down slowly up to 35 °C.

Calcium Sulfate: Brine 3

A volume of 700 mL of Brine 1 + CaCl2·2H2O (0.0536 mol) was mixed in a 1 L flask, with the aid of magnetic stirring, and heated up to 90 °C for few minutes. Note that coupons were immersed in the solution before heating. Once the temperature was reached, the heating was stopped, and Na2SO4 (0.0404 mol) was added. Ion content of Brine 3: [Na+] = 14,539.41 mg/L, [Ca2+] = 3,469.61 mg/L, [Mg2+] = 1,215.25 mg/L, [Cl−] = 25,673.04 mg/L, [SO42−] = 8,618.19 mg/L, and [HCO3−] = 122.034 mg/L. A white solid was formed almost immediately. Then, the solution was allowed to cool down slowly up to 35 °C.

Calcium Sulfate + Calcium Carbonate: Brine 4

The same procedure as in Brine 2 and Brine 3 was followed. However, NaHCO3 (0.0677 mol) and Na2SO4 (0.0404 mol) were slowly added to the mixture to obtain Brine 4. Ion content of Brine 4: [Na+] =16,762.85 mg/L, [Ca2+] = 3,469.61 mg/L, [Mg2+] = 1,215.25 mg/L, [Cl−] = 25,673.04 mg/L, [SO42−] = 8,618.19 mg/L, and [HCO3−] = 6,023.23 mg/L. A white solid was formed immediately after the addition of the sodium salts (Scheme ).

Scheme 1

Experimental Setup

(A) exhibits the preparation and characterization of the aluminum and steel alloys prior to their immersion in Brines 1–4. (B) shows the environments employed and the techniques employed for measurement and characterization of the alloys after immersion in the brines.

Results and Discussion

The solubility of the minerals present in brines, associated to oil and gas fields, depends on the particular set of physicochemical conditions such as supersaturation, temperature, pressure, ionic strength, evaporation, contact time, and pH.[48] CaCO3 solubility is greatly influenced by the carbon dioxide content (partial pressure) of the water and temperature. However, for CaSO4 solubility, the partial pressure of CO2 is not as important; instead, scaling of this salt increases as the result of mixing dissimilar waters or as temperature and pressure change.

It is worth mentioning that the kinetic and the thermodynamic aspects of CaSO4 and CaCO3 crystallization have been studied by many authors in the laboratory.[28,30,31,36,49−51] However, there is only a handful of reports on the mixed precipitation phenomena, which is likely because of the inherent complexity of the scaling process, showing that even a small amount of another precipitating salt affects the scaling structure and the thermodynamic and kinetics of precipitation.[40,52−54]

For the experiments presented herein, we set three different scaling environments to evaluate their scaling and corrosive influence on steel and AA coupons. It is expected that the employed metals exhibit a distinctive affinity for nucleation and agglomeration of scaling because of the superficial roughness of each material. As shown in Table , the average mechanical roughness of each coupon is presented, as well as the rate of scale growth per material and scaling environment. The profilometry, scanning electron microscopy (SEM) images, and composition analysis are shown in the Supporting Information (Figures S7–S10 and Tables S5 and S6).

Table 1

Roughness Data and Comparison of the Scaling Growth on Steel and Aluminum Coupons after Exposure to Brines 2, 3, and 4

material	roughness (Ra; μm)	CaCO3 scaling mass (kg/m2)	CaSO4 scaling mass (kg/m2)	CaCO3 + CaSO4 scaling mass (kg/m2)
steel 304-2B	0.022–0.028	0.0201	0.0091	0.0049
carbon steel 1045	0.030–0.038	0.0511	0.0572	0.0624
AA 2024-T3	0.066–0.077	0.0552	0.0540	0.0838
AA 3003-H14	0.094–0.098	0.2665	0.2452	0.2288

Calcium Scales on Steel and Aluminum Coupons Using Brines 2, 3, and 4

Calcium scales, from Brine 2, 3, and 4, were formed at 90 °C, which was evident by the immediate formation of a white precipitate (see Materials and Methods). The final pH of the solutions was 7.21, 7.26, and 7.27 for Brine 2, Brine 3, and Brine 4, respectively. PXRD confirmed the formation of calcite and gypsum crystalline phases (see the Supporting Information for characterization of the scaling products). The yield of the reaction was quantified at 63% (0.0383 mol) of CaCO3, 45% (0.0276 mol) of CaSO4 for Brine 2 and Brine 3, respectively, while for Brine 4, the yield was 59% (0.0369 mol) of CaSO4 and 50% (0.0352 mol) of CaCO3. Coupons were immersed in the respective brine for 3 h before the first electrochemical measurement (T0) was taken. The coupons remained in the solution all the time except when the electrochemical analysis took place. Table shows that the roughness of the materials employed is related to the scaling mass per exposed area. It is worth noting that aluminum coupons, 2024-T3 and 3003-H14, display higher roughness than 1045 and 304-2B steel coupons. The visual examination after 576 h of the test showed a trend in scaling formation on the surface of the coupons in the following order: 3003-H14 > 1045 ≈ 2024-T3 > 304-2B, regardless of the type of Brines (2–4), see Figure . For SEM/EDS analysis, samples were cleaned employing the standard G1–90, ASTM. Subsequently, the surface, texture, and composition of the coupons were analyzed.

Figure 1

Coupons of aluminum and steel alloys before and after immersion in Brines 2, 3, and 4.

In the case of steel 304-2B, which has the lowest roughness, the scaling mass formed is on average 0.0114 kg/cm2, while the carbon steel 1045 and the AA 2024-T3 exhibit very similar scaling masses, 0.0569 and 0.0643 kg/cm2, respectively, despite the fact that there is a 0.0397 μm difference in roughness between the two materials. Finally, the AA 3003-H14 exhibited the highest roughness and the most significant scaling mass of 0.2468 kg/cm2 (Table ).

The mechanical profilometry diagrams and the exposed area analysis revealed that even though the roughness of 304-2B and 1045 samples is similar (0.0243 vs 0.0333 μm), the amount of scaling on 1045 is between 2.5 and 13 times higher than that observed on 304-2B (Figures S7 and S8). Although the roughness difference between 1045 and 2024-T3 is more significant (0.0333 and 0.730 μm, respectively), the amount of scaling is comparable, which could be associated to the texture of the samples, as seen in Figures S1 and S3, where scanning electron microscopy displayed nonhomogenous surfaces at a scale close to 50 μm. Finally, 3003-H14 presented the highest roughness of all the materials and showed a more significant amount of scaling, 47 times higher than that of 304-2B.

The Brines (2, 3, and 4) employed in this study can be considered as coprecipitating and also exhibit inverse solubility, since the precipitation of these out of a solution takes place at high temperatures, 40 °C for CaCO3 and 80 °C for CaSO4. Note that the ratio CO32–:SO42– in Brine 2 is 1:0.46, for Brine 3 is 0.03:1, and 1:1 for Brine 4. The comparison of the scaling mass on the coupons (Table ) showed that Brine 4 produced an inferior amount of scaling compared to Brine 2 and Brine 3 for the 304-2B and 3003-H14 coupons. It is possible to assume that scaling tendency is dependent on the physical properties of the samples rather than the characteristics of the brine; however, studies, where the coprecipitation of carbonate and sulfate crystals was examined, have shown that the pure calcium sulfate scale is less adherent than those scales containing coprecipitated carbonates and sulfates. Nevertheless, in the presence of CaSO4, the CaCO3 scaling, which is usually very adherent, loses its strength and becomes less adherent,[40,53,54] explaining the lower scaling masses on 304-2B, 2024-T3, and 3003-H14 for Brine 3 in comparison with Brine 2.

The calcium scaling was removed from the coupons to determine the mass losses and the corrosion rate by the standard G1-90, ASTM.

Visual examination of the coupons showed that the 3003-H14 and 1045 coupons suffered more damage than the rest for Brines 2–4, see Table and Figure . For AAs, Brine 4 (supersaturated with CaCO3 and CaSO4) was the most detrimental environment. While for the steel 1045, Brine 3 (supersaturated with CaSO4) produced more damage to the material, 304-2B did not show evidence of corrosion under the conditions employed. The mass loss and the corrosion rate for 304-2B immersed in Brine 2 should be taken with caution since mass transfer could have taken place from the insulating coating.

Table 2

Mass Losses and Corrosion Rate Comparison for Aluminum and Steel Alloys

2024-T3	exposed area (cm2)	mass losses (%)	corrosion rate (mm/year)
Brine 2	11.1691	0.43	0.0649
Brine 3	10.9493	0.61	0.0929
Brine 4	10.9707	1.01	0.1534
3003-H14
Brine 2	6.0788	3.32	0.2513
Brine 3	6.524	2.80	0.2703
Brine 4	6.294	3.67	0.3670
1045
Brine 2	10.6928	0.19	0.0276
Brine 3	10.6352	2.25	0.3086
Brine 4	10.2148	0.15	0.0217
304-2B
Brine 2	6.4592	0.73	0.0355
Brine 3	6.5672	0.00	0
Brine 4	6.1264	0.00	0

Figure 2

Coupons of aluminum and steel alloys after cleaning.

After the comparison of the exposed materials immersed in different environments, it is possible to suggest that AAs suffered more damage in the supersaturated mixed solution of calcium carbonate and calcium sulfate. In comparison, steel 1045 exhibited more damage in a supersaturated solution of calcium sulfate, see Table .

SEM/EDS analysis showed that Brines 2–4 formed a nonhomogeneous scaling coating on the surface of 304-2B and 1045. The composition of the measured areas revealed the presence of calcium salts, according to the EDS analysis. Thus, it is clear that the dissolution (sodium hydroxide and zinc) employed for cleaning the coupons did not remove the calcium carbonate scaling from the coupons’ surface because it was a basic solution.

Figures A,B show that the phase distribution is not homogeneous; for instance, the darker regions correspond to the light elements (scaling products), while the brighter regions relate to heavy elements (coupon composition). The secondary electron micrography (Figure S9) showed the topography of 304-2B and 1045, and it was evident that 1045 is worn because of the exposure to the scaling and corrosive environment. 304-2B does not appear worn because it has a more uniform texture than 1045. We suggest that such scaling coating, although nonhomogeneous, might have protected the material surface, affecting the corrosion rate on the coupons.

Figure 3

Backscattered electron images for steel alloys (A) 1045 and (B) 304-2B and AAs (C) 2024-T3 and (D) 3003-H14 immersed in scaling conditions.

In contrast, the AAs 3003-H14 and 2024-T3 did not show evidence of scaling on the surface of the coupons immersed in any of the Brines (2–4), which could be explained by the fact that the cleaning solution employed was acidic (nitric acid), dissolving the calcium carbonate scaling. Figure C shows the backscattered electron images for 2024-T3, which exhibit a localized (pitting) corrosion with irregularly shaped cavities on the surface. In general, this type of corrosion is prone to pH close to neutral, covering all-natural environments such as seawater, surface water, and moist air, which matches the experimental conditions employed in this work. Figure D presents the micrography for 3003-H14 where surface fractures are visible by SEM. It is possible to infer the material worn due to corrosion, generating large voids. Even though calcium scaling grew on the surfaces of the AAs, the coupons suffered pitting and crevice corrosion for 2024-T3 and 3003-H14, respectively. So, it is feasible to assume that scaling speeds the corrosion process up.

Figure exhibits the backscattered electron micrography for the baseline experiments, also tested, for 576 h in a nonscaling environment (Brine 1). The secondary electron images are available in the Supporting Information (Figure S10). SEM/EDS analysis showed no damage for the 1045 and 304-2B steel alloys; conversely, the AAs exhibited a similar behavior compared to the scaling environments (Brines 2–4), meaning that pitting and crevice corrosion are present for 2024-T3 and 3003-H14, respectively; however, the damage on the coupons is less severe for the nonscaling environment.

Figure 4

Backscattered electron images for steel alloys (A) 304-2B@500 μm and (B) 304-2B@100 μm and AAs (C) 2024-T3 and (D) 3003-H14 immersed in nonscaling conditions.

Besides, we evaluated the corrosion of the coupons, employing EIS. For the sake of simplicity, only EIS data at 576 h is presented because it was observed that for each material Nyquist and Bode plots do not vary significantly from 0 to 576 h. Therefore, the discussion of the EIS data focuses only on 576 h (Figure ). The comparison for each sample at 0h and 576 h can be found in the Supporting Information (Figures S11 and S12).

Figure 5

EIS results for 304-2B and 1045 at 576 h of immersion in Brines 2 (CaCO3), 3 (CaSO4), and 4 (CaCO3 + CaSO4). (5A) corresponds to the Nyquist plot, and an enlarged plot is also presented to highlight the 1045 response, while (5B) shows the Bode plot data.

After exposure to Brines 2–4, 304-2B showed final impedance values between 104 and 105 Ohm·cm2 (at 0.1 Hz), while the impedance values of 1045 were close to 103 Ohm·cm2 (at 0.1 Hz). These results reveal that the 304-2B sample exhibited higher impedance during the whole test (576 h), meaning that it is significantly more corrosion-resistant than the 1045 sample. Although the final impedance values (5B) did not vary significantly, the Nyquist plot (5A) depicts a larger semicircle in Brine 3 compared to the semicircles in Brines 2 and 4 for 304-2B samples, which is indicative of a capacitive behavior. The 1045 coupons immersed in scaling Brines (2–4), exhibited lower impedance values (close to 103 Ohm·cm2) and smaller semicircles, regardless of the brine employed. All the above mentioned implies less corrosion for 304-B, which is in agreement with the visual, corrosion rate, and SEM/EDS analysis described above.

For the AAs 2024-T3 and 3003-H14 the impedance differences are less pronounced than in the case of steel alloys. As in the previous analysis, Brine 3 provided the highest impedance values for both alloys, see the Bode plot. The 2024-T3 sample showed final impedance values between 2 × 104 Ohm·cm2 (Brine 4) and 4 × 104 (Brine 3) Ohm·cm2 (at 0.1 Hz), compared to 3003-H14, which final impedance values are one order of magnitude lower, with impedance values between 3 × 103 Ohm·cm2 (Brine 4) and 6 × 103 (Brine 3) Ohm·cm2 (at 0.1 Hz). The Nyquist plots of the AAs (AA) exhibited lower slopes than the 304-2B samples, which means lower impedances for the AA samples, therefore higher charge transfer process.

Besides, the 2024-T3 samples presented larger semicircles in Nyquist plots for Brine 3 compared to semicircles in Brines 2 and 4. This feature proves higher impedances at a lower frequency, as seen in Figure B, which means that the CaSO4 scaling environment might protect the metal decreasing the corrosion phenomena compared to CaCO3 and CaCO3 + CaSO4 scaling environments.

Figure 6

EIS results for 2024-T3 and 3003-H14 at 576 h of immersion in Brines 2 (CaCO3), 3 (CaSO4), and 4 (CaCO3 + CaSO4). (A) corresponds to the Nyquist plot, while (B) shows the Bode plot data.

From the electrochemical data, it is possible to assume that a less homogeneous texture of the material (as presented in Table S5) might play a significant role in scaling and corrosion. In general, the corrosion behavior is related to the impedance response of each material tested in this study, that is, the higher the impedance after exposure to the brines, the less the corrosion. The decreasing order for the impedance values of the samples is 304-2B > 2024-T3 > 3003-H14 > 1045.

Besides, the electrical resistance and capacitance were estimated for the baseline experiments, (nonscaling environment, Brine 1). Figure shows the data corresponding to the oxide products on the coupons; the data for the bare metals are shown in Figure S14. As observed in 7A, the capacitance of the samples decreased with time meaning the continuous formation (non-permeable) of oxide products on the surface of the materials. By the end of the test, 3003-H14 exhibited the lowest capacitance value of all the samples, which is attributed to more stable oxide products. The 2024-T3 system has the highest capacitance, so the oxide products are less stable. Moreover, Figure B is useful to evaluate the protective character of the oxides for the baseline experiments meaning that the higher the resistance, the more significant corrosion. Again, 3003-H14 and 2024-T3 exhibited the highest and the smallest damage, respectively. Therefore, we assume that the scaling environment (Brines 2−4) increased the corrosion rate for the aluminum alloys. The steel alloys’ (1045 and 304-2B) behavior remained almost constant during the test.

Figure 7

Electrical capacitance and resistance of the formed oxides on steel and AAs for the baseline experiments (nonscaling conditions).

Finally, the potential diagrams (Figure ) represent the electrochemical state of the samples during the corrosion process in solution. The potential was measured under open circuit conditions prior to the electrochemical tests. According to Figure , the corrosion potential of the 304-2B sample showed higher OCP potential (more positive) among all samples. Correlating this value with the thermodynamic features of the iron Pourbaix diagram (pH = 7.21–7.27 with E ≈ −300 up to +150 mV), it is established that the sample was within the passivation region, which led the formation of a continuous oxide layer that protected the metal substrate at longer exposure times as evidenced the higher impedances values of the system (Figure ). The 1045 sample exhibited an average potential value of around −600 mV (SCE), which correlated the Pourbaix diagram again, corresponding to the lower part of the intermediate oxidation region of the Fe2+ where different equilibrium reactions took place. Therefore, this sample did not develop passivation conditions as the 304-2B sample did. For this reason, the impedance measurements registered lower values regardless of the brine solution at 576 h of testing. On the other hand, the AAs presented the lowest OCP potentials (less negative) from −780 to −610 mV; these conditions (potential and pH = 7.21–7.27) correspond to a complete passivation feature, which was more evident for 2024-T3 than for 3003-H14 because the impedance values were higher for the former sample (Figure ). According to SEM/EDS analysis (Figure ), the oxide corrosion products were porous calcium scales formed on the surface of the AAs with pieces of evidence of pitting and/or crevice corrosion, as reported elsewhere.[47,48]

Figure 8

OCP Potential over time for steel 304-2B, carbon steel 1045, AA 3003-H14, and AA 2024-T3 coupons immersed in Brine 2 (A), Brine 3 (B), and Brine 4 (C).

Conclusions

In this study, we have analyzed and compared the performance of steel and aluminum alloys in the presence of highly scaling and corrosive environments. The results presented herein show that the presence of mixed carbonate and sulfate scaling environments (Brines 2–4) increased the corrosion rate for the AAs and the carbon steel when compared to a nonscaling environment. However, the scaling environments did not affect the corrosion behavior of stainless steel greatly.

The 304-2B exhibited less roughness and less amount of scaling deposition with higher anticorrosion properties at longer exposure times. Although 1045 and 2024-T3 displayed similar roughness and scaling growth, impedance values differ in one order of magnitude, which shows that 1045 is less resistant to corrosion than 2024-T3 under scaling conditions. Visual examination of the 1045 samples after the 576 h of testing confirms the formation of significantly more oxides on the surface of 1045 compared to 2024-T3; the oxides on 1045 were more stable and protected the material better from further corrosion as the corrosion test advanced. It was notorious by all the techniques employed that 3003-H14 exhibited the highest roughness, the most significant scaling formation, and the lowest corrosion resistance compared to the rest of the samples. The baseline experiments (nonscaling environment) demonstrated that the scaling environments increase the corrosion rate for the AAs and carbon steel.

In general, the AAs (2024-T3 and 3003-H14) employed in this study did not perform better than the 304-2B; however, 2024-T3 performed better than 1045, which is commonly employed in oil and gas industry activities. Therefore, it would be desirable to test other AAs exhibiting better corrosion resistance under oil and gas production conditions. The 5xxx and 6xxx series are promising candidates for this purpose. Further studies in our group are evaluating such candidates to study corrosion and scaling phenomena.

Our results contribute to the understanding and study of a topic we believe is in its infancy—the simultaneous scaling and corrosion phenomena on materials employed in the oil and gas industry. We hope that the results presented herein draw attention to (a) the lack of research in the internal corrosion of steel and aluminum pipelines and (b) the need to evaluate AAs as materials for tubular manufacturing for the oil and gas industry.