The effect of a paraffinic model oil (LVT-200)-containing select surface-active compounds (myristic acid and acridine) on CO2 corrosion with and without intermittent wetting has been studied. Observations have shown that the presence of myristic acid in the oil phase does not affect the corrosion behavior due to its lack of partitioning in the water phase. However, after direct contact between the oil phase-containing myristic acid and the metal surface, there was a significant decrease in the corrosion rate. This phenomenon gradually diminished at pH 4.0 but was more persistent at pH 6.5. The presence of acridine in the oil phase was shown to have a strong inhibitive effect at pH 4.0, even during the partitioning step. The partitioning of acridine from the oil phase to the water phase at pH 4.0 was confirmed by ultraviolet-visible spectroscopy results. However, there was no inhibitive effect conferred by the presence of acridine on the corrosion rate at pH 6.5. An experimental methodology was developed that facilitated improved simulation of the effect of intermittent oil/water wetting on CO2 corrosion. The electrochemical current response during the oil/water intermittent wetting cycles showed that the persistency of model oil (without surface-active compounds) on the mild steel surface is only a matter of seconds. Corrosion rate measurements showed that the presence of myristic acid renders the oil layer more persistent after intermittent wetting compared to one-time direct contact.

Pipelines are the most effective way to transport oil and natural gas, particularly for their bulk transmission over long distances.1-2  Corrosion in oil and gas pipelines occurs because of the presence of dissolved corrosive gases, such as CO2 and/or H2S, in reservoir-derived brine, and contact between this brine and the steel surface. The oil phase itself does not cause corrosion and can even inhibit corrosion.3-6  Crude oils can be defined as naturally occurring liquid mixtures of hydrocarbons (83 wt% to 87 wt% of carbon, 10 wt% to 14 wt% of hydrogen) containing derivatives of nitrogen (0.1 wt% to 2.0 wt%), oxygen (0.05 wt% to 1.5 wt%), sulfur (0.05 wt.% to 6.0 wt.%), metals (less than 1,000 ppm), and other elements. In fact, crude oils naturally contain many surface-active compounds.7-9  These compounds can preferentially adsorb at steel/water, steel/oil, and oil/water interfaces, consequently altering the wetting properties and corrosion behavior of steel surfaces.10 

Therefore, if the water phase in a pipeline was entirely entrained by the hydrocarbon phase, instead of flowing at the bottom, no corrosion problems are expected to occur. Consequently, it is important to determine whether the pipe’s internal surface is wetted by oil or water depending on the fluid properties and the operating flow rates.10  This knowledge can help decrease costs and mitigate the potential for adverse environmental impacts caused by leakage from corroding tubular steels.

In two-phase oil-water flows through a horizontal pipeline operating at low flow rates, the water phase forms a separate layer flowing at the bottom of the pipe’s internal surface. This corresponds to a stratified flow regime because gravitational force dominates over turbulent force. By increasing the flow rate inside the pipeline, the turbulent energy increases and, as a result, the water phase becomes gradually entrained as droplets in the oil phase.11  An example of this can be seen through research conducted with 5% water cut and mixture liquid velocity of 0.5 m/s and 1.5 m/s, as shown in Figure 1. In Figure 1(a), a separate layer of water on the bottom half surface of the pipe is observed. However, in Figure 1(b), the water phase is mostly observed in the form of droplets. Therefore, water may not always wet the bottom surface of the pipe completely, leading to very different corrosion rates depending on the operating scenarios. The measurement of iron count showed the corrosion rate was lower when the mixture liquid velocity was increased from 0.5 m/s to 1.5 m/s if other conditions were kept constant.12 
FIGURE 1.

Images of horizontal flow patterns of oil/water flow (LVT-200 & water) for 5% water cut and at a mixture liquid velocity of (a) 0.5 m/s and (b) 1.5 m/s.12 

FIGURE 1.

Images of horizontal flow patterns of oil/water flow (LVT-200 & water) for 5% water cut and at a mixture liquid velocity of (a) 0.5 m/s and (b) 1.5 m/s.12 

Close modal

When the water content is low, it can stay entrained in the hydrocarbon phase, as a result, the oil phase wets the internal surface of the pipe, and there would be no corrosion problem. By increasing the water content, water breakout may occur, consequently, water may more easily wet the surface of the pipe, and corrosion is possible.13-15  Considering that production flow rates can vary significantly during the life of pipeline systems, different flow patterns can be expected in which oil and water can alternately wet the pipe’s internal surface. This phenomenon is identified as “intermittent wetting.”16  Flow rate and water cut can affect the degree of intermittent wetting.15  This phenomenon can influence the corrosion mechanism of the metal surface, which may differ from those commonly accepted for aqueous corrosion (i.e., full water wetting).6,17-18  Consequently, the wetting condition of the metal surface is a key parameter to predict corrosion behavior,19-22  and it is important to know the relation between wetting of the surface and corrosion processes.23-25  This knowledge allows for more accurate predictions and assessments of corrosion behavior in pipeline systems, contributing to effective corrosion management strategies.

Chemicals and Materials

The composition of crude oils is complex and contains many different chemical compounds. Therefore, model oils containing select dissolved species were used to simulate crude oil behaviors to isolate their influence. In this study, LVT-200 model oil, which is a hydrotreated light distillate petroleum fraction,26  was used with and without the addition of 0.1 wt% of two different surface-active compounds, myristic acid, and acridine. Myristic acid and acridine are representative of oxygen and nitrogen-containing compounds, respectively, which naturally exist in crude oils. The surfactant concentrations used in this study are in the common range for such types of naturally occurring compounds in crude oils. The molecular structures of these surface-active species were shown in Figure 2. Note the amphiphilic character of myristic acid and the aromaticity of acridine. Acridine, however, is insoluble in LVT-200 due to the oil’s low aromaticity. Therefore, acridine was dissolved in a 60:40 weight ratio of LVT-200 and Aromatic-200. Aromatic-200 model oil contains a complex mixture of aromatic hydrocarbons which are mostly derivatives of naphthalene.27  Although myristic acid has low solubility in water, acridine is soluble in water at low pH values related to its protonation.25 
FIGURE 2.

Molecular structures of (a) myristic acid and (b) acridine.

FIGURE 2.

Molecular structures of (a) myristic acid and (b) acridine.

Close modal

The metal specimens used for electrochemical measurements were machined from X65 carbon steel which is a pipeline steel material used in the upstream oil and gas industry. This X65 carbon steel has a uniform, fine structure of cementite in a ferrite matrix and conforms to applicable specifications for API 5L, Grade X65 seamless or welded pipe with the chemical analysis shown in Table 1.

Table 1.

Chemical Compositional Analysis of X65 Pipeline Steel

Chemical Compositional Analysis of X65 Pipeline Steel
Chemical Compositional Analysis of X65 Pipeline Steel

Experimental Apparatus and Procedure

Experiments were performed in a standard three-electrode glass cell with an X65 carbon steel rotating cylinder electrode (RCE) working electrode, a platinum-coated titanium mesh counter electrode, and an Ag/AgCl (KCl saturated) reference electrode. Before each experiment, the RCE was sequentially polished with 400- and 600-grit silicon carbide abrasive papers, cleaned with isopropanol in an ultrasonic bath, and air-dried. Both oil and water solutions were deoxygenated for 1 h by sparging with CO2 before the introduction of the working electrode. After the RCE was inserted into the glass cell, a precorrosion test was conducted to determine whether the initial corrosion rate was close to the blank test corrosion rate to ensure no contamination occurred from the previous test. Linear polarization resistance (LPR) measurements were taken with a scan range from −5 mVOCP to +5 mVOCP, a scan rate of 0.125 mV/s, and a B value of 26 mV using a Gamry Interface 1010B™ potentiostat. The RCE specimen was rotating at 1,000 rpm throughout the experiment, which is equivalent to a velocity of 1 m/s in a pipe with a diameter of 10 cm. This simulates representative mass transfer conditions experienced in oil and gas pipelines.28  The glass cell was sparged with CO2 throughout the test to prevent air (oxygen) ingress and to saturate the test solution with CO2. To minimize the noise in electrochemical measurements caused by CO2 sparging, the sparge tube was retracted into the headspace during data acquisition. The pH was adjusted by adding deoxygenated hydrochloric acid or sodium bicarbonate solution during each experiment. The experiments were performed using two different procedures: one-time direct contact or intermittent wetting.
  • One-time direct contact: The main glass cell was connected to a separate side glass cell used to change the level of the water phase in the main glass cell, enabling immersion of the specimen in oil and water, alternately. A schematic of this setup was shown in Figure 3. In this method, the experiments were performed in four steps:

    1. Precorrosion: The polished specimen was immersed in the water phase.

    2. Partitioning: 0.3 L of an oil layer containing a surface-active compound was added to the top of the water phase. The polished specimen remained immersed in the water phase and rotated for 1 h at 1,000 rpm. Corrosion rate by LPR was measured every 20 min.

    3. Direct inhibition by oil: The water was drained from the side port of the glass cell until the polished specimen was immersed into the oil phase and rotation continued for 1 h at 1,000 rpm.

    4. Persistency: After immersion in the oil layer, fresh brine, prepared and sparged in the side glass cell, was gravity fed into the main glass cell so that the specimen was returned to the water phase. Corrosion rate by LPR was measured every 20 min until reaching a stable value.

    A schematic of the procedure for these experiments was shown in Figure 4. The test matrix used to study the effect of direct contact with oil on corrosion rate was presented in Table 2.

    As acridine could partition into the water phase depending on the pH values, UV-visible spectroscopy (UV-Vis) measurements were performed to measure the concentration of acridine in the water phase at different steps of the experiments. UV-Vis spectroscopy is a widely used analytical technique that involves measuring light absorption in the ultraviolet and visible regions of the electromagnetic spectrum. In UV-Vis spectroscopy, a sample is exposed to light of a particular wavelength, and the amount of lightabsorbed by the sample is measured. The amount of absorption is related to the concentration of the sample and the molar extinction coefficient of the molecule being studied.29 

  • Intermittent wetting: The electrochemical glass cell was connected to another reservoir glass cell to change the level of the water phase in the main glass cell, enabling the placement of the specimen in oil and water, alternately. These two glass cells are connected using a pump with two solenoid valves. A simple schematic of the intermittent wetting setup was shown in Figure 5. Experiments were performed therein in four steps as well with the third step being different from the previous method. In this method, the “inhibition by oil” step consisted of alternating between oil wetting and water wetting multiple times instead of a single direct continuous contact with oil to facilitate improved simulation of intermittent wetting. The intermittent wetting step includes two substeps:

    • Oil wetting step: Brine is pumped from the electrochemical cell to the reservoir cell to lower the oil layer, so the oil layer completely surrounds the specimen surface.

    • Water wetting step: Brine is pumped from the reservoir cell back to the electrochemical cell to lift the oil layer well above the specimen surface.

FIGURE 3.

Schematic of main and side glass cells. The side glass cell was located on a shelf above the main cell.

FIGURE 3.

Schematic of main and side glass cells. The side glass cell was located on a shelf above the main cell.

Close modal
FIGURE 4.

Procedure of one-time direct contact experiments: (a) precorrosion, (b) partitioning, (c) direct inhibition by oil, and (d) persistency.

FIGURE 4.

Procedure of one-time direct contact experiments: (a) precorrosion, (b) partitioning, (c) direct inhibition by oil, and (d) persistency.

Close modal
FIGURE 5.

Simple schematic of the intermittent wetting setup.

FIGURE 5.

Simple schematic of the intermittent wetting setup.

Close modal
Table 2.

Test Matrix to Study the Effect of One-Time Direct Contact with Oil on Corrosion Rate

Test Matrix to Study the Effect of One-Time Direct Contact with Oil on Corrosion Rate
Test Matrix to Study the Effect of One-Time Direct Contact with Oil on Corrosion Rate

The wetting periods were initially conducted at regular intervals of 30 s for a minimum duration of 1 h. Due to the short duration of the intermittent wetting cycles, it was not possible to perform LPR measurements during these cycles. Therefore, an alternative approach had to be used to evaluate the corrosion response. In pursuit of this, the electrode potential was held at an arbitrary anodic potential of +60 mVOCP, while the current response was collected. The full example of data from this test is shown in the Effect of Oil/Water Intermittent Wetting on Corrosion Rates section. It is understood that the measured current does not correspond to the natural corrosion rate of the specimen, however, it provides indication of whether or not corrosion occurs.

Most of the experiments were performed considering wetting periods that were repeated every 60 s for 1 h; 60 s cycles proved to be more practical than 30 s and yielded no significant difference in the results. For these sets, LPR was performed after the cyclic wettings. A schematic picture of the procedure for these experiments was shown in Figure 6. The test matrix used to study the effect of intermittent contact with oil on corrosion rate was presented in Table 3.
FIGURE 6.

Procedure of intermittent wetting experiments: (a) precorrosion, (b) partitioning, (c) oil/water intermittent wetting repeated multiple times over 1 h, and (d) persistency.

FIGURE 6.

Procedure of intermittent wetting experiments: (a) precorrosion, (b) partitioning, (c) oil/water intermittent wetting repeated multiple times over 1 h, and (d) persistency.

Close modal
Table 3.

Test Matrix to Study the Effect of Intermittent Wetting on Corrosion Rate

Test Matrix to Study the Effect of Intermittent Wetting on Corrosion Rate
Test Matrix to Study the Effect of Intermittent Wetting on Corrosion Rate

Effect of One-Time Direct Contact with Oil on Corrosion Rates

Effect of Myristic Acid

The results of baseline experiments in the absence of an oil phase and myristic acid are shown in terms of corrosion rate vs. time in Figure 7 (blue line)—as expected, the corrosion rate is relatively stable over time at a value close to 4 mm/y. The results of the one-time direct contact experiments in the presence of LVT-200 containing 0.1 wt% myristic acid at pH 4.0 are also shown in Figure 7 (orange line). In the repeated one-time direct contact experiments, the corrosion rate did not change in the partitioning step, however, there was a major decrease after the direct contact with the hydrocarbon phase—this effect gradually diminished over time. The same set of results obtained at pH 6.5 are shown in Figure 8. A stable uniform corrosion rate of around 1.5 mm/y was observed in the baseline experiment (in the absence of oil and myristic acid). In the one-time direct contact experiment, there was again no effect in the partitioning step, a significant decrease in the corrosion rate after the direct inhibition step. Yet, the persistency step was much more stable and lasted until the end of the experiment, with no noticeable change in the corrosion rate.
FIGURE 7.

Corrosion rate in the absence and presence of LVT-200 containing 0.1 wt% myristic acid at pH 4.0, 30°C, 0.96 bar CO2, and 1,000 rpm.

FIGURE 7.

Corrosion rate in the absence and presence of LVT-200 containing 0.1 wt% myristic acid at pH 4.0, 30°C, 0.96 bar CO2, and 1,000 rpm.

Close modal
FIGURE 8.

Corrosion rate in the absence and presence of LVT-200 containing 0.1 wt% myristic acid at pH 6.5, 30°C, 0.96 bar CO2, and 1,000 rpm.

FIGURE 8.

Corrosion rate in the absence and presence of LVT-200 containing 0.1 wt% myristic acid at pH 6.5, 30°C, 0.96 bar CO2, and 1,000 rpm.

Close modal

The results, especially associated with the persistency step, are dependent on pH, which is a property of the aqueous phase. Myristic acid has a very low solubility in water and no effect on corrosion was observed during the partitioning tests at pH 4.0 or 6.5. Consequently, it can be postulated that the pH of the water phase affects the nature of the species present at the oil/water interface, which is in turn related to the observed corrosion behavior.

Looking first at the effect of pH, the pKa value for myristic acid is 4.9, which means at pH values higher than 4.9 it increasingly exists as myristate anions (Figure 9).
FIGURE 9.

Dissociation of myristic acid.

FIGURE 9.

Dissociation of myristic acid.

Close modal
In order to compare the results at different pH values, the Henderson-Hasselbalch equation is applied as follows:30 
formula
According to Equation (1), the ratio of myristate to myristic acid at pH 4.0 is equal to:
formula
And at pH 6.5 this ratio is equal to:
formula

Assuming that these comments are applicable to the oil/water interface, it can be hypothesized that the interface is mainly made of myristic acid at pH 4.0, while it is mostly comprised of myristate ions at pH 6.5. In addition, it can also be postulated that the layer adsorbed at the oil/water interface at pH 6.5 is denser and more compact because myristate ions cannot exist in the oil phase (as opposed to myristic acid).

During the wetting cycles, the steel specimen crosses this interface twice (water to oil and oil to water) and it is likely that the structure of the layer at the oil/water interface is replicated on the steel surface, as discussed in Effect of LVT-200 Containing Myristic Acid section. Following all of these assumptions, the effect of myristic acid on the corrosion rate could be explained by the nature of the oil/water interface:

  • At pH 4.0, the adsorbed layer is mostly comprised of myristic acid. The layer initially provides some protection but is quickly penetrated by water and is not persistent, probably due to the loose organization of molecules within the layer and weak van der Waals interactions with the metal surface.

  • At pH 6.5, the adsorbed layer may be made of more densely packed myristate ions, which is more surface active (provides lower interfacial tension value25 ) and interacts with the metal surface by electrostatic forces,31  and consequently offers superior corrosion protection and better persistency than myristic acid.

These postulates require further validation.

Effect of Acridine

The results of one-time direct contact experiments in the presence of a mixture of 60% LVT-200 and 40% Aromatic-200 containing 0.1 wt% acridine at pH 4.0 are shown in terms of corrosion rate vs. time in Figure 10. In this experiment, the corrosion rate started to decrease in the partitioning step until it reached a stable value, and it did not change after direct contact with the hydrocarbon phase. The results of one-time direct contact experiments in the presence of the mixture of LVT-200 and Aromatic-200 containing 0.1 wt% acridine at pH 6.5 are also shown in Figure 11. In this experiment, there was no effect on the corrosion rate either in the partitioning step or even after the direct inhibition step. The results of the potentiodynamic sweeps taken at the end of each experiment agree with the corrosion rate measurements. The potentiodynamic sweep at pH 4.0 is shown in Figure 12(a). Both cathodic and anodic reactions were retarded by the presence of acridine in the oil phase. At pH 6.5, however, both cathodic and anodic reactions remained unaffected, as shown in Figure 12(b).
FIGURE 10.

Corrosion rate in the absence and presence of the LVT-200 and Aromatic-200 mixture containing 0.1 wt% acridine at pH 4.0, 30°C, 0.96 bar CO2, and 1,000 rpm.

FIGURE 10.

Corrosion rate in the absence and presence of the LVT-200 and Aromatic-200 mixture containing 0.1 wt% acridine at pH 4.0, 30°C, 0.96 bar CO2, and 1,000 rpm.

Close modal
FIGURE 11.

Corrosion rate in the absence and presence of the LVT-200 and Aromatic-200 mixture containing 0.1 wt% acridine at pH 6.5, 30°C, 0.96 bar CO2, and 1,000 rpm.

FIGURE 11.

Corrosion rate in the absence and presence of the LVT-200 and Aromatic-200 mixture containing 0.1 wt% acridine at pH 6.5, 30°C, 0.96 bar CO2, and 1,000 rpm.

Close modal
FIGURE 12.

Potentiodynamic sweeps for experiments conducted in the presence of the LVT-200 and Aromatic-200 mixture containing 0.1 wt% acridine at: (a) pH 4.0 and (b) pH 6.5.

FIGURE 12.

Potentiodynamic sweeps for experiments conducted in the presence of the LVT-200 and Aromatic-200 mixture containing 0.1 wt% acridine at: (a) pH 4.0 and (b) pH 6.5.

Close modal
The results are once again pH-dependent. However, acridine is much more soluble in water, especially at pH 4.0, as the results of the partitioning step show. The pKa value for the acridinium cation is 5.6, which means at pH values lower than 5.6, acridine increasingly protonates to form acridinium cations (Figure 13). According to Equation (1), the ratios of acridine to acridinium at pH 4.0 and pH 6.5 can be readily calculated and used to explain the corrosion rate results. A possible explanation for the inhibitive effect of acridine at different pH values could be that acridinium ions can partition through the water phase and adsorb at the steel surface. The concentration of acridinium is high at pH 4.0 and the acridine:acridinium ratio is 0.0251. Consequently, it can inhibit corrosion and reduce the corrosion rate. At pH 6.5, however, the acridine:acridinium ratio is 7.94 and, due to the higher concentration of acridine, which is not soluble in water and alters the surface wettability toward more hydrophilicity compared to acridinium,25  there is almost no effect on the corrosion rate.
FIGURE 13.

Dissociation of acridinium.

FIGURE 13.

Dissociation of acridinium.

Close modal
UV-Vis spectroscopy data obtained for water phase samples during different steps of the experiments are shown in Figure 14. According to the literature the peaks observed at 350 nm and 400 nm are associated with acridine and cationic acridinium.32  The spectrum obtained for pH 6.5 (Figure 14[b]) shows no absorbance at the wavelength value of 350 nm, indicating that no acridine partitioned from the oil phase to the water phase. These results agree with the corrosion rate and polarization curves in Figures 11 and 12(b). On the other hand, Figure 14(a) shows high absorbance values at different steps of the experiment at pH 4.0, which means that there is acridine partitioned into the water phase. The concentration of acridine in both oil and water phases could be calculated based on the absorbance values at those specific peaks mentioned above and, consequently, the value of the partition coefficient (log P) is obtained by Equation (4). Comparison of the partition coefficients and corrosion rate results at pH 4.0 shown in Table 4 demonstrates a good agreement between the partitioning of acridine to the water phase and the corrosion mitigation effect. As the experiment proceeds, the partition coefficient increased indicating more acridine was present in the water phase. Simultaneously, the corrosion rate decreased due to the adsorption of dissolved acridine in the water phase.
formula
FIGURE 14.

UV-Vis spectra for absorbance of acridine from the oil phase into the 1 wt% NaCl solution at (a) pH 4.0 and (b) pH 6.5.

FIGURE 14.

UV-Vis spectra for absorbance of acridine from the oil phase into the 1 wt% NaCl solution at (a) pH 4.0 and (b) pH 6.5.

Close modal
Table 4.

Comparison of Partition Coefficients and Corrosion Rates for Acridine at pH 4.0

Comparison of Partition Coefficients and Corrosion Rates for Acridine at pH 4.0
Comparison of Partition Coefficients and Corrosion Rates for Acridine at pH 4.0

It is also important to mention that the nature of the oil/water interface played a much smaller role in the corrosion behavior, compared to the case of myristic acid. Acridine does adsorb at the oil/water interface but does not orient in an organized fashion, the way that amphiphilic myristic acid does. It can be postulated that the incoherent structure of the layer absorbed on the steel surface is similar to that interface, yielding little effect in terms of corrosion protection.

It was observed that acridine can partition from the oil phase to the water phase, adsorb on the specimen surface, and provide corrosion mitigation at pH 4.0. Therefore, performing cyclic oil-water wetting would not change its inhibitive behavior on CO2 corrosion. On the other hand, myristic acid did not partition from the oil phase to the water phase. Therefore, performing cyclic oil-water wetting experiments would test the persistency of the oil film containing myristic acid on the specimen surface during the water-wetting cycling. Moreover, the corrosion mitigation effect provided at pH 6.5 in the presence of myristic acid is relatively high and persistent. Consequently, only myristic acid was used, at pH 4.0, in the next series of experiments: alternate oil-water wetting experiments.

Effect of Oil/Water Intermittent Wetting on Corrosion Rates

Effect of LVT-200

An anodic potential of +60 mVOCP was applied to directly measure the electrochemical current response of the specimen to oil/water intermittent wetting. This method was used to clarify the results, i.e., the 60 mVOCP overpotential would generate a clear and measurable anodic current if the surface is wetted by water but no current if the surface is wetted by oil (the actual value of the current being irrelevant). The interest of this method is in the oil/water intermittent wetting steps when transient currents could give qualitative information about the persistency and effect of an oil (or water) layer on the steel surface. The transient current response, in the absence of the oil phase, is shown in terms of current vs. time in Figure 15. A stable uniform current response was observed under full water wetting conditions, which indicates a steady active dissolution of the specimen surface. An anodic potential of +60 mVOCP was also applied under full oil wetting conditions and the transient current response is shown in terms of current vs. time in Figure 15. A constant zero current response was observed under full oil wetting conditions. As shown in Figures 15 and 16, the electrode was immersed in water, and then in oil, alternating every 30 s. The anodic current response in these experiments changed periodically between a stable value and zero, which is in agreement with the literature.33-34  It was observed that the current response decreased gradually as the oil replaced the water on the specimen surface. However, it increased rapidly to the initial value as the water replaced the oil on the specimen surface. As seen from Figure 15, the influence of LVT-200 model oil with no additives shows no persistency on the specimen surface as the measured current increases and decreases similar to the amount of specimen surface area remaining in the brine phase. It is speculated that this very fast response indicates that a thin water layer always remains on the specimen surface.
FIGURE 15.

Transient current-time response under fully water wetting, fully oil wetting, and intermittent wetting conditions using LVT-200 model oil, blue, red, and yellow curves are currents (i[A]), oil wetting part of the intermittent wetting cycle.

FIGURE 15.

Transient current-time response under fully water wetting, fully oil wetting, and intermittent wetting conditions using LVT-200 model oil, blue, red, and yellow curves are currents (i[A]), oil wetting part of the intermittent wetting cycle.

Close modal
FIGURE 16.

Transient current-time response under intermittent wetting conditions in the presence of LVT-200 containing 0.1 wt% myristic acid.

FIGURE 16.

Transient current-time response under intermittent wetting conditions in the presence of LVT-200 containing 0.1 wt% myristic acid.

Close modal

Effect of LVT-200 Containing Myristic Acid

An anodic potential of +60 mVOCP was applied and the transient current response under intermittent wetting conditions in the presence of LVT-200 containing 0.1 wt% myristic acid at pH 4.0 is shown in terms of anodic current vs. time in Figure 16. A lower current response was measured in water-wet periods, compared to the current response for water-wet periods of intermittent wetting with model oil alone. It can be postulated that model oil containing myristic acid developed a persistent layer on the specimen surface which was not observed for the model oil alone. The pattern of changes in current during upward and downward movements of the oil layer was also observed to be different. Unlike the experiment with LVT-200, it cannot be assumed that a thin layer of water remains on the surface.

The current-time results agreed with the observed surface wettability. The specimen surface wettability in the presence of LVT-200 alone and LVT-200 containing myristic acid are shown in Figure 17. In the presence of LVT-200 alone, the specimen surface was observed to be hydrophilic, while in the presence of 0.1 wt% myristic acid dissolved in LVT-200, the specimen’s surface became hydrophobic.
FIGURE 17.

Surface wettability state in the presence of LVT-200 alone and LVT-200 containing 0.1 wt% myristic acid.

FIGURE 17.

Surface wettability state in the presence of LVT-200 alone and LVT-200 containing 0.1 wt% myristic acid.

Close modal
The results for corrosion rate experiments under intermittent wetting conditions in the presence of LVT-200 containing 0.1 wt% myristic acid are shown in Figure 18. In this experiment, the specimen was periodically immersed in each phase for 60 s and this cycle was repeated for 1 h. It was observed that the corrosion rate did not change in the partitioning step, however, a significant decrease was measured after the intermittent wetting cycles. The results of the potentiodynamic sweeps taken at the end of the intermittent wetting experiment compared with the baseline experiment agree with the corrosion rate measurements. The potentiodynamic sweeps are shown in Figure 19. Both cathodic and anodic reactions were retarded by the oil/water intermittent wetting.
FIGURE 18.

Corrosion rate in the presence of LVT-200 containing 0.1 wt% myristic acid at pH 4.0, 30°C, 0.96 bar CO2, and 1,000 rpm for one-time direct contact with oil (orange) and 1-h intermittent wetting (green).

FIGURE 18.

Corrosion rate in the presence of LVT-200 containing 0.1 wt% myristic acid at pH 4.0, 30°C, 0.96 bar CO2, and 1,000 rpm for one-time direct contact with oil (orange) and 1-h intermittent wetting (green).

Close modal
FIGURE 19.

Potentiodynamic sweeps for baseline and intermittent wetting (LVT-200 containing 0.1 wt% myristic acid) experiments at pH 4.0, 30°C, 0.96 bar CO2, and 1,000 rpm.

FIGURE 19.

Potentiodynamic sweeps for baseline and intermittent wetting (LVT-200 containing 0.1 wt% myristic acid) experiments at pH 4.0, 30°C, 0.96 bar CO2, and 1,000 rpm.

Close modal

The results for corrosion rate experiments under 1 h fully oil wetting condition in the presence of LVT-200 containing 0.1 wt% myristic acid is also shown in Figure 18. In this experiment, the specimen was continuously immersed in the oil phase for 1 h. The corrosion rate did not change in the partitioning step, while there was a major decrease after the direct contact with the hydrocarbon phase that was gradually diminishing. Different corrosion behaviors observed for these two experimental methods could be related to the structure of film formed on the specimen surface. This phenomenon can be explained by Langmuir and Langmuir-Blodgett concepts.35  Amphiphilic myristic acid molecules orient themselves at the oil/water interface and form a monolayer (similar to a Langmuir film). When the specimen crosses this interface, the monolayer is likely to adsorb to the specimen surface. During intermittent wetting experiments, the specimen crosses this interface 60 times, however, for one-time direct contact experiments the specimen crosses the interface only twice. A possible mechanism to explain the stable effect of myristic acid on corrosion rate after intermittent wetting cycles is that increasing the number of contact times between the specimen surface and the oil/water interface promotes the development of a more persistent layer or a multilayer (Langmuir-Blodgett film), as compared to a one-time direct contact.

  • The addition of myristic acid and acridine to a model oil decreased the CO2 corrosion rate through adsorbing on the steel surface and changing the wettability of the steel surface from hydrophilic to hydrophobic.

  • Varying pH of the aqueous phase above or below the pKa of myristic acid and acridinium will alter their properties through ionization processes. This will result in significant changes in the corrosion inhibition behavior of these surface-active compounds.

  • The presence of myristic acid in the oil phase does not affect corrosion behavior through partitioning from the oil phase to the water phase. The direct contact of oil with the specimen, however, caused a significant decrease in the corrosion rate, which gradually diminished to pH 4.0 but was more persistent at pH 6.5.

  • The presence of acridine in the oil phase has a strong inhibitive effect on corrosion at pH 4.0, through both partition from the oil to the water phase and direct contact of the specimen with oil. There was no effect on the presence of acridine on the corrosion rate at pH 6.5.

  • Persistency of model oil with no additives on the mild steel surface was in the order of seconds. It is speculated that water never leaves the specimen surface and may always exist as a thin layer (an initially water-wet metal surface stayed water-wet).

  • An LVT-200 model oil layer containing myristic acid replaced the water on the specimen surface and formed a much more persistent layer compared to LVT-200 model oil alone.

  • The presence of myristic acid renders the oil layer more persistent after intermittent wetting as compared to one-time direct contact.

Trade name.

The author would like to thank the following companies for their financial support: Ansys, Baker Hughes, Chevron Technical Center, Clariant Corporation, ConocoPhillips, ExxonMobil, M-I SWACO (Schlumberger), Multi-Chem (Halliburton), Occidental Oil Company, Pertamina, Saudi Aramco, Shell Global Solutions, and TotalEnergies.

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