Abstract: In oil and gas production environment, CO2, H2S and Acetic Acid (HAc) have become a main source that affect corrosion to the carbon steel and cause failures on the facilities. In order to minimize the failures, there must be a tool to predict those failures. This research proposed a prediction model incorporated with H2S and HAc in CO2 environments using a set of experiments. The experiments were conducted in the 3% NaCl solution saturated with 300 ppm of H2S in CO2 gas. LPR and EIS methodology were used to study corrosion mechanism and to calculate carbon steel corrosion rate. The results revealed that effects of H2S and HAc in CO2 environment have contributed in increasing corrosion rate. Those effects were caused by additional of cathodic activation reactions in the system as described by scan polarization. Analyzing through EIS showed that combined effect of CO2 and H2S promote to form film formation indicated by Warburg diffusion spectra. Although interaction of CO2 and H2S gas has a capability to produce film formation, the film can not inhibit corrosion reaction that may relate to the presence of HAc in the solution.
INTRODUCTION
Crude oil is a complex mixture of hydrocarbons species which determines oil characteristics. Analytical data shows that components in crude oil varied with main components classified as: carbon, hydrogen, nitrogen, sulfur and their compounds. Studies have demonstrated that those multi species factors can govern the corrosion process in many ways and in several mechanisms (Joosten and Hembree, 2002; Amri et al., 2008). Many efforts have been made for understanding CO2/H2S corrosion mechanism to improve the predictions. But, to date, the reported model available does not represent combining parameters such as HAc. H2S and CO2. There are limited studies in the literature to observe those mixed effects.
CO2 corrosion: In the CO2 corrosion of carbon steel, basically, there are two main corrosion mechanisms which are cathodic reactions and anodic reaction. In cathodic site CO2 dissolves to the water phase and hydrates to form carbonic acid as follows (Nordsveen et al., 2003; Gray et al., 1990):
| (1) |
| (2) |
Carbonic acid dissociates into further reactions which depends on pH. At pH 4 or lower carbonic acid dissociates into bicarbonate ions and carbonate ions in two steps (Parakala, 2005):
| (3) |
| (4) |
It was suggested that H+ ions are dominant species promoting corrosion. H+ ions are able to diffuse to the metal surface through boundary layer. On the metal surface, H+ ions involve in hydrogen evolution reaction. These additional charge transfer reactions are suggested as factors governing the corrosion rate (Parakala, 2005):
| (5) |
| (6) |
On anodic site, The mechanism of anodic reaction is the oxidation reaction to form ferrous (Fe2+) ions. The general reaction process is:
| (7) |
Nesic et al. (1996) reported that the presence of CO2 does not have any effects on the anodic dissolution of iron. Iron dissolution kinetics in CO2 environment involves chemical ligand that adsorbs at the metal surface and catalyzing the dissolution of iron.
H2S corrosion mechanism: Hydrogen sulfide is weakly acidic, when dissolved in water, H2S is involved in a series of chemical reactions:
| (8) |
| (9) |
| (10) |
| (11) |
| (12) |
The role of H2S in changing behavior of corrosion rate was studied by Brown (2004). In his experiment, he found that small concentration of H2S (less than 30 ppm) with CO2 saturated in water, corrosion rate will increase compare with no H2S. However, corrosion rate will decrease when he conducted a series of variable as; 15 % NaCl, 7, 9 bar total pressure, 100 ppm of H2S concentration, pH solution<5. It happened in single phase and multi phase flow.
Anderko and Robert (1999) reported that the corrosion rate drops significantly for partial pressures of H2S ranging from 2.10-6 to 10-4 atm and reaches a plateau in a relatively wide range of H2S partial pressures above 104 atm. Reduction in corrosion rates has been reported when the H2S partial pressure exceeds 10-3 atm in some systems. At substantial H2S partial pressures (above 10-2 atm), the aqueous H2S, and HS- species become sufficiently to increase in the corrosion rate.
Acetic acid corrosion mechanism: Acetic Acid (HAc), structural formula is represented as CH3COOH. It is a weak acid which is not completely dissociated in aqueous solutions. It has been reported that free acetic acid can increase corrosion rate (Veloz, 2002; Ismail et al., 2006). Mechanism of dissolved acetic acid in CO2 corrosion can be correlated to the concentration of undissociated HAc present in the brine (George and Nesic, 2007). The dissociated of the acid can modify the corrosion rate in the CO2 corrosion rate has been validated with laboratory tests by (George and Nesic, 2007). The dissociation process of acetic acid in water is given by (Nafday, 2004):
| (13) |
The aqueous of HAc, then partly dissociation into hydrogen and acetate ions:
| (14) |
| (15) |
The reaction mechanism and kinetics of the overall reactions are influenced by acetic acid concentration, CO2 partial pressure, pH and water contaminants. The effect of HAc on the corrosion of mild steel has been studied by a number of experimental results. Crolet (1999) make the point that CO2 induced acidification also can cause partial re-association of anions. Such weak acids then will increase the oxidizing of H+ by raising the limiting diffusion current for cathodic reduction. The presence of this acid also will tend to solubilise the dissolving iron ions.
Other effects of HAc are that it decreases pH, increasing the cathodic limiting current, and decrease Ecorr. In this condition, the cathodic reaction will become the rate determining step. The limitation is due to diffusion of proton to the steel surface rather than electron transfer. There was an agreement that HAc can increase the cathodic reaction rate (hydrogen evolution reaction (Crolet, 1999).
Garsany et al. (2002) argued that the increase of corroisn rate of HAc in CO2 environment must be proportional to the concentration of undissociated acetic acid in the brine. They emphasized that the electrochemistry of acetic acid at steel cannot be distinguishable from free proton (because of its rapid dissociation). The work of Crolet (1999) suggested that the presence of HAc inhibited the anodic (iron dissolution) reaction at this inversion point, HAc is the predominant acid compared to carbonic acid and is therefore the main source of acidity.
Experimental set-up:
Specimen preparation and test matrix: The working electrodes were
carbon steel and the chemical composition is as shown in Table
1. The cylindrical specimens have diameter of 12 mm and 10 mm length.
Before immersion, the specimen surfaces were polished successively with 240, 400 and 600 grit SiC paper, rinsed with methanol and decreased using acetone.
| Table 1: | Composition of 080a15 carbon steel used in the experiments |
| Table 2: | Experimental matrix used in the test |
| Table 3: | Gas composition used in the experiments. |
| Fig. 1: | Experimental arrangement for static test, 1 Glass cell, 2: Reference electrode, 3: Counter electrode, 4: Working electrode, 5: Co2 gas bubble |
The experiments were repeated at least twice in order to ensure reasonable reproducibility. The test matrix used to do the experiment is presented in Table 2.
Static test set-up: The typical experimental arrangement for the static test is illustrated in Fig. 1. The test assembly consisted of one-litre glass cell bubbled with CO2. The electrochemical measurements were based on a three-electrode system, using a commercially available potentiostat with a computer control system. The reference electrode used was a Ag/AgCl and the auxiliary electrode was a platinum electrode. The Linear Polarization Resistance (LPR) technique was used to measure the corrosion rate. The procedure was similar to ASTM Experimental test G 5-94 (ASTM, 2004).
Cell solutions: The experiments were performed in stagnant condition. The total pressure was one bar, the glass cell was filled with one liter of distilled water with 3% wt NaCl which was stirred with magnetic stirrer. Then, CO2 or CO2/H2S/N2 gas was bubbled through the cell (at least one hour prior to experiments) in order to saturate and de-aerates the solution. After the solution was prepared, the pH was adjusted to reach the pH set by using NaHCO3 as buffer solutions. During the experiment, constant concentration of gases was continuously bubbled through the electrolyte in order to maintain consistent water chemistry.
Gases compositions: The gas mixtures comprising 300 ppm H2S balanced with nitrogen was obtained commercially from MOX®. The mixture of H2S balanced with N2 and CO2 was adjusted using gas regulator and flow meter purged to the glass cell through mixing tube. The compositions of mixed gases planned were as follows (Table 3).
RESULTS AND DISCUSSION
Corrosion rate of carbon steel at varying HAc concentration in CO2/H2S system studied by LPR and EIS technique is presented in Fig. 2. In the experimental condition with HAc concentration ranged from 0-18 ppm, the presence of HAc increased corrosion rate consistently. In other word, HAc controlled the corrosion rate. HAc increased corrosion rate by 0.5 times with addition of 180 ppm of HAc.
In order to study surface characteristics of H2S/CO2/HAc corrosion, it was studied using EIS technique as presented in Fig. 3. In Fig. 3, It demonstrates characteristics of the Nyquist plots of 80 and 130 ppm of HAc concentration in saturated 300 ppm H2S/CO2 solution. As can be seen from the Fig. 3. The impedance diagram showed a depress semi-circle at high frequencies which indicating a double layer capacitance. This condition, as quoted by Bai et al. (2006) was suggested as there were heterogeneous surface and the non-uniform distribution of current density.
With the addition of 80 and 180 ppm HAc, the steady state impedance diagram demonstrated a smaller depressed semi-circle with similar characteristic. The decrease in polarization resistance Rp from EIS measurements indicated the increase in corrosion rate with increasing HAc concentration. Moreover, there was a tail observed in the experiments (Fig. 3). These results suggested that the mechanism is under diffusion process control in the presence of HAc. The same characteristic was found in the experiments without HAc. From the Fig. 3 description, it can be concluded that the corrosion reaction of H2S/CO2 system was dominated by HAc reactions.
From the EIS data, it can also be described several values of solution resistance, charge transfer resistance, capacitance film formed and corrosion rate in the form of equivalent circuit parameters as presented in Fig. 4.
| Table 4: | Circuit parameters values for representing EIS experimental data. |
| Fig. 2: | Corrosion rate at varying HAc concentration at conditions: total pressure 1 bar, 0.7 bar CO2, 0.3 mbar H2S, 22°C. (Comparison between LPR and EIS results) |
| Fig. 3: | Nyquist plot to calculate corrosion rate as a function of HAc concentration at conditions of total pressure 1 bar, 0.7 bar CO2, 300 ppm H2S, 22°C |
Table 4 showed the parameters values of solution resistance, charge transfer resistance, capacitance film formed and corrosion rate which was obtained from the experiments using EIS technique.
From the equivalent values, it demonstrated a decreased of charge transfer resistance with the increased of HAc concentration. It means that corrosion rate increased as concentration of acetic acid was added (Fig. 3). The effect is proportional to the amount of HAc added. As shown in Fig. 3, the charge transfer resistance decreased from 104 to 74.
| Fig. 4: | Typical equivalent circuit for a mixed diffusion and charge transfer control used to represent the experimental conditions |
| Fig. 5: | Potentiodynamic sweeps in CO2 solution with/without H2S. (Total pressure 1 bar, temperature 22°C, pCO2 0.7 bar, pH2S 0.3 mbar, pH 4 and stagnant) |
The polarization sweeps were conducted to study effect of H2S on CO2 corrosion. The result is presented in Fig. 5. From the figure showed that there were differences between polarization graph of carbon steel corrosion in CO2 and in 300 ppm H2S/CO2 system. H2S gas increased CO2 corrosion rate and also increase cathodic Tafel slope. This finding was also observed by Zhang and Cheng (2009), when they conducted experiments in condition with a constant H2S/CO2 partial pressure ratio of 1.7.
Kvarekval (1999) explained that the increased corrosion rate was caused by sulfide ions or by H2S acting as a catalyst for hydrogen evolution and govern diffusion proton donors. Further, He reported that H2S can also increase the hydrogen evolution rate without taking part in the net reaction. In further analyses, these H+ ions concentrations from H2S molecule can penetrate on steel surface to create a pitting corrosion which can increase corrosion rate.
In addition, additional of H2S also gave impact on diffusion limiting current density of CO2 corrosion. As can be seen from experiments using EIS technique (Fig. 3), there was tail in the Nyquist plot which indicates mass transfer effect in the process. However, from the scan polarization analyses, it showed activation reaction control reaction.
| Fig. 6: | Potentiodynamic sweeps in 300 ppm H2S/CO2 saturated solution at various HAc concentrations. (Temp. 22°C,Total pressure 1 bar, pCO2 0.7 bar, pH2S 0.3 mbar, pH 4, stagnant) |
Thus, the behavior of cathodic limiting current density consisted of chemical reaction and diffusion process.
Effects of addition of HAc on potentio-dynamic test in Fig. 6. Figure 6 indicated that there was no significant effect of addition of HAc on anodic Tafel slopes in 300 ppm H2S/CO2 system. However, the cathodic slope showed an increase of reaction process in the presence of HAc. Figure 6 also revealed that anodic polarization behavior did not change significantly with the additional of hydrogen sulfide. Anodic tafel slope was consistence with iron dissolution in CO2 solution. However, cathodic Tafel slope have increased significantly. It means that HAc was a the dominant factors that govern the reaction process.
CONCLUSIONS
| • | In the presence of 0.3 mbars of H2S in 0.7 bars of CO2, the average corrosion rate increased approximately 10% compared to free H2S |
| • | The H2S accelerate corrosion rate by increasing cathodic Tafel slope |
| • | The introduction of HAc to 180 ppm in the H2S/CO2 mixture gases caused corrosion rate increased |
| • | The anodic polarization behavior did not change significantly with the additional of hydrogen sulfide |
| • | HAc was a the dominant factors that govern the reaction process in CO2/H2S system. Behavior of cathodic reaction consisted of chemical reaction and diffusion process |
ACKNOWLEDGMENT
The authors are thankful to Universiti Teknologi PETRONAS for providing grant and facilities for the research.