Inhibition efficiency of asphaltenes on H2S/CO2 Corrosion of API 5L X52 Carbon Steel

Mahdi JAVIDI1, Benyamin AFSHANG2

1 Associate Professor, Department of Materials Science and Engineering, School of Engineering,

Shiraz University, Shiraz 7134851154, Iran, mjavidi@shirazu.ac.ir 2 Master of Science, Department of Materials Science and Engineering, School of Engineering,

Shiraz University, Shiraz 7134851154, Iran, Benyamin.Afshang@yahoo.com Abstract

Asphaltenes are the highest molecular weight and most polar constituents of crude oil. Previously asphaltenes have been identified to inhibit the internal corrosion of mild steel pipelines. The present study investigates the inhibition behavior of asphaltenes on corrosion of carbon steel in CO2 saturated 3.5 wt.% NaCl aqueous environments containing 0, 100, and 200 ppm H2S concentration. For this purpose and in order to evaluate the inhibition efficiency of asphaltenes and compare it with crude oil, asphaltenes was extracted from crude oil and then it was dissolved and diluted in toluene and heptol as carrying medium in different concentrations. Electrochemical investigations were performed via potentiodynamic polarization technique. It was found that in case of dilution of asphaltenes in heptol a significant decrease in corrosion rate was achieved which resulted in sharp decrease in corrosion rate in comparison with toluene and the whole crude oil. Also, in case of dilution of asphaltenes in heptol it was resulted in deposition of a loose layer of asphaltenes on the steel surface. In all experiments with 100 and 200 ppm H2S concentration a tarnish sulfide film was immediately form on the surface and then asphaltenes layer was formed. It was concluded that the use of asphaltenes as corrosion inhibitor prevents the localized corrosion associated with H2S and CO2.

Keywords: Asphaltenes, toluene, heptol, CO2 corrosion, H2S

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Introduction

It is believed that the crude oil inhibits the corrosion of carbon steel in CO2/H2S containing process environments. Inhibitive property depends on type of crude oil, water cut and hydrodynamic conditions. The polar organic compounds have the largest potential effects on corrosion behavior, as they are the most water soluble and surface active and can form a hydrocarbon film on steel surface[1]. Mendez et al. [2] found that corrosion inhibition of crude oil associated with a synergistic effect of saturates, aromatics, and polars (resins plus asphaltenes). Hernandez et al. [3] indicate that crude oil inhibition is a function of saturate, aromatic, resin, asphaltenes, acids, and metallic ions. All these investigations revealed that surface active species adsorb onto the metal surface and form a protective barrier. Furthermore, in a research on some Venezuelan crude oil it was found that asphaltenes has a significant effect on corrosion inhibition of crude oil [4]. Ayello et al. [5] investigated several surface active compounds like acids, mercaptans and basic nitrogen compounds and suggested that high molecular polar compounds provide natural corrosion inhibition. Stroe et al. [6] studied the species that are passing into the water from oil phase and found that nitrogen compounds has the best inhibitive effect in water phase.

The highest molecular weight and the most polar compound in crude oil are asphaltenes which are insoluble in heptane and soluble in toluene and including numerous S, N, O functionalities. Different sources of crude oil with different compositions result in different asphaltenes characteristics. Ajmera et al. [7] investigated the role of asphaltenes on inhibition of CO2 corrosion and wettability of the steel surface. The phase behavior of asphaltenes in toluene and heptol (70:30 mixture of heptane and toluene) was compared with crude oil. It was found that inhibition of asphaltenes improves with increasing concentration of asphaltenes in both heptol and toluene as a strong protective layer is formed on the carbon steel surface. However, inhibition property begins in lower concentration in heptol.

In this study the corrosion inhibition effect of asphaltenes which extracted from an Iranian crude oil has been compared with the whole crude via potentiodynamic polarization technique.

Experimental procedure

Asphaltenes were extracted from an Iranian type crude oil according to IP 143 standard and were dissolved and diluted in toluene and heptol as carrying medium in different concentrations in the presences of CO2-saturated 3.5 wt.% NaCl solution containing different concentrations of H2S ( 0, 100, 200 ppm).

Tafel polarization tests were conducted in a potential range of ±300 mV with respect to open circuit potential of the electrodes. The tests were performed in a three electrode glass cell with a rotating disc working electrode made of API 5L X52 carbon steel; a saturated silver/silver-chloride (Ag/AgCl) as reference electrode and a platinum electrode as a counter electrode. The glass cell was filled with 200 ml of 3.5 wt.% NaCl solution at room temperature and then the solution was deaerated with nitrogen gas for 45 min to be deoxygenated and then saturated with

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CO2 gas for 45 min to form a blank solution. Then the solution containing 100 and 200 ppm H2S was prepared by adding Na2S and H2SO4 0.1 N solution into the blank solution prior to electrochemical testing. The working electrode with a surface area of 0.38 cm2 were wet-ground with silicon carbide abrasive paper up to 2000 grit, then rinsed with acetone and dried. The extracted asphaltenes were dissolved in toluene and heptol in 0, 0.1, 1 and 5 wt. % concentrations. In all experiments the working electrode rotated at 1000 rpm at 25±1 °C. After a stable open circuit potential, the corrosion investigations were performed and consist of two steps:

First, baseline step

The working electrode was introduce to the blank solution and solution containing 100 and 200 ppm H2S and Tafel plot measurements were performed via potentiodynamic polarization technique. Purpose of this step was to determine the CO2 corrosion rate of X52 carbon steel samples in blank solution and also in solutions containing 100 and 200 ppm H2S.

Second, partitioning step

As the Tafel plot polarization technique is a destructive test, the cell was filled with new blank solutions and while the working electrode was rotating at 1000 rpm, 20 ml of oil phase (crude oil, and toluene and heptol solution) poured into the glass cell slowly which formed a layer on top of the solution. The oil phase means the crude oil, toluene and heptol which containing 0, 0.1, 1 and 5 wt.% asphaltenes. The aim of this step was to detect the surface active compounds that are present in oil phase which can partition into the water phase and adsorb on the steel surface and effect the corrosion behavior.

Results and Discussion

Figure 1 represents the polarization curves of API 5L X52 carbon steel in the blank solution and solution containing 20 ml oil phase (crude oil). Also, all of the electrochemical data extracted from Tafel polarization tests in this study are reported in Table 1. These results show the baseline for corrosion of X52 carbon steel in the solution. The curves indicate that whole crude on top of the blank solution has a significant effect on polarization curve and shift the corrosion potential about 140 mV to more negative values. In general, the potential change in the presence of inhibitors, show the performance of corrosion inhibitors. Shift in the corrosion potential to more negative values indicate that the cathodic reaction are disrupted in the presence of inhibitor (cathodic inhibitors). While, more positive corrosion potential in the presence of inhibitor indicates the inhibitor is mainly affected the anodic reaction (anodic inhibitors) [8]. The purpose of this step was to detect the surface active compounds which are present in oil phase and can partition into the water phase and adsorb on the steel surface. The results showed that this crude oil has the inhibition effect and have some surface active compounds that partition into the water phase and reduce the corrosion rate. The corrosion rate of blank solution was found to be 2.9 mm/y and in the presence of crude oil on top of the solution, it was decreased to 1.8 mm/y. According to Fig. 1 a limiting diffusion current density can be seen in the cathodic segment

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which might be attributed to use of rotating disc electrode and the adsorption of surface active agents which present in crude oil on the steel surface.

Figure 1. The polarization curves for API 5L X52 steel immersed in CO2-saturated 3.5 wt.% NaCl solution.

Table 1.Electrochemical data extracted from Tafel polarization tests

Figures 2 and 3 show the Tafel polarization curve for API 5L X52 carbon steel in Blank solution and also solutions with oil phase (crude oil and toluene and heptol with different concentration of asphaltenes) on top. Same as in case of crude oil, the presence of asphaltenes in toluene and heptol also shifted the corrosion potential toward negative potentials in comparison with the

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Blank solution. However, the shift in potential for toluene was more than heptol. In case of asphaltenes in toluene, it can be seen that the cathodic Tafel slopes are more affected in comparison with anodic ones. But in case of asphaltenes in heptol both anodic and cathodic are effected almost the same. In terms of asphaltenes in heptol, as asphaltenes is partially aggregated and flocculated in solution a three dimensional film is formed and corrosion rate decreases [7, 9-11].

Figure 2. The polarization curves for API 5L X52 steel immersed in CO2-saturated 3.5 wt.% NaCl solution containing different concentrations of asphaltenes in toluene.

Figure 3. The polarization curves for API 5L X52 steel immersed in CO2-saturated 3.5 wt.% NaCl solution containing different concentrations of asphaltenes in heptol.

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Figure 4 shows the effect of introducing 100 and 200 ppm H2S to the Blank solution. As can be seen, the presence of H2S shifts the corrosion potential of steel to more negative potentials. As previously mentioned, corrosion potential shift to more negative values indicates a disruption in the cathodic reaction same as crude oil and asphaltenes. A tarnish layer formed on the steel surface when the electrode was immersed in H2S containing solution. The formation of FeS immediately occurs on the order of seconds or fractions of seconds [12]. Sulfide film affects the corrosion rate. The corrosion rate of X52 steel in solution containing 100 and 200 ppm H2S was 4.3 and 3 mm/y, respectively.

Figure 4. The polarization curves for API 5L X52 steel immersed in CO2-saturated 3.5 wt.% NaCl solution containing different concentrations of H2S.

Figure 5 and 6 present the polarization curves for X52 steel in solution with toluene and heptol as oil phase on top of the solution with different concentration of asphaltenes containing 100 ppm H2S. All experiments show a limiting diffusion current density that is attributed to the species which adsorb on the steel surface. Same behavior for change in corrosion potential was seen. The results indicate that, corrosion rate increases in the presence of H2S and corrosion rate is the highest in 100 ppm H2S compared to 0 and 200 ppm H2S containing solution.

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Figure 5. The polarization curves for API 5L X52 steel immersed in CO2-saturated 3.5 wt.% NaCl solution with 100 ppm H2S containing different concentrations of asphaltenes in toluene.

Figure 6. The polarization curves for API 5L X52 steel immersed in CO2-saturated 3.5 wt.% NaCl solution with 100 ppm H2S containing different concentrations of asphaltenes in heptol.

Figure 7 and 8 show the polarization curves for X52 steel in solution with toluene and heptol as oil phase on top of the solution with different concentration of asphaltenes containing 200 ppm H2S. Observations indicate that increase in H2S concentration from 100 to 200 ppm results in decrease in corrosion rate. This reduction might be attributed to the formation of compact barrier layer iron sulfide on the steel surface. In some cases localized corrosion was seen which is attributed to un complete coverage of asphaltenes on the steel surface.

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Figure 7. The polarization curves for API 5L X52 steel immersed in CO2-saturated 3.5 wt.% NaCl solution with 200 ppm H2S containing different concentrations of asphaltenes in toluene.

Figure 8. The polarization curves for API 5L X52 steel immersed in CO2-saturated 3.5 wt.% NaCl solution with 200 ppm H2S containing different concentrations of asphaltenes in heptol.

Figure 9 shows the polarization curves for X52 steel in solution with crude oil as oil phase on top of the solution with different concentration of H2S in comparison with the Blank solution. It was mentioned that the presence of crude oil on top of solution decreases the corrosion rate due to immigration of surface active compounds from crude oil to water phase. When the crude oil was added on top of the Blank solution without H2S, corrosion potential was shifted to more negative potentials. However, when crude oil was introduced on top of the Blank solution containing 100 and 200 ppm H2S, the corrosion potential shifted to more positive potentials and approximately was same as each other,-526 and -514 mV, respectively. This effect indicate the anodic performance of H2S. Some authors also found that H2S either can accelerate or inhibit iron

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corrosion in different conditions. In fact H2S either can effect on iron dissolution or hydrogen evolution [13, 14]. In addition the corrosion rate also is the lowest value in this two case, 1 and 0.5 mm/y, respectively. It can be concluded that increase in the concentration of H2S in solutions from 0 to 100 and 200 ppm promotes the inhibition efficiency of crude oil.

Figure 9. The polarization curves for API 5L X52 steel immersed in CO2-saturated 3.5 wt.% NaCl solution and crude oil on top at different concentrations of H2S .

Conclusions

The asphaltenic crude oil which was investigated in this study has some kind of surface active compounds which immigrate into the water phase and resulted in corrosion inhibition of carbon steel in crude oil.

Asphaltenes has a significant effect on corrosion of carbon steel even in the presence of H2S.

The presence of crude oil, toluene and heptol as oil phase on top of the CO2-saturated 3.5wt.% NaCl solution resulted in shift in the corrosion potential to more negative potential that indicate a disruption in the cathodic reaction.

The presence of H2S increases the corrosion rate of carbon steel but in high concentration of H2S (200 ppm) reduced the corrosion rate compared to 100 ppm H2S witch is attributed to formation of a more compact barrier layer of iron sulfide on the steel surface.

H2S show an anodic behavior in the presence of crude oil on top of water phase while it shows a cathodic behavior in presence of asphaltenes either in toluene or heptol.

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