|Hary Devianto||Chemical Engineering Program, Faculty of Industrial Technology, Institut Teknologi Bandung, Bandung 40132 Indonesia|
|Isdiriayani Nurdin||Chemical Engineering Program, Faculty of Industrial Technology, Institut Teknologi Bandung, Bandung 40132 Indonesia|
|Pramujo Widiatmoko||Chemical Engineering Program, Faculty of Industrial Technology, Institut Teknologi Bandung, Bandung 40132 Indonesia|
This study explored the effectiveness of tobacco extract as a corrosion inhibitor for carbon steel in an H2S-containing NaCl solution. The experiments were conducted at room temperature and atmospheric pressure, with a variation in inhibitor concentration and rate of fluid flow. Results showed that the tobacco extract decreases the corrosion rate of carbon steel in the 3.5% NaCl solution. Increasing flow rate and inhibitor concentration significantly affect the corrosion rate of carbon steel. In the NaCl solution, crystalline corrosion product formation of magnetite (Fe3O4) was mainly detected. Furthermore, the presence of H2S leads to forming of a mackinawite (FeS) structure. The inhibition efficiency of 1000 and 2000 ppm tobacco extract in NaCl solution with the presence of H2S is in the range of 24 to 69%.
Carbon Steel; Corrosion; Flow rate; Hydrogen sulphide; Inhibitor; Tobacco extract
Chemical industries produce daily necessities as a vital part of human life. The involvement of corrosive fluid in chemical processing, both in static and dynamic flow, leads to corrosion of the process equipment. Corrosion is a spontaneous metal deterioration process due to environmental interaction (Roberge, 2008). In industries, carbon steel is common material widely used for various purposes (Kurniawan, Mitha, and Fikri, 2018) because of its satisfactory performance and attractive cost (Gandy, 2007). However, carbon steel is susceptible to corrosion (Dwivedi, Lepkova, and Becker, 2017). Components such as sodium chloride and hydrogen sulfide in oil and gas industries (Al-Janabi, 2020) increase the corrosivity of process fluids to the carbon steel. The corroded carbon steel leads to equipment failures, loss of containment, product contamination, and work accident. Therefore, a study on carbon steel corrosion in NaCl and H2S environments is crucial, especially in the petroleum industry, to reduce costly social, economic, and even human losses (Zúñiga et al., 2011; Sosa et al., 2003).
Adding corrosion inhibitors can reduce corrosion
risk. The inhibitors are a small quantity of a compound that controls, reduces
or prevents reactions between a metal and its surroundings (Fouda et al., 2014; Bentiss et al., 2002).
The inhibitor could be a synthetic organic corrosion inhibitor or a green
corrosion inhibitor extracted from natural Resources (Rani and Basu, 2012).
Organic inhibitors, such as thiourea derivatives, are effective in an acid
environment (Shetty and Shetty, 2017). In
seawater, sodium benzoate also has been used as an inhibitor (Nik et al., 2010). However, the
application of synthetic organic corrosion inhibitors is hindered by their
toxicity effect (Popoola, 2019), despite
their high effectivity. On the other side, green organic inhibitors have
recently become an attractive alternative in corrosion prevention research due
to their availability and relatively low cost (Kurniawan,
Mitha, and Fikri, 2018).
The green corrosion inhibitors are biodegradable and do not contain heavy metals or other toxic compounds. Extracts of a plant contain a wide variety of organic compounds. Most of them contain heteroatoms such as P, N, S, and O (Shalabi et al., 2014; Bhawsar, Jain, and Jain, 2013). A significant number of scientifical studies have been dedicated to the corrosion inhibition of carbon steel in acidic media by natural products as corrosion inhibitors (Shalabi et al., 2014; Bhawsar, Jain, and Jain, 2013; Ramananda, 2013; Majidi et al., 2011; Eddy and Mamza, 2009; Prabhu et al., 2008). In example, Nicotiana tabacum extract oil has been studied as a corrosion inhibitor of mild steel in 2 M H2SO4 solution, with an efficiency of 94.13% at 10 g/L, 303K. Atropa Belladonna extract has also been investigated on the corrosion of carbon steel in 1 M HCl. It shows 96.6% at 500 ppm using the Tafel polarization technique. Pradityana et al. (2017) also used 300 mg/L Myrmecodia Pendans extract as a green inhibitor of mild steel corrosion in 1 M H2SO4 media with an efficiency of 64.68%.
Tobacco plants produce more than 4,000 chemical compounds, including terpenes, alcohols, carboxylic acid, nitrogen–containing compounds, and alkaloids (Loto and Popoola, 2011). These components may inhibit electrochemical activity by chemisorption of the extract components onto the steel surface. It can reach up to 91.5% on inhibition efficiency, higher than black wattle (66-87%) and orange peel (80% IE). the chemisorption of tobacco extract components provides higher effective protection due to the reduction of the reactivity of metal at the attached sites (Guo et al., 2018; Shehata, Korshed, and Attia, 2018). Kurniawan, Mitha, and Fikri (2018) used tobacco extract as an inhibitor in a 3,5% NaCl environment with CO2 with excellent results. In this work, we also use tobacco due to its abundant availability in Indonesia. The H2S is used as an impurity instead of CO2. The tobacco extract was obtained from Virginia tobacco leaves using the maceration technique with demineralized water as solvent. We evaluated the efficiency of tobacco extract as a green corrosion inhibitor using electrochemical techniques.
2.1. Preparation of tobacco extract
The dried and ground Virginia tobacco leaves (100 g) were soaked in 100 mL demineralized water for 72 hours under a room temperature of 27°C. The extract solution was filtered through filter paper and heated at 50 °C for 3 hours. The extract of tobacco leaf obtained was used as an inhibitor.
2.2. Preparation of metal specimens
The material tested was AISI 1018 carbon steel. Two types of specimens were prepared, i.e. specimens to measure the corrosion rate (in the form of a solid cylinder with a diameter of 3 mm and a length of 20 mm) and specimens to determine the inhibition mechanism (in the form of a 10 mm x 10 mm plate). The carbon steel cylinder is coated with Teflon®, which acts as an insulator so that only one side of the surface is in contact with the fluid. The plate specimens are connected by cables and covered with resin so that only one side is exposed. Before testing, the electrode was polished using different abrasive papers (up to 2000 grit) and cleaned with demineralized water and alcohol.
2.3. Preparation of test media
All chemicals were obtained from Merck, Germany, without prior treatment. The electrolyte solution is NaCl solution at a concentration of 35,000 ppm. The solution volume is 50 mL, with and without adding 600 ppm H2S. Concentrations of tobacco extract are varied at 0, 1000 and 2000 ppm. As in previous research, the H2S gas is made by reacting Na2S solid with 3 M of HCl solution using Kipp's apparatus (Rachmawati et al., 2011). The formed H2S gas is flowed into the NaCl solution for a specific time to obtain the determined H2S concentration.
2.4. Corrosion test
Corrosion rate was measured using the
potentiodynamic method and determining the corrosion inhibition mechanism by
using Cyclic Voltammetry (CV), Electrochemical Impedance Spectroscopy (EIS),
X-Ray Diffraction (XRD), and Scanning Electron Microscope (SEM) analyses. The
conventional three-electrode cell is used for the polarization curve, CV and
EIS experiments. Pt-Ir electrode was used as the auxiliary electrode, a
saturated calomel electrode (SCE) was used as the reference electrode, and the
sample was used as the working electrode. All polarization curve tests were
conducted at a scan rate of 5 mV/s. The Gamry Framework and Echem® Analyst
software analyzed the polarization curves. The EIS measurements were performed
with frequencies ranging from 50,000 Hz to 1 Hz, and EIS results were analyzed
using the Gamry Echem® Analyst software. After the corrosion test, the surface
morphologies were observed using SEM. Corrosion phases were detected using XRD
and identified by matching peak positions automatically with Match! Software.
Corrosion rate measurement experiments were carried out by varying the flow rate, the presence of H2S, and the concentration of tobacco extracts under room temperature. Then, experiments to determine the corrosion inhibition mechanism were carried out by varying the immersion time. The studied carbon steel samples were immersed in 3.5% NaCl solution with the addition of tobacco extract as an inhibitor for an hour and a week. The corrosion mechanism was analyzed based on CV and EIS analyses.
3.1. Corrosion Rate of Carbon Steel in 3.5% NaCl Solution
The corrosion rate of carbon steel was determined using the potentiodynamic polarization method against variations in the flow rate of the solution, the presence of dissolved H2S, and the concentration of tobacco extract in the electrolyte solution. The result is shown in Figure 1.
Effect of H2S - The polarization curve of carbon steel in NaCl solution with the presence of H2S can be seen in Figure 2a. The presence of H2S in the 3.5% NaCl solution increased the corrosion rate. This is due to the dissociation of dissolved H2S into H+ and HS-. The increased concentration of H+ ions in the solution increases the corrosion rate because the H+ ions can oxidize carbon steel (Rachmawati et al., 2011).
Effect of flow rate - In general, increasing the flow rate (related to the rotational speed of the electrode) of the solution will increase the corrosion rate of the carbon steel. The polarization curve in Figure 2b indicates increased dissolved oxygen as the fluid flow rate increases (Roberge, 2008). The cathodic curve moves to a higher current with increasing rotational speed.
Effect of tobacco extract - The polarization curve of carbon steel in NaCl solution with the presence of tobacco extract is presented in Figure 3a. The addition of tobacco extract decreased the corrosion rate of the steel in the NaCl solution containing H2S. Adsorption of C-O, N-H, and O-H functional groups from the compounds in the tobacco extract onto the metal surface forms metal protective layers. The presence of these groups in the extract is detected in the Fourier-Transform Infrared Spectroscopy (FTIR) spectrum, as shown in Figure 3b.
Figure 3 (a) Polarization curve of carbon steel in NaCl solution with H2S and various concentrations of tobacco extract (b) FTIR spectra of tobacco extract sample
To be noted, the addition of tobacco extract into NaCl solution in the absence of H2S and under static conditions increases the corrosion rate. This condition is caused by the acidity of tobacco extract, which increases the corrosivity of the solution. The NaCl solution with 1000 and 2000 ppm tobacco extract has pH of 6.4 and 5.9, consecutively. Formation of a stable, protective layer due to the acid environment, however, is hindered due to the slow diffusion and adsorption of tobacco extract's protective compounds onto the steel surface. The addition of disturbances in the form of flow rate and the presence of H2S resulted in tobacco extract forming a better protective layer. It inhibits steel corrosion with an efficiency of 24% to 69% at tobacco extract concentrations of 1000 ppm and 2000 ppm, respectively.
3.2. Corrosion Inhibition Mechanism of The Tobacco Extract
Based on the voltammogram in Figure 4, the corrosion product is unstable since the current ratio at the cathodic peak to the current at the anodic peak is not equal to 1. The reaction is also considered irreversible because the potential difference at the anodic and cathodic peaks exceeds 59 mV. Two cathodic peaks in the voltammogram of samples in the NaCl solution with H2S and inhibitor solution indicate the reduction occurred through two reaction stages. Meanwhile, one anodic peak on the voltammogram indicates that the oxidation reaction occurs in a single stage.
Figure 4a shows that within the same potential window and 1-hour immersion, the carbon steels show low corrosion current, both in a solution of NaCl with 600 ppm of H2S and NaCl with an inhibitor. The low current indicates a slow corrosion rate of carbon steel. In Figure 4b, it appears that the anodic current of carbon steel immersed in a solution of NaCl with 600 ppm of H2S and 2000 ppm of inhibitor for a week (40 mA) is smaller than the anodic current of steel after immersion for 1 hour (160 mA). This indicates that the tobacco extract, together with the presence of H2S, can function as an inhibitor. Figure 4b also shows that within a long exposure time, the corrosivity of H2S is higher than in tobacco extract.
Testing using EIS is carried out to determine the electrical circuit equivalent of the ongoing corrosion process. Three equivalent electrical circuits are generated depending on the type of electrolyte solution used. The Nyquist plot is shown in Figure 5. Meanwhile, the equivalent circuit and correlated electrical double layers on the electrode's surface are presented in Figure 6. Table 1 lists the fitting of the electrochemical impedance parameters of each configuration.
Figure 6 Equivalent circuit of steel corrosion in 3.5% NaCl solution and correlated electrical double layers: (a) in the absence of tobacco extract as an inhibitor, both with and without H2S; (b) with inhibitor; and (c) with inhibitor and H2S
Electrolyte resistance increased by adding the non-conductive tobacco extract concentration to the electrolyte solution, decreasing electric current and corrosion rate. The addition of tobacco extract and H2S also resulted in increasing in Rct value. The Rct relates to the protective formation layer on the steel surface (Pradityana et al., 2017). A thicker and evenly distributed protective layer inhibits the charge transfer of corrosion reaction (Fouda et al., 2014). The corrosion products of carbon steel in NaCl solution containing H2S and tobacco extract after immersion for one week were analyzed using XRD and can be seen in Figure 7a. Magnetite (Fe3O4) is always present as a corrosion product for the three types of electrolyte solutions used. Immersed in an electrolyte solution of H2S, the steel forms an iron sulfide compound, namely mackinawite (FeS).
Figure 7 (a) X-ray Diffraction spectra of the corroded carbon steel after one-week immersion in 3.5% NaCl contains H2S and tobacco extract and morphology of (b) mackinawite after one-week immersion in 3.5% NaCl solution contains H2S and 2000 ppm of tobacco extract, and (c) magnetite after one-week immersion in 3.5% NaCl solution contains 2000 ppm of tobacco extract
The existence of mackinawite and magnetite was confirmed by SEM observations and compared the XRD to the literature, as shown in Figures 7b and 7c.
Tobacco extract can
reduce the corrosion rate of AISI 1018 steel in a flowing NaCl solution containing
H2S, with an inhibition efficiency of 24 – 69% for a concentration
of 1000 – 2000 ppm. The corrosion mechanism of carbon steel in NaCl solution
with and without H2S or tobacco extract starts from the oxidation of
Fe to Fe2+ to form an iron oxide compound in the form of magnetite
(Fe3O4). The presence of H2S causes the
formation of an iron-sulfide compound, namely mackinawite (FeS), as a corrosion
product. Future work will be conducted to study the
operating temperature's effect on tobacco extract inhibitors' stability and
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