CYSTEINE INHIBITION MECHANISM AND EFFICIENCY ON CARBON STEEL CORROSION IN THE CARBON DIOXIDE SATURATED ELECTROLYTE SOLUTION

• Main Author: SUNARYA (NIM 30503005); Pembimbing: Dr. Ing. Cynthia Linaya Radiman, Dr. Hj. Sadijah Achmad, D, YAYAN
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/13163
• Institution: Institut Teknologi Bandung

The crude oil produced in oil mining contains very corrosive substances, corroding the pipe lines of the mining production wells, namely organic acids, chloride and sulphate salts, and CO2. Corrosion on the outer surface of the pipes can be overcome by coating those pipes or by applying chatodic protection, while the inner surface can be controlled by adding corrosion inhibitors. One of mechanism of corrosion inhibitors is through forming single molecular layers from which an inhibitor is adsorpted. Corrosion inhibitors which are able to form such layers are heteroatomic carbon compounds containing mercapto, amine, and/or tiocarbamido functional groups.<p>Amino acids have functional groups potentially acting as a corrosion inhibitor. Some of those groups even have been reported to have high inhibition efficiency on iron, copper, Cu-Ni and Pb-Ca-Sn alloys. The proposed inhibition mechanism is generally through the physically or chemically formation of adsorpted layers on the metal's surface. Nevertheless, the proposed mechanism has not yet touched the role of each group in inhibiting the metal corrosion.<p>The use of amino acids as carbon steel corrosion inhibitor in the oil mining environment has not been yet investigated. Therefore, need to be done study to amino acids. In this research will be study seven kinds of amino acids and selected one amino acid belong to the highest potency as corrosion inhibitor will be studied. The seven of amino acids are glycine, alanine, cysteine, threonine, tryptophan, proline, and phenylalanine.<p>This research aims at studying the mechanism and efficiency of the selected amino acid on carbon steel corrosion in CO2-saturated NaCl solution at particular pH level and temperature appropriate for the oil mining conditions. Specifically, this research has three objectives, namely (a) to study the process of carbon steel corrosion and inhibition mechanism of the selected amino acid, (b) to study the roles of functional groups found in the selected amino acid, and (c) to study the conditions as well as the composition of media which enable the functional groups to be actively involved in the corrosion inhibition process.<p>In order to achieve the goals mentioned above, this research employs potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) methods. In addition to that, computer simulation using Zview program is also taken into account as one of the methods. Furthermore, this research is also supported by the carbon steel surface characterization analysis. The followings are the steps in carrying out this research, namely (a) determining the carbon steel corrosion rate at particular pH level and temperature in accordance with the condition of the oil mining, (b) selecting one of the seven kinds of amino acids to be the corrosion inhibitor, (c) deciding corrosion inhibition mechanism from the selected amino acid, and (d) settling on inhibition efficiency.<p>This research found that carbon steel API 5 L X65 is corroded in the CO2- saturated NaCl solution significantly. Its corrosion rate increases with the increase of its temperature and media pH, reaching the highest at 340K at pH 4.0.<p>The corrosion of carbon steel in test media can be inhibited by amino acids with the following inhibition ability: cysteine > threonine > alanine > tryptophan > glycine > proline > phenylalanine. Since cysteine has the highest corrosion inhibition ability of all, it was selected as the candidate of inhibitor of tested carbon steel corrosion.<p>The mechanism of cysteine inhibition through the formation of protective film was physically adsorpted on the carbon steel surface with ?Gads equals to -20.276 kJ.mol-1, which is in line with Langmuir adsorption isotherm. The film formatted is, however, not perfect. Functional group dominantly involved in the formation process of the protective film is -SH, whereas -NH3+ is insignificantly involved.<p>Temperature increase influences the role of functional groups in the formation of protective film, namely the role of -SH group decreases, whereas the role of -NH3+ increases. The increasing temperature leads the ?Gads value to become more negative whereas ?Hads and ?Sads become more positive. This phenomenon shows that corrosion inhibition process is more stable at higher temperature by increasing its potential barrier. High temperatures alter the carbon steel corrosion process inhibited by cysteine. Below 330K, the corrosion process is controlled by the kinetic of charge transfer whiles over 330K, the corrosion process is determined by the diffusion process.<p>The rise of pH controlled by acetic buffer does not changes the cysteine inhibition mechanism but influences the role of functional groups during the formation of protective film, namely, the role of -SH group increases while -NH3+ group decrease. In other words, when the pH is low, -NH3+ group plays more significant role than -SH group in the protective layer formation.<p>On temperature of 330K and exposure time of one hour, cysteine inhibition efficiency can reach 77% for 0.10 mM concentration. The increase of cysteine concentration to 0.40 mM can increase its inhibition efficiency by 89%. Furthermore, the longer of exposure time the higher the level of inhibition efficiency up to 90% for 24-hour. On pH 3.0-5.0 acetate buffer, the efficiency inhibition of cysteine increase with the increase of cysteine concentrations and pH media. In environmental appropriate for the oil mining conditions, especially at temperatures 360K and pH 4.0, the cysteine's level of inhibition efficiency can reach 88% for 0.10 mM cysteine concentration. Compared to commercial inhibitor (thiourea), cysteine is still better or more superior. <br />