ABSTRACT

Geothermal brine is a complex system containing a wide variety of dissolved salts resulting from the conditions in a geothermal well. These fluids lead to corrosion in pipes and other parts of geothermal system construction and necessitate intense research efforts in finding new suitable materials. Carbon steel is susceptible to corrosion in geothermal brine especially when it is exposed to a high temperature and high-pressure medium, which is considered to be an aggressive environment. An artificial geothermal water, based on a brine composition found in Indonesia, was used to investigate the performance of high alloyed materials. The electrolyte has pH 4 and contains 1,500 mg/l Cl-, 20 mg/l SO42-, 15 mg/l HCO3-, 200 mg/l Ca2+, 250 mg/l K+, and 600 mg/l Na+. In order to protect the bare material in geothermal application, it is necessary to either use high alloyed materials or coatings. In this research, a coating system consisting of polyaniline and silicon dioxide was investigated regarding its behavior to protect carbon steel. In detail, the effect of SiO2 and polyaniline (PANi) addition was evaluated by exposure and electrochemical tests for 7 days, i.e. electrochemical impedance spectroscopy (EiS) and open circuit potential (OCP) at room temperature and 150 °C with 1 MPa pressure.

INTRODUCTION

Geothermal energy is one of the promising renewable energy resources that can be used as an alternative to conventional fossil fuel energy. At the moment, geothermal energy has been widely developed because of its reliability, consistency and lower carbon dioxide emission.1-3 One challenging aspect of geothermal application is the performance and durability of the construction materials under exposure in its severe environment, which commonly causes corrosion and scaling.4-6 Geothermal water is known to be an aggressive medium due to its physicochemical complexity, the operational conditions including high temperature and pressure, and its constituent components containing dissolved salts from different elements.7-9 There are several efforts in finding suitable materials for geothermal applications to avoid operation failure, enhance the lifetime of materials, and obtain a cost effective material, such as material selections,10-13 using high alloyed materials,14-16 or protecting a lower resistant material.17

Corrosion protection is an essential part of material selection in geothermal application, and one of the methods is by using barrier layers or coatings. Coatings have several advantages in comparison with other protective methods, i.e. they are effective, versatile, and have a reasonable cost. However, protective coatings require a proper application, surface preparation and corrosion analysis.18

In this paper, a coating system based on alkyd binder containing polyaniline and silicon dioxide is introduced, using some components that are locally available in Indonesia. Many studies have investigated polyaniline, which is an electroactive polymer, as pigment in coatings to enhance the corrosion resistance.19-21 This study will be carried out in artificial geothermal water with high temperature and high pressure, with the addition of silicon dioxide particles in the coating system to enhance its thermal properties.

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