Propriedades eletroquímicas, mecânicas e tribológicas dos produtos de corrosão formados no aço APIX65 em ambientes de pressão supercrítica de CO2

Abstract

The most used steels in the oil sector are carbon steels, belonging to the class of high strength and low alloy steels (ARLB), due to their greater availability and lower cost. Among these steels, API5L X65 steel is one of the widely used in the construction of pipelines for the transport of oil and natural gas. However, these steels have low corrosion resistance that can cause pipeline failures as they are exposed to aggressive environments, as the oil can contain high levels of salt and acid gases such as CO2 and H2S. This exposure can generate failures and consequently high annual investments for restoration or even replacement of these pipelines, in addition to presenting environmental and human health risks. In the presence of CO2, the main corrosion product formed is iron carbonate (FeCO3) or mixed carbonates that, depending on the properties, can provide protection to steel, decreasing the corrosion rate over time. Several studies have been carried out in order to understand both the chemical composition and the microstructures present in the corrosion products formed after corrosion in solutions containing CO2, however, little attention is paid to the electrochemical, mechanical and tribological properties of these products formed at high pressures. The objective of this work is to characterize the corrosion product films formed and to determine the electrochemical, mechanical and tribological properties after being exposed to two different media: 3.5% NaCl and a medium that simulates pre-salt formation water, temperature of 40 ºC, pressure of 15 MPa for different periods of exposure (7 and 15 days). Several characterization techniques were used, such as scanning electron and atomic force microscopies, X-ray microtomography, X-ray diffraction, potentiodynamic polarization, electrochemical impedance spectroscopy, nanoindentation and wear tests. The products formed by FeCO3 showed properties superior to those of mixed carbonate (FexCa1−xCO3). The presence of Ca2+ in the brine favored the dissolution of the film, decreasing the steel's ability to protect against CO2 corrosion, reducing its mechanical properties and wear resistance.