Investigação do processo de corrosão do aço API 5L_X65 em meio de bicarbonato e cloreto utilizando serie temporal de imagens

Abstract

The carbon steel corrosion process used at in machines and pipelines of oil fields is mainly attributed to the presence of CO2, and H2S dissolved at solution. These compounds usually can to carry the corrosion type more common in this system: the pit corrosion. Depending on operation conditions of the pipe (pressure, temperature), the formation of different corrosion products can occur, as iron carbonate (FeCO3). So, this work presents a pit corrosion study of the API 5L_X65 carbon steel in carbonate solution and different chloride concentrations. The measurements were made at steel samples with and without any heat treatment. The corrosion process was evaluated using the techniques of chronopotentiometry of zero current, to measure the open circuit potential (Eoc), the polarization curve (PC) and chronoamperometry (CA). In all cases was evaluated in situ the metallic surface using an optical microscope (OM). The in situ images were separated into a sequence of micrographs and processed with Image-J software to convert the image into quantitative information. The pits were selected by adjusting the image histogram (Threshold) and then the images were binarized to separate the bottom (steel) from the object (the pits). This procedure makes it possible to quantify black (bit 1) and white (bit 0) regions, to determine which part has been corroded relate the area and number of punctual attacks. The time series of image was acquired at 3 and 120 frames min-1 during the Eoc and CA, respectively. The Eoc was monitored for 9000 s or until its stabilization and the CA was performed polarizing the electrode at 350 mV more positive than Eoc obtained. The electrolyte used was a solution of 0.1 mol L- 1 NaHCO3 and NaCl chloride at different concentrations. A factorial design was made by varying the concentration of chloride ions at 2.25, 2.5, 3.0, 3.5 and 3.75% (w / w) and the temperature was 18.5, 25.0, 40.0, 55.0 and 62.5 °C. In all measurements the Eoc initially presented abrupt drop and after stabilized at potential near -0.69 V. The micrographs in situ obtained during CA showed the formation of pits but it has not been possible to relate the corrosion rate increases with temperature and ion solution increase. Passivation of the surface was observed with the aid of OM considering a slight darkening of the metal surface. The characterization of the films was performed by scanning electron microscopy (SEM). It was observed that the formation of a film over whole surface and pitting occurs at some points, indicating that corrosion occurs in two forms, localized and generalized.