The effect of different brines and temperatures on the competitive degradation mechanisms of CO2 and H2S in API X65 carbon steel

Abstract

Produced water found in the exploitation and transport of oil and gas is composed by many contaminants such as chloride, CO2 and H2S. Nowadays, a very limited amount of studies in the literature have explored the effects of different salt concentration on the both corrosive mechanisms of CO2 and H2S. The aim of this work was to analyze the temperature and Cl− concentration influence on corrosion behavior of API X65 steel exposed to a CO2 saturated only and CO2/H2S (from thiosulfate ions) environments. The evaluation was carried out through Electrochemical Impedance Spectroscopy (EIS), Linear Polarization Resistance (LPR) and weight loss tests at 90 °C and 120 °C with 0.1% wt. and 3.5% wt. NaCl. The samples characterization was performed by Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD). The results showed that sodium chloride made it difficult to precipitate FeCO3 films at 90 °C both in environment with CO2 only and with thiosulfate-CO2. In the environments with addition of thiosulfate, the higher concentration of sodium chloride, favored the formation of the more stable sulfide films revealing pyrite as the product. The higher temperature favored the formation of both FeCO3 and FeS films, for all conditions tested. Sulfide films formed at 120 °C were identified as pyrite and greigite, in which are sulfide layers with better protective characteristics. However, due to the imperfections and brittleness of these films, it was not possible to have a significant reduce of the corrosion process in the steel. Only the condition with higher sodium chloride concentration at 120 °C, corrosion rates were reduced due to higher precipitation of mackinawite films together with more stable phases such as pyrite.