The Effect of Trace Amount of H2S on CO2 Corrosion Investigated by Using the EIS technique

A project has been initiated with the aim of extending the model to cover the effect of H 2 S on CO2 corrosion. This report covers one of the main building blocks necessary to complete the mechanistic CO2/H2S corrosion model, namely, electrochemistry of API 5L X65 carbon steel CO2 corrosion in the presence of small amounts of H2S (≤340ppm). The corrosion process monitored by Linear Polarization Resistance (LPR) and Electrochemical Impedance Spectroscopy (EIS) showed a significant decrease in corrosion rate in the presence of H2S due to the metal surface coverage by a sulfide film. This sulfide film was identified as mackinawite by X-ray photoelectron spectroscopy (XPS). Since the experimental results suggested that the mechanism is a retardation of the charge transfer process, the surface coverage was calculated from the corrosion rate. The Langmuir-type adsorption isotherm was successful in modeling the surface coverage by mackinawite in the presence of trace amounts of H2S. Introduction The advantage of EIS technique over other electrochemical experimental techniques in studying corrosion has been discussed in the previous work. This report extends the EIS investigation of the electrochemical reaction in saturated CO2 solution with slightly higher H2S concentrations (0~340ppm) in a low temperature, low pressure and low pH condition over longer immersion time (4~72 hours). It has been speculated previously that the protective thin sulfide film might have formed at the electrode surface via solid state formation, resulting in a lower corrosion rate in the presence of trace amount of H2S. Numerous reports have hinted that this thin sulfide film may possibly be mackinawite (FeS1-x). Hence, providing direct evidence that confirms the existence of thin mackinawite film on the electrode was one of the main goals of this study. However, this very thin film was not visible to the naked eye, as the electrode surface remained shiny throughout the experiment due to the low corrosion rate. Scanning Electron Microscopy (SEM), which examines the electrode surface, was incapable of observing such a thin layer of film. Although X-ray Diffraction analysis (XRD) is a popular technique that enables one to distinguish different types of thin iron sulfide on a flat surface, it is not suitable for a round surface on the cylindrical electrode. On the other hand, even though Electron Dispersion Spectroscopy (EDS) technique had detected sulfide elements on the electrode surface, it was not able to confirm the exact iron sulfide compound. In order to clarify the exact chemical composition of this thin protective sulfide film, the electrode surface was examined using X-ray photoelectron spectroscopy (XPS). Surface analysis by XPS is accomplished by irradiating the target surface with soft x-rays and analyzing the energy of the detected electrons. The x-rays interact with atoms in the surface region, causing electrons to be emitted by the photoelectric effect. The emitted electrons have measured kinetic energy given by: KE= hν – BE – φs (1) where hν (eV) is the energy of the photon (x-ray), BE (eV) is the binding energy of the atomic orbital from which the electron originates, and φs (eV) is the spectrometer work function. The binding energy is the energy difference between the initial and final states after the photoelectron has left the atom. Since each element has a unique set of binding energies, XPS can be used to identify and determine the concentration of the elements in the surface. Slight variation in the elemental binding energies (the chemical shifts) arises from the chemical potential of compounds. These chemical shifts can be used to identify the chemical states of the materials being analyzed and it is very effective for determining various types of iron sulfides. Experiment Experiments were conducted in a glass cell on carbon steel X65 at these conditions: T = 20C, pH = 5, P = 1 bar, ω = 1000 rpm, gaseous H2S concentration in CO2 = 0, 3, 15, 40, 160 and 340 ppm. In these experiments the corrosion process was monitored with different electrochemical measuring techniques: Electrochemical Impedance Spectroscopy (EIS) and Linear Polarization Resistance (LPR). Experiments for each H2S concentration were conducted at least twice to make sure that the results were reproducible and reliable.