The corrosion of carbon steel (CS) structures in the presence of H2S (sour environment) is a serious and ongoing challenge to the oil and gas industry. There is, however, a limited number of studies that cover H2S corrosion, compared to the literature available on CO2 corrosion. Sulfide scale formation has been acknowledged as a possible factor for preventing metal loss in sour environments. However, there have been inconsistencies in reports about the chemistry, structure, and formation mechanisms of these scales. In addition to the formed sulfide scales, different deposits are present that not only affect the corrosion of the alloy, but also the mechanism of film formation and its protectiveness. In this thesis, the mechanisms and kinetics of CS corrosion and sulfide scaling in sour oil and gas pipelines were investigated with particular emphasis on the phase and morphology of produced scale as a function of temperature and time.
A sequence of investigations were carried out to study the scale formation in two types of media: pure water and solutions with different chloride ion concentrations from various types of salts. The aim was to study the initiation and the propagation of corrosion in sour media. Many oilfield process parameters, for instance, time, temperature, chloride ion concentration, pH of corrosion environment, and other chemicals (such as corrosion inhibitors), which may influence the corrosion kinetics and this mechanism, were also studied. A comprehensive and systematic analysis of H2S corrosion performance of CS was conducted using an experimental system under the anaerobic condition that would simulate the forming of an iron sulfide scale: a set of electrochemical and ex-situ surface analysis techniques were used.
The morphology and chemical composition of the sulfide scale formed was found to be a porous structure, which is replicated across a change of time scales and at different temperatures. In comparison to the scale formed at room temperature, higher temperature formation results in a smaller pore size and thinner layer (50 oC and 80 oC). A three-layer porous structure is found to be shaped on the carbon steel surface, in contrast to other studies. Correlative solution analysis was also performed to assess the dynamic nature of the environment; the Fe2+ ion concentration increased considerably during the test; the pH values of the water solution fluctuated with time, and with variations dependent on the specific conditions, up until about 4500 s and then stabilized at similar values (of 3-4) until the end of the reaction.
The addition of salts significantly accelerates general corrosion, and the solution chemistry and temperatures play an important role in both the corrosion and FeS film formation. Corrosion rates (CR) were determined from the linear polarization resistance (LPR) test; the CR increased with high chloride ion concentrations in an aerated environment as well as at a higher temperature of 80 °C. The morphologies and Raman spectra indicated the iron sulfide producted under all conditions is dominated by the mackinawite phase.
Two commercial organic inhibitors were inspected for potential mitigation of CS in H2S. The inhibition efficiency (IE %) was found to rise with inhibitor concentrations as expected. Both Cls were found to behave as mixed inhibitors but were dominated by cathodic effects. The additional of the inhibitor before H2S exposure delayed the formation of FeS scales. However, polarization resistance further improved with rising inhibitor concentrations and temperatures, indicating that a protective sheet was formed on the surface. The addition of an inhibitor to an already scaled surface showed a marked effect on the electrochemistry, suggesting that the inhibitor molecule can access the corroding surface.