Formation And Evolution Of H₂S Corrosion Products And The Influence On Corrosion Mechanism Under High H₂S Partial Pressure

The increasing demand of energy and technology development have led to the exploration of all possible extraction options,which includes the deep-water and highly sour oil and gas fields.There have been increasing concerns related to the failure risks and safety challenges posed by H2S corrosion to the integrity of oilfield pipeline.Although the research on CO2/H2S corrosion has been investigated for many years,the corrosion mechanism of low alloy steel under high H2S partial pressure is still lack of theoretical understanding and adequate data support.The study herein presents an investigation into the overall corrosion behavior of mild steel in highly sour environments.In this thesis,high-pressure in-situ electrochemical methods and corrosion simulation experiments,combined with surface analysis,were used to investigate the formation and evolution of H2S corrosion products and their influence on the corrosion behavior of mild steel.An environmental boundary of formation of different corrosion products was also discussed.Firstly,the formation and evolution of H2S corrosion products under high H2S partial pressure were studied.A thin mackinawite film with preferred orientation initially formed on the steel surface by a solid-state reaction.When more ferrous ions were released from the steel surface,more mackinawite particles began to precipitate on the film surface because of the supersaturation of iron sulfide.As the Fe2+ migrated to the outer layer and HS-migrated to the matrix surface,new corrosion products formed above and under the original mackinawite layer.The amount of Fe2+migrated to the outer layer decreased with the corrosion rate and the rich sulfur phase,pyrrhotite,formed gradually.Secondly,environment boundary of different types of H2S corrosion products was established.A thermodynamic model was developed to clarify the environmental boundaries for the mackinawite and pyrrhotite formation.And it was validated on the basis of the experimental data.Mackinawite is mainly formed at low temperatures and low H2S partial pressures,whereas pyrrhotite coexists at higher temperatures and higher H2S partial pressures.Thirdly,the effect of the formation and evolution of corrosion products on the electrochemical corrosion behavior under high H2S partial pressure was investigated.And the role of pyrrhotite on localized corrosion was discussed.As mackinawite and pyrrhotite formed on the steel surface,corrosion potential shift to the positive direction and corrosion current decreased gradually.The potential difference between the steel surface covered by mackinawite and the surface with pyrrhotite was the driving force for a galvanic couple,which led to localized corrosion.Finally,the effect of thermophilic sulfate-reducing bacteria(SRB)on the corrosion behavior of pipeline steel under deposits was investigated based on the failure cased from field conditions.The porosity of CaCO3 particles and the anaerobic environment under the deposits provide a convenient substrate for the adhesion and growth of SRB.Inhomogeneous distribution of deposits initiated the localized corrosion and the formation of FeS by the metabolic activity of SRB would accelerate the anodic dissolution of pipeline steel.The study on the corrosion mechanism of low alloy steel under high H2S partial pressure will be expected to provide theoretical and experimental support for corrosion prediction and control in highly sour oil and gas fields.