Boivin, Joseph (Corrosion and Protection Center, Key Laboratory for Environmental Fracture (MOE), ) | Wen, Zhibin (Corrosion and Protection Center, Key Laboratory for Environmental Fracture (MOE), ) | Li, Xiangyang (China Petroleum Engineering Co., Ltd BEIJING Company,) | Ding, Jinhui (Corrosion and Protection Center, Key Laboratory for Environmental Fracture (MOE), ) | Lu, Minxu (Corrosion and Protection Center, Key Laboratory for Environmental Fracture (MOE), )
During exploitation and production activities in Sichuan highly sour reservoirs in China, phenomena of elemental sulfur deposition and corrosion were found. Since the characteristic of elemental sulfur depositing on the steel surface has been considered as one of the most important factors that control the corrosion behavior, much more research work is needed to evaluate and predict elemental sulfur corrosion in laboratory by simulating various contact forms between sulfur and steel. In this paper we report the effects of test method and temperature on sulfur corrosion of steel by simulation experiments under different conditions. The forms of sulfur during the tests such as suspending in solution with different content, depositing on the steel surface, paste coverage and molten coverage are investigated. Different corrosion type and rate are found among these tests. Increasing temperature may also change the corrosion type from general corrosion to localized corrosion. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) are also employed in order to reveal the formation characteristics of sulfide corrosion product and its effects on the localized corrosion.
With the increase of energy demand and exploitation and production activities in China, the highly sour oil and gas fields which have harsh corrosive environment are gradually being developed. The sulfur presented in sour gas reservoirs can deposit in the pipelines and transport systems. The more content of H2S in the gas, the more likelihood of sulfur precipitation would occur, especially in the gas wells with more than 30% H2S. The corrosivity of the environment with high H2S and CO2 can be further enhanced by the existence of sulfur, and even the nickel-based alloys may suffer severe pitting corrosion under the deposited sulfur, which will increase the risk of corrosion failure of tubing, casting and pipelines.1-2 Elemental sulfur corrosion mainly refers to the corrosion of S8-based elemental sulfur in the presence of electrolyte. Acidification in sulfur containing aqueous systems has been reported, with species such as H2S, H2SO4 and polysulfides being possible candidates as hydrolysis products. The corrosion process is influenced by the surface contact state of sulfur and metal, environmental temperature, pH value, H2S concentration, Cl- concentration etc.3-7 In this paper, the elemental sulfur corrosion behavior is studied under the conditions of different sulfur depositing characteristics and temperatures, which will provide theoretical basis for the corrosion risk prediction.
The test specimens are machined out of API X65 pipeline steel with chemical composition shown in Table1. For the corrosion experiments, four types of conduction between elemental sulfur and steel surface are investigated with 3.5% sodium chloride solution, including suspending in solution with different sulfur content (A), depositing on the steel surface (B), paste coverage (C) and molten coverage (D) on the steel surface, as shown in Figure 1. In case A, the sulfur powder is directly added into the solution with coupons at the bottom of glass cell.