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This paper studies the corrosion mechanism of the weld seam of the submarine pipeline’s spool. The types and causes of inner tube weld corrosion are simulated by EDX and XRD analysis of the on-site pipe corrosion products, combined with OLGA Software simulation pipeline flow pattern. The tensile testing, impact testing and hardness testing were carried out on the base metal and the weld by tensile tester, pendulum impact tester and Brinell hardness tester to analyze the mechanical properties of the base metal and the weld; the microstructural difference between the weld and the base metal were analyzed by optical microscopy; The results show that the liquid phase flow rate along the line is between 3.5m/s and 7.5m/s, which aggravates the mixing between the gas and liquid phases to form a bubble flow. When the entire weld area is immersed in the same simulated medium solution, the galvanic corrosion occurs in three parts of the weld zone. The weld seam and heat affected zone will be accelerated to corrode as the anode region of the galvanic couple. The weld seam has the lowest corrosion potential and is always used as an anode to accelerate corrosion.

The corrosion behavior of the Xinjiang oil field (China) gathering pipeline steel was studied by a weight-loss technique, microscopic characterization, an X-ray diffractometer, and a three-dimensional microscope. When the CO2/O2/H2S system contains a low H2S concentration (100 ppm), the CO2 and O2 concentrations increase and the corrosion rate of 20# steel increases. When the CO2 concentration increases (10–30%), the corrosion rate increases by 8.84%, and the corrosion product film thickness increases significantly by 231.01%, while when the O2 concentration increases (0.5–2.5%), the corrosion rate increases by 29.24%, and the thickness of the product film increased by 29.21%. The two mechanisms that promote the increase in the corrosion rate are different. CO2 promotes the cathodic reaction to increase the corrosion rate, while O2 accelerates the corrosion process by destroying the structure of the corrosion film. The morphology of corrosion products is more complex. The increase in O2 concentration increases the number of corrosion defects, and the change in CO2 concentration has little effect on local corrosion. The corrosion products mainly include FeCO3, Fe2O3, and FeS2, but S and FeS are not formed. The reason is that CO2 controls the corrosion process, and a large amount of O2 is consumed, which weakens the interaction between H2S and O2. [ABSTRACT FROM AUTHOR]

Multi-component thermal fluid flooding is an efficient thermal technology for heavy oil recovery. However, the oil and gas transported by the gathering pipeline exhibit the large temperature difference along with high O2, H2S and CO2 contents, which may easily bring serious corrosion problems to the pipeline. In this article, the present work aims to study corrosion behavior and corrosion mechanisms in a CO2/H2S/O2 coexistence system using a Huning 4 block gathering pipeline material, particularly clarifying the influence of temperature on the corrosion behavior and localized corrosion mechanism. The high-temperature autoclave was used to simulate the corrosion environments. The corrosion products at various temperatures were examined via three-dimensional optical microscopy, scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The maximum uniform corrosion rate of L245NS is 0.5149 mm/a (at 333 K), and the maximum local corrosion rate is 4.6258 mm/a (at 363 K). At lower temperatures (303–333 K), the corrosion product film possessed a double-layered membrane, where the bottom layer was enriched with S-based elements, mainly FeS or FeS2 and the upper layer was mainly covered by FeCO3 with the uniform corrosion state. Applying the higher temperatures (363–393 K) made the FeCO3 grain coverage more continuous and dense, leading to the formation of a dense double-layered film, where the uniform corrosion was suppressed, while localized corrosion emerged. The thermodynamics shows that an increase in the temperature was found to decrease the solubility product coefficient of FeS and FeCO3, inducing the appearance of a protective layer and inhibiting the corrosion process. [ABSTRACT FROM AUTHOR]