Corrosion behavior and mechanism of X80 steel in silty soil under the combined effect of salt and temperature

Effect of seawater salinity, pH, and temperature on external corrosion behavior and microhardness of offshore oil and gas pipeline: RSM modelling and optimization

This research aims to investigate the effects of seawater parameters like salinity, pH, and temperature on the external corrosion behaviour and microhardness of offshore oil and gas carbon steel pipes. The immersion tests were performed for 28 days following ASTM G-1 standards, simulating controlled artificial marine environments with varying pH levels, salinities, and temperatures. Besides, Field emission scanning electron microscopy (FESEM) analysis is performed to study the corrosion morphology. Additionally, a Vickers microhardness tester was used for microhardness analysis. The results revealed that an increase in salinity from 33.18 to 61.10 ppt can reduce the corrosion rate by 28%. In contrast, variations in seawater pH have a significant effect on corrosion rate, with a pH decrease from 8.50 to 7 causing a 42.54% increase in corrosion rate. However, the temperature of seawater was found to be the most prominent parameter, resulting in a 76.13% increase in corrosion rate and a 10.99% reduction in the microhardness of offshore pipelines. Moreover, the response surface methodology (RSM) modelling is used to determine the optimal seawater parameters for carbon steel pipes. Furthermore, the desirability factor for these parameters was 0.999, and the experimental validation displays a good agreement with predicted model values, with around 4.65% error for corrosion rate and 1.36% error for microhardness.

The effect of thermodynamic changes in the cooling of saline soils on the corrosion system of carbon steels

In this experiment, Q235 and X80 carbon steels, which are widely used in oil and gas pipelines and ancillary facilities, were selected to study the changes in the corrosion behaviour and mechanism of carbon steels in the process of natural saline soil cooling to a freezing state through electrochemical testing. The equivalent circuit model of carbon steel before and after the freezing phase transformation in the soil was determined. Based on the corrosion kinetic parameters and soil thermodynamic changes, the influencing factors of steel corrosion during the cooling process were systematically analysed. It was found that temperature mainly affected carbon steel corrosion by changing the properties of the solution. The main factors affecting the corrosion behaviour of the carbon steel were the thermal motion of molecules, ions, and electrons in solution, oxygen dissolution and diffusion, ion adsorption, diffusion mass transfer, and unfrozen water content change during the cooling process.

Electrochemical corrosion behaviour of four low-carbon steels in saline soil

In this paper, the electrochemical corrosion behaviour of Q235, X65, X70, and X80 low-carbon steel was systematically studied by a variety of test techniques using natural saline soil containing 1.1% salt under laboratory conditions. The electrochemical corrosion behaviour, macro-micro corrosion morphology, and corrosion product composition of these four low-carbon steels in saline soil were studied to explore their salt corrosion resistance and reveal their corrosion mechanisms. The research results showed that oxygen absorption corrosion occurred in all four low-carbon steels in the saline soil, and the corrosion types were all localised corrosion. The corrosion process of Q235 steel was controlled by mass transfer, while the corrosion processes of X65, X70, and X80 steel were controlled by charge transfer. The corrosion rates of these four low-carbon steels in saline soil followed the order Q235 > X65 ≈ X70 > X80. Variation in elemental composition was the main reason for this difference in corrosion behaviour. Finally, microscopic test results showed that local corrosion pits were present on the surface of the steel sheet specimens, and the uniformity and compactness of the corrosion product accumulation were poor.

Electrochemical Corrosion Behaviour of X70 Steel under the Action of Capillary Water in Saline Soils

In this paper, the electrochemical corrosion behavior of X70 steel in saline soil under capillary water was simulated by a Geo-experts one-dimensional soil column instrument. A volumetric water content sensor and conductivity test were used to study the migration mechanism of water and salt (sodium chloride) under the capillary water. The electrochemical corrosion behavior of the X70 steel in the corrosion system was analyzed by electrochemical testing as well as the macroscopic and microscopic corrosion morphology of the steel. The test results showed that the corrosion behavior of X70 steel was significantly influenced by the rise of capillary water. In particular, the wetting front during the capillary water rise meant that the X70 steel was located at the three-phase solid/liquid/gas interface at a certain location, which worsened its corrosion behavior. In addition, after the capillary water was stabilized, the salts were transported with the capillary water to the top of the soil column. This resulted in the highest salt content in the soil environment and the most severe corrosion of the X70 steel at this location.