INTRODUCTION

ABSTRACT:

The CO2 erosion-corrosion electrochemical behavior of X70 pipeline steel was investigated under jet impingement condition by high temperature and high pressure loop equipped with electrochemical measure equipment. The surface morphologies and profiles of the X70 steel after jet impingement experiment at different impinging velocities were characterized by scanning electron micrography (SEM) and morphology profile apparatus. The results showed that the resistance to erosioncorrosion of X70 steel decreased with the increase of impinging velocity. With the increase of impinging velocity, the cathodic reactions of X70 steel were accelerated and anodic reaction rates didn't change obviously. The corrosion behavior of X70 steel shifted from activation and mass-transport mixed-control to electrochemical-control. With the increase of the fluid velocities, the maximum phase angles of X70 steel decreased gradually due to the change of double-layer structure of electrode/solution interface. With the increase of the fluid impinging velocity, the constant phase element of double layer Qdl increased and charge transfer resistance Rt decreased, due to the increase of the mass-transfer coefficient and wall shear stress with the fluid flow velocity. The erosion-corrosion profile was axisymmetrical basically, showing the “W” shape. The depth of corrosion increased with the increasing of the radical distance, and then decreased.

CO2 corrosion is frequently encountered in oil and gas gathering and transportation systems, which could causes the common occurrence of the failure cases of pipelines and structure components resulting in significant economic loss as well as environmental pollution and ecological damage1-3. CO2 corrosion is very complex, and there are numerous factors influencing the corrosion process of steels 4, 5. Currently, such as elbows, tees, valves, sudden expansion pipes and other complex flow components were damaged by erosion-corrosion under high temperature and high pressure conditions 6, 7. With the existence of CO2 in oil and gas gathering and transportation facilities, CO2 pressure, temperature, and velocity of fluid are high, which cause to form complex high-speed turbulent area in the local positions of the elbows, tees, valves and other flow components and the mass transfer process is greatly accelerated and such damage of flow components would reduce their service life. The jet impingement test can simulate reliably and repeatedly high-turbulence conditions at liquid and multiphase turbulent system of high temperature and high pressure of CO2 gas, which is an important researching equipment to evaluate the corrosion resistance of materials under high-speed flow and investigate erosion -corrosion behavior and damage mechanism of steels 11, 12. Generally, the erosion -corrosion simulation tests and the corresponding in-situ electrochemical tests are conducted at room temperature and/or atmospheric conditions 13, the research works are far from enough under high-speed fluid impinging as well as the high temperature and high pressure conditions by now. Most considered that the flow pattern at submerged jet divided into three regions 14: stagnation region (laminar zone, x/d=1.5), transition region (high turbulence zone, 1.5<x/d=4) and wall jet region (low turbulence zone, x/d>4), as shown in Figure 1. Therefore, the corrosion profile of X70 steel at 20m/s was consistent with the flow pattern.

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