Effect of Dissolved CO2 on the Interaction of Stress and Corrosion for Pipeline Carbon Steels in Simulated Marine Environments

Abstract

Offshore pipelines are subjected to stresses (e.g., from fluid flow, mechanical vibration, and earth movement). These stresses, combined with corrosive environments and in the presence of trace gases (O2, CO2), can increase the pipeline’s corrosion rate and potentially lead to cracking. As such, the impact of trace gases such as CO2 (linked to enhanced oil recovery and carbon capture and sequestration) on corrosion is key to determining whether pipelines are at increased risk. American Petroleum Institute (API) 5L X70 and X100 were exposed as stressed C-rings (80% or 95 % of yield strength). The tests were conducted with either N2 (control) or CO2 bubbled through 3.5% NaCl, at either 5 °C or 25 °C. Linear polarization resistance was used to assess corrosion rate, while morphology and variation were determined using optical microscopy (generating metal loss distributions) and scanning electron microscopy. The control experiment (N2) showed that corrosion rates correlated with temperature and stress. In this low O2 environment, both alloys showed similar trends. Under CO2 exposure, all samples showed accelerated corrosion rates; furthermore, the morphologies generated were different for the two alloys: undercutting corrosion with discontinuous microcracks (X70) or deep, wide ellipses (X100). Understanding these changes in corrosion response is key when selecting materials for specific operational environments.

Influence of the Morphology of Eutectoid Steels on Corrosion Resistance in NaCl Aqueous Medium with and without CO2

“…Table 4 presents the results divided into levels of resistance to polarization for each sample according to its solution. The corrosion resistance of steels used in the oil industry was evaluated, depending on their morphologies [8]. It was possible to identify that those with globular (spheroidized) cementite obtained better results for corrosion when subjected to high temperatures (>60 • C) and with the use of CO 2 in a NaCl solution.…”

“…The evaluation of the tensile strength of wires subjected to severe corrosion conditions with CO2 and NaCl is of interest to the oil industry, in addition to appealing to companies manufacturing risers that develop materials with different compositions and heat treatments. The application of other materials and in environmentally unfavorable conditions are relevant factors for the industries that manufacture such components since, for these reasons, the optimization and quality of the final product are essential [7,8]. CO2 gas has a very damaging effect on the surface of steel, especially due to forming H2CO3, an acidic compound that can lead to the rupture of pipelines and other oil production structures [9,10].…”