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Effect of Dissolved Oxygen on Fatigue Crack Growth Rate of Low Alloy Steels in Saltwater with Cathodic Protection Environment

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Low alloy steels (LAS) are widely used in the marine and offshore oil and gas industry for various applications from bolting to large pressure containing heavy wall forgings. These materials are subject to various types of corrosion (general or uniform, pitting, crevice, etc.) and degradation in seawater environment. However, their selection for the applications, in comparison with stainless steels and corrosion resistant alloys, is justified due to their availability, manufacturability, proven service history, and lower cost.

Product Number: 51323-18913-SG
Author: Arshad Bajvani Gavanluei, Jenny Crump, Joseph Wilhelmi, Jack Smith, Paul D. Bunch
Publication Date: 2023
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Low alloy steels (LAS) frequently used for pressure containing and primary load bearing equipment in subsea applications are susceptible to various types of corrosion in seawater. Cathodic protection (CP) is usually applied to protect LAS materials from corrosion. However, applying CP can lead to Hydrogen Embrittlement (HE) and failure of the protected components, especially higher strength alloy steels, if CP design parameters are not carefully considered. HE occurs due to absorption and diffusion of atomic hydrogen into the steel. The source of hydrogen is a cathodic reaction of the reduction of water, which occurs at the surface of the cathode. The presence of Dissolved Oxygen (DO), which its amount may vary in seawater with depth and other parameters, can result in an additional cathodic reaction at the component surface that has the potential to influence the extent of HE.
Some standards recommend removal of DO from the test cell by deaeration to eliminate the occurrence of the oxygen reduction cathodic reaction and to allow only water reduction or hydrogen evolution cathodic reaction to occur causing more hydrogen generation. Theoretically, this can lead to a higher degree of hydrogen embrittlement and more conservative test results. Experiments have been conducted in deaerated and non-deaerated conditions, however, there has not been an inclusive study focusing on the presence versus absence of DO. In this study, fatigue crack growth rate (FCGR) tests were performed using two heats of a LAS, ASTM A182 (F22), to determine the effect of DO presence in a defined saltwater with CP solution. Increasing ΔK FCGR tests were performed at a constant R-ratio in deaerated and non-deaerated saltwater with applied CP using compact tension (CT) test specimens. FCGRs obtained at various ΔK values showed two distinct behaviors at lower ΔK and at higher ΔK ranges. Scanning electron microscopy (SEM) was used to study crack growth mechanism at varying crack depths and a change in morphology was observed as the cracks grew from the pre-crack to the final crack length.

Low alloy steels (LAS) frequently used for pressure containing and primary load bearing equipment in subsea applications are susceptible to various types of corrosion in seawater. Cathodic protection (CP) is usually applied to protect LAS materials from corrosion. However, applying CP can lead to Hydrogen Embrittlement (HE) and failure of the protected components, especially higher strength alloy steels, if CP design parameters are not carefully considered. HE occurs due to absorption and diffusion of atomic hydrogen into the steel. The source of hydrogen is a cathodic reaction of the reduction of water, which occurs at the surface of the cathode. The presence of Dissolved Oxygen (DO), which its amount may vary in seawater with depth and other parameters, can result in an additional cathodic reaction at the component surface that has the potential to influence the extent of HE.
Some standards recommend removal of DO from the test cell by deaeration to eliminate the occurrence of the oxygen reduction cathodic reaction and to allow only water reduction or hydrogen evolution cathodic reaction to occur causing more hydrogen generation. Theoretically, this can lead to a higher degree of hydrogen embrittlement and more conservative test results. Experiments have been conducted in deaerated and non-deaerated conditions, however, there has not been an inclusive study focusing on the presence versus absence of DO. In this study, fatigue crack growth rate (FCGR) tests were performed using two heats of a LAS, ASTM A182 (F22), to determine the effect of DO presence in a defined saltwater with CP solution. Increasing ΔK FCGR tests were performed at a constant R-ratio in deaerated and non-deaerated saltwater with applied CP using compact tension (CT) test specimens. FCGRs obtained at various ΔK values showed two distinct behaviors at lower ΔK and at higher ΔK ranges. Scanning electron microscopy (SEM) was used to study crack growth mechanism at varying crack depths and a change in morphology was observed as the cracks grew from the pre-crack to the final crack length.

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