고강도 API 라인파이프 강재의 수소 전해 장입 조건이 수소 취성 거동에 미치는 영향

다국어 초록 (Multilingual Abstract)

API linepipe steels have recently attracted significant attention for their application in hydrogen transportation pipelines, driven by the global transition toward carbon neutrality and the urgent demand for hydrogen energy transport infrastructure. In this study, the effects of hydrogen charging methods (Ex-situ and In-situ), electrolyte environments (NaCl, NaOH and H2SO4), and charging current density on the hydrogen embrittlement behavior of API X60 steel were systematically investigated. Comparative SSRT (Slow Strain Rate Test) results revealed that the ex-situ charging condition caused only negligible strength degradation due to the desorption of diffusible hydrogen prior to testing. In contrast, the in-situ charging condition exhibited pronounced embrittlement behavior, attributed to the continuous hydrogen supply to the crack tip during deformation and stress-induced hydrogen diffusion. Accordingly, the in-situ charging method was adopted to evaluate hydrogen embrittlement sensitivity under different electrolyte environments.
Among the tested electrolytes, the alkaline NaOH environment exhibited the lowest relative notch tensile strength (RNTS), indicating the highest embrittlement susceptibility (NaOH < NaCl < H₂SO₄). Despite the high hydrogen evolution rate associated with the low pH in the H₂SO₄ environment, irregular embrittlement behavior was observed due to severe bubble formation, surface shielding effects, and competing corrosion reactions that hindered effective hydrogen ingress Analysis of current density effects showed that, in both NaOH and NaCl environments, embrittlement severity increased with increasing current density up to 1 A m⁻², followed by a saturation-like behavior at higher current densities. This trend was attributed to enhanced hydrogen recombination, surface hydrogen saturation, and excessive bubble formation, which collectively reduced the effective hydrogen charging efficiency at high current densities.
The electrolyte-dependent embrittlement behavior was further supported by electrochemical hydrogen permeation tests. In the NaCl environment, corrosion reactions and the formation of surface oxide layers caused the applied cathodic current to be partially consumed by competing electrochemical reactions, while corrosion products partially blocked hydrogen transport pathways, resulting in reduced hydrogen ingress and diffusion. In contrast, the NaOH environment suppressed corrosion-related surface reactions, maintaining higher hydrogen diffusivity and permeation flux, which ultimately led to the most severe hydrogen embrittlement behavior observed in this study.

목차 (Table of Contents)

• 제 1 장 1
• 1.1 서론 1
• 1.2 연구 목적 8
• 제 2 장 이론적 배경 9
• 2.1 수소취성의 메커니즘 9
• 2.1.1 수소취성 9
• 2.1.2 HEDE (Hydrogen-enhanced decohesion) 11
• 2.1.3 HELP (Hydrogen-enhanced localized plasticity) 14
• 2.1.4 AIDE (Adsorption-induced dislocation emission) 18
• 2.1.5 Hydrogen internal pressure theory 20
• 2.1.6 라인파이프강에서의 수소 취성 21
• 2.2 전해액 환경 별 수소 장입 특성 및 취성 영향 22
• 2.3 수소취성 평가 방법 28
• 2.3.1 내부 수소 함량 및 수소 거동 정량화 28
• 2.3.2 저속변형률 인장시험을 통한 수소취성 거동 평가 36
• 제 3 장 합금 및 실험 방법 37
• 3.1 합금 37
• 3.2 저속변형률 인장시험 (Slow strain rate test, SSRT) 40
• 3.2.1 SSRT 시편가공 및 시험 장비 40
• 3.2.2 수소 장입 방식 비교 42
• 3.2.3 파면 분석 45
• 3.3 수소 투과 실험 (Hydrogen permeation experiment) 46
• 제 4 장 실험결과 및 토의 49
• 4.1 SSRT (Slow strain rate test) 거동 및 파면 분석 49
• 4.1.1 수소 장입 방식에 따른 수소 취성 거동 비교 49
• 4.1.2 전해액에 따른 수소취성 거동 57
• 4.1.3 전류밀도에 따른 수소취성 거동 59
• 4.1.4 전류밀도와 전해액에 따른 In-situ SSRT 파면분석 64
• 4.2 수소 투과 거동 분석 72
• 제 5 장 연구 결론 78
• REFERENCES 81