PSI - Issue 77

A. Hell et al. / Procedia Structural Integrity 77 (2026) 41–48 Author name / Structural Integrity Procedia 00 (2026) 000–000

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due to hydrogen embrittlement (HE), as the accumulation of hydrogen in metals can lead to catastrophic failure (Barrera et. al. (2018)). This is of particular concern in safety critical hydrogen storage and distribution applications like hydrogen pressure vessels and pipes, pointing out the need for extensive material qualification. Ductile low-carbon steels with a ferritic microstructure, e.g. used in pipelines and pressure vessels are considered comparably less susceptible to hydrogen, but may still suffer from hydrogen induced cracking in H 2 S-containing environments (sour service conditions) or mechanical degradation regarding fracture toughness and fatigue crack growth rate (Ghosh et. al. (2018), Hoschke et. al. (2023)). For instance, Nagumo et. al. (2001) observed a decrease in ductile crack growth resistance in banded ferritic-pearlitic steel, after electrochemical hydrogen charging with a sodium chloride solution and ammonium thiocyanate. The dimpled fracture in the H-charged state exhibited larger and shallower dimples and quasi-cleavage features. In HE testing, hydrogen charging of specimens is performed using gaseous or cathodic/electrochemical hydrogen charging, either before subsequent mechanical testing (precharging) or in situ (Zafra et. al. (2023)). Cathodic hydrogen charging is a simple, but effective method that can be implemented in most laboratories and is still widespread in research, with Bolzoni et. al. (2024) successfully applying it for charging of compact tension specimens of carbon and low alloy steel. However, progressive desorption of hydrogen from the sample volume during testing is challenging and has to be considered when interpreting results. The scope of this study is the investigation of hydrogen impacts on crack growth resistance of P355NH, by developing a suitable cathodic precharging approach and make initial statements about the steel’s hydrogen susceptibility and related damage mechanisms. Hydrogen uptake behavior of P355NH was determined and used as an input for simple H-diffusion calculations. Results from fatigue precracking and quasi-static fracture mechanical experiments are related to these findings, supported by microscopic investigations. 2. Materials and Methods 2.1. Steel grade and sample preparation The steel grade examined in this study was P355NH, a low alloy steel with ≤ 0.18 % carbon content and a normalized, fine-grain ferritic-pearlitic microstructure. Compact tension specimens (CT50) with W = 50 mm and B = 0.5* W were machined from a rolled slab in L-T direction. To increase stress triaxiality and ensure straight crack fronts, the samples were side grooved to a net thickness of B N = 20 mm. Additionally, in order to characterize the charging setup as well as hydrogen uptake properties of the material, flat sheets with the dimensions 60 mm x 30 mm x 1.5 mm were extracted from the steel plate and used for hydrogen concentration measurement. The specimen plane lies in the rolling plane, with the 60 mm edge oriented in rolling direction. To prepare a clean surface for charging, all CT50 samples were cleaned with ethanol and blasted with glass beads (70-110 µm size) with a pressure of six bar. To minimize potential measurement errors from remaining surface contamination, the sheet samples for hydrogen concentration measurement were wet grinded with SiC paper up to grit size P4000 i.e. with a grain size of 5 µm. The remaining sample thickness after grinding was determined to be 1.4 mm. 2.2. Hydrogen charging and measurement Cathodic hydrogen charging of compact tension specimens and sheet samples was performed in 0.25 mol/l sulphuric acid in distilled water with a set current density of 1.2 mA/cm 2 . The electrolysis voltage was limited to a maximum of 2.4 V to prevent decomposition of the electrolyte. Potassium iodide (KI) was used to promote hydrogen entry into the samples (Müller et. al. (2019)), acting as a recombination inhibitor due to the adsorption of iodide ions on the specimen surface, i.e. the cathode. The initial concentration of KI in the electrolyte was 0.005 mol/l. Graphite plates acted as anodes in the charging process. CT50 specimens were charged for 96 hours at room temperature. The sheet samples for hydrogen uptake measurements were charged for various durations until a stable plateau in measured hydrogen concentration occurred. If necessary, KI was refilled to prevent a diminishing promotor effect. All samples were stored in liquid nitrogen after charging to counteract hydrogen desorption until subsequent measurements, with the maximum storage time never exceeding 14 days. Hydrogen analysis was conducted in a Galileo G8 system from Bruker using the inert gas fusion method (IGF). For each measurement, one gram of material was cut from the sheet