Assessment of hydrogen embrittlement of stainless steel diaphragms applied in hydrogen compression

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School of Engineering | Master's thesis
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2025-11-24

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Engineering Materials and Product Development

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Master's programme in Mechanical Engineering
Konetekniikan maisteriohjelma
Magisterprogrammet i maskinteknik

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en

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161

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Abstract

Hydrogen embrittlement remains a critical challenge in the reliability and safety of components used in high-pressure diaphragm compressors. This focus of this thesis is on metastable austenitic stainless-steel diaphragms (EN 1.4310) in three conditions: as-supplied, as-supplied electrochemically H-charged (EC), and ex-service condition. A comprehensive characterization workflow was devised which included multi-directional tensile testing with DIC to assess anisotropy, microhardness mapping, microstructural analysis via optical microscopy, SEM/EBSD and inclusion analysis via EDS, surface characterization using optical profilometry, and thermal desorption spectroscopy (TDS). Damage in the ex-service diaphragm was evaluated via eddy currents and dye penetrant NDT techniques. FEM simulations of one compression stroke of the diaphragm operation cycle were also performed to assess the stress-strain field and relate it with the location of damage existing in ex-service diaphragm. The simulations showed that regions experiencing high tensile stresses, local sharp curvatures and hydrostatic tension accumulate more H and are more prone to crack initiation. Also, crack propagation initially depends on the microstructure and as it reaches a sufficient length, gets governed by the maximum in-plane principal stresses. Overall, the study results reveal two opposing trends. EC of the as-supplied diaphragm in 1N H2SO4 at -681mV decreases its ductility by 84% and promotes mainly transgranular quasi cleavage fracture, including some intergranular features. In contrast, the ex-service condition experiencing long-term gaseous H exposure combined with cyclic loading at ≈250 ℃ leads to microstructural recovery, shown by a decreased kernel average misorientation, lower martensite content and increase in ductility by 16.6-33.3% depending on the testing direction. Moreover, TDS revealed that the total H concentration accumulated during service corresponds to 6.25-15.85 h of equivalent EC in 3 wt.% NaCl at an applied potential of -1.2V. Depending on the region, the ex-service diaphragm showed a 103-243% increase in total H concentration compared to the as-supplied uncharged material.

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Supervisor

Vilaca, Pedro

Thesis advisor

Fangnon, Eric

Keywords

hydrogen embrittlement, thermal desorption spectroscopy, metastable austenitic stainless-steel, diaphragm compressors, finite element modelling, microstructure recovery

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https://urn.fi/URN:NBN:fi:aalto-202512169327

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[dipl] Insinööritieteiden korkeakoulu / ENG