Study warns cathodic hydrogen charging can crack stainless steel in acid and leave impurity deposits in other electrolytes.
Study: Insights into cathodic hydrogen charging - surface morphology evolution. Image Credit: Thaweesak Thipphamon/Shutterstock.com
This new research systematically investigates the changes to surface morphology during cathodic hydrogen charging. While hydrogen embrittlement resulting from this charging process has been extensively evaluated, research at the surface level has been less widespread.
The researchers evaluated how electrolyte composition, applied current density, and initial surface roughness influence hydrogen uptake in 316L austenitic stainless steel.
The findings show surface roughness plays a central role in regulating hydrogen recombination and absorption kinetics (with the study’s data pointing primarily to recombination/bubble evolution effects). Their results emphasize the importance of carefully controlling charging parameters and monitoring surface condition/chemistry in hydrogen embrittlement research.
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Hydrogen embrittlement in structural alloys arises from hydrogen ingress processes that begin at the metal surface.
During cathodic hydrogen charging, the hydrogen evolution reaction generates adsorbed hydrogen atoms that either recombine to form molecular hydrogen or diffuse into the lattice. Although this electrochemical method provides a safe and practical alternative to high-pressure gaseous charging, it also alters the material's surface condition.
These surface changes can influence hydrogen adsorption, absorption kinetics, and embrittlement behavior.
Previous studies have primarily examined mechanical degradation after hydrogen uptake. In contrast, researchers have not fully established the early-stage surface interactions that occur during charging. Electrolyte chemistry, applied current density, and initial surface roughness govern these interactions.
Together, these parameters regulate hydrogen recombination kinetics, local overpotential, and surface stability, thereby controlling hydrogen uptake, surface damage evolution, and unintended surface deposits.
By comparing acidic, neutral, and alkaline electrolytes under controlled current densities and surface conditions, the researchers established direct correlations between charging parameters, surface morphology evolution, and hydrogen uptake in 316L austenitic stainless steel.
Investigating Cathodic Hydrogen Charging
The researchers solution-annealed commercial 316L stainless steel at 1050 °C for one hour to obtain uniform grains of approximately 40 μm with random crystallographic orientation. After forming disc-shaped specimens, they systematically introduced surface roughness using sequential grinding papers and diamond suspensions, achieving a 1 μm finish.
This controlled preparation enabled direct evaluation of the influence of surface roughness on hydrogen uptake, while recognizing that roughness can also co-vary with near-surface deformation such as dislocation density and residual stress.
To introduce hydrogen, the team used a two-electrode electrochemical cell under galvanostatic control. They selected three electrolytes, 0.2 M H2SO4 (acidic), 3 wt% NaCl (neutral), and 0.2 M NaOH (alkaline), to assess the influence of pH and ionic environment.
Adding Na2HAsO4·7H2O as a recombination poison suppressed molecular hydrogen formation and enhanced atomic hydrogen absorption.
The samples were hydrogen charged over 10 hours at 80 °C under current densities of 0.3, 3, and 30 mA/cm2 to evaluate current-dependent effects. Separately, in situ optical microscopy was used to observe hydrogen bubble evolution under a dedicated condition (0.2 M H2SO4 at 25 °C and 0.5 mA/cm2 for ~10 minutes).
The researchers quantified the total hydrogen content using inert gas fusion analysis. They characterized surface morphology through optical microscopy and scanning electron microscopy. In situ optical imaging (under the separate condition noted above) monitored hydrogen bubble evolution during charging, while X-ray photoelectron spectroscopy identified oxide species and impurity-related surface deposits.
These techniques enabled direct correlation between charging parameters, hydrogen uptake, and surface morphology evolution, including deposit formation in neutral/alkaline media.
pH and Surface Morphology Play a Decisive Role
Hydrogen charging in 0.2 M H2SO4 led to substantial surface degradation, especially at higher current densities.
At 30 mA/cm2, polished samples developed extensive cracking, and crack density increased with applied current. Hydrogen concentration increased from about 2 ppm at 0.3 mA/cm2 to 17.5 ppm at 30 mA/cm2, confirming a strong relationship between charging intensity and hydrogen uptake.
Microscopic analysis showed crack initiation along slip bands and grain boundaries, consistent with hydrogen-induced localized plastic deformation and internal stress buildup.
The original surface condition played a clear role in hydrogen absorption behavior.
Roughened specimens produced more hydrogen bubbles during charging, indicating enhanced recombination of adsorbed hydrogen atoms and reduced hydrogen entry into the metal lattice. In contrast, smooth, polished surfaces produced fewer bubbles and exhibited significantly higher hydrogen concentrations.
These results indicate that surface roughness critically regulates hydrogen recombination kinetics and subsequent damage evolution. The authors interpreted the higher bubble density on rough surfaces as a direct indicator of greater recombination and reduced absorption.
Hydrogen uptake depended strongly on electrolyte composition. Among the tested environments, H2SO4 yielded the highest hydrogen content, NaOH produced intermediate levels, and NaCl resulted in the lowest uptake. This behavior is likely caused by variations in hydrogen fugacity, which increases with greater overpotential and decreasing pH.
As a result, acidic conditions create elevated effective hydrogen pressure at the metal surface, enhancing hydrogen absorption and intensifying surface damage. The study does note, however, that uptake is not solely determined by pH: differences between NaCl and NaOH are discussed in terms of hydrogen source availability (H from water in NaCl versus contributions from water and OH- in NaOH) and the role of surface films/deposits.
A key nuance is that “low-cracking” conditions can still create chemically significant artifacts.
In neutral 3 wt% NaCl, the authors reported surface deposits attributed to Mg(OH)2 from trace impurities and noted surface changes such as inclusion detachment/pitting under some conditions. In alkaline 0.2 M NaOH, they identify deposits consistent with lead-rich hydrated lead carbonate, most likely hydrocerussite (2PbCO3·Pb(OH)2), again linked to trace reagent impurities.
These deposits can modify the effective surface state during charging and should be considered when interpreting hydrogen uptake and embrittlement outcomes.
Looking Ahead
Cathodic hydrogen charging, the researchers say, should be treated as more than a way to “add hydrogen”. Their results show it can alter the steel’s surface and chemistry simultaneously, making hydrogen embrittlement tests harder to interpret, even if the aim is as simple as reaching a target hydrogen level.
The study shows the answer is a more controlled approach to charging: choosing electrolyte and current conditions that reduce unintended surface damage and avoid chemical “side effects” such as impurity deposits.
The team says this could help labs develop more consistent, comparable charging protocols and offers lessons on how surface states govern hydrogen entry. This is important for alloys used in hydrogen-energy infrastructure, where reliable embrittlement assessment is key to safe operation.
Journal Reference
Zhang, P. et al (2026). Insights into cathodic hydrogen charging - surface morphology evolution. International Journal of Hydrogen Energy, 220(43), 154155. DOI: 10.1016/j.ijhydene.2023.01.149
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