Local Hydrogen Measurements in Multi-Phase Steel C60E by Means of Electrochemical Microcapillary Cell Technique

Abstract

By utilizing hydrogen as an eco-friendly energy source, many metals are exposed to gaseous (pressurized) hydrogen. High-strength steels with an ultimate tensile strength of 800 MPa and above are especially susceptible to hydrogen-induced fracturing, also referred to as hydrogen embrittlement (HE). Both the microstructure and phase fractions within the steel, as well as lattice distortion, carbide precipitation, residual stress, etc., significantly affect the susceptibility to HE. Among others, one important cause for this observation is found in the locally varying hydrogen solubility within different microstructural phases such as martensite, bainite, pearlite, and ferrite. Both a thorough understanding of the HE mechanisms and taking countermeasures in the form of alloying design require an accurate analysis of local diffusive hydrogen concentrations within the material. Thermal analysis methods such as Thermal Desorption Mass Spectrometry only display an integral hydrogen concentration throughout the whole sample volume. To analyze the local diffusive hydrogen concentration, novel measuring techniques with a high special resolution must therefore be utilized. The current research presents first-of-its-kind hydrogen analyses by means of the electrochemical microcapillary cell. Using a 10 µm tip opening diameter allows for conducting local diffusive hydrogen measurements within individual grains of multi-phase carbon steel C60E (1.1221). The results confirm that hydrogen is distributed heterogeneously within multi-phase steels. Considering the individual phase fractions and the respective local diffusive hydrogen concentrations, a total diffusive hydrogen concentration can be calculated. The obtained value is in good agreement with reference thermal hydrogen analyses. Our results suggest that electrochemical microcapillary cell measurements offer great potential for further studies, which will provide a better understanding of HE and local hydrogen accumulation.

Effect of Hydrogen Charging on the Mechanical Properties of High-Strength Copper-Base Alloys, Austenitic Stainless Steel AISI 321, Inconel 625 and Ferritic Steel 1.4511

“…Hydrogen as an eco-friendly energy source is considered a cornerstone for meeting the climate targets stated within the "Paris Agreement" by reducing carbon dioxide emissions when used as a substitute for fossil fuels [1][2][3][4][5]. However, large-scale hydrogen technologies are still in the development phase since several different challenges must be addressed, such as the production and generation of sufficient amounts of pure hydrogen, hydrogen storage and transportation as well as the actual combustion processes.…”

“…HE is generally caused by diffusive hydrogen [3,[6][7][8][9]11,22]. By providing external activation energy in the form of elevated temperature or mechanic stresses, trapped hydrogen may become diffusive and therefore contribute to HE [9,[23][24][25][26].…”

“…The process of HE is explained via two main mechanisms referred to as Hydrogen-Enhanced Decohesion/HEDE and Hydrogen-Enhanced Localized Plasticity/HELP [3,7,22,[31][32][33][34][35]. Within the literature, there are additional theories of HE [36][37][38] which are heavily debated [39] and probably not sufficient to solely explain all hydrogen-related effects in metals.…”

“…This is explained by very fast hydrogen diffusion velocities within BCC steels between 1.0 × 10 −7 for ferritic microstructures [43] and 2.0 × 10 −9 cm 2 /s for martensitic microstructures [44,45] as well as low hydrogen solubility. High-strength steels endure higher (elastic) operating stresses, which act as driving forces for hydrogen diffusion [3]. Therefore, hydrogen predominantly diffuses into locally strained lattice areas such as crack tips, radii, thread bases, etc.…”