The SMS research group has gained a lot of expertise and international recognition in hydrogen/material interaction related research. Specialized equipment is at our disposal for which reliable and consistent methodologies have been developed.
The available hydrogen characterization techniques include:
Electrochemical and gaseous hydrogen charging
The SMS group has long-lasting experience in electrochemical hydrogen charging (potentiostatic and galvanostatic) required for further hydrogen characterization. The main expertise includes various steel types and titanium alloys. Electrochemical hydrogen charging can be performed from room temperature up to 80°C at ambient pressure. Recently installed infrastructure enables gaseous hydrogen introduction under elevated pressure (200 bar) and temperature (300°C) as well.
Hot/melt extraction
The hydrogen uptake capacity of metals can be measured by heating specimens to a fixed temperature. Melt extraction heats the specimen to 1600°C (above the melting temperature of steel) and gives the total hydrogen content in the material. Hot extraction uses lower temperatures (300°C-950°C) and is typically used to measure the diffusible hydrogen content. Both techniques can be used to evaluate the time needed to obtain saturation of the material with hydrogen.
Electrochemical hydrogen permeation
- Basic:
The apparent hydrogen diffusivity can be evaluated via the electrochemical hydrogen permeation technique. The principle is schematically illustrated in the figure below. The setup consists of two electrochemical cells separated by the metal specimen (WE). At the left side (cathodic cell), hydrogen is produced through electrochemical reactions. Hydrogen is subsequently absorbed and diffuses through the specimen towards the other side. At the right side (anodic cell), hydrogen is detected. This leads to a hydrogen permeation transient that can be analysed to quantify the apparent hydrogen diffusivity.
- Under load:
The SMS research group developed a unique extension to the permeation setup enabling the evaluation of the hydrogen diffusivity of materials under load since hydrogen diffusivity can alter when a material experiences mechanical stress. As schematically illustrated below, the setup is similar to the basic electrochemical hydrogen permeation technique, however, the specimen is clamped in a load ring which is able to put the specimen under a constant load.
Thermal Desorption Spectroscopy (TDS)
Thermal desorption spectroscopy (TDS) is a technique used to characterize hydrogen trapping in metals. During TDS, a specimen is gradually heated in an infrared furnace with a fixed heating rate from room temperature up to 950°C. Hydrogen effusion from the specimen is simultaneously measured by a mass spectrometer. The hydrogen flux as a function of temperature can be analysed to obtain activation energies of hydrogen trapping sites. For materials characterized by a low hydrogen diffusivity (<10-14 m²/s), trapping analysis is not possible, however, TDS can be coupled with numerical analysis as an alternative to the electrochemical hydrogen permeation technique to determine the apparent hydrogen diffusivity.
In-situ and ex-situ mechanical testing
Evaluation of the influence of hydrogen on the mechanical performance of materials can be performed in different in-house developed setups. Specimens are either precharged with hydrogen and subsequently mechanically loaded (ex-situ approach) or electrochemically hydrogen charged during the mechanical test (in-situ approach) with the possibility of precharging as well.
- Tensile test:
Constant extension rate tensile tests are used for ductile materials to evaluate the hydrogen embrittlement sensitivity at room temperature. Both flat specimens and round bars can be tested. Moreover, notched specimens (in both geometries) can be tested as well. Different deformation rates can be applied from 0.0001 mm/min up to 500 mm/min with a maximum load capacity of 50 kN. The results of tests performed on DP steel in various charging and loading conditions can be found below.
- Bending test:
Three-point bending tests are used for intrinsically brittle materials, such as martensitic steels, to evaluate the hydrogen embrittlement sensitivity at room temperature. The tests can be performed at different deformation rates as well. The developed methodology is limited to flat specimens without notch.
- Post mortem analysis of mechanical specimens:
Analysis of fracture surfaces and side surfaces to reveal the deformation and failure mechanisms in the presence of hydrogen belongs to the main expertise of the SMS group. Some examples are given below. The first figure represents SE images of fracture surfaces of a martensitic steel tested through bending with hydrogen revealing different fracture types. The second figure illustrates an EBSD analysis of the deformation mechanisms in austenitic steel in the presence of hydrogen. Research into the initiation and propagation of hydrogen-induced and hydrogen-assisted cracks to determine microstructurally vulnerable features can be performed as well. The reader is referred to material characterization and failure analysis for more information on the available characterization techniques.
