Hydrogen susceptibility tests
Electrochemical permeation
The electrochemical permeation technique is used to determine the hydrogen diffusion coefficient. For this, a Devanathan-Stachurski set-up is used. This set-up consists of two independent electrochemical cells, having a thin metal membrane placed between them. Hydrogen is generated and introduced into the metal by diffusion on one side of the set-up via galvanostatic or potentiostatic electrochemical charging. The hydrogen then diffuses through the sample and is measured on the other side of the membrane. Using Fick’s laws, or other diffusion theories, the corresponding diffusion coefficient of hydrogen through the metal is calculated.
Thermal-desorption spectroscopy
To perform a thermal-desorption spectroscopy (TDS) analysis, the samples are heated at different heating rates (e.g., 600 °C/h, 900 °C/h and 1200 °C/h) inside an oven chamber under a constant nitrogen carrier flow up to 900 °C. The amount of H released at specific temperatures is measured by a mass spectrometer. By performing TDS analysis at different heating rates, it is possible to apply the Kissinger method to calculate the activation energy (Ea) for each peak and link them to specific microstructure features that could be acting as hydrogen traps. In this way, accurate TDS-fitting can result in a detailed hydrogen trap analysis of a material.
Melt/Hot extraction
Melt and Hot extraction are similar techniques to measure the hydrogen content (in weight parts per million - wppm) of a material – usually metals. Melt extraction is performed at 1600 ºC and it measures the total amount of hydrogen present in the sample. Hot extraction is performed usually at 350 ºC and it measures the amount of diffusible hydrogen present in the sample. Both techniques can be used to determine the hydrogen saturation curve by measuring the hydrogen uptake after different charging time intervals.
Electrochemical hydrogen charging
Electrochemical hydrogen charging is used to introduce atomic H into the metallic samples. The charging experiment is performed in a H charging cell in which the sample acts as a cathode and is symmetrically positioned between two platina anodes. The charging conditions are a combination of electrolyte (e.g., 0.1 M NaOH, 0.5 M H2SO4) with or without the addition of recombination poison and current density for galvanostatic charging or applied potential for potentiostatic charging.
Gaseous hydrogen charging
Gaseous hydrogen charging is another technique used to introduce atomic H into the samples. The charging experiment is performed in a high-pressure autoclave at elevated temperatures in which the sample is placed directly inside a sealed chamber exposed to a hydrogen-rich atmosphere. The charging is performed under specific conditions pre-determined, i.e., temperature, pressure and time span. In contrast to electrochemical charging, gaseous hydrogen charging typically induces a uniform hydrogen concentration profile and present milder hydrogen charging conditions with lower sub-surface concentrations