Basics of Hydrogen Permeation
Terminology
Here you will find an overview of typically used terms.
The terms used are based on the standard DIN EN ISO 17081:2014 - 10
(Electrochemical methods for the measurement of hydrogen permeation and for the determination of hydrogen uptake and transport in metals).
Term
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Alternative Terms
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Description
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|---|---|---|
Oxidation cell | Oxidation side, fuel cell side | The permeation current is measured in this part of the cell. |
Reduction cell | Reduction side, electrolysis side, pickling side | The hydrogen is produced in this cell section. |
Test piece | Sample, membrane, metal membrane | The workpiece to be analysed represents the working electrode in this measurement. |
Counter electrode | Auxiliary electrode | Depending on the test medium, the nickel or mixed metal oxide electrode on the reduction side is essential for electrochemical hydrogen generation. |
Oxygen electrode | Oxygen consumption electrode, oxygen gas diffusion electrode O2-GDE | This electrode on the oxidation side is used to measure the oxidation current on the oxidation side. |
Reference electrode | Indicator electrode | The Mini-HydroFlex hydrogen reference electrodes are optionally used as indicator electrodes to check the potentials of the test piece. |
Etch | Test medium, pickling | The etch or test medium is the electrolyte on the reduction side, in which the hydrogen is developed chemically or electrochemically. |
Inhibitor | Restraint | Inhibitors or restraints are special additives that minimise or even prevent the development of hydrogen. |
Passivation current | Background current, base current | This current occurs on the oxidation side due to the passivation of the test piece. |
Permeation current | Diffusion current | This current is caused by the oxidation of the hydrogen atoms on the oxidation side of the test piece. |
Transient decay current | Decay current, transient current | The decaying current after hydrogen evolution has ceased is known as the transient decay current. |
Oxidation current | Total current | The oxidation current is the total current on the oxidation side, i.e. the sum of the passivation current and permeation current or passivation current and decay current. |
Diffusion of Hydrogen
In pure metals with few defects at interstitial sites, the transport of atomic hydrogen is determined solely by lattice diffusion. However, in alloys and metals with dislocations, grain boundaries or inclusions, these can become traps for the hydrogen atoms. A distinction can be made between reversible and irreversible traps.
Reversible traps are places in the metal where hydrogen atoms remain longer than at a lattice site. Compared to the time required to reach a stationary permeation state, however, this dwell time is short.
In irreversible traps, on other hand, the hydrogen atoms remain at the respective temperature for a very long time or even continuously compared to the duration of the permeation measurement. The hydrogen atoms are therefore only mobile at the interstitials (lattice sites) and at reversible defects. The diffusion rate of the hydrogen atoms through the metal during a permeation test can therefore be influenced by both irreversible and also reversible traps.
In the steady state, all irreversible traps are filled. If the moving hydrogen atoms are removed during a first permeation measurement and the sample is then immediately subjected to a second measurement, the difference between the first and the second permeation transient can be used to determine the influence of the irreversible traps on the transport.
With regard to the transport through the sample interior, the surface-related processes become less important for the transport rate with increasing sample thickness. Measurements on samples of the same material but different sample thicknesses can thus provide indications of the extent to which hydrogen transport is controlled by the sample interior or by surface processes such as adsorption kinetics or transport through an oxide layer. If the thickness of the samples varies, double the thickness should be used.
Palladium coatings on the oxidation side or on both sides of the sample are allowed. However, this can influence the hydrogen concentration near the surface in the base material and thus also the permeation current. It must be verified whether the palladium coating has an influence on the calculated diffusion capacity of the sample. On the oxidation side, the palladium can increase the oxidation rate.
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How to Measure Hydrogen Permeation
Our concept is characterised by less technical equipment. The measuring cell consists of a reduction cell (charging cell) and an oxidation cell (fuel cell). The test piece is located between these two subcells. To detect the permeation current, an oxygen consumption electrode (oxygen gas diffusion electrode O2-GDE) is used on the oxidation side of the test piece.
Evaluation of the Measurement
The passivation current at the beginning of the measurement decreases to a small constant offset. For an exact calculation of the permeation current, you have to correct the measured current by this offset.