Thin-film-on-steel sensors, on the other hand, have a significantly thicker membrane. However, it is usually made of a material unsuitable for hydrogen (17-4PH or 1.4542), i.e. a martensitic high-performance steel with a rather low nickel content. Austenitic steels with a nickel content of more than approx. 13 percent are considered hydrogen-compatible. Therefore, alternative steel alloys must be used. Due to the principle, a high yield point is required so that the membrane and thus the sputtered resistance bridges can stretch so much that a usable signal is generated. Unfortunately, many H2-compatible steels such as AISI316L do not meet this requirement. If steel sensors with AISI316L membranes are used, they are usually not equipped with the long-term stable sputtered resistance bridge, but have a coating that shows a greater change in resistance with the same strain that is often more susceptible to signal drift.
A major challenge is to find suitable steel alloys that are H2-compatible and at the same time suitable for the construction of thin-film sensors. For thin-film-on-steel cells with sputtered resistors, there are some austenitic steel alloys with a high nickel content that also have a sufficient yield point and are therefore fundamentally suitable. But for the sensor manufacturer, the difficulty with these steels is to get them from a manufacturer with the material quality that allows long-term stable, low-drift sensors to be made from it. Critical parameters are usually the homogeneity of structure, alloy and thermal treatment. Tests by Trafag with its own sensors made of various alloys and sensors from competitors have shown that many of the solutions offered today have significantly more long-term drift than conventional sensors for air or oil. Thanks to extensive experience, many years of intensive research and countless tests, Trafag has succeeded in developing a thin-film-on-steel sensor made of hydrogen-compatible steel, the long-term stability of which is significantly better than that of most of its competitors.
Long-term stability performance criterion
The long-term stability of hydrogen pressure sensors is now the main criterion when evaluating pressure transmitters. Because the design and size, electronics and mechanical structure are mostly taken from proven industrial pressure sensors, they almost always meet the requirements of hydrogen applications. The long-term stability of the sensor, i.e. that the measurement accuracy does not change, or changes only slightly over the period of use, is critical in hydrogen applications in particular. Poor long-term stability is primarily reflected in the zero point drift, which means that the signal no longer shows zero when there is no pressure. During testing, the embrittlement, which is very often mentioned in the literature as the biggest problem, did not occur with Trafag sensors. The bursting tests of the standard sensors, i.e. made of non-hydrogen-compatible material, did not show any measurable reduction in the burst pressure, even after prolonged use in a hydrogen environment, but the signals did show masive drift. In the application, three parameters in particular have a major influence on the long-term stability of hydrogen pressure sensors:
- Pressure: The higher the pressure, the stronger and faster the
diffusion effects. Alternating load cycles can also accelerate the effect because the movement of the structure facilitates the mobility of the hydrogen that has penetrated.
- Temperature: The higher the temperature, the faster the harmful effect of hydrogen manifests itself. The embrittlement decreases again from about 60°C, but the diffusion continues to increase.
- Time: The duration of exposure to hydrogen is critical. The signal deviations only become apparent after a certain time and are not linear.