The sensor chip of piezoresistive pressure transducers is usually surrounded by a steel membrane. For the housings of these measuring instruments, stainless steel is also used in most applications. But should contact with hydrogen occur, this material can become brittle and then crack.
Hydrogen embrittlement affects not only steel, but also other metals. This is why the use of titanium offers no alternative in regard to hydrogen applications.
What is meant by embrittlement?
Hydrogen embrittlement refers to a loss of ductility in the material. Ductility describes the property of materials to plastically deform under stress before they finally fail. Depending on its type, steel can deform by more than 25 percent. Materials that do not have this ability are termed brittle.
But ductile materials can also become brittle, or frail. When this embrittlement of the material is the result of hydrogen absorption, this is then termed hydrogen embrittlement.
Hydrogen embrittlement occurs when atomar hydrogen diffuses into the material. The prerequisite for hydrogen embrittlement itself is usually hydrogen corrosion.
Hydrogen corrosion, also known as acid corrosion, always takes place whenever oxygen deficiency exists and metal comes into contact with water. The end product remaining from this redox reaction is pure hydrogen, which then oxidizes the metal. The metal goes into solution as ions and causes the material to be evenly degraded.
The hydrogen released by this redox reaction diffuses into the steel due to its small atomic size of only about 0.1 nanometer. The hydrogen directly occupies the metal lattice of the material as atomic interstitials. Lattice imperfections arising here then increase the absorption capacity. This leads to a chemical fatigue in the material, which can ultimately cause cracks from the inside to the outside, even at low loads.
Hydrogen and pressure transmitters
Because of its very tiny dimension, hydrogen can not only penetrate the material, but can actually penetrate it completely. For this reason, not only an embrittlement of the material can occur. The metal membranes of piezoresistive pressure sensors are very thin – the thinner they are, the more sensitive and accurate the sensor becomes. If hydrogen diffuses into and through the membrane (permeation), it can then react with the transfer fluid surrounding the sensor chip. As a result, changes in the metrological properties of the measuring bridge occur due to hydrogen adsorption. At the same time, an increase in pressure can also occur as a result of these deposits, with outcomes ranging from a buckling of the sensor membrane through to its complete destruction.
Besides using a thicker but somewhat more inaccurate membrane, this process can be greatly retarded by using a gold alloy and the unit lifespan thus optimized. You can read more about this here.