The global community and governments continue to fund development of alternative fuels to reduce dependency on oil and improve the environment. This has resulted in a continuous improvement and increased activity in the usage of hydrogen. Hydrogen has a large potential to be the fuel carrier of the future since it is environmentally friendly. However, the generation and storage of hydrogen poses problems that has to be addressed by ensuring correct material compatibility for safety and performance.
Hydrogen is the simplest and smallest atom element and is found in water, acids, bases and organic compounds. Hydrogen atoms are not considered corrosive, however, if the atoms split into hydrogen ions (two H+ atoms), it can penetrate through thin metal diaphragms. It is important to ensure material compatibility when selecting components for hydrogen application since it will dictate the two major forms of corrosion: hydrogen permeation and embrittlement.
Pressure sensors employing thin isolation diaphragms with fluid fill are prone to hydrogen permeation. Hydrogen ions will penetrate the diaphragm and will be trapped in the fill fluid, producing hydrogen bubbles. These bubbles will cause the zero and span readings to change, degrading the performance of the pressure sensor. Over time the build up of hydrogen will cause outward expansion of the isolation diaphragm, leading to cracks and sensor failure through loss of fill fluid. Hydrogen permeability occurs in both pure and non-pure hydrogen applications involving high pressure-temperature in pure hydrogen applications, galvanic reaction such as seawater and steam at high temperatures. To reduce or eliminate hydrogen permeation in thin diaphragms, sensor manufacturers employ expensive plating process involving gold or platinum.
Another form of corrosion involving hydrogen is hydrogen embrittlement that results in reduction of mechanical properties of the metal, leading to failure of the pressure sensor. Material selection is very important and emphasis on material compatibility and hazardous analysis must be considered prior to hydrogen service. Factors for hydrogen embrittlement are environmental, internal absorption and chemical reaction in the presence of hydrogen ions. Susceptibility of embrittlement increases with increase in tensile stress and ultimate strength of the metal as a function of hydrogen purity levels. To reduce or eliminate embrittlement, material selection together with thickness, surface finish, weld free joints and conservative design stresses must be considered. Avoiding metals such as 410 stainless steel, 1040 steel, 17-4 / 17-7 stainless steels, and Inconel 718 since these are extremely embrittled in hydrogen. Metals such as 310 and 316 stainless steel offer negligible embrittlement.
To overcome hydrogen permeability and embrittlement, AST offers a complete line of pressure sensors for hydrogen service that employ one-piece thick diaphragm 316L diaphragms, free on internal O-rings, welds or fill fluids. Using Krystal Bond Technology, AST pressure sensors are constructed with the most advanced MEMS process in the pressure sensor industry.
AST2000H2 Hydrogen Pressure Sensor - Click the image for more details
“This approval allows AST to serve the growing market of hydrogen powered vehicles. We are the only EC79 approved sensor supplier in the world.” - Karmjit Sidhu, Vice President Business Development and AST Co-Founder
The 79/2009/EC approval was obtained for the 2MPa range of the Model AST2000 and Model AST4000 pressure sensors that are used as components on the vehicle hydrogen regulator controlling hydrogen flow to the fuel cell stack. Also approved are the 35MPa range of pressure sensors that are typically used on Class 3 hydrogen tanks for use on buses, forklifts, cars, farm machinery, off-road vehicles and stationary back-up power systems.
To obtain approvals, AST pressure sensors were tested according to Regulation EU 406/2010 to verify compliance with Regulation EC 79/2009 of the European Parliament and the Council of the European Union (see http://eurlex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2009:035:0032:0046:en:PDF). The requirements and test procedures were established to meet integrated system safety requirements. With greater world emphasis on green technology and sustainability, hydrogen is highly considered an alternate clean fuel carrier for powering surface transportation.