Hydrogen in its liquid form causes materials to become brittle. To avoid this, stainless grade 316L, which is resistant to H embrittlement, is often used for the wetted parts that come into contact with liquid H, such as pressure transducers, heat exchangers, humidifiers, combustors, and fuel cell plates. Heat, high pressure and humidity (in the form of water, the only waste product) also tend to cause corrosion, another reason to use stainless steel in fuel cells
. Other materials, especially graphite, are sometimes used, but stainless is favored because of its low cost, thin gauges and machinability, plus the fact that vehicle manufacturers are already familiar with it.
Stainless steel is used on fuel cell bipolar plates, as they have to be chemically resistant. To increase corrosion resistance, these are coated with materials
such as graphite, noble metals, metal nitrides or metal carbides. A disadvantage is that the oxide layer can become so thick that it impedes contact between the electrode plate and the membrane backing. However, 349TM austenitic and 446 ferritic have proven effective in overcoming this. Sandvik provides precoated strip steel for the production of bipolar plates and interconnects for different types of fuel cells, made of ferritic type 441 (EN. 1.4509). Another application for stainless could be in the bodies of the hydrogen buses themselves. Because stainless steel is stronger, lighter and offers more crash resistance than ordinary steel, the frames of modern buses, however they are fuelled, are often made of such grades as 304 austenitic, ferritic or duplex.