Hydrogen for use as fuel gets more and more attention and stainless steels can be used for tubing as and fittings or for the boss in the gas tank. The susceptibility to hydrogen embrittlement of three austenitic stainless steels was tested by different methods for use in high pressure hydrogen. Two versions of UNS S31603 stainless steel with high and low nickel content as well as UNS S31675 and UNS S21900 both with a higher mechanical strength were investigated. The tests included a variety of conditions, e.g., with and without hydrogen environment, in-situ charging and pre-charging, cathodic hydrogen charging and high-pressure hydrogen gaseous charging. The high-pressure hydrogen gas SSRT was conducted at –40°C and 85 to 87.5 MPa. The results showed similar ranking for the three materials to hydrogen embrittlement. The UNS S31603 stainless steel with 11.5% Ni is prone to hydrogen embrittlement while UNS S31603 with 13% Ni, UNS S21900 and UNS S31675 showed good resistance. The 316L stainless steel with 11.5% Ni is more susceptible to hydrogen embrittlement at lower temperatures.
Today hydrogen for use as fuel for vehicles is getting more and more attention as an alternative to vehicles using fossil-based fuels. Hydrogen is used in both electric fuel cell cars and in heavy vehicles using direct combustion of hydrogen. Such vehicles require stainless tubing for transport of the hydrogen and for bosses to composite tanks in addition to tubing and fittings in hydrogen fueling stations. These systems operate at high pressures, typically 35 or 70 MPa. To avoid overheating of the gas during filing the vehicle, cooling to temperatures as low as –40°C may be performed. SAE(1) J2579 prescribes using Slow Strain Rate Testing, SSRT, to qualify material. Testing should be performed at –40 to –50°C at 125% of the design pressure which results in 87.5 MPa for a 70 MPa system. Requirements for passing SSRT in SAE J2579 is an elongation above 12%, yield and tensile strength above specified minimum values and work hardening ratio above 1.071. SSRT is a common method for testing the resistance against hydrogen embrittlement. However, the ratio of elongation or reduction of area in hydrogen gas compared to air is often used when evaluating the resistance to hydrogen embrittlement.
