[ oxygen ] to those of pressure and concentration. As the system temperature rises, materials burn far more readily and rapidly in a gaseous oxygen environment.
Given that hydrogen electrolysers commonly operate at temperatures above 80 ° C, this presents an additional risk that must be considered during system design. Table 4 shows how an increase in temperature affects the rate at which a material burns.
Unlike concentration and pressure, the temperature of an oxygen-enriched environment does alter the Maxwell-Boltzmann distribution curve. This is because an increase in gas temperature increases the velocity of particles, meaning that more particles exceed the activation energy required for a successful oxidation collision / reaction.
Material thickness
Extensive testing and real-world incidents have shown that thinner materials ignite and burn more easily in oxygen-enriched environments, whereas thicker ones may selfextinguish. 6, 7, 8, 9 The thickness at which a range of conical alloy test samples self-extinguish was
Fig. 3. As temperature increases, more particles exceed the activation energy threshold( temperatures in ° C). 5
demonstrated by WHA International and is shown in Table 5.
Given the shape of the test samples( conical, with combustion started at the apex), it can be shown that as the thickness of the alloys reaches a certain point, it can no longer sustain combustion, leading to the conclusion that material thickness is an important parameter when designing equipment for oxygen-enriched environments.
The issue of material thickness in relation to an electrolyser is particularly significant when considering coalescing filter media inside gas-
Table 3. Increase in burn rate at elevated pressures ²
Flammability of Metals by Pressure in an Oxygen Environment a, b, c
Material Test Pressure Rod Burn Length
Burn Rate( in / s) [ mm / s ]
Bronze / Aluminium Mixture |
50 psi( 345 kPa) 100 psi( 689 kPa) |
|
|
0”( 0 mm) 12”( 305 mm) |
0.0( 0.0) 0.3( 7.6) |
Aluminium 4043 |
25 psi( 172 kPa) 50 psi( 345 kPa) |
4.4”( 111.8 mm) 12”( 305 mm) |
0.6( 15.2) 1.1( 27.9) |
316 Stainless Steel |
500 psi( 3447 kPa) 1000 psi( 6895 kPa) |
1.7”( 43.2 mm) 12”( 305 mm) |
0.3( 7.6) 0.4( 10.2) |
304 Stainless Steel |
500 psi( 3447 kPa) 1000 psi( 6895 kPa) |
3.8”( 96.5 mm) 12”( 305 mm) |
0.3( 7.6) 0.4( 10.2) |
316L Stainless Steel |
250 psi( 1724 kPa) 1000 psi( 6895 kPa) |
2.6”( 66.0 mm) 12”( 305 mm) |
0.2( 5.1) 0.4( 10.2) |
a
Data for pressure effect comparison only; not to be considered standard values for the listed materials. b
Data based on rods tested per ASTM G 124 and ISO14624-4, with rod dimensions of 12” in length and 0.125” in diameter. c
Data derived from testing at NASA’ s George C. Marshall Space Flight Centre.
Hydrogen Tech World | Issue 21 | April 2025 45