Ignition of flammable hydrogen/air mixtures by mechanical stimuli. Part 2: Ignition under conditions of rust and surface pyrophoric material contamination

•We investigate ignition of H2/air by Mg Fe–O ignition with sliding friction.•The complexities involved in estimating surface temperatures are discussed.•Ignition probability can be estimated from surface & thermite threshold temperatures.•A mechanism is suggested for ignition process with pyrophoric/thermite material.

Magnesium and its corrosion products represent a major constituent of nuclear wastes which produce hydrogen. A study has been carried out to provide a useful background for determining the conditions necessary for ignition of hydrogen in air atmospheres with magnesium and radiologically hazardous pyrophoric materials present. Over 500 tests have been performed to investigate ignition of these flammable mixtures with frictional sliding stimuli. The uncertainties involved in determining the surface temperatures and ignition probabilities with surface substances present are clearly complex and not fully understood. These are discussed in detail and a mechanism is suggested for the ignition of hydrogen in air atmospheres by pyrophoric (Mg/iron oxides thermite) reaction resulting from surface contamination and sliding mechanical stimuli. It is concluded that it is possible to obtain an estimation of ignition probability if there is some knowledge of the ignition temperature of the pyrophoric mixture and the likely surface temperatures being generated.