Reactivity of H2/air mixtures over hot metal surfaces

This paper reports on a systematic study of hydrogen oxidation in the presence of different materials, ranging from quartz to carbon steel. The reactivity tests conducted at low hydrogen concentration revealed significant promotion of the oxidation reaction by carbon steel in a temperature range (300–400 °C) where gas phase reactions occur to a limited extent. This effect is strongly enhanced by surface modifications induced by acid corrosion. Stainless steel surfaces behaved quite differently under similar reaction conditions, showing rather an inhibiting role in H2 oxidation.The physicochemical characterization of carbon steel samples before and after acid treatment revealed that the surface area undergoes a strong increase due to corrosion, and also the structure is modified with surface enrichment of trace metal components, which can exert a catalytic role.The potential risk represented by the oxidation properties of these widely used materials is mitigated by the low value of the heat of combustion of hydrogen per unit volume; with a composition below the lower flammability limit, weak thermal effects were in fact observed by infrared measurements of the surface temperature. However, using a simplified kinetic expression, it can be estimated that when hydrogen-rich mixtures come into contact with an overheated metal surface, the exothermic reaction can generate dangerous hot spots.