On the Thermodynamics of Aluminum Cladding Oxidation: Water as the Catalyst for Spontaneous Combustion

Abstract

A recent article (Casey et al. in J Failure Anal Prev 22:1252–1259, 2022) finds a thermodynamic explanation for the catastrophic effect of water as an extinguisher in aluminum-clad tower block fires (Entropy 22: 14, 2020) to be “unsubstantiated hypotheses” and “suppositions unsupported by data”. The article by Casey et al., however, is misleading because it is based upon a false premise that it is the hydrolysis of solid aluminum panels that produce hydrogen (H₂), which was not detected in their experiments. The combustion of aluminum (Al) to alumina (Al₂O₃) reaction is highly exothermic to the extent that it can be explosive, but the reaction is inhibited, for all temperatures of solid and liquid Al, below the melting point of alumina (2250 °C), by the formation of a thin nanometre skin of alumina that prevents combustion. In tower block infernos of Al-plastic cladding materials, there cannot be production of detectable hydrogen gas, as wrongly assumed by Casey et al., who only investigate the laboratory hydrolysis reaction of solid aluminum in cladding samples, and not the combustion conditions at temperatures exceeding 1500 °C. Here we show that H₂ is an intermediate in the Al-combustion mechanism of cladding fires, and that water (H₂O) is the catalyst. This is one of two possible reaction mechanisms that enable combustion by circumventing direct oxidation of Al. For cladding with adjacent plastic insulation material, water provides an alternative mechanism via methane and the carbide Al₄C₃ as intermediates, also with H₂O as the catalyst.