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

• Published in: Journal of failure analysis and prevention 2022-08, Vol.22 (4), p.1771-1775
• Main Authors: Maguire, John F., Woodcock, Leslie V.
• Format: Article
• Language: English
• Subjects: Aluminum; Aluminum carbide; Aluminum oxide; Catalysts; Catastrophic failure analysis; Characterization and Evaluation of Materials; Chemistry and Materials Science; Cladding; Classical Mechanics; Corrosion and Coatings; Exothermic reactions; Hydrolysis; Materials Science; Melting points; Oxidation; Quality Control; Reaction mechanisms; Reliability; Safety and Risk; Solid Mechanics; Spontaneous combustion; Technical Article > -Peer-Reviewed; Thermodynamics; Tribology
• ISSN: 1547-7029; 1728-5674; 1864-1245
• DOI: 10.1007/s11668-022-01471-0

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 (H 2 ), which was not detected in their experiments. The combustion of aluminum (Al) to alumina (Al 2 O 3 ) 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 H 2 is an intermediate in the Al-combustion mechanism of cladding fires, and that water (H 2 O) 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 Al 4 C 3 as intermediates, also with H 2 O as the catalyst.