Modeling Hydrogen Chloride and Aluminum Surface Interactions for Spacecraft Fire Safety Applications

Experiments and modeling were performed to determine the surface kinetics of gaseous hydrogen chloride (HCl) with aluminum surfaces subjected to various treatments. HCl and other acid gases are a spacecraft fire safety concern because they are commonly found in products from electrical wire pyrolysis. Three types of aluminum surfaces were considered: surface with chromate conversion coating (iridite), anodized, and untreated. A test cell made of polytetrafluoroethylene was used to measure the difference in the HCl concentration from the inlet to the outlet. A simple one-step global surface reaction model that accounts for active surface sites was proposed. The kinetic constants were calibrated using the measured data. The calibrated model was validated against experiments with different flow rates and HCl inlet concentrations. The results showed that anodized aluminum had the most HCl uptake, followed by the iridite, and then the untreated aluminum. The amount of HCl uptake appeared to correlate well with the thickness of the oxide layer on aluminum. The relevance of these findings is discussed with respect to the design of large-scale fire safety experiments in space and various fire safety application scenarios.