Effect of Hygrothermal Aging on the Pyrolysis of Magnesium and its Mixture

Abstract

Magnesium (Mg) is one of the notable metal fuel alternatives to fossil fuel often used for enhancing the combustion in rocket motors. However, the effect of extended storage conditions such as heat and moisture on the pyrolysis characteristics of pure Mg and its mixture are unknown. This study delves into understanding the hygrothermal aging effect of pure Mg and its compound “Magnesium-Teflon-Viton” (MTV). In particular, pure Mg and MTV are aged under relative humidity (RH) ranging from 0-95% for varied durations, and the thermal stability tests using calorimetry are performed to identify the reaction mechanism that leads to thermodynamic performance deterioration due to extensive aging.  Both pure Mg and MTV exhibited formation of oxide film (MgO, Mg (OH)2) on the surface of Mg during aging, observed by a decrease in Mg signals and an increase in oxygen signals from the surface analysis using Energy Dispersive X-ray Spectroscopy. Comparing the result of pristine and aged Mg shows that under hygrothermal aging, the Mg oxidation accelerated, thereby reducing the heat of reaction and activation energy. However, MTV undergoes multi-step reactions involving one endotherm (melting of Teflon), and two exotherms (pre-ignition reaction (PIR) facilitating the liquefaction and vaporization of Mg and MTV combustion reaction in the condensed phase) with a distinct trend in both pristine and aged samples. Under hygrothermal aging, due to the increase in the amount of MgOH2, the melting of Teflon was strengthened, while the PIR was relatively weakened. As a result, the heat transferred to the condensed phase was reduced, which in turn further weakened the MTV combustion reaction. This phenomenon is intensified with the increased humidity, leading to the increased oxide layer which was evident from a decrease in the heat of reaction and activation energy. Therefore, understanding such adverse effects of moisture and heat on the thermochemical properties of Mg and its compound is essential for predetermining the anticipated performance of industry-standard rocket pyrolants.

Keywords: Magnesium, MTV, Pyrolant, Rocket motor, Hygrothermal aging, Reaction kinetics