Oxidation of Magnesium: Implication for Aging and Ignition

Abstract

Magnesium is widely used in pyrotechnic formulations; it is also a component of reactive alloys, e.g., AlMg and BMg, which are potential fuels for explosives and propellants. Despite its widespread applications, the kinetics of oxidation of Mg powders are not well quantified. Such kinetics are of fundamental importance for the models aimed to describe thermally induced ignition of metal powders. In addition, for Mg the issue of aging is important because its oxide, MgO, is porous. In this work, magnesium oxidation by both oxygen and steam was studied by thermo-analytical measurements for micron-sized spherical powders. Heat flow calorimetry and thermogravimetric analysis were used to quantify reaction rates for low and elevated temperature ranges, respectively. Experiments with spherical powders with different but overlapping particle size distributions were used to identify the location of the reaction interface. The reaction was found to occur at the interface of metal and the growing oxide layer for all oxidizing conditions. The reaction is thus rate limited by diffusion of oxidizer to the metal surface. Kinetics of oxidation for both dry and humid oxidizing environments were quantified using thermo-analytical measurements and different data processing techniques. The activation energy of magnesium oxidation in humid environments at low temperatures is close to 60 kJ/mol. Activation energy for oxidation of magnesium in oxygen is 148 kJ/mol. For oxidation of magnesium in steam at elevated temperatures, the activation energy increases linearly from approximately 130 to 360 kJ/mol, while the thickness of the oxide layer is growing up to 2.4 μm. Simplified diffusion-limited reaction models were developed for oxidation of magnesium in both oxygen and steam. The models enable one to predict both preignition reactions occurring upon heating of Mg particles and the time of Mg powder aging when exposed to moisture or oxygen at different temperatures.

Oxidation of differently prepared Al-Mg alloy powders in oxygen

“…Thus, Case I describes the reaction best for the Al-Mg alloys and oxidation of both Mg and Al is governed by the inward diffusion of oxidizer. This is similar to the oxidation mechanism identified for pure Mg [18], but different from the oxidation mechanism for pure Al [17]. We can speculate that diffusion on the oxygen sublattice is easier in the presence of Mg, perhaps due to point defects.…”

“…When a powder with different particle sizes is oxidized in a TG experiment, the mass can increase at different rates for particles with different sizes, particularly when the oxidation reaction occurs at the particle surface. In order to account for this, and to allow for a more meaningful interpretation of TG measurements, a methodology was developed in previous research [16][17][18]22] that distributes the weight change from the TG measurements among particles of different sizes using the measured particle size distribution. This methodology was applied to the measurements for the spherical atomized powder in this study.…”

“…Such descriptions are important for reactive materials for prediction of both aging and ignition behaviors of the relevant compositions. Recently, thermo-analytical studies were used to clarify reaction mechanisms and describe them quantitatively for both Al [16,17] and Mg [18] powders. Here, a similar experimental approach and data processing techniques are applied to characterize high temperature oxidation in Al-Mg alloys with equal mass fractions of aluminum and magnesium.…”

“…Quantitatively, following previous work [16][17][18], functions of mass as a function of time for all overlapping particle sizes were compared to each other using the parameter, Ern, calculated for each particle size bin, n:…”

Atomic‐Scale Observation of Irradiation‐Induced Surface Oxidation by In Situ Transmission Electron Microscopy

“…In stage II, however, the CdO growth speed starts to decrease. It seems that the transition from CdS to CdO is inhibited by the already transformed oxide layer, signifying a diffusion‐limited growth . To summarize, real‐time observations reveal that an electron beam can stimulate a structural transition from CdS to CdO at the surface.…”