Oxidation of Magnesium: Implication for Aging and Ignition Hongqi Nie, Mirko Schoenitz, and Edward L. Dreizin* New Jersey Institute of Technology, Newark, New Jersey 07102, United States 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. 1. INTRODUCTION Magnesium is widely used in pyrotechnic formulations and other energetic materials. 1-3 It is also well-known that magnesium powders are prone to aging, in particular in humid environments; 4,5 however, we were unable to find references describing this aging quantitatively. Two main oxidation products form during oxidation of magnesium: magnesium hydroxide, Mg(OH) 2 , and magnesium oxide, MgO. In low-temperature, humid environments, magnesium hydroxide forms as a result of hydration of a naturally grown thin MgO film uniformly adhering to the Mg metal core. 6,7 Multiple studies characterized the respective bilayer structure including a relatively dense layer of Mg(OH) 2 on top of a porous layer of MgO. 8,9 As the temperature reaches or exceeds the range of 280-450 °C, and depending on the partial pressure of water, Mg(OH) 2 decomposes, resulting in the growth of thicker MgO films. 10,11 Low-temperature rates of formation of layers of oxidized magnesium need to be known in order to develop handling guidelines for or predict the lifetime of magnesium powders stored under different conditions or included as components in a composite structure. The rates of reaction at elevated temperatures become important for predicting the ignition behavior of magnesium and related energetic systems. Addi- tionally, kinetics of magnesium oxidation may serve as a useful reference for assessments of reactivity of alloys employing magnesium, e.g., Al·Mg alloys prepared by mechanical milling for advanced energetics. 12-14 This work is aimed at characterizing oxidation of Mg powders experimentally in both dry and humid gas environ- ments. Separate experiments address different temperature ranges. Location of the reaction interface is first established. Reaction rates are then quantified. 2. EXPERIMENTAL SECTION 2.1. Material. Spherical magnesium powder with a nominal size of -325 mesh, and 99.8% purity was provided by Magnesium Elektron. This powder was passed through a 550 Mesh (25 μm opening) sieve to obtain coarse (as received) and fine fractions for high-temperature oxidation measurement in oxygen and steam. The actual particle size distributions for both size fractions were measured using a Beckman-Coulter LS230 Enhanced Particle Analyzer. Results are shown in Figure 1. For low-temperature oxidation measurements, in order to obtain a more distinguishable discrepancy between the reaction progresses of two size fractions, an even finer size fraction was prepared by passing the as-received powder through a 650 mesh (20 μm opening) sieve. Sieving was carried out using ASTM certified 12-in. test sieves by Advantech Manufacturing. Powders were placed on the sieves in an argon-filled glovebox, and shaken by hand for several minutes. It was only necessary to obtain small quantities with sufficiently different size Received: September 10, 2015 Revised: December 12, 2015 Article pubs.acs.org/JPCC © XXXX American Chemical Society A DOI: 10.1021/acs.jpcc.5b08848 J. Phys. Chem. C XXXX, XXX, XXX-XXX