Ageing of magnesium; Influence of particle size and particle shape

Many pyrotechnic compositions contain magnesium as a fuel. Ageing as a result of the influence of moisture leads to a decrease of the amount of metallic magnesium and the formation of hydrogen gas. A model that predicts the conversion of metallic magnesium in an MTV (Magnesium, Teflon, Viton) based composition as a function of temperature and relative humidity was presented at the ICT conference in;1995 [1]. In this paper the influence of particle size and shape on the ageing of magnesium is studied. At one set of conditions the conversion of various magnesium powders with different particle sizes and shapes was determined from their mass increase. The conversion rate is quantified in order to investigate whether it is worthwhile to change particle size and shape of magnesium in a composition to reduce the ageing rate. It appears that the conversion of magnesium (pyrotechnic magnesium as well as atomised magnesium) into magnesium hydroxide as a function of time can be described by an S-function. The curve of the conversion rate has a similar shape as described in ref. [1]. The conversion and conversion rate of both atomised magnesium and pyrotechnic magnesium are modelled on the basis of experimental data obtained with various particle sizes. This provides the opportunity to predict the ageing rate for any size of magnesium particles. The use of atomised magnesium instead of pyrotechnic magnesium significantly reduces the conversion rate by a factor of 4 to 9, depending on the particle size. We conclude that it is worthwhile to optimise a composition not only with respect to performance but also with respect to ageing.