Nitridation of magnesium in the temperature range 400–650°
The reaction between sheet magnesium and gaseous nitrogen has been studied gravimetrically in the range 400–650° on a thermal balance and a vacuum microbalance of the Gulbransen type. The behaviour is complicated since weight is lost by evaporation as well as gained by nitridation. Evaporation is promoted by traces of water vapour but inhibited by oxygen, or by an unidentified impurity in nominally pure nitrogen, or by mechanical damage caused by abrasion or sawing. Evaporation is found with electropolished magnesium in very pure nitrogen at 500 and 525°(and probably above 525°) on the microbalance, but not on the thermal balance, where the characteristic effect is “breakaway”—a sudden increase in the rate of weight gain. The results can be explained in terms of the formation and growth of cavities at the nitride–metal interface, together with the rupture of the film covering the cavity. If at rupture the film is thin the magnesium vapour will escape, but if thick the vapour will react with nitrogen within the cracks to form the nitride.