Two aspects of water in self-heating risk of aluminium powders: calorimetric study

Two samples ("O" and "C") of aluminium-based powders were calorimetrically investigated with respect to the self-heating risk to water and aerial oxidation. Powder "C" was rich in aluminium (ca 85%); however, its particles were coarse, proving nominal size of ca 190 mu m. On the other hand, sample "O" contained only 23% aluminium, but it was evidently fine-grained (nominal size about 3 mu m). The fine character of the sample proved to be crucial for appearance of hydrolysis reaction with water. During the hydrolysis, about 36% of the aluminium of sample "O" was converted to aluminium hydroxide, while only about 0.15% it was for sample "C". Studying aerial oxidation, unexpectedly high increase in self-heating risk was found for the pre-wetted samples. Both samples, regardless of the difference in their grain size, represent quite comparable self-heating risk. A probable explanation of the finding is suggested.