Evaluation of the compatibility between aluminum tank material and ammonium nitrate under fire conditions

On 17 December 2013 an explosion occurred at Drevja in Nordland county, Norway. The explosion was the consequence of a prolonged fire in a mobile explosives manufacturing unit (MEMU) loaded with materials for the on-site production of ammonium nitrate based explosives. During the investigations of the accident, concerns have been raised on the compatibility between the aluminum in the cargo tanks and ammonium nitrate (AN) during fire. This was investigated by using methods such as thermal analysis (differential scanning calorimetry (DSC)) combined with fire experiments. The experiments were designed to resemble fire conditions in such a way that high temperatures could be maintained throughout the experiment. None of the experiments indicated any serious incompatibility between melted aluminum and AN. Instead, a relatively controlled and familiar decomposition of AN into visible water fumes was observed in all experiments. This also correlates with similar reported experiments, which conclude on a relatively poor reactivity between the two materials. This is due to the rapid formation of a protective oxide layer on the metal surface caused by AN, which prevents further reactions at the interface. Thus, when solely considering the compatibility, we suggest that the use of aluminum tanks when transporting AN does not aggravate the already present risk of explosion in AN during fire. The thermolysis of AN is especially dependent on temperature and pressure, which these experiments also suggest. Thus, these parameters are of particular importance when evaluating the risk of explosion of AN during fire. During the experiments, some distinct changes in the decomposition of AN could at times be observed with apparent flames, accompanied by orangebrown fumes, characteristic for nitrogen dioxide formation. This was observed in particular when melted AN spattered out and landed on the hot steel surfaces, which correlates well with the formation of nitrogen oxides from AN being more prevalent at high temperatures and pressure. Decomposition products such as nitric oxide and nitrogen dioxide are not only known to have sensitizing effects on AN, but can also react further by highly exothermic and possibly explosive reactions if maintained at these extreme conditions. However, the role and importance of the formation of nitrogen oxides in explosions of AN resulting from fire is yet to be more thoroughly investigated.