Ammonium nitrate (AN, NH4NO3) remains classified as an oxidising agent for transport, storage and handling purposes. The safe use of ammonium nitrate requires strict procedures as AN is incompatible with many materials. This study investigates the thermal decomposition of AN on surfaces of rust, potentially present on steel in storage facilities of AN. Infrared spectroscopy and X-ray diffraction served to characterise the mineralogy of the rust, relatively to neat iron (III) oxide. Furthermore, simultaneous thermogravimetric measurements and differential scanning calorimetry afforded the isoconversional analysis that yields the activation energies of the decomposition process. The results, in conjunction with the molecular modelling involving oxygen-deficient Fe2O3 clusters, elucidate the effect of rusts, existing on the surface of corroded steel, in reducing the ignition temperature of AN. This process manifests itself by lowering the activation energies of the initiation channels of the decomposition reactions. On the contrary, pure Fe2O3 does not influence the decomposition of AN. The dehydroxylation of hydrated iron (III) oxide, present on surfaces of rust, exposes the Fe sites that react exothermically with AN, before the material assumes the ordered Fe2O3 phase.