Thermodynamic study on redox reactions of boiling nitric acid solutions

Kato, Chiaki   ; Kiuchi, Kiyoshi; Sugimoto, Katsuhisa*

In order to understand corrosion of metals in nitric acid solutions, it is necessary to know the generation mechanism of high equilibrium potential in the solutions, especially under boiling conditions. The Raman spectroscopic analysis showed that the existing amount of un-dissociated HNO increased with increasing nitric acid concentration and solution temperature. The existing amount of NO also increased by thermal decomposition. The thermodynamic calculation showed that the important nitrogen oxides in nitric acid solutions are HNO, NO, HNO, NO, and NO. The equilibrium potential of nitric acid solutions is, however, mainly decided by the HNO/HNO equilibrium. The thermodynamic calculation also suggested that the increased oxidization potential on the heat-transfer surface is attributed to the reduction of nitrous acid concentration by the thermal decomposition of nitrous acid on the surface and the continuous removal of decomposition product from the solutions by boiling bubbles.