Effects of nitric acid passivation on the physicochemical properties of stainless steel 316 and its reactivity with highly concentrated hydrogen peroxide

Abstract

Stainless steel 316 (SUS316) tanks and tubes for the storage of highly concentrated hydrogen peroxide, which is used as a clean propellant in rocket systems, are passivated with nitric acid to suppress hydrogen peroxide decomposition and increase its storability. However, the physicochemical changes to the surface of passivated SUS316 due to nitric acid passivation remain unexplored, and the surface properties of SUS316 that determine the rate of hydrogen peroxide decomposition have not been clearly identified. Therefore, developing the optimal passivation process to maximize the storage of hydrogen peroxide is challenging. To this end, we investigated the effects of nitric acid passivation on the physicochemical properties of the SUS316 surface and the decomposition rate of hydrogen peroxide stored using SUS316. Moreover, a novel method, which measures pressure changes caused by the decomposition of hydrogen peroxide, enabled a quantitative evaluation of the decomposition rate that was ≥10 times faster than the conventional measurement of active oxygen loss. Scanning electron microscopy and contact surface profiling analysis revealed that nitric acid passivation reduced the roughness of the SUS316 surface, rendering it smooth, and reduced the surface area that promoted the decomposition of hydrogen peroxide. X-ray photoelectron spectroscopy showed that nitric acid passivation decreased the ratio of Cr(VI)/Cr(III) on the SUS316 surface. Cr(VI) and Cr(III) were dominantly in the CrO3 and Cr2O3 phases, respectively. In addition, direct reaction studies indicated that CrO3 promoted the decomposition of hydrogen peroxide at a rate of approximately 100 thousand times higher than those of Cr2O3 and Fe2O3. Therefore, a small surface area and low ratio of Cr(VI)/Cr(III) on SUS316 may be key in enhancing the storabilities of hydrogen peroxide in SUS316 tanks or tubes.