Corrosion of Metals by Nitric Acid-Dinitrogen Tetroxide-Fluoride Mixtures: AISI 321 Stainless Steel

Abstract The inhibiting effect of fluoride on the corrosion of AISI 321 stainless steel (SS) by mixtures of 100% nitric acid (HNO3) and dinitrogen tetroxide (N2O4), used as the oxidizer components of liquid rocket propellants, has been investigated, with particular regard to the influence of the N2O4 content of the mixture and the nature of the solid corrosion products formed in such systems. It has been established that the effectiveness of fluoride inhibition in these mixtures decreases with increasing N2O4 content and, for one of these N2O4 mixtures [high-density (nitric) acid (HDA)], with increasing water concentration. Investigation of the surface film formed on AISI 321 SS in the latter mixture has shown it to be primarily an oxide layer but some fluoride is incorporated also. It has been found that the long-term storage of fluoride-inhibited HNO3-N2O4 mixtures in contact with AISI 321 SS results in the eventual precipitation of an insoluble corrosion product identified as a previously unknown hydra...