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

The inhibiting effect of fluoride on the corrosion of AISI⁽¹⁾ 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 hydrate of iron (III) fluoride.