Corrosion rates of cold reduced and annealed plain carbon steel AISI 1020 tubing in fuming nitric acid (FNA) were obtained by measuring the change in electrical resistance of the tube with time. Corrosion rates decreased with increasing nitrogen dioxide (NO2) concentration in FNA in the range 0 to 14 weight percent and also decreased with increasing H2O content in the range 0 to 3.5 weight percent. This behavior indicates the possibility that nitronium ion (NO2+) or nitrogen pentoxide (N2O5) are involved in the rate-controlling step in the corrosion process.
Tentative or short-time passivation of the steel was obtained when the NO2 concentration was greater than about 9 weight percent and H2O concentration was 3.5 weight percent at the upper concentration range studied. Short-time passivation was also obtained when an oxide film formed under appropriate conditions was initially present on the specimen surface. Inhibition in the FNA media occurred only in those cases where the maximum rate of metal removal was less than 0.025 mil/min, corresponding to a weight of metal removed of approximately 0.1 mg/cm2 of apparent surface. Adding 1.5 weight percent of either perchloric acid (HClO4) or sulfuric acid (H2SO4), and 0.5 weight percent hydrofluoric acid (HF) produced inhibition in short-time experiments. 4.3.2
