Thermodynamic Study of Corrosion of Iron in NaNO3 ‐ NaNO2 Mixtures

Abstract

The influence of oxoacidity on the corrosion of iron in molten sodium nitrate‐nitrite mixtures has been demonstrated by means of potential‐pO2− diagram. Oxoacidobasic titrations of iron(III) have been carried out by means of potentiometric measurements with an yttria‐stabilized zirconia membrane electrode, indicator of pO2−. They have shown the formation of three different species of iron(III) and have allowed the determination of experimental oxoacidity constants in the temperature range from 693 to 773 K. These results and the thermochemical data of the literature have been used to establish the potential‐pO2− diagram relative to iron. This diagram permits one to forecast that three different compounds may be formed, according to the pO2−, when iron is in contact with the salt: or in strongly basic media, or in moderately basic media, in acidic media. A good correlation exists between our predictions and the corrosion tests reported in the literature.