Influence of temperature and sulphate ion on corrosion of mild steel in molten NaNo 3

Abstract

The influence of temperature on the hot corrosion behaviour of mild steel in molten NaN 0³ has been investigated in the temperature range 350–600°C by gravimetric and potentiostatic polarisation techniques, scale and melt analyses, and scanning electron microscopy. The oxidation kinetics follows the parabolic rate law and the corresponding activation energy of the process was found to be 105·8 kJ mol⁻¹. At temperatures <500°C, the oxide film was found to be adherent, compact, and protective in nature, consisting mainly of Fe₃o₄ and Fe₂O₃. Incorporation of sodium in the scale and dissolution of iron oxides started at temperatures >450°C. A comparative study of the results indicated that the addition of 7·5 wt-%Na₂SO₄ to the NaNo₃ melt under the same conditions moved the corrosion potential in the active direction, increased the corrosion rate, decreased the passivation potential range, reduced the activation energy, and lowered the temperature at which spalling of the scale took place from 600 to 550°C. An attempt is made to interpret the results in the light of the available models of corrosion of iron and steel in molten salt systems.