Anodic Polarisation of Mild Steel in Molten NaNO 3 -KNO 3 Eutectic Containing Acid and Base Additions

Abstract

The effect of NaPO₃ and K₂Cr₂O₇ as acids and Na₂O₂ and KOH as bases on the anodic polarisation of mild steel in NaN03-KN03 melt has been investigated at 400°c. At a given current density (c.d.) the anodic polarisation in the presence of acidic additives increases linearly with the logarithm of acid concentration, the rate of this increase itself increasing at higher c.d.s. At all c.d.s. examined the anodic polarisation increased more rapidly with log C of NaPO₃ than with that of K₂Cr₂O₇. This has been attributed to the higher acidic strength of NaPO₃, but may also be related to the formation of a thick layer of insoluble iron phosphate on the electrode surface, especially at high c.d.s. In the case of added Na₂O₂ and KOH the anodic polarisation decreases linearly with the logarithm of base concentration.