S.S. Mahmoud *  

Chemistry Department, University College of Girls for Arts, Science and Education, Ain Shams University, Heliopolis, Cairo, Egypt

Abstract

The corrosion and passivation behaviours of iron in ternary molten Li₂CO₃-Na₂CO₃-K₂CO₃ mixture at different temperatures (475- 550 ^oC) were studied. The techniques of measurements were: open- circuit potential, galvanostatic anodic polarization, and cyclic voltammetry. The amounts of iron dissolved in the melt were determined by atomic absorption spectroscopy after each experiment. Iron undergoes spontaneous passivation upon its immersion in the melt and the thickness of the oxide scales increases with the increase of temperature. During the passivation potential range different oxides and spinals are formed. These include a cubic solid solution of FeO and a-LiFeO₂, a-LiFe₅O₈ and a-Fe₂O₃. At high anodic potentials the decomposition of carbonate takes place leading to passivity break down and CO₂ and O₂ gases evolution. Also, the spoliation of oxide scales in the carbonate melt occurs at the passivity breakdown. The values of corrosion parameters (R_p, i_o, i_corr, i_p) were calculated. The different values of activation energy of corrosion process were calculated. The results of cyclic voltammetric investigation indicate the formation of different oxides and spinals, and the cathodic and anodic decomposition of carbonate. The corrosion tests in 0.05 M HCl solution indicate that the oxide scales formed on iron are multilayered. The outer layer is less protective than the inner one.

Keywords: molten alkali carbonate, corrosion, passivation, iron.

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* Corresponding author. E-mail address: drsohairr@hotmail.com