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Electrochemical dissolution and passivation behavior of iron in ammoniacal caron leaching solution Roy, Subrata


The electrochemical passivation of pure Fe in ammoniacal solution was investigated to determine the stability of both Fe-oxides and Fe-ammines during anodic polarization. The potentiodynamic experiments were done in 6M total NH₃[3M NH₄OH and 1.5M (NH₄4)₂CO₃] solution. Different experimental parameters such as temperature (15°, 25°, 35°, 45° and 55°C) and pH (6, 8, 10 and 12) were used. Pure oxygen and argon gas was sparged during the experiment to oxygenate or de-oxygenate the solution with no stirring in either case. Polarization plots show that both active anodic dissolution and passive regions are present for pure iron in ammoniacal solution. It also shows that as the temperature increased the dissolution rate increased in both anodic and passive regimes. At the same time, the active region for iron dissolution is present across a wider potential range. For pH 10 the highest dissolution rate is around 0.025A/cm² or 260 g/m²hr¹ at 55°C and passivation of iron generally occurs at ca. -0.36 V (SHE) irrespective of the temperature. The peak anodic dissolution rate (0.2 A.cm₋² or 2080 g/m²hr¹) surprisingly occurs at pH 6. Potentiostatic experiments were done at different fixed potentials at pH 9 and 25°C. The highest current density was registered at -0.6 V. The peak dissolution current observed for the potentiostatic tests is roughly two orders of magnitude lower than that observed in the potentiodynamic test for pH 9 and at 25°C. Solution and morphological analyses were done by ICP and SEM, respectively for pH 6 and 9 solutions. The current efficiency for pH 6 is far lower than at pH 9 which implies that the current registered at pH 6 is used for the formation of a product film. Speciation calculations indicate that this film may be siderite (FeCO₃) at low potential. From XPS analyses, it is believed that the passive layer formed at higher potentials (more than 0.40V) is Fe₂O₃. Speciation diagrams point to the stability of iron tetra-ammines at pH 10. It was shown that metastable Eh-pH diagrams for Fe/NH₃/CO₃/H₂O system can be generated through potentiodynamic measurements aimed at active and passive behavior of iron.

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