The passivation of iron in ammoniacal solutions containing copper (II) ions

In the present study it was found that the presence of millimolar amounts of copper (II) ions in ammoniacal solutions leads to the spontaneous formation of a stable passive layer on metallic iron and iron alloys with nickel and cobalt, under conditions in which they would otherwise remain in active dissolution. This finding may be of significance to industrial processes which employ ammoniacal solutions to leach metal values from materials rich in metallic iron, such as, for example, laterite ores which have undergone a reductive pre-treatment.The spontaneous passivation is found to take place regardless of the presence or practical absence of dissolved oxygen, and occurs more readily the higher the copper concentration and the lower the ammonia–ammonium bicarbonate concentration, though it does not take place in ammonia-free aqueous solutions of similar copper (II) ion concentrations.Electrochemical and scanning electron microscopy / energy dispersive X-ray spectroscopy (SEM/EDX) investigations conducted as part of this study showed that the observed passivation is promoted by the cementation of copper onto the actively dissolving iron surface, and subsequently by its re-dissolution. Such process is thought to create favourable conditions which promote the formation of a stable passive layer on the iron surface.

► Iron, Fe-Ni, and Fe–Co alloys readily passivate in ammoniacal cupric ion solutions.
► The cementation of copper onto iron precedes and promotes passivation.
► As copper redissolves it is replaced by a stable passive oxide layer.
► The delay to passivation increases for decreasing [Cu(II)] or increasing [NH3]T.
► Passivation occurs regardless of the presence of dissolved oxygen.