Passivation of reactive metals using novel chemical treatments

There is an urgent need to find a replacement treatment, as efficient as the toxic hexavalent chromium systems, for corrosion inhibition and passivation of metals, particularly for alloys used in aerospace applications. For example rare earth conversion treatments (particularly based on cerate) have recently been developed for aluminium alloys by Hughes and others (1). In our work we have looked at two novel chemical methods to decrease the corrosion susceptibility of metals including steel, AA2024, AA7075 and various magnesium alloys. We have been designing and evaluating a family of compounds based on rare earth metal ions such as Ce(III), La(III), Pr(III) (and mixtures of these in an unrefined form known as Mischmetal) and an organic ligand which is itself a likely inhibitor. The group of compounds based on organocarboxylates and organophosphates coupled with these rare earth metals (REM) ions has shown high corrosion inhibition efficiencies both in aqueous chloride environments and in epoxy coated specimens. In parallel work, we have discovered that some ionic liquids (organic liquids composed entirely of ionic species) containing bis(trifluorosulfonamide) or organophosphates anions produce thin films on magnesium alloys which provide a high level of corrosion protection for these alloys. In some cases these films appear to have self-healing properties and may contain an organic oily layer on the uppermost surface. This paper presents an overview of the work to date, including some typical examples of key measurements used to evaluate the effectiveness of the chemical treatments in corrosion mitigation and to determine their mechanism of action.