Passivation and Activation of Iron in the Presence of Molybdate Ions Available to Purchase

In extension of previous studies of the mechanism of the action of XO₄ⁿ⁻ inhibitors of corrosion, the behavior of MoO₄²⁻ ions has been investigated. This inhibitor was found to exert a noncathodic action in facilitating anodic passivation, but the phenomena are more complex than those exhibited by CrO₄²⁻, TCO₄⁻, and MnO₄⁻. The transition between active and passive potentials comprises several well-defined stages which were identified in potentiostatic measurements. The character of surface films was studied as a function of potential, both by analytical procedures and by electron microscopy and diffraction. The existence of mixed complex species containing Fe(II), Fe(III), and Mo(VI) in solution was established spectro-photometrically and solids having such compositions were synthesized. A super passive state was found to be produced at noble potentials when ferrous ions were present in sufficient concentration, and the dependence of all transition potentials upon pH was established. An interpretation of the processes occurring in the different regions of potential is presented, and deductions from the super passive behavior are extended to a generalized theory of the passive state.