Final Report - Strategic Environmental Research and Development ...

Figure 2.34. SEM/EDS after in situ AFM scratching in 0.2 mM Pr 3+ .
5.2.5 Discussion of Inhibition Mechanism by Selected Inhibitors
5.2.5.1 Molybdate
Equilibria in aqueous solutions of molybdate have been studied in detail [5-7]. It has been found
that at concentrations above 10 -3 M the dominant species in solution at pH 7-12 is MoO 4 2- . In
acidic solution, polymerization condensation occurs as the ion becomes protonated resulting in
the formation of heptamolybdate ion Mo 7 O 24 6- at pH 3-5, and the octamolybdate ion Mo 8 O 26 4- at
pH less than 2:
-
At lower molybdenum concentrations, the presence of significant amounts of HMoO 4 and
H 2 MoO 4 has also been reported [8]. Figure 2.35 shows the specie diagram for MoO - 4 in 0.1 M
-
NaCl aqueous solution generated using MEDUSA. The plot shows that the solubility of MoO 4
significantly decreases below pH 6 as the molybdate polymerizes to form protonated species of
the Mo 7 O 6- 24 ion which were shown to reduce in a fashion that is not self-inhibiting (Figure 2.8).
The mechanism by which molybdate imparts inhibition is not well understood and different
mechanistic proposals have been put forward over the years [2,9-10]. According to Moshier et al.
molybdate forms an adsorbed layer on the surface that is concentration dependent [2]. At
molybdate concentrations less than 100 ppm, a thick film forms consisting mostly of MoO 2 . In
contrast, at higher concentration, the ratio of Mo 4+ to Mo 6+ concentration decreases and the
molybdate anion MoO 4 2- is preferentially adsorbed on the surface thereby impeding the ingress
of chloride ions. An alternative mechanism proposed by Breslin et al. involves an oxidationreduction
process whereby aluminum is oxidized and molybdate is reduced [9]. It is suggested
that there is competitive adsorption between molybdate and chloride ions at flawed regions in the
passive film. At sufficiently high molybdate concentrations, reduction of Mo 6+ to Mo 4+ is
favored resulting in the formation of a protective MoO 2 film:
(2.1)
(2.2)
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