Final Report - Strategic Environmental Research and Development ...

concentration and the silicon to cation ratio (i.e. SiO 2 /Na 2 O). In this study, SiO 2 /Na 2 O was equal
to 1. The solution pH was measured after progressive addition of Na 2 SiO 3 to 0.1 M NaCl
solution. As illustrated in Figure 2.38, increasing the silicate concentration linearly increases the
solution pH. At SiO 2 /Na 2 O =1 the solution can be described as an aqueous solution of silicic
acid and hydroxyl ions as illustrated in the following chemical equation:
29 Si-NMR has been recognized as the most valuable method to study the anionic speciation in
silicate solutions [12-13, 15-16]. However, performing these measurements is notoriously
difficult. Attempts to conduct 29 NMR in this particularly study failed due to intense background
resonance. Nevertheless, Gaggiano et al. used solid-state 29 NMR to study anionic species in
silicate solution at 0.1 M concentration. It was found that, at SiO 2 /Na 2 O =1, the predominant
species in solution is the anionic monomer of the type .
Based on the above results on silicate and on the speciation information gathered through
literature, inhibition mechanisms are proposed for alkaline and acidic solutions. At high pH, the
aluminum-oxide film is chemically unstable and dissolves to form ions in solution:
(2.6)
(2.7)
The silicate ions in solution will then react with the
chemical reaction as proposed by Swaddle [14]:
species through the following
It has been reported that the isoelectric point of oxide-covered aluminum is 9-9.5 [17]. Therefore,
in alkaline silicate solution both the oxide surface and aluminosilicate anions are negatively
charged. Sodium detected in the film by XPS suggests that Na + ions in the solution adsorb on
the negatively charged oxide surface behaving as a coagulating agent [18]. The Na + ions on the
surface coordinate with the oxygen atoms of the aluminosilicate anions acting as a bridge
between the anions and the surface. Furthermore, Na + is observed to behave this way all
throughout the film as indicated by XPS depth profiling (Figure 2.14). The chemisorbed layer
provides a protective film over the aluminum alloy surface as supported by the electrochemical
polarization curves (Figure 2.9) and the impedance data (Figure 2.10). To summarize, at high
pH the dissolution of the oxide film on the surface promotes the formation of a protective
aluminosilicate film that is physically adsorbed via a coupling mechanism with Na + ions. This
mechanism has been proposed recently in the literature by Gaggiano et al. [16].
(2.8)
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