Final Report - Strategic Environmental Research and Development ...

esults in the formation of silicate- and silica-based particulates. At even lower pH, the main
form of species in solution is silicic acid. However, it was shown that activation of the
aluminum surface promotes the formation of monomeric silicate ions and the deposition of
aluminosilicate film across the whole surface, which was shown to provide some corrosion
inhibition in comparison to the inhibitor-free case.
Praseodymium imparted cathodic inhibition at near-neutral pH, lowering the oxygen reduction
kinetics by a factor of 10 at an optimum concentration of 0.2 mM. It is proposed that an oxide
characterized as
forms over the ORR supporting IMC particles as the
local pH increases. As the cathodes on the surface become blocked by a thick-oxide, new sites
are activated resulting in the formation of a thick-film across the whole surface. In low and high
solution pH, praseodymium renders poor inhibition. In the latter case, it was shown that
praseodymium still forms a thick-oxide film over the IMCs as a result of ORR but is unable to
stop the uniform dissolution of the Al-oxide film.
Lastly, in situ AFM scratching experiments reveal that at their respective optimum inhibition
concentrations, all three inhibitors reduced the removal rate of the S-phase particles in
comparison to the inhibitor-free case. In the case of molybdate, no form of attack was observed
even after scratching at high forces. Similarly, almost no attack was observed in the scratched
region in the presence of silicate ions in solution. In the case of praseodymium, attack in the
form of trenching was observed around all IMC particles, however ex situ SEM analysis showed
the larger particles to be still intact and Mg and Al were still detected over the S-phase particles
by EDS.
This work sheds light on the inhibition mechanisms of selected chromate-free inhibitors in hopes
of helping to facilitate improvements in chromate-free protection schemes. More specifically,
this work can be useful in studies involving inhibitor combinations that incorporate the inhibitors
looked at in this study. However, one must consider the effect of these inhibitors beyond the
realm of bulk solution inhibition. For instance, implications based on the AFM scratching results
coupled with electrochemical experiments would suggest that molybdate may be a suitable
replacement for chromate. However, the large ratio of molybdate-to-chloride ion concentration
may not be adequate for organic coating applications since the high concentration of inhibitor
release necessary for corrosion inhibition may lead to coating degradation via osmotic blistering
[27]. Therefore, caution must be exercised when extending the application of these selected
inhibitors into the realm of coating systems.
5.2.7 Acknowledgements
The authors would like to gratefully acknowledge the Strategic Environmental Research and
Development Program (SERDP) for funding this work. The authors also acknowledge the help
of Dr. Lisa Hommel from The Ohio State University for assistance with XPS measurements.
5.2.8 References
1. P. Schmutz and G.S. Frankel, J. Electrochem. Soc., 145, 2295 (1998).
2. W.C. Moshier and G.D. Davis, Corrosion, 46, 43 (1990).
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