Final Report - Strategic Environmental Research and Development ...

hydrotalcite as surface pretreatments and paint pigments [19, 33, 37]. Hydrotalcites are anionic
clays composed of alternating layers of a compound like Al-Zn hydroxide and exchangeable
anions. When such hydrotalcites are exposed to an aggressive electrolyte containing Cl - , anion
exchange occurs, releasing the anions from the hydrotalcite interlayers, which could act as a
corrosion inhibitor. The exchange is chemical in nature and is controlled by an equilibrium
constant and exchange isotherm. It was shown that hydrotalcites produced with Zn and vanadates
imparted good corrosion protection to AA2024-T3 [33]. Most vanadate salts are extremely
soluble, which limits their use as a pigment. However, hosting vanadates within hydrotalcite
seems to reduce the effective solubility and modulate the release. The simplicity of the process
and the potential control of releasable anions make hydrotalcites commercially very attractive.
Moreover, since replacement of the vanadate anion with Cl - is followed by a change in the
crystalline structure of the hydrotalcite, x-ray diffraction of the coatings could be used to monitor
the early stages of corrosion [33].
Organic pigments: Organic pigments based on the chemistry of alkanethiols C n H 2n+1 SH (such as
perfluoroalkyl amideethanethiols) are promising. Alkanethiols can be strongly chemisorbed on
metallic surfaces and tend to form dense structures by self-assembly [38, 39]. Several other
organic compounds such as diphosphone-alkanes and dimercapto thiodiazole also can form selfassembled
films on different metallic substrates and they have been the subject of investigations
[40]. Self-assembly of alkanethiols occurs spontaneously on Cu and Au surfaces [38, 41].
Alkanethiol molecules consist of two functional groups; one is responsible for the chemisorption
and the other determines the hydrophobicity of the chemisorbed molecule [40]. This process is
attractive because of the possibility of tailor-making films in short time by spray or simple
immersion [38, 40, 42, 43]. However, this technology is still in its earliest stages and the
investigations reported so far mainly cover theoretical aspects of the film formation mechanisms
on Cu, Au and Ag thin films rather than more applied work on real commercial substrates.
Rare Earth Metal (REM)-based pigments: Following successful lines of research in the area of
soluble rare earth metal (REM) inhibitors and conversion coatings, such as those based on Ce
ions, new REM pigments for organic coatings have become commercially available in the past
several years [44]. A distinctive aspect of this coating formulation is pigmentation with
nanoencapsulated REM inhibitors. These coatings have performed well in standard testing and
are receiving a great deal of attention from the military user community. However, at the present
time, the mechanistic understanding underlying important aspects of inhibitor function and
applicability appears to lag the understanding of its performance profile in accelerated testing.
REM inhibitors have also been incorporated in cation-exchanging clay particles [45]. These
particles have been dispersed in organic resins as corrosion inhibiting pigments, and the resulting
corrosion resistance has been demonstrated. This line of research has been fundamental in nature
and has revealed some important attributes and limitations of REM inhibition [46]. Taken
together, these two inhibitor development activities show the need to increase the understanding
of REM inhibition in coatings so that corrosion protection technologies are selected and
deployed with a proper recognition of their advantages and limitations.
2.4 References
1. M. W Kendig, A. J. Davenport, and H. S. Isaacs, Corros. Sci., 34, 41 (1993).
2. W. Clark and R.L. McCreery, J. Electrochem. Soc., 149 B379 (2002).
3. P. Schmutz and G.S. Frankel, J. Electrochem. Soc., 146, 4461 (1999).
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