Environmental Engineering Reference
In-Depth Information
performances to Cr(VI) with respect to the obtained anti-
corrosive properties.
Petrunin and his coworkers [20] studied the formation
and properties of siloxane monolayer on metal surfaces
using dif erent silanes. h e AlOSi covalent bonds occur in
the presence of adsorbed water on the aluminium surface,
and the silane monolayer on aluminium has, as a result, a
decrease of the amount of adsorbed water on the surface
leading to an inhibition of the hydration of the metal-oxide
surface i lm. In the case of aminopropyltrimethoxysilane,
the positively charged layer of the silane i lm activates local
metal corrosion when exposed to chloride-containing
media. Methacryloxypropyltrimethoxysilane (MAOS) [13]
shows a good corrosion protection of Al alloys when
immersed in NaCl solution due to the synergistic ef ect of
Aluminum-oxides and of Al-siloxane compounds forming
a polymeric passive i lm, together hindering ion dif usion
to the surface.
b.
According to their hydrophobicity:
water-based
(i.e., bis-
[trimethoxysilylpropyl] amine (bis-amino silane, (OCH
3
)
3
Si(CH
2
)
3
NH(CH
2
)
3
Si(OCH
3
)
3
) and vinyl-triacetoxysilane
(VTAS, CH
2
=CH(CH)
3
Si(COCOCH
3
)
3
)
or
alcohol-based
silanes
(i.e., bis-[3-(triethoxysilyl)] ethane, BTSE, (OC
2
H
5
)
3
Si(CH
2
)
2
Si(OC
2
H
5
)
3
and bis [3 (triethoxysilyl)propyl] tetrasul-
i de - bis-sulfur silane, (OC
2
H
5
)
3
Si(CH
2
)
3
S
4
(CH
2
)
3
-Si(OC
2
H
5
)
3
.
h e most important advantages of these types of water-
based silanes is that alcohol-free solutions can be prepared,
that they hydrolyze instantaneously and completely, and
that their corrosion protection performances are compa-
rable with alcohol-based silanes and chromates, especially
with topcoats. h ey have a broader compatibility with a lot
of paints compared to monofunctional silanes, due to the
fact that they have more organic functional groups which
are responsible for paint adhesion [32, 33].
2.2.2
Surface Treatment and Silane Chemistry
Silane surface treatment is a simple process. h is is accomplished by dip-
ping metals into diluted silane solutions (e.g., 2-5 vol%) for a few seconds,
followed by drying in air and curing by dif erent temperatures. Silanes
are only active in the hydrolyzed form (Si-OH). An access of SiOH in the
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