Environmental Engineering Reference
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on the surface. h e ef ect of surface pretreatment on aluminium was also
studied by Franquet [41], who concluded that the amount of hydroxyl
groups on the surface, as a result of dif erent pretreatments, inl uences
the initiation and the formation of the silane i lms [28]. At er alkaline
degreasing of the metal samples, it is important that a water-free sur-
face is present due to the fact that remaining contaminants may block
the active sites by reacting with the Si-OH groups. Several studies have
shown that the silane i lm thickness on metals increases with the con-
centration of the silane solution [31]. h e relationship between the silane
i lm thickness and the concentration of the solution is nearly linear [24].
It should also be noticed that in comparison to the conventional chro-
mate layers (which are about 3 μm thick), silane i lms are typically much
thinner. For example, a i lm from a 5% bis-amino/VTAS mixture formed
silane i lm has a thickness of about 250 nm, while a i lm from a 2% silane
solution is about 50 nm in thickness. Also, the thickness of the silane i lm
and its chemical composition show a strong dependence on the nature
of the used silane. In the case of bis-silanes the obtained surface i lm
is thicker and more homogenous compared to one from a monosilane
formed i lm, which is thinner and more heterogeneous.
h e presence of sulphur in bis-sulfur silanes plays an important role in
the improvement of the corrosion properties, which assures the formation
of the silane i lm via the Cu-rich precipitates [43].
In the case of bis-sulfur silane (PSS) the possible Reactions 2.4-2.5 are:
xCu + R-S-S-S-S-R
Cu x S + R-S-S-S-R
(2.4)
h e breakdown of the sulfur bridges of the silane molecules on the sul-
phide surface can occur according to:
nCu x S + R-S-S-S-S-R
n 1 (Cu x S)-S-S-R + n 2 (Cu x S)-S-S-R
(2.5)
where, n = n 1 + n 2 (n 1 and n 2 - corresponding stoichiometric coei cients).
2.2.3 Experimental Procedure
h e silanization of metal surfaces can be accomplished by various experi-
mental techniques, e.g., deposition from solution, from vapor, plasma and
electrodeposition [44, 45]. Gandhi and van Ooij [46] demonstrated that
by electrodepositing silanes on the metal substrate surface the obtained
i lm is uniform and the interfacial layer is strong. Nevertheless, the most
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