Improvement of the Corrosion Resistance of Aluminium Alloys Applying Diff erent Types of Silanes - Advanced Materials for Agriculture, Food and Environmental Safety - page 38

commonly applied method remains the deposition from solution because
it is a very fast and cheap method and the i lm thickness does not vary sig-
nii cantly when the immersion time is in the range of 30 s to 30 min. Silane
i lms inhibit corrosion attacks primarily because they act as a hydrophobic
barrier, preventing the transport of water/ions to the metal/coating inter-
face [25]. Also, the crosslinking of the siloxane i lm can be accelerated by
increasing the curing temperature and time. Curing of the silane i lm also
results in a decreased i lm thickness and an increase of the i lm density.
h e selection of a suitable silane for the surface modii cation is inl u-
enced by one or a combination of the following listed factors [47]:

• desired concentration of surface hydroxyl groups;

• type of surface hydroxyl groups;

• hydrolytic stability of the bond formed; and

• physical dimensions of the substrate or substrate features.

h e silane ei cacy on inorganic substrates thereby varies from poor
(carbon black, graphite) < slight (barytes, gypsum, marble) < good (lead,
zinc, nickel, asbestos, steel, iron, inorganic oxides, talc, tin) < excellent
(copper, silicon, aluminium-silicates, aluminium (AlO(OH)), glass,
quartz, silica) [47].
Another major advantage promoting the usage of silane i lms in addi-
tion to the anticorrosive ef ects is that the mechanical properties such as
resistivity against mechanical damages caused by impact, stretch and wear
are improved. h ere are also some studies concerning the improvement of
the mechanical properties of silane i lms by loading them with nanoparti-
cles into the i lms. For example, to improve the corrosion protection of AA
5005 a mixture of bis-amino silane, VTAS 5% containing 50 ppm alumina,
was used. h e samples were exposed for 336 h in the SST and the results
obtained were similar to the case of chromate corrosion protection [34].
h ere are also silane types, such as the bis-amino ones, which cannot be
applied for corrosion protection of aluminium as a single agent because of
the formation of protonated amine groups (R-NH 3 + ) promoting the dif u-
sion of Cl - into the i lm, and therefore accelerating the occurring corrosive
attack even though they act as a good primer for a subsequent painting step.
h e application of bis-silanes (bis-1, 2-[triethoxysilyl] ethane (BTSE),
bis-[triethoxysilylpropyl]) tetrasuli de (bis-sulphur or PSS) leads to thicker
and more homogeneous i lms, whereas the usage of mono-silane (mer-
captopropyltrimethoxysilane (MPS)) leads to thinner and more hetero-
geneous i lms. For example, BTSE is used in dif erent commercial silane
mixtures for thin-i lm deposition.