Improvement of the Corrosion Resistance of Aluminium Alloys Applying Diff erent Types of Silanes - Advanced Materials for Agriculture, Food and Environmental Safety - page 46

is possible that they hydrolize back to much more hydrophilic silanol
groups (2.6) according to:

Si-O-Si + H 2 O ↔ Si-OH + HO-Si (2.6)

h ere are dif erent possibilities for improving the water resistance of the
i lm, the most common being fully crosslinking the silane i lm and improv-
ing the intrinsic i lm hydrophobicity by using silanes with hydrophobic
organic-inorganic groups, i.e., a sulphur chain -S4- (in bis-sulfur silane).
Bis-[3-(triethoxysilyl)-propyl]-tetrasuli de used for conversion conferred
hydrophobic properties on the surface, resulting in a protective action,
which was proved also by the values of the contact angles and by SST (no
pits for 192 h continuous salt spray exposure).

2.5.2 Characterization with SEM/EDX - FIB

h e addition of tartaric acid to the anodization bath improved the layer
structure. h e layer structure consisted of an initial inner uniform barrier
type oxide layer (not visible in Figure 2.5), a relatively thick middle layer
(A) with a i ne-structured morphology with enhanced corrosion resist-
ance, and an outer oxide layer (B) with a coarse morphology increasing
the adhesion of organic primers (Figure 2.5). h e presence of Cu 2+ in the
anodic layer grown on aluminium copper alloys not only alters the ordered
morphology of the anodic layer, but also changes its composition and die-
lectric properties. As stated in the introduction, Cu present in the solid
solution oxidizes and leads to the generation of oxygen bubbles during
i lm growth, including i lm disruption. A lateral porosity is caused by the
changing direction of pore propagation. h is contributes to the three-di-
mensional structure of the pore morphology.
h e TSA anodic layers were measured by FIB and the layer thicknesses were
between 2.5 μm (Figures 2.7 and 2.8), 2.6 μm (Figure 2.6) and 3.6 μm (Figure 2.9),
appropriate for aerospace industry requirements. h e TSA anodic layer could
not protect the surface completely; the aluminium oxide layer was damaged by
the action of Cl - , assuring a gradual penetration of Cl - through the barrier oxide
layer. h e silane conversion layers were very thin and the TSA pores' i lling was
dependent on the silane nature. For BTSE and bis-sulfur silane, the pores were
not i lled (Figures 2.4 and 2.5). We proposed a conversion coating base of silane
mixture at er HTS (the pores at er HTS are i lled) (Figures 2.10 and 2.11) for
more improvement in corrosion resistance; e compared the results of the con-
version coating with silane mixture with bis-amino silane:VTAS (Figure 2.8)
and bis-amino silane with bis-sulfur silane (Figure 2.9).