Improvement of the Corrosion Resistance of Aluminium Alloys Applying Diff erent Types of Silanes - Advanced Materials for Agriculture, Food and Environmental Safety - page 39

Table 2.3 Silane mixture solution; pH as a function of the content of
bis-aminosilane in the bis-amino silane/VTAS mixtures (at a concentration
of 5%) [32].

Bis-amino: VTAS pH Solution stability

VTAS 2.8 Hazy in a few minutes

1:5 3.0 Hazy in a week

1:2 3.3 Hazy in a week

1:1 3.5 Hazy in a week

2:1 4.0 Clear over one year

5:1 5.8 Clear over one year

6:1 7.5 Clear over one year

Bis-amino silane 10.8 Hazy in a few minutes

h e quality of the obtained conversion coatings based on silanes also
depends on some physical parameters such as curing time and curing tem-
perature, as well as on the chemistry of the silane molecules themselves.
h us, the optimization of dif erent processing variables is the foremost
objective to obtain dense and functional protective silane i lms. Standard
mixtures of silane applied for the aluminium alloy corrosion protection
treatment are: bis-sulfur:bis amino = 3:1 and bis-amino:VTAS = 5:1 [31].
h e pH value and the stability of the solution of the water-based silane
mixtures strongly depends on the mixture ratio as shown in Table 2.3.
h e treatment of aluminium and its alloys by silanes is less complicated
compared to other metals, due to the fact that the applicable pH window is
wider and aluminium oxide has a strong ai nity to silicates. Furthermore, alu-
minium is a trivalent ion and can crosslink a Si-O-Si bond by inserting into it.
h e most important properties of silane i lms are their hydrophobicity
and barrier properties.
h e factors which inl uence the ability of an organic silane to generate
a hydrophobic surface are: organic substitutes, the extent of surface cover-
age, the residual groups (unreacted groups from silane and from the sur-
face) and the distribution of the silane of the silane surface [47].
h e silane groups are not electrochemically active in solution or in
solid state, but some silanes contain functional groups with electrochemi-
cal activity. Due to the hydrophobicity of silane i lms, they reduce the rate