Improvement of the Corrosion Resistance of Aluminium Alloys Applying Diff erent Types of Silanes - Advanced Materials for Agriculture, Food and Environmental Safety - page 43

from 100 kHz to 10 mHz, with an AC excitation amplitude of 10 mV. A
standard calomel electrode (SCE) was used as the reference electrode and
coupled with a glassy carbon counter electrode. h e surface area exposed
to the electrolyte was 1.00 cm 2 . All the measurements were performed
at er 30 min of open circuit potential (OCP) measurements and the
measurement potential was the OCP at that time.

2.5 Results and Discussion

2.5.1 Contact Angle

h e contact angle measurements represent a simple method to dei ne the
relative degree of interaction of a liquid with a solid surface. h us, if the
contact angle of water is less than 30 , the surface is considered to be hydro-

philic, and in the case where the value of contact angle is greater than 90

°

°

it is considered to be hydrophobic [42]. h us, the contact angle of water
on a substrate is a good indicator of the relative hydrophobicity. h e con-
tact angle value depends on the chemical composition (polar or non-polar
surface) but also the roughness of the surface has a great inl uence. h us,
polar (hydrophilic) surfaces show a better wettability with polar liquids,
such as water or water-based (or polar solvent-based) primers. h e results
of the contact angle measurements from the AA 2024 samples at er being
treated with dif erent silanes are given in Table 2.4 and their wettability is
presented in Figure 2.3.
h e factors that have an important role in the ability of organosilanes
to generate a hydrophobic surface are its organic functional groups, the
extent of the surface coverage, possible residual unreacted groups (from
the silane and/or the substrate surface), and the homogeneity distribution
of the silane on the surface. h e silane surface treatment depends on the
number of hydroxyl groups on the treated metal surface and on their acces-
sibility for bonding. A simple explanation is that the organosilanes react to
form a monolayer and, therefore, in the case where all hydroxyl groups are
covered by the silanes (which mean that the surface is protected) a hydro-
phobic surface is obtained.
In the case of the mixture of bis-amino silane:bis-sulfur silane, the wet-
tability and the hydrophobicity could accurately be controlled, resulting
in a i lm that wets very well and which is also sui ciently hydrophobic to
provide excellent corrosion protection.
A comparison of the contact angle results obtained leads to the con-
clusion that the values increase as follows: HTS < BTSE < bis-amino