aluminium surface could effectively enhance its surface free energy and hydrophilicity. Results show that the Zr treatment of aluminium could further decrease the contact angle and increase the surface free energy compared to the chemically etched sample.The Zr conversion coating could enhance the wettability of the aluminium surface by two main ways. First, the Zr conversion layer includes oxides/hydroxides components which could produce strong hydrogen bonds with the water molecules, leading to better water molecules spreading on the aluminium surface. Second, the zirconium layer increased the surface roughness, helping the water molecules to spread on the aluminium surface easier[15,32].
Table 4
Adhesion strength values obtained from pull-off test (recovery adhesion was
obtained after 30 days immersion in 3.5% w/w NaCl solution).
Adhesion loss
(%)
Recovery adhesion
strength (MPa)
Dry adhesion
strength (MPa)
41.3 2.6
±
0.06 4.5
±
0.32 Degreased Al
29.9 4
±
0.30 5.60
±
0.07 Surface prepared Al
16.6 5.2
±
0.18 6.3
±
0.33 Zr treated Al
3.3. Epoxy coating properties on the Zr treated samples
3.3.1. The effects of Zr conversion coating on the epoxy adhesion properties It has been shown that the surface morphology and chemistry of aluminium were significantly affected after chemical etching and Zr treatment of the samples. The epoxy coating was applied on the aluminium samples before and after Zr treatment. The effects of Zr treatment on the epoxy coating adhesion to the surface of aluminium were studied. Dry and recovery (after 20 days immersion) adhesion strengths were measured by pull-off adhesion test. The adhesion loss values were then calculated according to Eq. (3).
Adhesion loss (%) =(1- recovery adhesion dry adhesion)×100
(3)The type of adhesion failure occurred on different samples are presented in Fig. 4. The adhesion strength values are also given inTable 4.According to Fig. 4 it can be clearly seen that two different types of adhesive and cohesive failures occurred on different samples after pull-off test. The coating detachment from the surface of the coating applied on the degreased aluminium sample was completely in the form of adhesive failure. It can be also seen from Table 4 that the dry adhesion strength of the epoxy coating Fig