2. Experimental
2.1. Materials
Aluminium alloy 1050 (AA1050) specimens (having thickness of 2 mm) with the composition given in Table 1 were supplied by Arak Al Co.Acetone, nitric acid (HNO3) and sodium hydroxide (NaOH) (pre-pared from Mojallali Co. and Merck Co.) are the materials used to prepare the Zr conversion coating bath.
2.2. Surface preparation of AA 1050 The surface of aluminum panels was degreased by acetone followed by chemical etching. The chemical etching was done by immersing the degreased aluminium samples in 5% w/w NaOH solution (alkaline etching) at 50.C for 3 min [1]. In this process,the oxide and any deformed layers presented on the surface of aluminium can be carefully removed. Then, the etched samples were washed with distilled water and then were immersed in the acid etching (desmutting) solution composed of 50% v/v nitric acid solution for 1 min (at ambient temperature) [1]. Finally, the desmutted samples were washed with distilled water. All prepared samples were kept in a desiccator before coating application, for preventing the occurrence of further corrosion on these samples.
2.3. Surface treatment of AA1050 with Zr conversion coating The surface prepared aluminium samples were dipped in hexafluorozirconic acid (H2ZrF6) based solution with Zr concentration of 100 mg/l in deionized water. The pH of the solution was adjusted to 4.5 using sodium hydroxide (NaOH 0.1 M). The surface treatment was done at ambient temperature for 3 min.
2.4. Epoxy coating application The epoxy/polyamide coating was applied on the surface of aluminum panels before and after Zr treatment. The epoxy resin used was Araldite GZ7 7071X75 (based on bisphenol-A in a xylene solution). The solid content, epoxy value and density of the resin were 74.76%, 0.1492.0.1666 Eq/100 g, and 1.08 g/cm3, respectively. The epoxy coating was prepared through mixing the epoxy resin with polyamide hardener (in which the ratio of epoxy resin to hardener was 70:40 w/w). Moreover, additives like leveling agent (BYK-306)and defoamer (Efka-2025) were added to the coating formulation to enhance its film formation performance. Finally, the coatings prepared were applied on the aluminium samples with and with-out Zr conversion coating. The epoxy coating was applied on the aluminium surface via film applicator. Samples were then cured in an oven at 120.C for 30 min. A dry film thickness of 40 } 5 m was measured on the cured samples.
2.5. Methods
2.5.1. Surface characterization of Zr treated samples The topography and chemical composition of the AA 1050 samples with and without Zr conversion coatings were studied byFE-SEM (Mira) equipped with EDS model SAMx. An Ambios model AFM was used in order to investigate the topography of the Zr treated sample. Static contact angles were measured on different
Table 1
Chemical composition of AA1050 (% w/w).
Others Zn Mg Mn Cu Fe Si Al
0.03 0.01 0.03 0.03 0.01 0.23 0.08