Investigation of anodised surface complexity and its correlation with surface hydrophilicity using fractal analysis

Abstract

Anodising is a process of developing oxide coating on surface of metal electrolytically to improve its corrosion resistance. However, there has been inadequate research on the impact of the anodised surface characteristics on hydrophobicity or hydrophilicity. Surface roughness and porosity are often used to quantify surface characteristics, while each alone could not be enough to reflect all complexity of the surface. Hence, the fractal theory utilised to quantify the complexity of the anodised surface and decode its relation with the hydrophobicity or hydrophilicity of the surface. In this study, experiments of anodising for aluminium 6061 were carried out with three concentrations of electrolyte (3.5, 2.5 and 1.5 mol) and three current densities (1.0, 1.5 and 2.0 A/dm2). In addition, the surface characteristics after anodising was studied by using Scanning Electron Microscopy (SEM) analysis. Furthermore, surface roughness measurement was performed to obtain the surface roughness profile for fractal analysis. Moreover, a hydrophobicity test was implemented on anodised samples to obtain the contact angle between the water droplet and the surface. The outcomes showed that higher fractal dimension of anodised surface resulted in a smaller contact angle presenting a more hydrophilic surface. Furthermore, anodised samples with higher current density and concentration of electrolyte will result in higher fractal dimension and hence lower contact angle indicating the anodised surface will become more hydrophilic. Therefore, it is shown that measurement of fractal dimension is a reliable indicator for predication of surface wettability after anodising in order to improve the paint adhesion.