Anodizing with Simultaneous Silanization for Hidrophobicity Enhancement and Corrosion Protection of Aluminum Alloys

In refrigeration systems, aluminum sheets are continually submitted to condensation of water, which provides a corrosive environment and eventually results in the formation of ice and impairment of heat exchange. Therefore, hydrophobic surfaces are highly desirable, in order to extend life and improve performance of refrigeration systems. In this work, we propose a novel anodizing method combining porous anodic oxidation with simultaneous incorporation of hydrophobic organosilanes. Commercially pure aluminum (AA1100) was used as substrate and anodizing baths consisted of different portions of sulfuric acid, water, ethanol and hexadecyl-trimethoxysilane (HDTMS). In some cases, HDTMS hydrolysis was started in controlled environment before anodizing. Anodizing was performed galvanostatically, starting at different temperatures with a current density of 15 mA/cm2, for 25 minutes. The anodic layers obtained were characterized by scanning electron microscopy, Raman spectroscopy and attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR). Contact angles were measured by the sessile drop technique. The anodic layers obtained are circa 10 µm thick with incorporation of silanes inside pores and on the surface, partially blocking the pores. Hydrophobicity was significantly enhanced by adding organosilanes, achieving contact angles greater than 130°. The effects of ethanol concentration, HDTMS concentration and curing process on contact angles and potential transients during anodizing were studied. Corrosion resistance of anodic layers was evaluated using potentiostatic methods and standard salt spray exposure and compared to other traditional anodizing methods. Corrosion resistance was greatly increased by the developed process.