Fabrication of superhydrophobic surfaces on aluminum substrates using NaNO₃ electrolytes

A superhydrophobic surface with a water contact angle of 166.0° and a tilting angle of 1.5° was fabricated on an aluminum substrate by electrochemical machining using neutral NaNO₃ electrolytes, followed by fluorination. The fabrication process is based on the fact that the grain boundaries and dislocations on aluminum are anodic dissolved before the grain itself by an applied electric field. Using scanning electron microscopy to analyze surface morphology, micrometer scale caves, and protrusions were found on the surface, with numerous nanometer mastoids contained in the protrusions. These binary micro-nano rough structures, which are similar to the micro-structures of a lotus leaf surface, play an important role in achieving superhydrophobicity. The effects of processing time, processing current, and electrolyte concentration on superhydrophobicity were also examined. The results show that electrochemical machining does not require rigid processing parameters, uses a simple device, and is highly efficient and environmental friendly. The optimum processing conditions are a processing time of 60 min, a processing current of 250 mA, and an electrolyte of 0.15 mol/L.