Techniques for Surface Treatment of Light Metals. Super water-repellent aluminum by electrolytic etching and chemical adsorption.

In order to decrease frost on an air conditioner's fins, super water-repellent aluminum is required. Two requirements for the super water-repellent surface are reported, i.e., (1) roughness surface (large surface area) and (2) water-repellent functional group on the surface. Using both electrolytic etching and anodic oxidation made the aluminum rougher. During anodic oxidation, small pits were made in the large pits due to electrolytic etching. During the process of water-repellent treatment, a silane surfactant was used. The surfactant was attached to the aluminum by the Al–O–Si unit. For water-repellency, a fluoroalkyl group is better than an alkyl group. Fluoroalkylsilane, whose fluoroalkyl group is longer than C = 6, show super water-repellency, because many CF2 functional groups appear on the surface.