A Novel Polishing Process for Ultra-Smooth Aluminum Surfaces via Anodizing in Sodium Metaborate

A novel ultra-smoothing process for aluminum surfaces was developed using porous alumina formation and subsequent oxide dissolution. A submicron-scale periodic dimpled aluminum surface with an arithmetic mean roughness of 31.5 nm was prepared by anodizing in an etidronic acid solution. This dimpled aluminum specimen was then anodized in a sodium metaborate (NaBO 2 ) solution to form a unique porous alumina film with an extremely flat barrier layer, which differs from the typical hemispherical barrier layer. The outer porous layer became thicker with time during anodizing, whereas the thickness and smoothness of the inner barrier layer were maintained without oxide breakdown. As the porous alumina film was chemically removed in a CrO 3 /H 3 PO 4 solution, a smooth aluminum surface was exposed. The mean roughness of the aluminum surface drastically decreased to 0.5 nm by short-term anodizing for 15 min and slightly decreased with further anodizing. As a result, an ultra-smooth aluminum surface measuring 0.4 nm in roughness, which is much smaller than that of an electropolished aluminum surface (1.3 nm), was successfully obtained via anodizing in NaBO 2 and subsequent oxide dissolution. Our smoothing process was compared with conventional smoothing processes such as electropolishing and barrier oxide formation.