Anodization Behavior of Barrier-Type Al₂O₃ Films in Aqueous Sulfonic Surfactant Electrolytes

Abstract

Barrier-type Al₂O₃ films were formed in aqueous solution containing sulfonate-based electrolytes by anodization of aluminum substrate without any particular surface treatment on it. The galvanostatic formation behavior of such “barrier-type” Al₂O₃ films was studied by measuring anode potential-time curves. As the electrolytes, the authors selected sodium salts of n-dodecylbenzenesulfonate (DBS), butylnaphthalenesulfonate (BNS), and n-dodecylsulfate(SDS), mono, di, and trivalent naphthalenesulfonates, sulfonate polymers, and inorganic sulfate. Among them, DBS and BNS showed high coulombic efficiency (η > 90%) to form a barrier-type Al₂O₃, which is comparable to that of ammonium adipate. The η was found to be high when HLB (Hydrophile-Lipophile Balance) of sulfonates is less than 40. Above 40(HLB > 40), the η became lower than 50%. It is concluded that higher hydrophobicity of the electrolyte leads to higher capability to form a barrier-type Al₂O₃.