Anodization behavior of barrier-type Al2O3 films in aqueous sulfonic surfactant electrolytes

Barrier-type Al2O3 films were formed in aqueous solution containing sulfona te-based electrolytes by anodization of aluminum substrate without any part icular surface treatment on it. The galvanostatic formation behavior of suc h "barrier-type" Al2O3 films was studied by measuring anode potential-time curves. As the electrolytes, the authors selected sodium salts of n-dodecyl benzenesulfonate (DBS), butylnaphthalenesulfonate (BNS), and n-dodecylsulfa te(SDS), mono, di, and trivalent naphthalenesulfonates, sulfonate polymers, and inorganic sulfate. Among them, DBS and BNS showed high coulombic effic iency(eta >90%) to form a barrier-type Al2O3, which is comparable to that o f ammonium adipate. The eta was found to be high when HLB (Hydrophile-Lipop hile Balance) of sulfonates is less than 40. Above 40(HLB > 40), the eta be came lower than 50%. It is concluded that higher hydrophobicity of the elec trolyte leads to higher capability to form a barrier-type Al2O3.