The electrical properties of the anodic oxidation coatings formed in alkaline baths (NH₄OH-NH₄F bath and NaOH-H₂O₂ bath) were investigated and compared with those of an anodized film formed in an H₂SO₄ bath.
NaOH-H₂O₂ film showed a lower value of electric resistance than anodized coatings formed in NH₄OH-NH₄F or H₂SO₄ baths, and the value (20μm, 200°C) decreased in the order of NH₄OH-NH₄F film (1.7×10¹⁵Ωcm), H₂SO₄ film (1.3×10¹⁵Ωcm) and NaOH-H₂O₂ film (6.6×10¹³Ωcm). The anodized coatings formed in the H₂SO₄ bath showed higher resistance than the coatings formed in the alkaline baths, and electric resistance of these films increased as the temperature increased.
In the same manner, breakdown voltage of the films (5μm, 15°C) decreased in the order of H₂SO₄ film (75V), NH₄OH-NH₄F film (45V) and NaOH-H₂O₂ film (40V). NH4OH-NH4F film (20μm, 100°C: 11.5) showed a higher dielectric constant than H₂SO₄ (20μm, 100°C: 8.5).
From the above results, it is concluded that the pore walls of the NaOH-H₂O₂ film were non-uniform and branched, and that aluminum hydroxide containing a considerable amount of water was deposited on the pore walls.