Sulfuric acid anodizing produces a porous film. A thin film resembling a barrier layer is generated at the onset of oxidation. As the oxidation progresses, holes develop in the film due to dissolution in sulfuric acid, leading to the growth of the film. The thickness of the film is directly proportional to the passed electrical charge, i.e., it is proportional to the current density and oxidation time. The oxidation film thickness is calculated by the formula δ=kIt, where δ is the oxidation film thickness (μm), I is the current density (A/dm2), t is the electrolysis time (min), and k is a coefficient typically set at 0.3. The actual value of k is determined through experimentation and is closely related to process parameters. Under the same process conditions, thick and thin films exhibit variations. Additionally, the thickness of the oxidation film cannot indefinitely increase with anodizing time; it has a limit associated with process conditions. For producing thicker films, it is advisable to slightly reduce sulfuric acid concentration and temperature, increase current density, and introduce air agitation.
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