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After aluminum alloy casting production, the voltage enters a more orderly and slow rising stage. It shows that the non-porous layer is continuously decomposed and the new non-porous layer is growing at the same time, that is to say, the transformation efficiency of the non-porous layer in the oxide film is balanced with the decomposition rate, so the thickness of the non-porous layer is not easy to rise again, and the voltage change is small.

However, at this time, the transformation and decomposition of the oxide film at the bottom of the hole can not be stopped, and they are still continuing to develop, resulting in the bottom of the hole gradually moving into the metal matrix. With the increase of oxidation time in aluminum alloy casting production, porosity occurs as holes increase, and the film layer with porosity gradually thickens. When the film conversion rate and decomposition rate achieve dynamic balance, even when the oxidation is promoted, the thickness of the oxide film will not rise again, and the anodizing stage should be stopped.

Within the first few seconds to tens of seconds of switching on electricity, the surface of aluminum is directly transformed into a layer of tight, with high insulation properties of oxide film, thickness of about 0.01 ~ 0.1μm, for a layer of continuous, non-porous film layer, called the non-porous layer or barrier layer, the film occurs when the current passes through and the film layer thickens again.

The oxidation process in aluminum alloy casting production is easy, the liquid is orderly, the operation process is convenient, the debris composition is allowed to be wide, the power consumption is less, the cost is reduced, and it can almost be more suitable for the processing of aluminum and various aluminum alloys, so it has been widely used in China