Neither the national standard for aluminum alloy building profiles in China nor the Japanese industrial standard describe the method principle of the alkali drop test, but in order to better grasp the operation method of the alkali drop test, it is necessary to understand the method principle of the alkali drop test. The drop alkali test is mainly used to investigate the alkali corrosion resistance of the anodic oxide film.For the anodic oxide film, its alkali corrosion resistance is relatively poor. When a certain concentration of sodium hydroxide solution drops on the surface of the anodic oxide film, the anodic oxide film will be quickly eroded. If the alkali corrosion resistance of the anodic oxide film is poor due to poor hole sealing or loose oxide film, the erosion rate will be faster. Therefore, calculating the penetration time of the anodic oxide film can be used to evaluate the alkali corrosion resistance of the anodic oxide film.However, due to the fast erosion rate of the oxide film by the sodium hydroxide solution, it is difficult to evaluate the alkali corrosion resistance of the oxide film.At present, there are two main test methods for alkali drop test, one is visual observation method and the other is instrumental measurement method.The visual observation method is based on the fact that when the sodium hydroxide solution drops on the surface of the oxide film, the oxide film will slowly dissolve. The chemical reaction equation is as follows：

Al2O3·χH2O＋2NaOH＝2NaAlO2＋（χ+1）H2O

During the dissolution of the oxide film, the sodium hydroxide solution continues to erode inside the oxide film. When the sodium hydroxide solution erodes to the surface of the base metal, the metal aluminum and the sodium hydroxide solution undergo a replacement reaction. During the reaction, hydrogen will precipitate and produce corrosion and bubbling.The chemical reaction equation is as follows：

2Al＋2NaOH＋2H2O＝2NaAlO2＋3H2

The instrumental measurement method is based on the electrical insulation of the anodic oxide film. The aluminum matrix is a good conductor of electricity, and the aluminum anodic oxide film is a high-resistance insulating film. Its insulation is related to the thickness of the oxide film. During the dissolution of the oxide film by sodium hydroxide solution, the resistance of the oxide film will slowly decrease as the thickness of the oxide film decreases. When the resistance is reduced to a certain value, it can be considered conductive, that is, the oxide film is considered to be dissolved.