There are two methods available for testing the alkali corrosion resistance of alumite film. One is known as the alkaline dropping corrosion resistance test and the other as the electromotive force measurement.

(1) Alkaline dropping corrosion resistance test

This test is used only in Japan. An alkaline dropping corrosion resistance testing machine shown in Fig. 1 is used to measure the alkali corrosion resistance of a type of alumite based on the time (in seconds) elapsed until the complete melting of the alumite films on a series of specimens placed in a thermostatic chamber at 35ºC ± 1°C and with a 100 g/L sodium hydroxide solution drip. This testing method is unsuitable for measuring the alkali corrosion resistance of alumite films 20 μm thick or thicker due to the small amount of sodium hydroxide solution that can be dripped.

Fit the measurement test probes of an ohmmeter to cleaned and dried specimens to determine whether the alumite films have completely melted. If the ohmmeter reads a DC resistance of 5 kΩ or lower under a load of 50 g, regard the specimens as conductive.

Failure to ensure stringent control of the test environmental conditions will result in incorrect test results. The test environmental conditions refer to the concentration and temperature of the test solution, the temperature of the specimens, and the temperature and humidity inside the testing machine. Fig. 2 shows the influence of the temperature inside the testing machine on the corrosion resistance of the alumite films:

In recent years, nickel-based and fluorine-based sealing additives have been used. Keep the following point in mind: depending on the sealing method used, the alumite film on the area dripped with sodium hydroxide solution may not melt uniformly or corrosion product may remain on the substrate, making conductivity testing impossible.