Alkaline corrosion resistance is defined with two testing methods, the alkaline dropping test and the electromotive force measurement test.

(2) Alkaline corrosion resistance test by Electromotive force

This testing method is also unique to Japan and is an improved variant of the Alkaline Dropping Test, based on a principle of time measured for a predetermined area of a film to dissolve in sodium hydroxide solution.
The test uses an electromotive force measurement device shown in [FIg.1] where the tested specimen kept at 35°C is made to contact 1mL of 100g/L sodium hydroxide solution and measuring the 1mV voltage between the measurement cell and the base metal to investigate the corrosion resistance.

Compared to the alkaline dropping test, this method is easier to operate, eliminates human induced errors, and since it uses a relatively large amount of test solution of 1mL a film as thick as 20μm or more can be measured.
[Fig.2] shows testing results of boiling water sealed sulfate film corrosion resistance compared between alkaline dropping test and electromotive force measurement test. They show virtually identical results.

[Fig.3] shows a comparison of results of an alkaline dropping test and an electromotive force measurement test of a sulfate bath sealed film using a commercially available nickel based sealing agent. The alkaline dropping test yields higher result.

As it can be seen, it should be noted that alkaline corrosion resistance measurements are strongly effected by the pore sealing process and electrolytic coloring process.