October 2011 Archives

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.

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.

(4) Alkali corrosion resistance

Table 4 shows the allowable alkali corrosion ranges specified for the respective types of alumite film subjected to either the alkaline dropping corrosion resistance test or the electromotive force measurement test. (See the next section for the procedural details of the alkaline dropping corrosion resistance test and the electromotive force measurement test.)

[Table 4] Alkali corrosion resistance ranges (seconds)

(5) CASS corrosion resistance

Table 5 shows the CASS corrosion resistance ranges specified for the respective types of alumite films as subjected to the CASS corrosion test.

[Table 5] CASS corrosion resistance ranges

(6) Wear resistance

Table 6 shows the wear and abrasion resistance ranges for the respective types of alumite films as subjected to the sand-falling abrasion resistance test, the abrasive jet test, or the reciprocating wear test. (See

[Table 6] Wear resistance

(7) Stain resistance

Table 7 shows the stain resistance range specified for all types of alumite films as subjected to the chromic-phosphoric solution immersion test, the dye spot test, or the admittance test:

[Table 7] Stain resistance

(8) Designation convention for alumite films

The designation of an alumite consists of a prefix for type, followed by letters for corrosion resistance, wear resistance, and stain resistance in this order:

Alkaline dropping corrosion resistance test = KS; electromotive force measurement test = KG; CASS corrosion test = L; sand-falling abrasion resistance test = WF; abrasive jet test = WJ; dye spot test = SD; chromic-phosphoric solution immersion test = SP

Example 1: AA15/L-WJ: 15 μm-thick film, having passed the CASS corrosion test and the abrasive jet test
Example 2: AA6/KS-WF: 6 μm-thick film, having passed the alkaline dropping corrosion resistance test and the sand-falling abrasion resistance test

JIS standards specify anodic oxide films on aluminum and its alloys as follows:

(1) Classification of anodic oxide films

Table 1 shows the classification of anodic oxide films by film thickness. "AA" stands for an alumite film on an aluminum material, and the numeral suffixes indicate the film thickness:

[Table 1] Classification of anodic oxide films

(2) Test items by use

Table 2 lists the test items required for anodic oxide films by use. When an anodic oxide film does not fall into a single category of use, the test items required for the anodic oxide film shall be determined by agreement between the interested parties.

[Table 2] Test items by usage environment (marked with as applicable)

(3) Film thickness ranges

Table 3 shows the allowable film thickness ranges determined from microscopic coating thickness measurements, the eddy current method, the split beam method, and the coating mass measuring method. Any film with a thickness less than 80 percent of the lower limit of the applicable value range shall be rejected.

[Table 3] Film thickness ranges (μm)

(To be continued)