Hard Anodizing Specifications for the Industry

Sometimes it is difficult to find out how to and what to use in an anodizing specification. There are several specifications but MIL-A-8625 F is probably the most common standard used both in the US and Europe for hard anodizing.

This specification does not restrict the process condition, but only which requirements the hard anodic layer has to pass.

Below you will find the four tests which you should always ask your anodizer about.

Coating thickness

If not specified the coating thickness should be no less than 2 mils (50 ┬Ám) and the thickness must not vary more than ± 20%. One of the test methods recommended is ASTM B244 measured on each load run through the process. The size of the test samples should have a width of 3 inches (7.63 cm), minimum length of 3 inches (7.63 cm), and a minimum nominal thickness of 0.032 inches (0.081 cm).

Coating weight

The coating weight should be measured according to ASTM B 137 and must not weight less than 4320 mg/ft² for every 1 mil of unsealed coatings. The frequency of this process control test to be conducted is at least once every month and the number of test samples to be tested is three. The size of the test samples should have a width of 3 inches (7.63 cm), minimum length of 3 inches (7.63 cm), and a minimum nominal thickness of 0.032 inches (0.081 cm).

Abrasion resistance

Abrasion resistance should also be tested on three anodized unsealed test samples at least once every month. The test should be performed according to FED-STD-141, Method 6129. The test samples shall be composed of either 2024-T3 or the alloy used most frequently during the month and should be, whenever possible, anodized on a production load. The size of the test samples should have a width of 4 inches (10.16 cm), minimum length of 10 inches (25.4 cm), and a minimum nominal thickness of 0.032 inches (0.081 cm).

The maximum wear index of 3.5 mg/1000 cycles on aluminum copper alloys with a copper content of 2% or higher. The wear index for all other alloys shall not exceed 1.5 mg/1000 cycles.

Corrosion resistance

The most common corrosion test will be described here, the salt spray test ASTM B117-07 in accordance to MIL-A-8625F.

A lot of the hard anodized parts are not sealed to give the highest wear performance. To get the highest corrosion resistance the parts should be sealed either in hot water or dichromate seal.

A test sample with a nominal thickness of 2 mils must not show any corrosion pits except those within 0.06 inches (1.5 mm) of jigging marks or corners after 336 hours of exposure to neutral salt spray. There should not be more than a total of 15 isolated pits, and the size of the pits should not exceed 0.031 inch in diameter on a test group of samples of five, and not more that 5 isolated pits on each.

This test is the most commonly used, but it is important here to mention that the corrosion test should be application based, especially if the application is used in alkaline environment. The evaluation of the corrosion test should always be in accordance to end user specification.

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