Hardness of Hardcoat Anodize


What would be the hardness of the Alcoa Alumilite 226 hardcoat finish? The substrate is 6061 alloy aluminum tubing. G.S.

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If you follow the manufacturer’s processing guidelines for this finish, I would expect the Rockwell ‘C’ scale hardness to be somewhere in the range of 50–68. This is in the same range as the hardness of nitrided steel. For the curious, Rockwell ‘C’ 50 –68 converts to Vickers or Diamond Hardness of approximately 500–942. The Knoop Hardness equivalent is 530–920. Incidentally, Rockwell ‘C’ of 68 is the top of the scale.

The processing guidelines for the Alumilite 226 process are sulfuric acid concentration of 12 wt.% and 1–2% oxalic acid. This is the same as 130–140 g/L sulfuric + 10–12 g/L oxalic. Of course, percent is by weight, not by volume. The temperature of the anodizing bath is 50°F (10°C) + 2°F (+ 1°C). Current density is 36 ASF, or 3.9 A/dm2.

Anodizing under these conditions will yield 1 mil (25 ?m) of anodic film thickness in 20 min, 2 mil in 40 min and so forth. If you’re anodizing high-copper aluminum alloys (2000 series), expect slightly less coating; perhaps 20% less with these processing conditions.

In general, if the coating is to be sealed, expect hardness up to 30% less than for unsealed coatings.


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