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Hardcoat problem on 6061-T6 tubing

I have an ongoing problem with HardCoating on the I.D. OF 6061-T6 alum. cylinders.(I.D. Size is 10.5") The Problem: A percentage of the parts have a vertical line through the HardCoat, parallel the grain flow. The lines are approximately .625 - 1.0 inches in length and are a knife edge wide. The HardCoat tapers to the knife edge. The marks are random. Some parts only one mark while a couple of parts have two marks. The Anodizer does not tool off the I.D. The Anodizer had said that this has always been a problem. The material is 6061T-6 per QQ-A-200/8 The HardCoat process is per MIL-A-8625 / MIL-PRF-8625 [on DLA] , TYII , CL.1 The parts were received in this condition.

ED GAGG
- Westbury, New York, USA
2002

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2002

These are something like stress cracks. Anodizing builds vertically from the plane. If the plane is curved, as the inside of a cylinder, then the vertical growth crashes into the adjacent vertical growth and stress cracks.

Your anodizer can get some minor help by raising his temperature, but the only complete relief is to find the thickest oxide (which will have to be thinner than you get now) and then reduce the thickness to a point where the cracks do not occur.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina