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topic 5562

Fatigue improvement over hard coat anodize


Q. I need to improve the fatigue strength of a hard coat extruded part (6063-T5). I have been told that a Dichromate seal will greatly improve fatigue strength. Is the published studies of fatigue improvement with Dichromate seal on hard coat anodized aluminum?

John Schnell
- Jackson, Tennessee USA


A. You have it backwards, sealing hardcoat aluminum actually reduces its strength, but does however increase its corrosion resistance. If you require the utmost fatigue strength, leave your coatings in the unsealed condition. If you require the highest possible fatigue strength with better corrosion resistance considering using boiling DI water as it tends to have the least amount of degradation of all the commercially available sealing mediums. Nickel products such as nickel acetate and nickel fluoride will only degrade the strength of your finished part. Estimates are anywhere from 15-50% loss of strength depending on sealing medium you use.

Matthew Stiltner
- Toledo, Ohio


A. Wernick, Pinner, and Sheasby's The Surface Treatment and Finishing of Aluminum and Its Alloys does report that a dichromate seal over hard anodize reduces the fatigue strength loss associated with hard anodizing aluminum. As you would expect, they also note that the wear resistance is decreased when the part is sealed. This is on pp 718-720 of the 5th edition.

Sjon Westre
- Minden, Nevada

July 2013

A. Hi John S. I think Matthew may have confused fatigue strength with wear resistance. While sealing does soften a hardcoat and reduce it's abrasion resistance, apparently the right sealing can restore a portion of the loss of fatigue strength which hard anodizing causes. If anyone has the original reference that would be great! Good luck.

The quote in Wernick, Pinner, and Sheasby goes:
quote from pinner 1

quote from pinner 2


Ted Mooney,
Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey

July 31, 2013

A. For fatigue strength, try 7075 Aluminum and electroless nickel plating.

John Halonen
- Minnetonka, Minnesota

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