Summary for fatigue improvement over hard coat anodize Discussion
fatigue improvement over hard coat anodize is discussed as a tradeoff between fatigue strength, corrosion resistance, and wear resistance. John Schnell asks whether a dichromate seal can greatly improve fatigue strength on hardcoat anodized 6063-T5 extrusions. Matthew Stiltner replies that sealing hardcoat aluminum generally reduces strength, though it improves corrosion resistance, and says unsealed coatings give the highest fatigue strength; he adds that boiling DI water is the least damaging commercial seal and that nickel acetate or nickel fluoride seals can reduce strength by 15-50%. Sjon Westre cites Wernick, Pinner, and Sheasby, saying the book reports that a dichromate seal over hard anodize reduces the fatigue-strength loss caused by hard anodizing, while also reducing wear resistance. A follow-up note suggests Matthew may have confused fatigue strength with wear resistance, and that the right seal can restore some fatigue strength lost to hard anodizing.
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
2000
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
2000
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
2000
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:
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com – Pine Beach, New Jersey
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July 2013
A. For fatigue strength, try 7075 Aluminum and electroless nickel plating.
John Halonen
– Minnetonka, Minnesota
July 31, 2013