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Fatigue strength loss due to hard anodize treatment

I'm looking for published data on fatigue strength loss from hard anodize treatment of 7075-T6 aluminum. The rule of thumb that I keep running into (including on this website) is %30. Can anyone direct me to a text book or specific test results to elaborate on this number? I have a high stress medical application where raw aluminum parts survive the required life, but anodized parts fail prematurely. At this point in the design cycle I'm hesitant to change geometry or plating processes unless absolutely necessary. Would lowering the overall thickness of the coating (from .0020" to .0004") increase the life? Would cryogenic processing help?

Ralph Schriver
New Products Mechanical Engineer - Pittsburgh, Pennsylvania, USA
2004

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2004

See The Surface Treatment and Finishing of Aluminum and Its Alloys by S. Wernick, R. Pinner, P.G. Sheasby, 6th Edition, Volume 2, Pages 805-809 and Pages 1102-1104.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

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I am not sure, but would shotpeening improve fatigue strength for aluminium before anodizing.

Khair Shishani
aircraft maintenance - Al Ain, UAE
2004

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2004

I just got a book-Russian-corrosion and protection of aluminium alloys-V.CS.Sinievskie=Moscow1986-page308. bending test Al -B95T1 An alkali etching lowered fatigue resist up to 40%,after anodizing in H2SO4 FOR 3,5,AND 10 MICRONS THICKNESS,FATIGUE RESIST WENT DOWN FROM THE ,AFTER ETCHING CONDITION,TO 10,30,AND 40%.

HOPE THIS WOULD HELP.

Khair Shishani
aircraft maintenance - Al Ain, UAE

Ralph, reducing the thickness of the hard anodize layer will help, as it reduces the size of the initial crack that starts your fatigue problem. Plus I concur with the shot peening suggestion, although it may make your finish look different.

It has also been my experience that 6000 series alloys are better in fatigue than 7000 series, even though the strength is lower, as they have less second phase particles. Of course, 6061 may not have the required initial strength.

Good luck!

Lee Gearhart
metallurgist - E. Aurora, New York
2004