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Anodized titanium, strength and fatigue effect

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Q. Titanium 6al4v anodising:

We should like to chromic acid anodise a titanium casting.

We have been told that this process lowers the fatigue-strength of the product -- has anyone ever heard of this phenomena?

a.c.mulder ten kate
- hoogeveen, the netherlands

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A. It absolutely will, but not much. A couple of %, depending on the process. You are actually removing (converting) metal with any anodize process.

James Watts
- Navarre, Florida

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January 23, 2012

Q. Application is a tensile force on an anodized titanium rod. What is the relative "ballpark" difference in tensile failure strength of the titanium oxide layer versus the titanium base metal? I would think that the oxide layer would fail at less tensile force than the underlying substrate, but does it, and just how much earlier? An approximate factor would be great!

Mike Palatas
Management - Gardena, California

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January 25, 2012

Hi Mike. I can't give engineering advice on an application that I don't understand, and out of my field, but I don't think that's quite the way to approach it. I think the thing to find out may be the allowable strain in the titanium oxide coating, i.e., how much it can stretch without cracking. Then you determine what stress the raw titanium would be under at that same strain. My feeling is you'll probably find that stress level so low that fatique issues won't be a problem. Assuming that is a safe stress level for the titanium under the cyclic loading conditions, multiply it by the net section of raw titanium to determine the load.

Regards,

Ted Mooney, P.E. RET finishing.com Brick, New Jersey