Ti6Al4V vs. Aluminum Alloy in steam sterilization

[BiPolarMoment]

I've been searching for an answer to this with only finding a few hints and little else.

I'm in the process of laying out a steam sterlization case and generally the individual components (SS instruments, titanium implants) are segregated from other metal contact thru either silicone or vinyl coated brackets. Most of our previous cases employ a screw rack that is vinyl coated aluminum--presumably for both aesthetics and galvanic corrosion protection.

This particular project is low-quantity, so in the interest of possible cost reductions the manufacturer suggested creating this part out of bare (no vinyl coating) aluminum alloy sheet (I'm not sure which, I'd guess 60xx)--possibly anodized if that was preferred. My question is--these titanium screws will be in minor contact with this rack during multiple steam sterilization cycles at the hospital (270 deg. F for 6 min.); what sort of corrosion should be expected?

From what I've read the aluminum should corrode preferentially but I've also read that the titanium could absorb some hydrogen in the process and eventually suffer from hydrogen embrittlement--but in both those cases I believe the environment was highly acidic.

No corrosion is obviously preferred; the Titanium needs to retain it's material properties even if the aluminum slowly erodes to nothing.

 

[CoryPad]

If the titanium and aluminium are both passive, then I don't think 6 minutes will generate noticeable corrosion.

For the aluminium, use a low copper concentration to minimize corrosion.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[BiPolarMoment]

Thanks Cory, however my concern isn't much with one cycle as it is with (possibly) tens or maybe hundreds. The screws in the rack are used and replaced but the less commonly used ones may persist for months. The amount of corrosion in the aluminum is mostly immaterial (heh), except as it pertains to hydrogen absorption by the titanium due to the reaction. Negligible doesn't necessarily mean acceptable if I use that term to QA. The primary concern is that compromised material performance of the titanium causing a failure during/after surgery (that's what they're for BTW) in which we identify hydrogen embrittlement as root cause and then get questions from the FDA on our packaging (or how we didn't test an intentionally embrittled screw as "worst case").

I personally agree with you based on the likelyhood of this having occured in the past due to vinyl wear and tear exposing aluminum on our different [old] existing racks and the [to my knowledge] absence of any failures in a mode that would be described by brittleness at the area of contact with the rack--doesn't eliminate the concern, however.

 

[CoryPad]

What is the temp/humdity cycle? If it is constantly 270 [°]F and 100% humidity, then maybe you might see some corrosion at contact points (although I don't think it will be enough to cause sufficient hydrogen to be generated and absorbed into the titanium pieces). If it is 6 minutes at temp/humidity followed by ambient for a long time, I don't think there will be much of anything going on.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[sreid]

If you hard anodize the aluminum, you are unlikely to ever have electrical contact with the titaniun.

 

[BiPolarMoment]

sreid: this is precisely what I was thinking as well, but I have been unable to find sources (thru google mind you) suggesting alumina as an electrical insulating barrier in galvanic couples. Probably because in most situations there's abrasion/fasteners involved that mitigates it's usefulness, but that isn't really present in this case.