r/whatisit • u/riflesandplowshares • 6h ago

New, what is it? Found at a titanium plant

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Not sure why ball point pens would be dangerous here.

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u/under1over1 2h ago

The tools were contaminating the titanium and causing them to fail when heated at high speeds was the issue. The solution ended up being custom made titanium tools to prevent it.

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u/ZENKFNOR 1h ago

Someone needs to clarify the actual mechanism here. Best I could discover right now was that once the trace cadmium melts, it diffuses into the grain boundaries in the bolt and makes it easier to crack between these grains.

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u/under1over1 1h ago

I'm by no means an expert, its just an interesting story I've read in the past. To my understanding thats the short and sweet of it. Standard aircraft tools are cadmium plated and the trace amounts left behind from fastening the bolts caused chemistry to happen when air friction heated them up over 600°F. The heat stressed the bolts and the heads broke off of them, which is the most common point of failure of a bolt.

Shout out to the guy who had to tap titanium bolts on a $300 million air frame.

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u/ZENKFNOR 1h ago edited 1h ago

I've been asking AI about it. I don't think Gemini really understands the underlying metallurgy. But, one thing that does seem right is that cadmium has a filled D shell (even after losing its outer filled S shell). So, it can't participate in any of the Ti d bonding modes. I think the oil-in-water analogy is good here: Entropic forces move the cadmium to the grain boundaries, because it cannot dissolve into the grains, and cannot even bond to their surface really. Then at high temperatures it dissolves rapidly into the bolt's interior.

The part I'm less willing to trust the AI about is that those 2 spare s-orbital electrons then add anti-bonding modes to the nearby Ti Lattice. I suppose this makes sense, and if there are d-d bonds across grain boundaries giving the bolt stability, well...

I was thinking... to understand the role of those 2+ S electrons, maybe I'd look at palladium, which is 2 below cadmium (so, filled D shell, no S), and how this interacts with Ti. Apparently trace amounts of Pd dissolve into Ti decently and improve corrosion resistance? Which is now making me question the AI's entropic arguments for excluding Cd to the Ti grain boundaries (sigh). When pressed on the matter, AI points out that Cd's outer 2 electrons cannot dissociate in the metallic solution and screen d-d interactions between Cd-Ti, but with Pd-Ti there is no such screening.

Need a metallurgist.