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Comment by mjb | Hacker News Reader

Titanium fires sure are scary. But there's a good amount of chicken and egg here: expensive material limits demand, which limits progress on manufacturing techniques, which keeps part prices high. I would expect that significant manufacturing method progress would be made if there was a step change in the price of titanium stock.

And I wouldn't overstate the machining difficulty. Sure, it's a pain in the rear, and expensive, but can be done on regular machines with the right tools, techniques, and processes. I've made a couple of titanium parts myself.

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There’s a significant history of government effort to improve working with titanium. Construction physics wrote a nice review [0].

The current level of workability and cost and alloying is after that chicken and egg. Titanium is expensive because it is hard to manufacture, not just hard to work with, which limits demand. Titanium, to what we now know, is what it is. It’s the nature of the material not a lack of investment.

More realistically, the ROI isn’t there for most applications. Good aluminum is pretty darn good, massively easier to work, cheaper, etc. newer super steels have even made serious inroads on titanium parts because of workability and toughness.

[0] https://www.construction-physics.com/p/the-story-of-titanium

Magnesium is similar.

I used to have a magnesium campfire starter. It was a little ingot of magnesium, with a long flint, embedded along one side.

You used your knife to shave some magnesium, then the flint, to set it ablaze.

Worked a treat.

Titanium - Chlorine fires are even more magnificent than titanium-oxygen fires. Wet chlorine (>150ppm water) is too corrosive for ferrous metals and titanium is often used for pipes carrying wet chlorine.

If something happens that ignites one of these pipelines there’s absolutely no way to put it out - it has the fuel (titanium) and oxidizer (chlorine) and burns mega-hot until one of them is fully consumed along the entire length of the pipeline. The pipelines can sometimes be shockingly long (1 mile-ish).

But there’s also the base chemistry: titanium doesn’t behave like steel, and the chemical differences are why it is such a pain to work with, not inexperience.

The chemical difference between titanium and steel is mainly that titanium has a much higher reactivity with oxygen and nitrogen, the main constituents of air.

Like with aluminum, this high reactivity is masked in finite products made of titanium, because any titanium object is covered by a protective layer of titanium dioxide.

What is worse in titanium than in aluminum is that titanium has a low thermal conductivity, so a small part of the titanium can become very hot during processing, which does not happen with aluminum, where the remainder of the aluminum acts like a heatsink.

The hot spots that exist on titanium during processing, which do not exist on aluminum during processing, make titanium much more susceptible to reacting with the air or even to starting a fire.

Titanium, even as "commercially pure", has a much higher strength than aluminum, which requires higher forces for machining and increases even more the chances for overheating.

> Like with aluminum, this high reactivity is masked in finite products made of titanium, because any titanium object is covered by a protective layer of titanium dioxide.

My understanding is that rust fails to protect iron the same way. Is that right? If so, why the difference?

Rust can protect iron in that way, bluing is a common process to create a protective rust coating. However rust is fragile and often flakes off thus allowing the process to continue. Other metals their oxide is strong enough to protect the pure inner layers.

This depends on the alloy involved as well. In general though rust is not a good iron protection.

Once rust starts, it is porous & flaky and allows more oxygen to infiltrate and hit the next layer of iron. The reason it is porous & flaky is due to creating a mix of FeO and Fe2O3 which have different crystal structures so it doesn't create a nice protective barrier.

Yes, it is right. The difference is that in the case of aluminium and titanium (but also stainless steel), the oxide grows in a uniform way, covering all the metal. These protective layers are very thin and act as barriers stopping oxygen from reaching the metal underneath.

In case of iron, oxidation occurs at different points on the surface and the oxide layer initially leaves most of the metal exposed. The oxide is also not effective at stopping oxygen, so the rust layers keeps growing until it forms flakes that fall, exposing more of the metal. The process repeats until all the metal is consumed.

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