Does a nitrox tank rust more inside?

[Doc Harry]

What was in error?

I posted some info, but when I went back and checked on the information at its source, I was wrong.

 

[ianr33]

.
2. Rust is flammable and is a potential fuel source (rusty steel wool makes great kindling)

Rust is not flammable. The iron in rust is in its highest oxidation state (+3) so will not be oxidised any further.
Rusty steel wool will burn but it is the iron,not the rust (iron oxide,Fe2O3) that is burning.

Rust - Wikipedia, the free encyclopedia

 

[8thElementDiver]

Rust is not flammable. The iron in rust is in its highest oxidation state (+3) so will not be oxidised any further.
Rusty steel wool will burn but it is the iron,not the rust (iron oxide,Fe2O3) that is burning.

Rust - Wikipedia, the free encyclopedia

Ian is absolutely correct about the rust not burning. The chemical reactions in that Wikipedia article about rust, though, are pretty horrid (The key reaction is NOT the reduction of oxygen; in fact, that's a half-reaction, which is not even observable as a real reaction.)

Basically, the chemistry of rust formation is this:
4Fe + 3O₂ + 2H₂O --> 2(Fe₂O₃*H₂O)

Having more oxygen (as from a Nitrox fill, rather than just air) will drive the reaction to produce more rust.

However, the reaction won't start unless there is also water available, and if there is water available to start the reaction, you'll have enough oxygen in the tank to form rust whether you have air in it or Nitrox. Having Nitrox in the tank might give you a bit more rust, but the main problem will be that you have rust in the tank. And that doesn't depend on whether you have Nitrox--it only depends on whether you have water in your tank to initiate the reaction.

So keep your tank innards dry, and it won't matter whether the tank is filled with air or Nitrox--rust won't form.

BTW, I think one big reason to not suck a tank dry is that you lose the audible feedback when the valve isn't completely closed. With some air pressure in the tank, you'll hear the hiss if the valve is partly open and therefore you'll close it all the way. If the tank is drained so it's down to ambient air pressure, you could accidentally leave the valve partly open because there'd be no hiss to warn you. That would allow outside air (with its evil moisture) to get into the tank, and that would lead to rust. But if you suck a tank dry, completely close the valve while your reg is still attached, and take care not to introduce water during your air fills, you don't have to worry about rust forming in a sucked-down tank.

 

[DA Aquamaster]

So when I put a match to really rusty and degreased steel wool, what exactly starts burning? It make superb kindling regardless of the chemistry behind it.

 

[SDAnderson]

So when I put a match to really rusty and degreased steel wool, what exactly starts burning? It make superb kindling regardless of the chemistry behind it.

There's a big difference between mostly rusty and all rusty.

There's still a lot of unoxidized iron available. As steel wool rusts, the molecular bonds break down and it loses it's structural integrity. "Really rusty" steel wool will be a pile of dust on the table, not a wad of red-brown fuzz. What you're burning is "really rusty, sort of."

 

[scubadude79]

I agree with the rust not being able to burn.
While it is true that oxygen is required for combustion, it is also true that fuel (in this case iron or aluminum with lack of oxidization) will bond with the oxygen to a degree that it can no longer accept any more oxygen molecules thus becoming rust and inflammable.

Back in high school chemistry, we did this exact experiment with steel wool. We introduced flame in an O2 rich environment with new, rusty, and completely rusted (powder consistency as reefraff mentioned). The new and rusty still burned, but the completely rusted steel wool did not.

 

[ianr33]

IF rusty steel wool burns more easily than nice clean shiny steel wool then my GUESS would be that the iron surface has been roughed up by the rust. Bigger surface area,faster rate of reaction.

Another guess would be that nitrox tanks would rust faster than tanks filled with air but it depends which step of the reaction is the "Rate Limiting Step" If the hydration (adding water) is the slow step then it would make no difference if it was 1% O2 or 100% O2

Ian (Ph.D. Chemistry )

 

[DA Aquamaster]

There's a big difference between mostly rusty and all rusty.

There's still a lot of unoxidized iron available. As steel wool rusts, the molecular bonds break down and it loses it's structural integrity. "Really rusty" steel wool will be a pile of dust on the table, not a wad of red-brown fuzz. What you're burning is "really rusty, sort of."

Ok then, unless my tank is totally reduced to a pile of rust (in which case no one is going to certify it as O2 clean, even though it oddly enough won't burn (ior hold gas)) then the mostly rusty areas of the tank may have iron that is more prone to burn.

Basically, we are picking at nits and rust is still bad in an O2 clean tank and at least indicates a potential source of fuel.

 

[spoolin01]

IF rusty steel wool burns more easily than nice clean shiny steel wool then my GUESS would be that the iron surface has been roughed up by the rust. Bigger surface area,faster rate of reaction.

Another guess would be that nitrox tanks would rust faster than tanks filled with air but it depends which step of the reaction is the "Rate Limiting Step" If the hydration (adding water) is the slow step then it would make no difference if it was 1% O2 or 100% O2

Ian (Ph.D. Chemistry )

I'm guessing stoichiometry is the significant effect that is limiting rate. A little moisture gets in, rusting begins. It goes faster with higher O2 but eventually stops because the moisture is used up. Since all the of several electrochemical reactions inolving iron and oxygen benefit rate-wise from water, the rate of any particular formal hydration step isn't limiting in the sense of it being slow despite it being present, but in the sense of, there's no more water to participate. If EAN tanks rust more quickly than air, it should either be because for the tank fill/drain cycle, the process doesn't go to completion for the amount of water present (say in regularly cycled tanks where additional water is introduced) and the rate acceleration of higher O2 is therefore noticeable, or because the rate slows so much as the water is reacted away that the rate acceleration of higher O2 is noticeable, (or both, or because EAN fills are wetter, if that could be...). (just trying to milk some value from that esoteric over-priced education)