Rust in a sealed tube.

[Gritineye]

Need some input of wisdom to settle an argument, can anything more than a tiny amount of surface rust form inside a small diameter (say 1"-3") tube that is hermetically sealed at both ends?

I've never seen it myself and I think the reason is because all the oxygen and water vapour within is converted into minute surface rust.

Others disagree, what say you lot?.

 

[Gareth J]

That'd be my guess. Unless steel itself is somehow minutely permeable to the necessaries.

In reality, a tiny pin hole or a stress crack will draw in fresh air/water as the air inside expands then shrinks as the object heats/cools.

 

[Dee J]

I'd think that in practical terms over reasonable timescales there's likely to be minimal rust. But a truly hermetic seal is questionable - vacuum engineers pay a lot of attention to grain structure in metal because of gas permeability along the grain of the metal.

 

[julianf]

Remember, the reason why your radiators dont just rust through is not because they're not wet, but because the water has no oxygen in it.

Rust is ferrous oxide. Loose the oxygen and there is nothing to make the oxide from.

What I mean is, if you only have a handful of oxygen molecules, you are only going to make as many Fe2O3 molecules as they allow.

If there's no wormholes from other dimensions or porosity, that will then be that. All oxygen converted to rust, and then it stops.

 

[Maker]

I'd think that in practical terms over reasonable timescales there's likely to be minimal rust. But a truly hermetic seal is questionable - vacuum engineers pay a lot of attention to grain structure in metal because of gas permeability along the grain of the metal.

That's the kind of thing only considered in the absolute highest vacuum systems, if all the oxygen in the container were converted to rust, the pressure would only drop to about 80KPa which is barely a vacuum.

I don't think a 20KPa is enough to squeeze molecules along grain boundaries, especially at room temperature (High vacuum systems are "Baked" to shift volatile contaminants faster), but assuming it is (And the argument is purely academic ) , missing oxygen is replaced with air containing only 21% oxygen, so after however long that takes, there's only 4% O2 in there, which gets used up and replaced with more air bringing the fraction down to less than 1%, clearly this geometric progression approaches zero. After millions of years, assuming the container hasn't rusted through from the outside, or a meltdown at one of Fizzy's nuclear power stations hasn't destroyed it (), the tube is left slightly rusty inside and contains pure nitrogen, plus a bit of CO2 and argon.

 

[Gritineye]

Thanks chaps.

 

[Brad93]

Condensation Johnny

 

[bricol]

Cutting open a Dutton kit car chassis from the early 80's last year didn't reveal much rust, surface or otherwise, and I can't imagine it was used or stored under ideal conditions in the intervening years. So assuming it was welded up on a nice hot dry day so even the risk of condensation was low, your theory sounds okay to me.

 

[Ross365]

Rust tends to be a bag of worms, with lots of Fe2O3 but a bit of FeO and some quantity of iron hydroxide (OH) in one form or another.

Some expensive or otherwise special vehicles are put in to storage in huge plastic bags which have an inner layer of aluminium on them (a high-grade version of a crisp packet?) and the ali minimises diffusion of oxygen and water in to the bag.

I was looking at a picture of a museum display of a bar of angle iron half embedded in a hefty block of concrete, but they must have left some intentional holes to let the iron rust (and perhaps fed it with water), and the jacking effect of the rust (1 volume of iron gives ~2 volumes of rust) has cracked the concrete wide open.

 

[Cobbler]

I was looking at a picture of a museum display of a bar of angle iron half embedded in a hefty block of concrete, but they must have left some intentional holes to let the iron rust (and perhaps fed it with water), and the jacking effect of the rust (1 volume of iron gives ~2 volumes of rust) has cracked the concrete wide open.

look what happens to reinforced concrete when the rebar goes rusty, can break it wide open

 

[Maker]

I was looking at a picture of a museum display of a bar of angle iron half embedded in a hefty block of concrete, but they must have left some intentional holes to let the iron rust (and perhaps fed it with water), and the jacking effect of the rust (1 volume of iron gives ~2 volumes of rust) has cracked the concrete wide open.

 

[jimbo84]

Remember, the reason why your radiators dont just rust through is not because they're not wet, but because the water has no oxygen in it.

Rust is ferrous oxide. Loose the oxygen and there is nothing to make the oxide from.

What I mean is, if you only have a handful of oxygen molecules, you are only going to make as many Fe2O3 molecules as they allow.

If there's no wormholes from other dimensions or porosity, that will then be that. All oxygen converted to rust, and then it stops.

Isn't it because of rust inhibitor added into the system. Water does contain oxygen, H2O.

 

[julianf]

Isn't it because of rust inhibitor added into the system. Water does contain oxygen, H2O.

Nope. It's the disolved oxygen that creates the ferric ( @not done it yet : ) oxide.

The easy way to look at it is which is more reactive, hydrogen or iron?

You're never going to split oxygen off hydrogen using iron. So the oxygen for the rust *must* come from somwhere else, in this case, disolved oxygen.

Incidentally, hot water (as in 80c plus) contains virtually no disolved gas.

Talking about rust, the classic thermite reaction is splitting the oxygen off iron oxide using a more reactive metal (alluminium). Again, there's no chance you can split oxygen off water with iron.

 

[Ross365]

A central heating system has got steel, copper and brass in it and we'd like them to last as long as possible. I'm very happy to pay Mr Fernox for his System Protector additive once every ~3years.

A friend on mine had a small leak form in the copper pipe under his kitchen floor (concrete); bad news! It's not just iron that corrodes.

 

[Seadog]

Particularly important for gas pipes.

You should tell that to the idiot plumber that recently installed two gas meters near to me. And then proceeded to cement them into a trench. No protection at all. I couldn't be bothered having a conversation, let alone the probable argument that might have ensued.