[marwan]

Hi guys,

I would like to know what happens to the Aluminum oxide when it is emerge in a high pressure and temperature CO2.

I know it will corrode because of the high temperature, but what the effect of the high pressure.

Thanks in advance
Best
Mar

[Enthalpy]

Hi Marwan, welcome here!

That's hard to tell, because corrosion is a dynamic process, not a matter of equilibrium, and is essentially experimental.

For an equilibrium, high pressure would have favoured the reaction of CO₂ with the metal, because this reduces the volume.

Most impurities (carbon as well?) make aluminium's oxide layer less protective.

Do you really need aluminium? You say "high temperature", but aluminium is usually not chosen above +200°C and even less. Then, can you choose the alloy, like AA5083 or AA1080? Can you grow a thick and densified oxide layer by anodization prior to use?

[marwan]

Hi Enthalpy,

Thanks for your replay. As I know (since I am a mechanical engineer not chemical),  Aluminum Oxide is a ceramic and it can be stable in the room temperature, but I do not know about the high temperature.

I have done so much research about this, bit know enough information. I think I have to go a head and do an experimental.

Best

[Enthalpy]

The layer of aluminium oxide is extremely stable at heat, but it may not protect the metal beneath.

Heat lets oxygen diffuse more easily through the oxide. That's one reason to densify the oxide in water vapour, so the layer is thicker and more compact right from the beginning. You may try to anodize a very good alloy like AA5754, AA5083 or AA1080.

Though, I fear hot CO₂ will let the layer fail, as do so many compounds.

Experiment, sure. I've never seen data for CO₂.

[curiouscat]

Can you be more specific? What's the application?

What temperature? What pressure.

[Enthalpy]

Without additional details, we might even imagine a Magnox nuclear reactor
http://en.wikipedia.org/wiki/Magnox
meant to produce plutonium from natural uranium, a badly proliferating technology.

[marwan]

curiouscat, thanks for replay. they are 650 C and 20 MPa. There is no application just a study.

[Enthalpy]

Metallic aluminium is not useable at 650°C. That's a simple case.

Pure aluminum melts at 660°C. Alloys use to melt before. Anyway, the metal softens far before its melting point. Mechanical uses stop before 200°C for the best alloys. It's not even a matter of oxidation.

650°C is a high temperature for metals. If needing no mechanical property you might try some very special steel formulations, perhaps the NiCrMoTi 25-15 (I didn't re-check). Allegheny gives 146MPa yield strength for the CrNi 22-05 but over a short time; creeping should kill it over time.

650°C is a typical temperature for nickel alloys, like Inconel 718 and all the variants and copies developed for gas turbines. Water vapour shouldn't harm them; extensive data exists.

[marwan]

Enthalpy,
I really thank you so much for all the useful information. I just started this topic. Let me go deeper and I am sure I need your help in the future.

Thanks again

Marwan