Metallurgists Wanted. The Effect Of Aluminium Cans On A Cast Iron Fire Grate?

[Vimto]

I have an open fire in my lounge with a cast iron fire gate in which the fire burns. I noticed some years ago that empty aluminium cans when put on the hot embers of a coal fire degrade, the can, first the sides and then the ends. The ends lastly as they are thicker. All that remains is a very small amount of ash. This seemed a satisfactory way of disposing of them. Over a period of a few months the cast iron grate on which the fire sits tends to degrade itself and the bars become weak and brittle and finally drop off. This only happens when I have burnt aluminiun cans. If no cans are put on the fire then the grate will last many years.

What is happening here?

[Ellipsis]

Probably the temperature at which the aluminium burns is not good for your cast iron ...

[douglas9401]

It's a message from beyond telling you to recycle.

[Hopkirk]

I don't know the answer, but I would urge you to recycle your aluminium cans. Far better for the environment.

[Vimto]

Yes ellipsis, I rather gathered that but what is the chemistry involved or maybe it's a question of temperature.

[Hopkirk]

It could be a small scale version of how they produce iron in a blast furnace.



Thermite reaction
The blast furnace is used to produce iron on a large scale. Sometimes, it is necessary to produce a small amount of iron more quickly, for example if railway workers need to produce molten metal to fix a broken rail.

The thermite reaction uses aluminium powder and iron(III) oxide. When ignited, the mixture reacts vigorously because of the large difference in reactivity between aluminium and iron. The heat produced in the reaction melts the iron produced.

Iron(III) oxide + aluminium → iron + aluminium oxide

Fe2O3(s) + 2Al(s) → 2Fe(l) + Al2O3(s)

In this reaction, iron(III) oxide loses oxygen to form iron - so iron(III) oxide is reduced. Carbon gains oxygen to form carbon dioxide - so carbon is oxidised.

[ToraToraTora]

yep, I think Hoppy has it!

[Vimto]

Yes, I agree. Well done Hopkirk and I do now recycle my empty cans as advised if only to save me purchasing new grates every year!

[douglas9401]

Grate news, Vimto.

[Hopkirk]

I'm not sure if my suggestion is the answer.

The thermite reaction converts iron oxide (aka rust) into iron.
That doesn't explain the loss if iron from the grate, it should rejuvenate it. I suppose though that the added heat might melt away more of the grate.

[ToraToraTora]

I agree it's not conclusive but aluminium is an alkali metal and that will in the right circumstances create a thermite and that could explain it. maybe completely wrong but seems feasible on the face of it.

[theprof]

Cast Iron is carbon rich compared to standard steel. Typically it has a content of about 4% carbon. However, cast iron is not just an alloy of iron and carbon. It contains silicon as well in the type of cast iron we are talking about here, but suffice to say, I won't discuss the different grades of cast iron here.
Anyhow, molten aluminium in contact with the cast iron creates intermetallic compounds in layers at the interface, typically made from Fe2Al5 and FeAl3. The aluminium diffuses into the cast to produce areas that are high in aluminium. An exchange occurs with the silicon moving upwards into the intermetallic compound layers while the aluminium moves down.Now the next part is very complex but suffice to say that aluminium carbide, Al4C3, is produced is produced in the grate bars where the grate surface has been in contact with the aluminium. This aluminium carbide is extremely brittle and degrades readily due to the crystalline structure involved and is the reason the bars become weak and brittle and eventually drop off.

[theprof]

Many moons ago, I took one of the popular "sandwich" chemistry degrees and I elected to spend the placement in a metallurgy lab in a British Steel Corporation works. The metallurgists there were first rate guys and taught me a lot which has been useful now and again.

[theprof]

I've just re-read what I said above and think that I need to clarify that exchange business. What happens at the near atomic level is that small tubes are created between the interface of the molten aluminium and the grate. The silicone atoms have 14 protons in them whilst the aluminium have 13 and because of this and the atomic configuration( don't ask!), the silicon atoms go up and the aluminium atoms go down on more or less the same pathway effectively replacing the silicon sites.
That 13 and 14 are the the atomic numbers of aluminium and silicon respectively and the two are next to each other in the periodic table. It's a straightforward swap.
That concludes today's lesson on atomic structure (Who said thank God!) It's time for the port.

[Hopkirk]

I knew that

[theprof]

Hopkirk, I'll keep you in mind next time I need a right hand man lol

[Gizmonster]

30+ years ago, I could have probably given you a very thorough explanation (I graduated with a degree in Materials Science / Metallurgy) ..... however, that was then ...... my career path has since taken a VERY different route and as such, to quote Manuel from Faulty Towers, I know nothing !!