# Aluminum flux + cast iron crucible



## MysticColby (Feb 22, 2012)

So, I've been casting aluminum from a cast iron pot for a while now. Every now and then, a small hole forms in the crucible and needs replacing (looks like extra-intense heat right there, along with oxygen to expedite rusting - it flakes away at the hole).
Anyways, the second-to-the-last time I poured a casting, I added a little flux and degas from BCS 
http://www.budgetcastingsupply.com/Aluminum_Flux_Asbury.php and 
http://www.budgetcastingsupply.com/Aluminum_Degas_Tablets.php
Maybe 2 tablespoons total. The metal was especially gorgeous, and the dross floated like balloons on a pool and clumped together. Good results. The last time I went to melt metal, there was still some dross/flux left in the pot, and I added some scraps of aluminum, put on fire (coal furnace), and waiting for it to melt before adding more metal. When it looked molten, I added some more aluminum, then a bit more, then realized there was hardly any metal in the pot. I swear under my breath that there's a hole. I pick up the pot with tongs to salvage what I can (though I designed the furnace to allow for this - the metal simply drips through the coals and a grate and falls to the ground beneath), and found that the entire bottom of the pot had detached from the sides. I used tongs and was able to pick out small pieces of the bottom from the pot of the furnace, but for the most part it had dissolved into nothing. The pot wasn't new, but it was still in good condition.
My question: flux is supposed to bind oxides. Might it of bound the iron oxide, expediting rusting and effectively dissolving the bottom of the pot?
flux Ingredients: Sodium Fluorsilicate, Sodium Carbonate, Sodium Chloride, Potassium Chloride
degas ingredients: hexachloroethane, sodium nitrate, quartz (degas)
If I want to keep using this flux, do I need to switch to a graphite crucible?


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## Harold_V (Feb 22, 2012)

I'm mystified why anyone who hopes to cast aluminum with success would even consider using a metallic vessel as a crucible. You should know, at least by now, that aluminum absorbs iron, which alters its structure, and not for the better. 

If you must use a metallic vessel, the least you can do is coat it with a refractory. My personal choice would be to switch to a refractory crucible, either graphite/clay or silicon carbide, assuming it's compatible with molten aluminum. 

Harold


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## element47 (Feb 22, 2012)

I am by no means an expert on this, but once, I DID use a flux with an iron melting pot. This was a solder pot, so the temperatures are a lot lower than they would be to melt aluminum. I only used "solder-type" flux, which I believe is pine resin, not borax or anything like the chemically-reactive flux one would use for higher-melting-point metals. (meaning higher than lead or tin) With a solder pot, you are not supposed to use flux, you are supposed to dip the wires in flux

The iron pot immediately fell apart. Immediately. It cracked, it opened up, it failed in about 3 different ways. Iron is quite a reactive metal when it gets right down to it.


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## MysticColby (Feb 22, 2012)

After joining here, I had that same thought, but I've been using cast iron as a crucible for almost 2 years now and iron absorption has never seemed to be a problem (plus, it seems all alloys of aluminum have ~0.5% iron in them). The pots have always worn from the outside in - I've never seen evidence of the inside deteriorating (the top lip looks the same as the bottom once all the slag is removed). I usually get about 30+ melts in one pot before the coal melts/oxidizes a hole through the bottom. Thinking back, the aluminum doesn't even stick to the sides, only the slag does. I think somewhere on here it says to never use cast iron crucible as it dissolves in molten metal, except for melting aluminum (and maybe lead? I was only concerned with the aluminum, so I don't remember the others).


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## MysticColby (Feb 22, 2012)

element47 said:


> Iron is quite a reactive metal when it gets right down to it.



hehe, yeah. I'm sure I'm not the only one here who's made thermite :mrgreen: 
kinda funny to think how close I come to that reaction each melt - but the rust is never on the inside of the crucible, phew.


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