On 11/21/2009 04:03 AM, Erik Christiansen wrote:
> On Fri, Nov 20, 2009 at 05:10:18PM +0000, Leslie Newell wrote:
>> Carbon dioxide is 66% oxygen (CO2). As aluminum is very active it will
>> strip oxygen out of the CO2. That is also the reason why you should
>> never use a CO2 fire extinguisher on magnesium fires.
>
> However, it is extensively used in fire extinguishers precisely because
> it does not give up its oxygen even at hundreds of degrees C. I'm not
> sure of how many thousand degrees magnesium burns at, but it is more
> than 1500, because thermite (magnesium and iron oxide) combustion melts
> the iron produced by the reduction of the iron oxide.
>
> If the cutting operation is hot enough to dissociate CO2, then there'd
> better not be any oil about, especially as mist, unless Gene has his
> detonation-deadening earmuffs on tight. ;-)
>
> Wikipedia appears to be self-contradicting:
>
> "Carbon dioxide also finds use as an atmosphere for welding, although in
> the welding arc, it reacts to oxidize most metals."
>
> The closest I've come in a quick search is:
>
> http://www.hitech-inst.co.uk/pdfs/technical/heat_treatment.pdf which
> says:
>
> "The graph shows cell output against carbon monoxide/carbon dioxide
> ratio. this is plotted at 634°C and 812°C, ..."
>
> i.e. CO2 is still so completely undissociated at 812°C that ratios can
> be measured for metallurgical analysis. So the aluminium would melt
> long before the CO2 dissociated to any measurable degree.
>
> Erik
This may true in theory, but not in practice. Welding Aluminum with
a CO2 or AR/CO2 mix will give bad results. Guess how I found that out.:)
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