On Saturday 21 November 2009, 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
True, but those temps are not the temps of the arc, by about a factor of 10. My single sample that was badly decomposed, blackened and porous, I blamed on the 75% argon/25% co2 in the shielding gas I had. I theorized that the o2 was used up by the alu's reaction, leaving the c to blacken and contaminate the alu puddle. I resolved to get some purer argon, but locally it is not available. The mixed bottle, about 4" in diameter and maybe 18" tall, is $125 at TSC. Samson's in Clarksburg are similarly priced. And that makes it a very expensive toy for me. One that I only drag out when there is no other choice. -- Cheers, Gene "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) The NRA is offering FREE Associate memberships to anyone who wants them. <https://www.nrahq.org/nrabonus/accept-membership.asp> Power corrupts. Absolute power is kind of neat. -- John Lehman, Secretary of the Navy, 1981-1987 ------------------------------------------------------------------------------ Let Crystal Reports handle the reporting - Free Crystal Reports 2008 30-Day trial. Simplify your report design, integration and deployment - and focus on what you do best, core application coding. Discover what's new with Crystal Reports now. http://p.sf.net/sfu/bobj-july _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
