[TheForge] a question which probably has a simple answer

[Bruce .]

I'm no expert in magnesium or its casting.  I've done a very little bit of
aluminum casting.

You are right that there's be an oxide coat on magnesium -- it's just too
reactive a metal for there NOT to be one.  Quothe Wikipedia, "It tarnishes
slightly when exposed to air, although unlike the alkali metals, an
oxygen-free environment is unnecessary for storage because magnesium is
protected by a thin layer of oxide that is fairly impermeable and difficult
to remove."

Magnesium metal conducts heat much better than does iron, and almost as
well as does aluminum.  It's ignition temperature is nearly 900 F.  Bottom
line -- it takes some doing to get a large block of aluminum up to its
ignition temperature, even if the oxide coating is penetrated.

Note that to use one of those little fire-starter magnesium blocks, you
first shave off tiny pieces with a knife.  The tiny pieces can be got to
ignition temperature moderately easily.

As to the casting question, I'd wondered the same for aluminum, which is
also quite reactive.  The answer (for aluminum) seems to be that the heat
of oxidation is dissipated through the melt so fast that it makes a
negligible contribution to the temperature of the melt, and, the oxide
layer forms a protective coat even on the melt.  I don't have numbers
handy, but the molten metal may be more thermally conductive than the
solid, because the atoms are free to move around in the melt, so, I expect,
the temperature in the crucible would tend to be fairly uniform for
aluminum or magnesium.

When one casts aluminum, one should not stir the melt (much) as this
exposes the molten aluminum to air, producing slag (oxide - and maybe
nitride?)  Ideally, one would melt aluminum under an inert atmosphere (like
helium or argon, NOT under CO2 or N2, which are reactive under these
conditions).  In practice, some pretty nasty fluxes are used -- fluorides,
for example.  (In the case of aluminum and magnesium, floating ordinary
"glass" flux atop the melt may not be feasible because the glass may be
denser than the molten metal.)








On Tue, Oct 1, 2013 at 11:19 PM, terry l. ridder <terrylr at blauedonau.com>wrote:

> hello;
>
> a 16 yrs old young man was helping with various projects to earn some
> spending money. his parants are locked in a bitter divorce and being
> here gives the young man time away from the combantants.
>
> we were discussing small engines and i happened to show him two engines
> which have magnesium engine blocks. He pondered that for a moment and
> asked the question:
>
> "What keeps the magnesium from bursting into flames like in chemisty
> class?"
>
> his next question was:
>
> "How do they melt it to make the castings without it bursting into
> flames?" It is melted and poured either in a vacuum or inert atmosphere
> like argon, xenon, neon, etc.
>
> I may have known the answer at one time but have forgotten it over the
> years. I assume that it is passivated with an oxide which has a high
> ignition temperature.
>
> any one have a different or better answer.
>
>
> --
> terry l. ridder ><>
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-- 
Bruce
NJ