Was RE: [Leica] DSLR choice - magnesium/metallurgy

[abridge at gmail.com (Adam Bridge)]

Most of the corrosion resistant alloys form a very dense layer of
corrosion that protects the rest of the material. In stainless steel,
the only thing I know about except for inconel, the chromium that is
alloyed with the steel forms a very dense and almost impervious finish
to the surface of the material.

In nuclear power plants one would run the entire primary system at
operating temperature and pressures before the core was loaded in
order to firmly establish the oxidation layer. Inconel works the same
way that's an allow of nickle-chromium-iron (600 is the one I'm
familiar with).

Aluminum forms the same sort of layer as well, as I vaguely remember.
The more exotic light metals weren't good for power plant use and so I
don't know diddly about 'em. It's been decades since I worked in this
area though so take it all with the various grains of salt.

Adam


On 1/20/06, Douglas Sharp <douglas.sharp@gmx.de> wrote:
> Thanks Frank,
> so I assume the future will be carbon fibre sandwich materials, resins
> ,glass derivatives or industrial ceramics (TiO2) - or something totally
> new in the way of plastics.
> I read recently that one  company is even already making tripods on the
> basis of basalt- was it Manfrotto?.
> Interesting to read that titanium also "protects" itself with an
> oxidation surface layer, I thought the stuff was resistant to just about
> anything nature could throw at it.
> cheers
> Douglas
>
> Frank Dernie wrote:
>
> > Hi Douglas,
> > It is true that very few metals are the used as the pure element,
> > AFAIK, and certainly the metals I use, are all alloys. Ally wheels
> > for cars are mainly aluminium, though there is a small percentage of
> > magnesium and other alloying elements. Ferrari use magnesium wheels
> > on their road cars but they have to be immediately surface treated
> > and repainted to avoid corrosion if scratched.  "Ally" wheels corrode
> > readily if used on salt de-iced roads, unless painted, but not as
> > fast or badly as magnesium.
> >
> > Here is an excerpt from a materials handbook, on the engineering use
> > of magnesium alloys.
> >
> > "Magnesium, with a density of ?1.7gcm-3 is significantly lighter than
> > most other structural metals. The mechanical properties of the pure
> > metal are however very poor such that only alloys of magnesium are of
> > practical value in engineering applications. Under "ordinary"
> > atmospheric conditions magnesium and its alloys have a good
> > resistance to corrosion due to the formation of a passive oxide layer
> > as happens with aluminium and titanium. Unfortunately this layer is
> > not completely impervious particularly in the presence of damp air
> > containing salts. Corrosion resistance can thus be quite poor under
> > adverse conditions and care must be taken when selecting magnesium
> > based alloys for certain applications.  The low density of the base
> > metal ensures that the alloys are also extremely light. Aluminium
> > alloys are significantly stronger than magnesium alloys but the low
> > density of the latter affords higher specific strength and stiffness.
> > Magnesium alloys also possess good machinability and can be readily
> > welded using TIG. Having said that they are extremely soft and a
> > "bulk" penalty must be paid as a large volume of material is required
> > to achieve equivalent properties to other structural metals.
> > Furthermore they are expensive and their resistance to corrosion is
> > not as good as that of aluminium alloys.
> >
> > There are three primary groupings of magnesium alloys in common use:
> > casting alloys, wrought alloys and those which can be used in both
> > ways. For example;
> >
> > 1. Magnesium-aluminium-zinc alloys may be precipitation hardened and
> > are used in casting, extrusions and forging.
> >
> > 2. Magnesium-manganese alloys are readily weldable and mainly used in
> > sheet fabrication processes.
> >
> > 3. Magnesium-zirconium-rare earth alloys can be used in both cast and
> > wrought forms. They have reasonably high proof stresses, resistance
> > to impact and corrosion and may be easily fabricated."
> >
> >  The alloys used on cameras will be type 1, the others would be too
> > expensive.
> >
> > Many metals which corrode form a thin passive layer of oxide on their
> > surface almost immediately, this prevents further obvious corrosion.
> > On aluminium and titanium the oxide layer is thin enough to be
> > "invisible". On brass it tarnishes then stabilises, which makes it
> > good for engineering but tiresome for shiny ornaments!
> >
> > Basically magnesium is "the new titanium" in the camera marketing
> > game. It is nowhere near as good as titanium but massively cheaper
> > and easier to manufacture from.
> >
> > It is an unfortunate fashion which will hopefully not last long.
> >
> > Frank
> >
> >
>
>
>
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