First of all, sorry for asking questions a bit off the forum topic, and a bit silly.
It's a question around to me for quite a while. I knew we can easily polished Aluminium parts to mirror shiny by removing the Aluminium Oxide on the surface. However, is it possible to just polish the Aluminium Oxide Surface to a smooth surface, instead of removing it? What I'm doing is that I have a parts that requires a smooth surface and I want to make it with Aluminium.
I tried to polish some Aluminium bars with Autosol Metal Polish. The polished surface did become smooth (not mirror, just smooth like axles), and it stays smooth for months now. Here is the questions, is the smooth surface Aluminium metal? Or, is the smooth surface Aluminium Oxide? Will the smooth surface stay smooth?
The surface of aluminum is always oxide, if you remove it, the oxide will reform in a matter of milliseconds.
This metal is very reactive, fortunately the thin layer of oxide inhibits the further oxidation, otherwise you would just end up with a heap of powder.
I suspect that you are just not getting the surface smooth enough for your mirror finish.
The surface of aluminum is always oxide, if you remove it, the oxide will reform in a matter of milliseconds.
This metal is very reactive, fortunately the thin layer of oxide inhibits the further oxidation, otherwise you would just end up with a heap of powder.
I suspect that you are just not getting the surface smooth enough for your mirror finish.
Then... will the smooth surface stay smooth? One of my friends told me that Aluminium Oxide surface is rough, so the surface of Aluminium parts will have a rough surface overtime, even they start smooth.
I'm really confused. If Aluminium Oxide surface is a strong surface and it can resist oxidation, why would it become a rough surface overtime? Is my friend simply wrong? Or, is Aluminium Oxide will undergo some forms of corrosion other than oxidation?
When aluminium is anodised a much thicker oxide layer is formed (which can be dyed various colours)
and this protects soft aluminium from scratches and corrosion.
Aluminium oxide is a very hard ceramic though, polishing it probably needs specialist materials (its
commonly used in grinding wheels and the like itself, also known as corundum, sapphire, ruby, Polishing
risks thinning the coating unevenly exposing areas of bare metal.
Aluminium corrodes, not the oxide. Certain aluminium alloys are much more susceptible to
pitting and corrosion from the atmosphere than others (there are hundreds of aluminium alloy
formulations in several major classes). Aluminium items kept indoors aren't usually subject to
corrosion, outdoor aluminium fittings are routinely anodised to give long life, but do become dull and
rough with age eventually I believe.
The anodised coating isn't high-quality crystaline oxide, perhaps it can be polished with corundum
itself? Wikipedia has some good information about anodizing.
Aluminum oxide is VERY hard,
so polishing in itself is difficult but not impossible.
thats you direct question,
but
So Al is very reactive, and the air is full of oxygen and water vapour.
so Al "rusts" ( oxidises ) almost instantly its exposed to the air ( as said above )
and forms a layer of Al2 03 on its surface.
luckily, Al2 03 is very none reactive, and resistant to O2 and H2O penetration.
which slows the rest of the Al reacting. Else we would not have any Al in the world.
Great mirrors are formed by depositing Al on to glass under a none reactive atmosphere,
the glass gives the protection required to stop the Al reacting. The back of the mirror is then covered in something to stop the back oxidising through.
Car light reflectors are commonly made with plastic and a Al cover.
Some food packages have a shiny Al coating.
So you friend is right, as are you.
Al will keep reacting, and will eventually pit and dimple.
Give Al the right conditions, it reacts so strong it burns,
It just a matter of the environment your in, and the protection you can give it.
There's youtube video showing how to polish the bottom of an aluminum can with a piece of cloth and a bit of chocolate. Same as polishing leather to a mirror shine, the shine depends on filling the pores or cracks with a single layer of polish. Polish fills the cracks, optics takes care of the image. When you can tune your radio in the reflection is that good enough?
Great mirrors are formed by depositing Al on to glass under a none reactive atmosphere,
the glass gives the protection required to stop the Al reacting. The back of the mirror is then covered in something to stop the back oxidising through.
Even greater mirrors are made for telescopes, yes they are deposited on glass but the Al side is left exposed with no coating, this is known as a front silvered mirror.