Re: How thick is the aluminum oxide layer on a piece of pure Al?

Hi, Tess – hope you had a good Christmas! Here I am at home during the 
break, and I’m trying to answer your question about the thickness of the 
oxide film on pure aluminium. Not having a reference library to hand, and 
my own books at home being not too helpful, I may not be able to give you 
an exact answer. However, I suspect that there really isn’t one.
A pure, nascent, surface of aluminium will immediately corrode in moist 
air to form a surface film, which when formed, as you already know, 
prevents – or significantly hinders further access of oxygen to the metal 
and hence further corrosion. If the surface was initially very smooth – 
i.e. being the surface of a single crystal face of the pure metal, I think 
that the initial mono-layer formed might last for a time, but even in 
those unusual circumstances would slowly grow in thickness. Surfaces are 
rarely that smooth. As a matter of practice, aluminium is often treated 
electrochemically in a process known as anodisation to increase corrosion 
resistance, by thickening the layer of oxide. A layer as thick as 1000 
angstroms can be formed this way, but it not usual to do this on pure 
aluminium – usually alloys with other metals are used, even if aluminium 
is very much the predominant component.
So, the thickness of the layer will depend on surface smoothness, and on 
whether any treatments have been applied. At minimum it could be a 
monolayer – probably of the order 20 to 50 Angstroms thick, but this would 
be unusual, and not very stable in an atmosphere containing any water and 
oxygen. Much more usual would be a layer as thick as 500 angstroms or more.

By the way, angstroms are unusual units to use these days, and more 
commonly people use fractions of metres – such as nanometres or 
micrometers for very small units of length. One angstrom is one tenth of a 
nanometre and one ten-thousandth of a micrometre.