[Zyklonb]

After 15 hours, the sample to which chloride ion was added is reacting very slowly, like one tiny bubble per five seconds.
The pure sample has undergone no reaction to my knowledge. This suggests that even impure aluminum pasivates entirely in pure fuming nitric.
My scale is not accurate enough to see if there was any loss of mass, but everything looks the same.
 

[curiouscat]

So far as I remember SS316 piping & tanks were routinely passivated by successive washes of 10% NaOH followed by 33% HNO3.

[Enthalpy]

Alloys behave very differently, with little copper ruining the corrosion resistance of aluminium, allegedly because it makes the oxide layer porous.

Strange. As far as I can tell duralumin is still quite corrosion resistant, despite containing around 4% of copper (and some other elements).

It depends on the use. Upon regular skin contact, Al-4Cu is just fine. Under occasional rain as well. But put permanently outside, it won't resist rainwater - my own experience confirms docs. And in seawater or in the soil, it fails quickly.

Then you have surface treatments, that is mainly anodization. Al-4Cu are not the best alloys for that, but it does improve a lot. The older AA2017A, now AA2014 and AA2024 are used for aeroplane parts with anodization, and then they behave very well under rain. Same for Al-5Zn AA7075.

[marty63]

An interesting thing about the statue of eros in Piccadilly circus London is that it was cast in an alloy of aluminium. I believe that this alloy contained some copper which explains how the statue has the dark patina that it does. It was restored some time ago and found to have surprisingly little corrosion given the effects of pollution and external exposure that might be expected.

Here is a link to an excerpt from an article looking at the technicalities involved in the restoration:

http://ihbc.org.uk/context_archive/42/clarke_dir/clarke_s2.htm

Just thought it might be of interest to this discussion.

[curiouscat]

Just thought it might be of interest to this discussion.

Indeed very interesting! Thanks!

[Enthalpy]

An interesting thing about the statue of eros in Piccadilly circus London is that it was cast in an alloy of aluminium. I believe that this alloy contained some copper which explains how the statue has the dark patina that it does.

Successful TIG welding indicates that any copper contents is low. Dark patina is an indication of zinc or silicon in aluminium; for instance Al-Cu4% gives a clear anodization layer.

Presently, the preferred cast alloys are Al-Si13% and small variations, but maybe they weren't known then.

[marty63]

Ah! that is interesting. I looked it up and found that it is 99.1% Al, 0.027% Fe, 0.6% Si, & 0.01%Cu. So yes it looks like it is the silicon as you say. Found this out here:

http://books.google.co.uk/books?id=o7yJAgAAQBAJ&pg=PA108&lpg=PA108&dq=piccadilly+eros+statue+alloy+composition&source=bl&ots=eSI9WcTFBn&sig=Z5KMw2IXLn8sbFWrlfYofhu4PEg&hl=en&sa=X&ei=v409VMeKJezd7QbKmoDYBQ&ved=0CGIQ6AEwDg#v=onepage&q=piccadilly%20eros%20statue%20alloy%20composition&f=false

[Enthalpy]

Or just the result of air pollution. The composition may have been as pure as was possible then, and 0.6% Si would have about no effect on the oxide colour; for instance metallic frames for windows contain this proportion of Si and Mg. At 13% Si, the colour is quite visible.

Very pure aluminium (this case) is one good receipe for longevity under rain, an other being 3% or 5% Mg (boat hulls), still an other 0.6% Mg and Si (window frames), and an other just a bit of Mn. As opposed, Cu is very detrimental to the corrosion resistance, and Zn is bad also - pity, since these two are the best aluminium alloy hardeners.

[marty63]

I've cast a few sculptures in aluminium over the years and always found that it is really hard to cast if you don't have the right alloy. The 'right' alloy as far as the kind of thing available as scrap is old car wheels. The 'skin' of oxide generally impedes the flow but with alloy wheels it flows much better. Also, interestingly enough, it is best not to pour into red hot moulds - as in bronze casting. For some reason the aluminium flows better if the mould is less hot. I've never thought of this in terms of chemistry before but I guess that is how these things could be explained. A bit off topic I know. Sorry.