jps
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| posted on 23/7/18 at 06:24 AM |
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Sierra diff case is made from..?
Is it aluminum or an aluminium based alloy?
I ask because I have one which was once painted. After a few years sitting in a damp shed the paint is flaking badly with white powdery residue
everywhere...
I don't intend to repaint it but want to know what I'm leaving exposed.
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mcerd1
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| posted on 23/7/18 at 07:18 AM |
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its a cast Aluminium Alloy (don't ask me exactly which one though) - pure Aluminium isn't used for much, its far too soft (although it is
pretty good for corrosion resistance)
these were never painted on the original cars and they've lasted all this time in all weathers - so I don't think you need to worry much
paint doesn't stick well to to aluminium and its alloys, it needs really good prep work and/or etching primer to last
[Edited on 23/7/2018 by mcerd1]
[Edited on 23/7/2018 by mcerd1]
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jps
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| posted on 23/7/18 at 07:41 AM |
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quote:
Originally posted by mcerd1
these were never painted on the original cars and they've lasted all this time in all weathers - so I don't think you need to worry much
Cheers, I did not paint it myself and so my plan was to just get the loose and flaking bits off and then leave it. Currently it looks like something
Mr Whippy would paint thanks to the random way the paint has peeled...
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nick205
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| posted on 23/7/18 at 08:11 AM |
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quote:
Originally posted by mcerd1
its a cast Aluminium Alloy (don't ask me exactly which one though) - pure Aluminium isn't used for much, its far too soft
these were never painted on the original cars and they've lasted all this time in all weathers - so I don't think you need to worry much
paint doesn't stick well to to aluminium and its alloys, it needs really good prep work and/or etching primer to last
[Edited on 23/7/2018 by mcerd1]
Ditto!
They lasted for years underneath Sierras and Granadas unpainted. I didn't' paint mine when I moved it from donor Sierra to MK Indy
(replaced the driveshaft seals though) and no harm came to it wharsoever. Unless you have time (can be bothered) to strip the rest of the paint off
then I'd leave it as it is. IMHO it's more import that the internals function correctly than what the outside looks like (you can't
see it inside the car anyway.
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Angel Acevedo
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| posted on 23/7/18 at 11:30 PM |
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Painting aluminium without proper preparation/Materials is worse than leaving bare.
minor defects on coating and or chips will allow moisture to enter the space between paint and metal and this in turn will start filiform (sp?)
corrosion.
Which looks as you describe.
Regards
AA
Beware of what you wish.. for it may come true....
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907
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| posted on 24/7/18 at 04:26 AM |
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Anorak on.
It's almost certainly Aluminium Silicon, as silicon lowers the melting point and improves fluidity making it easier to cast.
To a certain degree it increases strength, although strictly speaking it makes the casting more rigid, and less ductile.
It decreases the coefficient of expansion (shrinkage and distortion) which is another bonus when casting.
It's also cheap as chips, being one of the most common substances on/in the planet.
The white powdery surface is aluminium oxide.
When steel oxidises it forms flakes, the flakes curl up and allow air and moisture under the flake so that a new flake forms underneath.
That's why left to it's own devices steel corrodes all the way through.
When aluminium oxidises it doesn't flake, in fact quite the reverse. The surface oxide forms a protective layer thus preventing
further oxidation of the aluminium underneath.
At this very moment my car is in the garage forming its very own protective layer. 
Anorak off
Paul G
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Angel Acevedo
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| posted on 24/7/18 at 12:43 PM |
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quote:
Originally posted by 907
...
When aluminium oxidises it doesn't flake, in fact quite the reverse. The surface oxide forms a protective layer thus preventing
further oxidation of the aluminium underneath.
...
Paul G
On bare aluminium that is true, but on coated aluminium, under certain conditions, the corrosion overcomes the protective layer and produces
filiform corrosion as stated above.
Case in point the white powdery substance that jps describes.
Changed coating to corrosion
[Edited on 7/24/2018 by Angel Acevedo]
Beware of what you wish.. for it may come true....
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jps
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| posted on 24/7/18 at 04:20 PM |
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quote:
Originally posted by Angel Acevedo
quote:
Originally posted by 907
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When aluminium oxidises it doesn't flake, in fact quite the reverse. The surface oxide forms a protective layer thus preventing
further oxidation of the aluminium underneath.
...
Paul G
On bare aluminium that is true, but on coated aluminium, under certain conditions, the coating overcomes the protective layer and produces filiform
corrosion as stated above.
Case in point the white powdery substance that jps describes.
Interesting - I had always assumed that the white reside was simply the aluminimum equivalent of rust (and behaved accordingly) whereas this suggests
it behaves somewhat differently...
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Angel Acevedo
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| posted on 24/7/18 at 07:52 PM |
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quote:
Originally posted by jps
quote:
Originally posted by Angel Acevedo
quote:
Originally posted by 907
...
When aluminium oxidises it doesn't flake, in fact quite the reverse. The surface oxide forms a protective layer thus preventing
further oxidation of the aluminium underneath.
...
Paul G
On bare aluminium that is true, but on coated aluminium, under certain conditions, the coating overcomes the protective layer and produces filiform
corrosion as stated above.
Case in point the white powdery substance that jps describes.
Interesting - I had always assumed that the white reside was simply the aluminimum equivalent of rust (and behaved accordingly) whereas this suggests
it behaves somewhat differently...
Modified my previous post to make more clear.
The white powder is indeed aluminium oxide.
This type of corrosion is common in lacquered rims and looks like wood worms.
...
Beware of what you wish.. for it may come true....
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