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What is the white rust in aluminum alloys composed of?

April 30, 2010


I'm currently working at a die casting of 4000 series aluminum alloys and some problems with corrosion are appearing lately.
in letter 11535 on this site it was said that the "white rust" normally seen in galvanized corrosion sites are zinc oxide. No problem with that, but what about the "white rust" in aluminum alloys? is it aluminum oxihydroxide? is it aluminum hydroxide?
thank you for your time.

Gabriel Isidore Barbejat
student - Florianopolis, Brazil

May 5, 2010

Dear Gabriel Isidore Barbejat
After casting of Aluminium, normally we are able to see some pores, in those pores water will deposit and react with Aluminium, in my knowledge that is Aluminium trioxide, that is Al2O3

Kannan Boopathi
- Salem, Tamil Nadu, India

May 5, 2010

The conventional naming for Al2O3 is aluminum oxide, because the valence of aluminum is a given of +3, so you delete the tri as it is the only possible combination.

James Watts
- Navarre, Florida

May 7, 2010

But doesn't aluminum create a thin and adherent layer of aluminum oxide in the moment it is cast? Isn't that the reason why it's so resistant to atmospheric corrosion? What makes the aluminum oxide turn into dust (white rust)?

Gabriel I. Barbejat
- Florianopolis, Brazil

May 10, 2010

But isn't aluminum oxide attached to the aluminum surface because it's pilling-bedworth ratio is 1.28? why would it detach as white rust? the resistance of aluminum to environment is due to this oxide layer so it doesn't make sense the aluminum oxide simply come of from the surface without chemical attack... the fact that the white rust appears faster in humid places doesn't proves that it needs an OH part?

Gabriel Brbejat
- Florianopolis, Brazil

May 10, 2010

The aluminum oxide coating on aluminum is very tenacious as long as it is thin. After it gets thicker, it is subject to fracturing into powder even tho it is nearly as hard as a diamond.
Rust on iron is iron oxide, and it requires water to form and it is NOT an hydroxide.
A salt environment such as sea side,promotes an even more rapid formation of the oxide, but does not react with the material or for a chloride product. Look up the definition of catalyst.

James Watts
- Navarre, Florida

May 11, 2010

Aluminum is said to be noncorrosive. But many years back we had an aluminum bucket. It was being used to heat water with an immersion coil. Often it would make mild crackling noise. Within a couple of months it developed whitish blisters all over. That was in Ahmedabad where water was perhaps hard.
Could this kind of corrosion be due to impurities or alloying elements in aluminum?

H.R. Prabhakara - Consultant
Bangalore Plasmatek - Bangalore Karnataka India

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