Does aluminum oxidize with water?

[miufahkiu]

From my Chem 12 knowledge, aluminium oxidizes with water, spontaneously. So, if I have bare aluminum exposed to water, it will eventually corrode, right?

I'm asking because some of the paint on my chainstays got scraped off from the rotor. I'll probably put some nail polish over it to be safe, but I just wanted to know if water would eat away at it.

The other thing is, is it common for the rotor to scrape paint off the frame? I think it happened from trying to take off/put on the rear wheel. I don't remember scraping the paint myself, but from the times I've had to remove/replace the rear wheel, it's been pretty rough. The 8" rotor only gives me about 1/4" clearance from the chainstay, so I have to be extremely delicate when taking off or putting on the rear wheel to avoid contact with the frame. Aargh, three days new, my bike's already scratched up, and it hasn't even seen any trails yet!

[shmity]

Aluminium will oxidize, but no where near the rate of steel. Dont worry about your frame corroding, you'll outlive the frame in that regard.
As for the rotor, alot of frames leve quite a bit of clearance, others dont. If it doesn't interfere while your riding, dont worry about it, just be carefull when removing or installing the wheel.

[AviateR]

Aluminium oxidation is good, aluminum oxide is actually stronger then regular aluminium, some frame manufacturers will "age" their frames to give them an oxide coating... geez I feel like Bill Nye the science guy

[Ralph Wiggum]

Originally posted by AviateR
geez I feel like Bill Nye the science guy

i love that guy :thepimp:
i used to watch him all the time on tv

[miufahkiu]

Originally posted by AviateR
Aluminium oxidation is good, aluminum oxide is actually stronger then regular aluminium, some frame manufacturers will "age" their frames to give them an oxide coating... geez I feel like Bill Nye the science guy
Right! I remember that now. Mr. Scodellaro would be ashamed of me forgetting his redox lessons so soon!

[UFO]

steel rusts because of the carbon content, which is why high end light weight steel is more prone to oxidation, higher carbon content

[IFO]

i wont worry about your Alloy frame rusting out...

unless you plan to kep your bike for a 100 years or so...

:D

u should see how many open wounds my BIGHIT has...:eek:

[FuManChu]

Originally posted by miufahkiu
From my Chem 12 knowledge, aluminium oxidizes with water, spontaneously. So, if I have bare aluminum exposed to water, it will eventually corrode, right?

I'm asking because some of the paint on my chainstays got scraped off from the rotor. I'll probably put some nail polish over it to be safe, but I just wanted to know if water would eat away at it.

The other thing is, is it common for the rotor to scrape paint off the frame? I think it happened from trying to take off/put on the rear wheel. I don't remember scraping the paint myself, but from the times I've had to remove/replace the rear wheel, it's been pretty rough. The 8" rotor only gives me about 1/4" clearance from the chainstay, so I have to be extremely delicate when taking off or putting on the rear wheel to avoid contact with the frame. Aargh, three days new, my bike's already scratched up, and it hasn't even seen any trails yet!


don't worry aluminum oxide just forms like a covering on the aluminum and prevents the aluminum underneith from further corroding not like steal though.

[switch]

Originally posted by UFO
steel rusts because of the carbon content, which is why high end light weight steel is more prone to oxidation, higher carbon content

Rust is iron oxide. Where does carbon figure in this. Please explain. Waiting for input...

[Craz]

Originally posted by switch
Rust is iron oxide. Where does carbon figure in this. Please explain. Waiting for input...

I thought that too...

Maybe more carbon = less iron, but it may still oxidize at the same rate, and will thus rust out the whole frame in a shorter time period?

Aluminum frames you need not worry about. Steel isn't THAT much of a biggy either.

[Fruit]

Meh, ask someone who knows...... like me:) (chemistry degree - slacking off lab work at the mo!)

Aluminium oxidises the same as steel but the oxide is similar in density to raw alu, and therefore forms a thin coating on the surface. iron on the otherhand is different in density and the oxide bubbles up and reveals raw metal. This is a crap description but my brain isnt working at the mo, bloody spectroscopy data analysis:(

Alu oxide is a protective layer, and anodising is a way of promoting a consistent layer of oxide, with fewer flaws, which prevents further oxidisation from occuring.

[TheGiggler]

yeah, raw Alu will oxidize naturally in air, very quickly, it is a rapid process.

the oxide layer is very hard but thin, so it is not at all like steel rusting.

[FuManChu]

Originally posted by Lunatik
I thought that too...

Maybe more carbon = less iron, but it may still oxidize at the same rate, and will thus rust out the whole frame in a shorter time period?

Aluminum frames you need not worry about. Steel isn't THAT much of a biggy either.


nah i dunno how carbon plays into this, as it has nothing to do with the rusting IMO, since like surgical stainless steel has very little carbon you would think it would rust really bad but it don't.

[UFO]

Originally posted by switch
Rust is iron oxide. Where does carbon figure in this. Please explain. Waiting for input...

sorry for being too vague, it was late last night :D

everything sounded so right last night....

expensive steel (cromo) is lower iron, and higher carbon content...which is more prone rusting. cheap steel (hi-tensile) is lower carbon content, higher iron, and is less prone to oxidation.

thinking about it now, it doesnt make sense either. but i remember reading about this way back in highschool. can somebody explain this to us? hehe

[TheGiggler]

iron itself will rust to form iron oxide as mentioned.

when you start adding other chemicals to the mix, the properties of the whole material change.

e.g. stainless steel has iron/carbon/nickel (and other stuff) but due to the chemistry and probably crystal structure, it does not easily react or oxidize.

when you start looking at different versions of cro-mo, the properties of the material will be different depending on the concentrations.

rusting, oxidization, is a chemical reaction of iron and oxygen. so you can actually put certain metals onto the iron (zinc?) that will oxidize first and act as a sacrificial rust inhibitor to protect the iron.

[Dave K]

Originally posted by statix
iron itself will rust to form iron oxide as mentioned.

e.g. stainless steel has iron/carbon/nickel (and other stuff) but due to the chemistry and probably crystal structure, it does not easily react or oxidize.



What makes stainless steel corrosion resistant is the addition of chromium. The high chromium content within stainless steel means that there is a thin layer of chromium oxide that forms on the outside of the part. This is the layer that protects the part from further corrosion.

There are other ways of making steel more corrosion resistant. One is the addition of copper to the alloy.

Titanium and aluminum (and copper) are somewhat similar in the way that they protect themselves. They quickly form an oxide layer on their surface that is strong and very unreactive. This protects the base material from corroding further. Different alloys are more corrosion resistant though. 6061 is more corrosion resistant than 7005 for example (especially if the 7005 is not heat treated properly).

Dave

[EndoKing]

Originally posted by statix
iron itself will rust to form iron oxide as mentioned.

when you start adding other chemicals to the mix, the properties of the whole material change.

e.g. stainless steel has iron/carbon/nickel (and other stuff) but due to the chemistry and probably crystal structure, it does not easily react or oxidize.

when you start looking at different versions of cro-mo, the properties of the material will be different depending on the concentrations.

rusting, oxidization, is a chemical reaction of iron and oxygen. so you can actually put certain metals onto the iron (zinc?) that will oxidize first and act as a sacrificial rust inhibitor to protect the iron.

That sounds about right. I know that carbon is added primarily to increase strength (ie. you get a different, stronger crystal structure than if it was pure iron), but it may also help to prevent oxidization. Nickel may be the "sacrificial" element in stainless steel.

This is totally unrelated, but all this talk about crystal structure just reminded me of something cool a friend in engineering did in a lab once. They took a little square of titanium & shot it with a .22 calibre rifle. The bullet goes right through. Then they took another little square of titanium, dipped it in liquid nitrogen (-80 degrees C) which causes a change in the crystal structure of the metal (to face-centred-cubic, I think, for all you scientists out there) & then shot it. This time the bullet dented it, but didn't go through. My friend kept the little pieces of metal to show me, it was pretty cool.

[shmity]

It is actually Chromium in stainless steel that makes it resistant to corrosion, not nickel. It doesn't act as a sacrificial anode, it mearly prevents the iron from oxidizing. A sacrifical anode is easist to think of a sheet of steel with a zinc block or magnisum attached to it through wiring or galvanised on (depends on the application, think burried pipes vs shed door), or various other methods. What happens is the the couple of these two materials, one becomes cathodic, one becomes anodic. The cathode oxidises slower than it would by itself, the anode oxdises faster than it would by itself. The only way to determine weather something can be used for an anode is what sort of metal you are wanting to protect. Essentially the two metals when joined, attempt to become in equilibrium, by exchanging ions, due to the difference in electrode potentials.


We now return you to your regular program.

[switch]

Originally posted by shmity
It is actually Chromium in stainless steel that makes it resistant to corrosion, not nickel.

Moly helps too. :)

When it comes to oxidation, think "valence".

[Dave K]

Originally posted by switch
Moly helps too. :)



Actually molybdenum is added to increase strength and hardenability in steels. It's not really added for it's corrosion resistance.

Chromium is also added to steels to improve their hardenability as well. 4130 is a chrome-molybdenum steel which is commonly used in bikes (hence cro-moly bikes of course).

When there is above 12% chromium in the steel alloy, then this becomes stainless steel (by comparison 4130 steel is 0.95% by weight chromium).

Dave

[*GiMpY_jR*]

thats all crazy talk i hope im smart some day...

[switch]

Originally posted by *GiMpY_jR*
thats all crazy talk i hope im smart some day...

It's all about attraction and stability.

Take chemistry in grade 11/12. Those courses will make it all clear.

[miufahkiu]

Wow, some of you guys are pretty wise. You sound like you've received a formal education of some sort. Mind if I ask you what professions you are employed in?

I'm considering going into metals and materials engineering next year; perhaps if any of you have knowledge about that, you could let me know what to expect. Thanks!

[Dave K]

Originally posted by miufahkiu
Wow, some of you guys are pretty wise. You sound like you've received a formal education of some sort. Mind if I ask you what professions you are employed in?

I'm considering going into metals and materials engineering next year; perhaps if any of you have knowledge about that, you could let me know what to expect. Thanks!

Dunno about the wise bit. It's easy to look things up in text books. I took mech engineering at UBC. There were a couple courses in metals and materials that we had to take (they were super interesting).

MMAT looks to be an interesting area. It was my second choice of options to take. Make sure you're math is strong. The first couple of years in engineering are heavy in math and calculus. They definately try to weed out the herd in the first 2 years (and the course load is heavier than standard science or arts course loads). At least that is what it was like at UBC. I would think it would be the same at most institutions. Good luck!

Dave

[switch]

Originally posted by Dave K
...and the course load is heavier than standard science or arts course loads

Engineering was a lot heavier, especially when compared to a standard arts program. With labs, almost double the classroom time. Then there's the homework load.

[miufahkiu]

I'm going to UBC at the moment too. I'm considering mech and MMAT as my top two choices. Competition getting into mech will be stiff, since most of the people I know are considering that. I like the sound of MMAT, and the fact that it's a smaller department.

As for the course load, it is intense! I'm doing alright in my math classes, and my calculus is pretty strong. But, by no means are my marks stellar. I'm holding down around a 75% average at the moment, I think. First term was good, but I'm getting slaughtered this term with the two physics courses and two math courses. Homework load is insane, as well as the lab schedule. Even with two midterms in a week, I'll have four other assignments to worry about. Thursdays I have 5 hours of labs in a row. I can't wait till finals are over!

I can't even imagine how good it will feel when I get my degree...

[Strupat]

aluminum oxide is actually stronger then regular aluminium

aluminum oxide is harder, not stronger. i think another name for it is crystal but don't quote me on that.

another thing is if the water hitting the aluminum has salt from the road etc it will corrode worse than steel.

[ralph]

as already mentioned, steel rusts b/c of iron... the carbon really has very little to do with it.

its not really accurate to say that steel has a lower iron content when there is a higher carbon content... all steel's have a carbon content of less then 6%... so, there is actually very little carbon in steel.

the material properties change with impurity content (carbon, molybdenum, si, etc...), but they are also highly dependent on heat treatments and things such as cold working.

for instance 4130 steel. famous for being flexy but strong. this is a low carbon steel (0.28 to .33 % carbon) Typically this steel is quenched from high temperature state (creating an extremely hard but brittle metal), then tempered to reduce internal strains- and depending on the temperature that the metal is tempered at the metal can go into different phases (ie form different crystal structures or meta-stable phases) and can have very different properties. its this tempering phase that provides the flex in the 4130 steel. actually, 4130 is a designation for the type of steel and some of the treatments that it will have undergone....

kinda off the subject from oxidation... the thin coating of oxidation on aluminum will prevent further oxidation (unless oxygen can diffuse through the oxide layer and further react, which may happen very slowly over time, but i don't believe this is something that you would even notice or have to worry about- its such a slow process).

-chelsey