who knows why?
 

Dear all, I noticed a very interesting phenomena. If you cover a cup of hot water with a very thin slice of Aluminium, after several minutes, the Aluminium will turn light brown, if you use cold water this change will take longer time. How comes the change of color? Is that because the phase change of Aluminium?

Thank you!

Linda

Re: who knows why?
 

Hi Linda,

at a first glance this can be due to some kind of heterogeneous reaction: the surface of aluminium is reacting with the fluid with which is in contact.

The temperature can make the reactions on the aluminium surface faster, accelerating therefore the formation of surface compounds which have the brown color.

Comments to my idea are welcome.

I hope this helps

Ciao

Maurizio

Re: who knows why?
 

The surface could be reacting with the water vapor rather than the fluid. The most likely reaction could be Aluminium oxidation in which case Al2O3 has to be brown in color (which I am not sure of).

Re: who knows why?
 

Hi, Maurizio and kalyan:

Thank you for your responses. I first thought of chemical reaction too but later I feel it is a little strange. You see, water is composed of H and O, if Aluminium reacts with water why cann't it react with air?(because I found that the thin piece of Aluminium will not change color if it is in the air.)

Is it possible that water made the Aluminium some sort of physical change? I am not sure of that.

Thank you & Regards!

Linda

Re: who knows why?
 

Hi

I presume it also depends on the quality of water. Water is not generally 100% pure in its natural available state. Added to that the potential of the Al alloy or metal does change in the presence of some elements and it shall help in the formation of the substrate which you might be seeing. The potential of the metal depends on the alloying elements present in the Al alloy.

I have a query. Is the plate perfectly clean? No biological reagents on the surface?

I hope i was able to clear the picture a bit.

Best Regards

Ram

Re: who knows why?
 

Hi,Ram:

Sorry replying late, I didn't access the web because of the net fail.The plate is clean and there is no biological reagents on the surface too. I guess what you said is quite reasonable but I can not find out whether there is some elements of the water caused the change.

Thank you!

Best Regards!

Linda

Re: who knows why?
 

The reaction may be quite similar to rusting. Iron exposed to water vapor/steam is likely to get oxidized much faster than in air. I am not fully sure but I think if the air is fully dry (0% humidity), it can not rust iron.

The only problem with the above argument is I am not sure if any one phase of Aluminium oxide is brown. So the Aluminium could be contaminated with iron which can rust due to oxidation.

Re: who knows why?
 

Unless the piece of aluminum metal you have is freshly polished or etched, there will already be a thin layer of aluminum oxide due to contact with air and moisture. The oxide is probably present as the alpha-oxide and the trihydrate. The alpha-oxide is transparent and the hydrate is white, but the layer is too thin so the piece looks like clean metal to the naked eye.

The alpha-oxide has very stable chemical properties, and protects oxidation of the natural aluminum metal. Otherwise aluminum metal can dissolve in water, although I am not sure of the temperature conditions required for this reaction.

The hydrate is amphoteric and could react with trace ionic species to produce certain aluminum salts, but I cannot tell you about the colors of aluminum salts. I don't think this is the cause of the observed discoloration anyway because the surrounding water and air are reasonably clean.

One possibility is that the brown stain you see is due to impurities actually present in the aluminum metal. When trace amounts of the cations of certain metals are embedded in the alpha-oxide, coloration seems to be enhanced. This is what gives some gem stones their brilliant colors. I am not sure what impurties may be present in your sample, although iron and calcium are likely suspects. I guess the warmer temperature just happened to bring out the color of whatever trace impurity is in the metal, and it the color is further enhance by the alpha-oxide.