[antfrogboy]

Hi there

apologies if this question is posted in the wrong place, but I'm no chemist and materials sounded kind of right!

I am refurbishing a bicycle.  The bike frame is a steel alloy: chromium-molybdenum (reynolds 501).  There is a metal fixture which i believe is made from aluminium which has corroded fast into the frame.  no amount of hitting it with a big hammer seems to work. 

so, the chemistry question: can i safely use caustic soda (hydrochloric acid, right?) to dissolve the aluminium fixture without damaging the cromoly?  The second question would be how can i even be sure it's aluminium? 

Any help on this would be greatly appreciated!

Thanks

T

[typhoon2028]

Caustic Soda is not hydrochloric acid.  Caustic soda = sodium hydroxide

How do you know it is aluminium?  Aluminium is pretty resistent to corrosion.  What color is the corrosion?

[antfrogboy]

i guessed it's aluminium because when i've seen the same part for sale it's been aluminium!  the corrosion is hidden, as it's where the part butts up against the frame.

so actually, it's almost the opposite of hydrochloric acid, my bad.  it does say on wiki that it will eat aliminium

"Strong bases attack aluminium. Sodium hydroxide reacts with aluminium and water to create hydrogen gas. The aluminium takes the oxygen atom from sodium hydroxide(NaOH), which in turn takes the oxygen atom from the water, and releases the two hydrogen atoms."

but based on the rest of the entry, it sounds like quite a slow reaction..

i suppose then my biggest question is what will it do to the steel alloy?

Tom

[typhoon2028]

I should say the oxide aluminium is coated with makes it corrosion resistent.

Do you have a picture?  There may be a better way to remove the piece.

[antfrogboy]

there should have been a picture in my post

http://www.flickr.com/photos/48511104@N00/3808464197/

it's the more pale circle of metal (i've now removed the black thing and the square sticking out bit).

I've exhausted mechanical methods, which is why i've ended up thinking chemistry instead.  I've tried heat, but i'm not sure about the dfferences between ali and chromoly when each expands due to heat... should i perhaps be dry icing it and not heating at all.

i've been told by an engineer friend that the corroding ali method works, but isn't pleasant...

[typhoon2028]

I cant open your picture at my work. 

NaOH will work but you will generate a lot of hydrogen gas

[antfrogboy]

if i try it, i'll make sure no one in the vicinity is smoking!

[Borek]

I should say the oxide aluminium is coated with makes it corrosion resistent.

Whenever two metals different are used in construction, there is a risk of electrolytical corrosion. could be that's what have happened here. Note that Al is usually passivated with oxide - but when the elemnt is put in place oxide layer can be scratched and there can exist quite good electrical contact between steel (iron) and aluminum.

no amount of hitting it with a big hammer seems to work.

WD40?

[antfrogboy]

i suspect it is electolyte corrosion, it's locked solid. 
I have tried wd40, then ramped up my disassembling lubes and used a mix of paraffin and diesel.

I'm going to have another go at mechanical means (building some kind of tool involving a huge lever) otherwise i'm going to resort to chemical and caustic soda seems to be the answer

[Borek]

Note: in the presence of air iron is going to corrode in NaOH as well (although not that fast as Al), not much you can do about it.