Sciencemadness Discussion Board

Reaction of Iron and Alum KAl(SO4)2

gregxy - 11-4-2011 at 12:26

The other day I was drilling holes in a piece of aluminum
and broke off the bit inside. I looked on the internet to
find a way to get it out and found some articles recommending
the use of alum (KAl(SO4)2) to dissolve the bit without
hurting the aluminium.

I'm not sure why this would react. Iron is less electropositive than K or Al?

I did not have any alum so I used dilute H2SO4. The following day
I found that the bit had disintegrated and a bunch of black
stuff had formed (iron oxides?) I think some of the Al dissolved too but not enough to matter. So maybe the alum just
promotes the oxidation of the iron by air? If that is the
case I would think plain salt would work as well.

[Edited on 11-4-2011 by gregxy]

blogfast25 - 12-4-2011 at 05:24

Dissolving iron without dissolving aluminium is a pipe dream: aluminium is more soluble in acids than iron (but there’s not a great deal of difference). If whatever you’re using dissolves iron, it ill also dissolve the aluminium. Many of these ‘chemical DIY articles’ are hog wash or old wives tales.

Mechanical recovery of the broken bit is your only real chance…

gregxy - 12-4-2011 at 12:03

I would not say it's a pipe dream.
The H2SO4 worked for me. The drill bit was reduced to
slivers, and the hole was at most a couple thousands larger.
In addition the aluminum chips that were in the flutes of the
drill were left behind too.

Many have reported success with alum, although it is reported to take a couple of days. Nitric acid is supposed to work the fastest and the best to remove steel parts from Al
without harming the Al.

While both metals are reactive, the oxide coating on the Al
does a much better job of protecting the underlying metal.

blogfast25 - 13-4-2011 at 12:21

Quote: Originally posted by gregxy  
While both metals are reactive, the oxide coating on the Al
does a much better job of protecting the underlying metal.



Not really. Try it with any old piece of Al: the passivation layer protects the metal against air but not against acids or alkalis. Al dissolves real quick in these. By contrast your drill bit is likely to be an alloy, alloyed for hardness and toughness and the alloying metals tend also to increase it's corrosion resistance.

No, something else is at play here but I don't know what. I was thinking maybe something electrogalvanic but that would only speed up the dissolution of the Al.

ThatchemistKid - 13-4-2011 at 22:13

Actually, from what I have read dilute nitric dissolves aluminum but aluminum is not readily attacked by the concentrated acid as it passivizes. And from experimentation concentrated nitric quickly reacts with Iron, although, Blogfast25 is right drill bits are made to be corrosion resistant so it would really depend on what kind of drill bit is stuck.

blogfast25 - 14-4-2011 at 04:42

Quote: Originally posted by ThatchemistKid  
Actually, from what I have read dilute nitric dissolves aluminum but aluminum is not readily attacked by the concentrated acid as it passivizes. And from experimentation concentrated nitric quickly reacts with Iron, although, Blogfast25 is right drill bits are made to be corrosion resistant so it would really depend on what kind of drill bit is stuck.


Yes, passivation of Al by concentrated nitric acid may be possible.