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Disclaimer: I have very limited knowledge of chemistry

My problem is the following: I have motorcycle part made from some sort of Aluminium alloy - most probably 6005, 6061-T6 or 7005. And inside of this aluminium part there is steel part which is simply stuck there and it's really hard to get it out physically without damaging the aluminium part.

After a bit of googling, I've found out several ways of doing that:

  1. By putting aluminium part with stuck steel part into water and start adding concentrated nitric acid until steel part starts to dissolve. But I've also read that dillute nitric acid (which will occur after mixing with water) will also dissolve aluminium or I'm wrong here?

  2. By using some sort of Alum: either by using Potassium alum or Ammonium alluminium sulfate. Some guy on the forum suggested to dissolve above alum in hot tap water and then put aluminium part with steel part into resulting solution and let it stay there for a day. It should dissolve or at least damage steel part without affecting aluminium part which may ease physical removal of steel part.

Could you please advise which of above solutions is better? And in case of using alum - which of them should I buy - potassium or ammonium one?

Thanks in advance for any help!

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The part of the steel insert that is stuck to the aluminum is not accessible to water. Let's look at linear coefficients of thermal expansion,

http://www.engineeringtoolbox.com/linear-expansion-coefficients-d_95.html

Aluminum is 22.2 ppm/K. Steels are smaller, the largest being 304 austentic at 17.3 ppm. Ambient temp is around 24 C, a 400 F kitchen oven is around 204 C. That should not be hot enough to lose temper in the alloys. If the aluminum hole is an inch in diameter, worst case increasing the temp 180 K times 4.9 ppm/K net gives you 882 ppm or 22 microns of slack. The steel insert pushes out.

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  • $\begingroup$ Thanks for your response! Looks like it's easier than I thought :) However, for more complex cases like broken steel screws inside aluminium parts which cannot be extracted - do methods in my question at least make sense? $\endgroup$ – Yuriy Nakonechnyy Apr 8 '14 at 15:28
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    $\begingroup$ According to chemwiki.ucdavis.edu/Under_Construction/… you are in bad position for any type of etching process. Aluminum will be oxidized preferentially over iron. $\endgroup$ – ssavec Apr 8 '14 at 16:34
  • $\begingroup$ @UncleAl tried to heat aluminium part yesterday and managed to push steel part out - thanks again for your help! Accepting your answer $\endgroup$ – Yuriy Nakonechnyy Apr 10 '14 at 8:20
  • $\begingroup$ @ssavec thought I didn't understand the article, thanks for mentioning that any type of etching won't work in my case :) $\endgroup$ – Yuriy Nakonechnyy Apr 10 '14 at 8:25
  • $\begingroup$ @Yura: As Uncle Al wrote in the other answer, there can exist specific conditions, under which you can etch iron and keep aluminum "intact". It will result in something like elox surface. Which you probably do not want inside your motor. $\endgroup$ – ssavec Apr 10 '14 at 11:51
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"However, for more complex cases like broken steel screws inside aluminium parts which cannot be extracted" Iron touching aluminum is a shorted electrolytic cell. The aluminum will preferentially erode in a wet corrosive environment. Hot concentrated nitric acid will passivate an aluminum surface by oxidation. Even if it preferentially goes after the steel, that is extremely hazardous - including inhalation.

Screw removal/extractor sets are tapered left-threaded hardened drill bits. As they drill into the broken stub they unscrew it. It still helps to heat the piece to equilibrium to disengage aluminum from steel. You don't want galling as you force out the steel.

http://www.harborfreight.com/12-piece-screw-extractor-set-40349.html
http://www.sears.com/craftsman-3-pc-screw-out-174-damaged-screw/p-00952154000P
http://www.liquidwrench.com/

Add a drop of silicone oil, Kroil, WD-40, Liquid Wrench, or Mobil 1 0w20 before starting. Silicone ends up everywhere. This is a disaster in a high vacuum lab. A machine shop is different.

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  • $\begingroup$ yes, already got screw extractor sets and used them with partial success. Just was curious whether it's possible to apply chemical compounds in this case which will react with steel but not aluminium... Regarding nitric acid - yes, I've read about it and I'm not so eager about using it because of hazards. The other solution with alum seemed more appropriate for me, but it seems it won't work as well... Thanks a lot for your time and help! $\endgroup$ – Yuriy Nakonechnyy Apr 8 '14 at 17:55
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This is an old post but these methods sound like overkill. If I couldn't get it out I would drill it out and re-tap the hole or insert a helicoil. Similar to what is done when a spark plug strips an aluminum head. Ask any old shop guy. They have been dealing with problems like this long before anyone thought of dissolving the steel.

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  • $\begingroup$ Yes, now I'm just re-tapping the hole and use bigger bolts if it's impossible to drill out the bolt. I was just curious if it's generally possible and feasible, since this looked quite effective to me :) $\endgroup$ – Yuriy Nakonechnyy Aug 29 '16 at 7:39
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If steel is really stuck in a blind hole in an aluminum part, immerse in a heated alum solution for several days. There wil be very little solid ferrous material left but a crust.

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The alum in water method works wel;. It is safer, cheaper, and can be accelerated with a small amount of heat, such as from a low wattage incandescent lamp.

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