1
$\begingroup$

I have a number of metal files that, to my regret, I have used on aluminum which gets stuck to the teeth. As suggested on several mechanics forums, I have tried soaking the files in concentrated NaOH solution without result.

Would connecting a DC source to a file anode and a copper cathode in an acidic CuSO4 solution preferentially remove the aluminum from the file? If so, what pH and voltage would work best?

If not, is there another method that would work?

$\endgroup$
5
  • 3
    $\begingroup$ You will be at the edge of a problem. Files are very hard steel ( eg Rockwell C 64), Steels hydrogen stress crack when the hardness is above about Rockwell C24. Acids generally put hydrogen into steel , cathodic charging really puts hydrogen into steel. I would try very hard to remove the aluminum with wire brushing. If you try acid and the files do not appear cracked , bake them at 300 F for an hour to remove hydrogen. That temperature is unlikely to soften/temper the files . ( 400 F is more typical bake but that may cause softening ). $\endgroup$ Oct 29, 2021 at 20:22
  • $\begingroup$ It requires time ( minutes to days) for the hydrogen to enter . diffuse to some location and concentrate enough to cause a crack. So it could be possible to clean the files then immediately bake them . Unfortunately , I don't think anyone could say how much time. $\endgroup$ Oct 29, 2021 at 23:05
  • 3
    $\begingroup$ Have you tried mechanical removal by using a file card? $\endgroup$ Oct 30, 2021 at 12:00
  • $\begingroup$ @the last line of the question: how about ultrasonic cleaning? Some powerful units can induce cavitation that might knock off aluminum particles. $\endgroup$ Oct 31, 2021 at 17:01
  • $\begingroup$ How about phosphoric acid, it will dissolve the aluminum and put a phosphate coating on the iron at the same time. Not sure what best conc would be, it could be looked up. $\endgroup$ Mar 26, 2023 at 8:46

2 Answers 2

1
$\begingroup$

Commercial cleaning agents containing nitric acid will passivate non-stainless steel, but nitric acid doesn't dissolve aluminum readily, so plain nitric acid (at concentrations ~27% or more) won't attack the steel or the aluminum.

However, the addition of a little fluoride will allow a small degree of corrosion of the steel. In the case of stainless steel, the steel will remain passive after extraction (and rinsing) from the solution, even tho a small amount of steel may have been removed.

On the other hand, aluminum can be cleaned with a relatively low concentration of nitric acid if the solution contains a significant amount of fluoride.

Granted, fluoride (as HF solution or NH$_4$HF$_2$) is dangerous, but can be handled safely if sufficient care is used.

It would seem that some balance of water, fluoride and nitric acid could result in rapid corrosion of the aluminum while at the same time barely touching the steel. I'm going to make a stab at 27% HNO$_3$ + 2% HF (or 3% NH$_4$HF$_2$) + 70% H$_2$O for a starter attempt. This is guaranteed to fizz up, dissolving the finely divided aluminum. If the steel is affected too much, increase the nitric acid concentration (yeah, counterintuitive, but higher HNO$_3$ passivates steel better).

I could name a company that sells these materials.

$\endgroup$
0
$\begingroup$

Try a variation of the classic bleach battery which has an anode of aluminum in aqueous NaOCl/NaCl. The latter is commonly sold as disinfecting chlorine bleach prepared from the action of chlorine on aqueous NaOH.

However, in place of the usual noble cathode of copper, I would try instead a freshly prepared inexpensive high surface area carbon cathode. For example, I have experienced good results from burnt food, to quote a source:

In cooking utensils, burnt food may produce carbon deposits with resulting rapid galvanic corrosion from the aluminum-carbon couple [474, 475].

Consider burnt bread with which I have had some success. Studies on advanced oxidation processes (AOP) interestingly cite, per my recollection, also the use of burnt coconut shells (likely reflecting local access).

Also, applying heat should promote the rate of the anodic corrosion of the aluminum (alloy?), which normally involves an inception period (which is characteristic of an electrochemical reaction).

Rationalization: Your Al contamination on the steel file may consist of an aluminum alloy, making it less anodic (so that, its tendency to corrode is reduced). Also, the relative surface area of the Al-steel anode to the cathode may not be small, which also reduces its rate of corrosion. Hence, the recommended use of a high surface area carbon cathode. Now, one could purchase activated carbon but I would try my home recipe if your problem is expected to be ongoing and you are looking for an efficient low-cost solution.

Note: In the current situation, the iron/steel file is the cathode which is coated with aluminum. As such, the cathode has little surface area relative to the anodic Al, which is the reverse of what one should have for fast electrochemical dissolution.

$\endgroup$
1
  • $\begingroup$ @AJOER Thanks for your answer. I will give your recommendations a try. It seems less perilous than concentrated HCl, HNO3, etc. A few questions: Would a charcoal briquette work as the cathode in place of the burnt toast or coconuts? Could I speed up the reaction by connecting the electrodes to an external power supply? (Fe/Al +, C-?) $\endgroup$ Nov 12, 2021 at 18:43

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.