[niehls]

Hi. I have a question regarding the etching of Cr in pure HCl. I am a physicist and have very limited knowledge (unfortunately) about chemistry, so please go easy on me.

I am etching a thin Cr film using pure HCl. In order for the process to start I need to have a piece of aluminum in the HCl, with the tweezer holding the Al touching the tweezer holding the Cr (metal tweezers). There are a lot of bubbles forming at the Al.

I assume there is a redox process going on at the Al with the oxonium ions in the solution with hydrogen gas as a product. My guess is something like:

2Al + 6H(+)_3O -> 2Al(3+) + 3H_2 + 6H_2O

I also suspect the tweezers need to touch in order for electrons to flow from the Chromium to the positively charged Al.

Now: What is happening to the Cr? I read somewhere that the Al has to be there to create an electrochemical potential between the Cr and the Al, and thereby remove the native oxide from the Cr, but I really have no idea how this would work. I would be very grateful if someone could explain this to a physicist trying to do stuff he doesn't really understand.

Regards,
Kristian

[Kellen]

I'm not an expert, but I saw no one has responded so I thought I'd pitch in.

In electrochemistry, electrons will tend to stick with the metals that want them more. Generally speaking, metals won't adopt a negative charge, so it's just a matter of which metal wants the positive charge less. If you look at this link: http://www.engineeringtoolbox.com/electrode-potential-d_482.html you can see a set of standard electrode potentials with some explanation of how they work. The electrons will prefer the aluminum over the chromium, which is why the chromium is "etched" or corroded. However, the aluminum doesn't want a negative charge, so it will dump the electrons off to the positively charged hydronium ions, creating hydrogen gas . [Note: in the link I provided, the series only applies to the elements in a 0 oxidation state. If you have ions of any of these elements, the potential will change drastically.] In this sense, both of the metals are being reduced, but with the aluminum the electrons are being replenished (oxidation) so there is a constant reaction without loss of Al metal. In other words, the reaction you provided is taking place, but the ionized Al will have its electrons replenished immediately by the chromium, causing the aluminum to plate out again. The pH of the solution will increase until the reaction reaches equilibrium. Thus, the acidity of the solution will affect the amount of etching. You may know that already, but I just thought I'd throw it in there.

Hope that was helpful.