rust at anode or cathode?

[SamuelTesla]

Rust formation basically has two steps: iron is oxidized to Fe2+, which is then again oxidized to Fe3+.

Kaplan says rust is formed at the cathode, because F2+ ions formed at the anode always migrate to the cathode (the example is that of a ship in water). But I don't understand how or why. Isn't the exposed iron the anode, because iron is getting oxidized there, and the water/oxygen the cathode? And how is the exposed iron serving as the cathode? Iron is getting oxidized, so how can it be formed at the cathode?

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[vsl5]

i think rust formation depends on the oxidation of iron atoms and the reduction of oxygen to hydroxide and then the iron reacts with hydroxide to form iron hydroxides (solid). Could you post the full question with relevant equations occuring at the anode and cathode?

 

[kehlsh]

Electrons travel into the cathode, where it crashes into the cations on the surface of the cathode. This is because reduction occurs at the cathode to receive electrons. M+ (aq) + e- → M (s).

 

[ModerateMouse]

Here is one site that somewhat explains rust.

It seems that you can't really state "rusting occurs at cathode" or "rusting occurs at anode". Everything is mixed together.

But here is a site that suggests that it occurs at the cathode.

It seems that the O2 is the species reduced at the cathode (to OH-). Since the OH- is now at the cathode, it precipitates with Fe2+, forming rust.

So, you are right in that the Fe2+ is produced at at the anode, but this species then needs to 'diffuse' to where there is a high OH- concentration (the cathode) to precipitate out.

Presumably the O2 in the air then oxidizes this further to a compound containing Fe3+ (as seen in the picture above).

 

[ModerateMouse]

Ironically enough, I was doing a TPR passage on rust and completely failed.

Instead of making a new thread, I thought I would post my question in here.

TPR Physics Passage #87

The set-up was done this way:

Reaction 1 (occurs between the water drop and the metal surface)

Fe(s) + 2H2O --> Fe²⁺(aq) + H2(g) + 2OH^-(aq)


Reaction 2 (drawn at the drop/atmosphere/metal interface)


2Fe²⁺(aq) + O2(g) + H2(g) + H2O --> Fe2O3(s) + 4H⁺(aq)


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Here is a breakdown of the reactions that occur (done by me, not included in the passage).

Reaction 1 oxidation

Fe(s) --> Fe²⁺(aq) + 2e^-

Reaction 1 reduction

2H⁺ + 2e^- --> H2(g)

Reaction 2 oxidation (I)

2Fe²⁺(aq) --> 2Fe³⁺(aq) + 2e^-

Reaction 2 oxidation (II)

H2(g) --> 2H⁺(aq) + 2e^-

Reaction 2 reduction

O₂(g) + 4e^- --> 3O^2-


The question and correct answer were:

1. With respect to the summation of reaction 1 and reaction 2, identify the cathode and the anode in the schematic above.

"The iron/atmosphere/droplet junction is the cathode and the iron/droplet surface is the anode"


This really frustrated me.

The reasons:
1) At the "iron/droplet" junction, the iron is oxidized (from 0 to 2+). So, it is an anode. But, the hydrogen from H₂O is also reduced to H_2(g).
The same location is therefore also a cathode!

2) At the "iron/atmosphere/droplet" junction, the O_2(g) is in fact reduced to 2O^2-. So, it is a cathode. But, the Fe²⁺ is also oxidized to a 3+ iron at the same junction.
The location is therefore also an anode!

How can they so completely ignore the existence of the other reactions?

Or am I missing something?

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One possibility I can see:

What must happen at the interface itself? Maybe the Fe(s) loses electrons at the interface, but these free electrons can diffuse in solution and reduce the H2O(l) further away.

So, the reduction happens at some distance away.

I don't know if this is possible, but if it is then it would make the "iron/droplet" junction just the anode (and not the cathode).

But then this logic fails when it comes to the "iron/atmosphere/droplet" junction, since this junction would also only be an anode.

I am out of ideas....
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[4563728]

Bump the last post, I have all the same questions/confusion the last poster did.

If iron is being oxidized and H2O/ O2 is being reduced, how can we specify which is the anode and which is the cathode? Wouldn't the iron surface be the anode since that is what is being oxidized and the atmosphere and water is the cathode since they are being reduced? (This is for the PR passage)