Cathodic protection (1 Viewer)

[crammy90]

Sacrificial anode: a more active metal is connected to the metallic hull to form an electrochemical cell for which it is the anode and oxidises. Its loss of electrons are gained by the cathodic steel hull to reduce the hull. As corrosion of a hull is when the metal hull is the anode, no corrosion occurs and the hull is protected. E.g. Zinc to protect a steel hull

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ANODE (-): Zn(s) --> Zn2+ + 2e—

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CATHODE (+): O2 + 2H2O(l) + 4e-- --> 4OH-
why is this the cathode reaction and not iron reducing?

 

[crammy90]

please some1 ive looked everywhere

 

[Azreil]

Have a look at your table of standard potentials (on the back of the periodic table). You'll find that water is more easily reduced than iron, hence is reduced in preference to the iron.

 

[henry08]

Lol , I thought this thread was going to be something about condoms.

 

[crammy90]

so pretty much nothing happens to the iron as the as iron is loosing electrons and the water is gaining them in preference.
so the iron isnt involved at all?
but i guess when the Zn goes the iron oxidises to reduce the water yeh
and then rusting occurs?
my teacher said the Fe was gaining the electrons from the Zn which was oxidising, so the Zn looses electrons and iron gains them. I guess what u said makes sense as well iron and Zn cant both oxidise lol.
like i mean Zn wouldnt oxidise, Fe oxidise, and then the Fe gain the 2e from the Zn. So yeh thanks for that.

 

[Azreil]

Iron isn't losing electrons. Zinc is.

The iron is WHERE reduction is occuring but it is not actually involved.

If the Zinc was allowed to be completely degraded, then the iron would eventually oxidise and hence rust. If someone's gone to the trouble of protecting a hull in this way though, they aren't going to let the zinc run out.

Hope it helps.