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Author Topic: Small quesion about sacrificial protection  (Read 279 times)

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UnprofessionalCoder

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Small quesion about sacrificial protection
« on: January 06, 2017, 09:16:01 AM »

Good day,
Now i just want to make sure my understanding of this topic is correct. When iron is galvanized with zinc and this zinc layer is scratched such that the iron is exposed, iron is still resistant to corrosion. This is because when it oxidizes, zinc immediately reduces it back because zinc is more reactive. Is that right? Or have I got something wrong?
Thanks in advance  ;D
PS. This captcha is driving me nuts :D
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thetada

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Re: Small quesion about sacrificial protection
« Reply #1 on: January 06, 2017, 09:24:47 AM »

Pretty much. The zinc only needs to be attached to the iron by an electrical conductor to be preferentially oxidised. Zinc will donate its electrons more readily than iron to any species in the market for receiving electrons.
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Enthalpy

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Re: Small quesion about sacrificial protection
« Reply #2 on: January 07, 2017, 11:55:32 AM »

As an alternative view, you might observe that the contact with zinc puts iron at an electric potential versus the electrolyte that prevents the attack of iron.

But... I tried once to experiment about it, with aluminium wrap foil alone, or in contact with copper, in a salt solution as concentrated as the Ocean, and in gaseous drinking water as well. The result was that the aluminium, despite being little alloyed to be corrosion-resistant, was punched through within few days, and the contact with copper made no obvious change. So:
  • "Aluminium" doesn't resist corrosion. Some special alloys may, under precise conditions.
  • The story of electrochemical contacts should be taken with much mistrust.
  • Maybe it works when the more corrodible metal is very pure. Indeed, very pure zinc in batteries corrodes little when the user draws no current. A tiny addition of mercury to zinc helped in the past.
  • Books forget such circumstances hence are almost always wrong. Never believe books.
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