[kenny1999]

my solution book said calcium cannot be a possible metal to do sacrificial protection for iron. Why??? Has it made wrong??

[DevaDevil]

while you are right that calcium has a lower reduction potential than iron, it has SUCH a low reduction potential (similar to lithium) that it is impossible to keep calcium a metal under ambient conditions. It oxidizes too easily.

[kenny1999]

while you are right that calcium has a lower reduction potential than iron, it has SUCH a low reduction potential (similar to lithium) that it is impossible to keep calcium a metal under ambient conditions. It oxidizes too easily.

sorry I don't understand :<

[DevaDevil]

no it cannot be used.

it oxidizes easier than iron, which would fulfill one of the requirements for "sacrificial metal", but it oxidizes so easily that it is not stable enough to be used in practise.

[kenny1999]

no it cannot be used.

it oxidizes easier than iron, which would fulfill one of the requirements for "sacrificial metal", but it oxidizes so easily that it is not stable enough to be used in practise.

then how about potassium or sodium? Can they be used for sacrificial protection/

If not, do they have the same reason as Calcium?

thank you!!

by the way, I'd like to know why Calcium ion is less easy to turn back to Ca(s) in Electrochemical series, in comparision with Sodium.. while Sodium is higher than Calcium in the reactivity series, which means sodium ion is less easy to return back to Na(s)

[DevaDevil]

then how about potassium or sodium? Can they be used for sacrificial protection/

[/quote]

cannot be used, for the same reason.


the standard reduction potentials for both calcium (2+) and sodium are fairly close.

[Enthalpy]

Hence people use zinc or (forbidden by RoHS?) cadmium.

The need is similar to batteries: a battery should consume the electrode only if current is drawn, not spontaneously. Hence zinc, manganese instead of potassium.

Yes, lithium makes good batteries now, but the electrolyte is not water - this was the difficult enabler.

Though, I'm less than convinced by anodic protection... At least in the very (very!) crude experiments I made, such a protection made little difference. Could this be one more case where a seducing theory is preferred over ugly facts?

O, and "iron" isn't available as a construction material. Steel is, in tens of thousands of different alloys, which behave in varied ways under corrosion - something a chemical potential won't tell you.

If you have a practical need, don't rely on sacrificial protection. Cover steel with an authentic layer (nickel, chromium, maybe zinc, not phosphate), made by a specialized company. This is nothing that can be predicted or re-invented in a day nor even a year.