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Cadmium coatings, how do they work?

With reference to standard cell potentials: I understand how zinc acts as a cathodic protector of steel, becoming the anode and hence corroding. But cadmium has a cell potential the other side of iron, so how does it become the sacrificial layer? Data I have as cell potentials is Zn... -.76V, Fe... -.44V Cd... -.4025V Thanks for any understanding anyone can give.

Andrew Graddon
student - Newcastle, NSW, Australia
2003

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2003

There is a little bit of confusion about what the "standard series" means. I believe you dissolve a mole per liter of the metal into solution, then you put a bar of that metal into the solution and measure its potential to either further dissolve or to plate out. While this gives you some general trends about cathodic protection, these are somewhat coincidental and occasionally analamous because you are not really measuring one metal against another in a real-world environment, just one metal against a high concentration solution of itself. Further, because of the difficulty of dissolving a mole/liter of some metals, some of the values are extrapolated from measurements taken at lower concentrations.

What you actually could look at is the "seawater series" where the solution is seawater (more realistic) rather than a high concentration solution of the metal. When you do that you will find that cadmium is cathodic to iron, as you would like to see. letter 671 will steer you towards some charts.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey