[VUBstudent]

I understand what is happening if you look at battery reactions and corrosion reactions. But one of the questions in a model exam was: What is the difference between them.

Sorry for my bad english. I hope you understand what my question is.

[chenbeier]

In a battery chemical energy is converted to electrical energy. At corrosion and also electrolysis the opposite takes place electrical energy is converted to chemical energy.

[VUBstudent]

Thank you!

[Enthalpy]

Imagine a usual battery where the negative electrode is consumed. This should happen only when the user draws current, that is, a good battery should have no self-discharge. It is a difference between corrosion where a metal is degraded without producing any current.

This behaviour of long shelf life is difficult to obtain. It works only with some metals and electrolytes. For instance with lithium and watery electrolyte, no chance. It also need metals of high purity, and sometimes carefully controlled additives. Hg was added to Zn years ago, making used batteries polluting, but meanwhile a higher purity of Zn avoids Hg completely.

When an alloy contains different phases of varied compositions, often precipitates that serve to strengthen it, the phases use to have different redox potentials and are electrically in contact, which creates many tiny batteries in short circuit. This is said to accelerate corrosion, and indeed, the precipitation hardened alloy families tend to resist corrosion less well.

A frequent recommendation to prevent corrosion is to avoid contacts of alloys with different redox potentials. A made very few trials and saw no difference. Possibly the explanation based on metal couples applies only to very pure metals. Commercial metals are usually alloyed, so they contain already many local corrosion couples, and adding one global corrosion couple more changes nothing. That would be my explanation attempt.