What Role Does Seawater and HCl Play in Electrochemical Corrosion?

[Alvine Alvin]

Hi there,

So this isn't actually my homework, I've been assigned to TA an electrochemistry lab. Problem is, I know almost nothing about electrochemistry and although I've tried to learn I'm still a bit rusty (har har!). The lab involves immersing different electrodes in seawater in a single beaker and measuring the potentials across them, and I have a few questions:

1)What exactly does the seawater do? If the potential is higher because of it then presumably it increases ionic conductivity. But the metals will almost certainly both be more cathodic than sodium and what should happen is that the anode is oxidised and then its ions move to the cathode and are reduced, right? So where does the ionic content of the seawater come into it? does it just shield the free ions and make it easier for them to move?

2)There is a 'discussion question' which asks what would happen if I joined gold and copper electrodes in a beaker of HCl. What happens here? Presumably copper is oxidised at the anode and moves to the cathode? What does the HCl achieve?

3)Then they ask if 'actual corrosion' would happen in this case? I don't know what this means. They ask if it would happen if we bubble air though the solution. If we did that presumbly the Cu(II) ions could react with O2 and form deposits of CuO on the gold electrode? But even if we did not I think that copper would still be degraded and deposited onto the gold electrode?

If you guys can provide any guidance on these, that would be wonderful!

Thanks,

Alvin

 

[Borek]

Seawater is just a (relatively) good conductor - you need to close the circuit.

Gold and copper in HCl - IMHO nothing should happen, both are noble metals.

No idea what they mean by 'actual'. But as I don't expect any oxidation to take place, I bet there will be no corrosion, unless it will be called 'nonexistent' or 'imaginary'.

 

[Alvine Alvin]

So the Na+ and Cl- ions don't do anything, they just improve conduction of the metal ions?

Surely some potential is established between gold and copper though, as they have different positions in the emf series?

 

[Borek]

So the Na+ and Cl- ions don't do anything, they just improve conduction of the metal ions?

Conductivity of the solution.

Surely some potential is established between gold and copper though, as they have different positions in the emf series?

Yes, but regardless of that surfaces of both are still at the safe potential (above hydrogen half cell). That's not the case when you connect iron/magnesium, iron/zinc, copper/zinc and so on - but it is always at least one metal that is more active than hydrogen.