Galvanic Protection of Metals

All non-zero potentials.

It depends upon the wetted metals only.

Platinum electrodes, for example, won't corrode in seawater, whetever the potential difference applied to them.

Don't expect the magnesium in the magnesium/steel system to last very long. Magnesium is often used as a sacrificial anode to protect steels (marine outboard engine propeller-shafts, for example) that are immersed in seawater.

Galvanic corrosion chart. Search for it on the web.

I could see the potential values in the galvanic corrosion chart. It is obvious that, the closer the values, the better the galvanic corrosion protection. The question is:

how big the potential difference is that could be critical or maximum in deciding material combination?

Any potential is too much.

Perhaps a look at the electromotive series will help.

Standard electrode potential (data page) - Wikipedia, the free

You will have to decide how much material loss over time you can tolerate.

Thank you all for the comments!