The root of the problem is old, dirty water in the cooling system. On the other hand if you use a proper coolant mix, there are zero corrosion issues, no earthing of the radiator is needed The main reason to use coolant instead of plain water is for its anti-corrosion properties. If the cooling liquid doesn't conduct electricity, there will be no electrolysis problem

Just a hint make sure you earth the block and not the alloy head as Nissan was wont to do in earlier days it cuts down on electrolytic corrosion of the alloy cylinder heads if you earth the cast Iron Block instead of the head and that occurs regardless of what coolant you use.

You would be amazed at what stray electrical charges you can find in cars when you go looking, That is why Police and Military vehicles tend to have many extra braided earth straps compared to Civilian variants.