Grounding question

OK, with the copper/brass rad, then the corrosion moves away from the rad and goes to the block & heads. In any electrolysis situation, one material is 'sacrificial', depending on where it falls on something called the galvanic scale. With brass & iron, it is the iron. With AL & iron, it is the AL. So your brass rad is safer from corrosion long term. Again, good, regular changeout of the coolant will keep the corrosion under control.