The fact is, older copper radiators are made up of (4) four different materials, not just copper. The copper tubes are bonded to the fin with solder (lead) and that has very poor conductive properties. The tanks are made of brass and the side channels are steel. An aluminum radiator is 100% aluminum and the tubes are much wider and closer
Copper is a better conductor of heat than aluminum. Copper/brass radiators usually have copper fins, but brass tubes (70% copper, 30% zinc). The bond between the fins and the tubes may be made with soldier (A tin/lead alloy, or high-tin alloy) or with a braze material (mostly copper). Engine Cooling.
The coolant acts like electrolyte in the battery and I remember a test where you would use a voltmeter with the + end in the coolant and the - end to ground and check for voltage. More that a certain amount and the coolant needs changing. I don't remember the exact voltage, but it is a measure of electrolytic activity. Frank
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