Cars and trucks have aluminum, iron, copper, brass and lots of other metals wetted by the coolant. This multiplicity of metals and their consequent galvanic action makes the coolant's anti-corrosion additive package work hard, at least until it's exhausted. Add in a stray electrical current, and you've got problems.
Here's a quick check for stray voltage. Set your high-impedance voltmeter to a low DC voltage range, and connect the negative probe to a good ground on the engine or to the negative battery post. With the engine cold, remove the radiator cap. Insert the positive probe into the liquid coolant through the radiator filler neck--but don't touch the filler neck or the fins. Start the engine. Hold the probe so it contacts only the coolant. A voltage reading of more than a few millivolts could be the source of your problem. Engine manufacturers have reported issues with aluminum-alloy engines with sustained voltages of 15 millivolts. Repeat this test with the meter set to the low AC voltage range. Turn the engine off before it heats up enough to boil over and cook you like an Easter egg.
Where is this stray voltage coming from? Check ground straps and connections for excellent electrical contact. Marginal-looking ground straps should be replaced, then connections cleaned and tightened, and covered in Vaseline, lithium chassis grease or dielectric paste to prevent moisture-driven corrosion.
