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If you system is creating 12V, you have a new perpetual motion machine and free electricity. Start you car, run an inverter off it and power your electronics simply from the corrosion in your system.

A galvanic reaction is in the mV, not volts. I am not denying you have a problem, but I think the magnitude is wrong. I have never heard of this and I been messing with cars 30 years. I am a shade tree mechanic, not an expert, but I am a professional electrical engineer. You need to watch how the snake oil salesman is presenting the snake oil. Make sure he did not take the "oil" out of a rain barrel.......

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Even if you had the perfect chemistry to form a primary cell, you would have from 1.2 to 2 Volts maximum. There is something else wrong. Disconnect the battery and measure to ground again. It should go away. If it does, I would look for a water temperature sensor with a broken insulator. It probably will not be easy to remove because of the electrolysis welding it into its socket.
Craig