Boiler Trouble - 0.5VDC coming from where?

Hi all,
I posted a message in December (I believe) about some sacrificial magnesium anodes in a small boiler that were being eaten like candy. After more
investigation (and removal of some aluminum faceplates & coverings), we discovered that there is a constant 0.5VDC between the rod (after we isolated it with plastic for the test) and the boiler. This voltage has a current source of about 2 or 3mA. This doesn't appear to be related to any electrical connections since we shut the power off, disconnected the grounds and it's still there. The problem appears as if it is chemistry-related. The owner has had his water tested and they say it has a "high conductivity". Any ideas on how to verify and remedy this situation? The anodes are rather expensive and at the rate they are being eaten, it's taking all the benefit of the boiler and tossing it out the window.
Thanks for any additional advice.
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