So I had a great local plumber replace galvanized with PEX five years ago. All the pipes they cut out were filled with corrosion like in Tuttle's photo. We chose to route the PEX like the old galvanized pipes.
Soon thereafter, the gas water heater failed. It was a decent Bradford White, still under warranty, installed by the same firm. The plumber contacted Bradford White, who authorized a new control unit, at no cost to me.
Within a week the water heater failed again. The plumber and I got on the phone with the pro support tech at Bradford White. After 20 minutes the tech said, "As long as the unit is grounded properly to the pipes this shouldn't be happening."
Bingo. The copper ground r0d was at the electrical service panel on the far side of the house, and the old galvanized pipes had been strapped to ground nearby. When they replaced the galvanized pipes, the gas water heater in the garage was no longer grounded through the plastic PEX. I installed a new ground strap and it has worked perfectly ever since.