Strange hot recirc. issue

I have a customer with a 75 gallon Bradford. Everything is installed correctly with a timer & electronic thermostat controlling the pump. Brass nipple & tee before copper recirc. line connects to the heater. I was there on second pinhole leak in the 1/2" C piping adjacent to the pump (Grundfos UP15-29SU). Pump was very hot as well. You can see channeling in the pump unions, pipe & fittings. Everything is chamfered. There was a solid block of "something" where the brass nipple connected to the tank. I was able to hammer it out & everything seems to work fine now. I've run into this before. Any idea as to what's going on. Stray currents? Cavitation of some kind from the blockage? No soft water on this install. I'm in Newport Beach, CA

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