Yes, Gerhard, I figured that out when I started cleaning it up. The base was also starting to disintegrate. l cleaned then descaled the tank with CLR, and inspected the tank and am putting it back in with a new element/thermostat, and zinc. I have two extra zincs and will check it every year from now on.

Anyone have any ideas how to check for an electrical leak that may be causing the zinc to disintegrate?  I think I changed the half-gone zinc 3 years ago when the old element developed a short.  This time all was working well and the element has appropriate resistance, so I don't think that is the problem. The tech at QUICKUSA said when the zincs go it's usually due to an electrical leak. It's a 220AC heating element, connected to sea water only by the heat exchanger tubing and coolant.  The 220 ground wire is connected to the boat's 220 ground.  I don't have it connected to the bonding system.  I have a galvanic isolator but rarely ever use shore power for brief periods. Because it's a magnesium anode, it would be used up before the boat's zincs...if indeed there is a connection somehow.