Whale Marine 74556 Whale Seaward Magnesium Anode

What the brand says

The Whale Seaward Magnesium Anode (part 74556) serves as a sacrificial replacement rod for Seaward by Whale water heaters. Constructed from magnesium with 3/4" NPT threads and measuring 4 inches long, it prevents tank corrosion by eroding first in harsh water conditions.