Hot water is by its nature pretty corrosive. Overly high water pressure can also do in a water heater prematurely. My granddaddy was having to get a new one about every five years back about 30 years ago, and it turned out his residential water pressure was off-the-chart too high. A pressure regulator took care of the problem. He put in a new water heater in 1988 or '89, and I replaced it last summer when it finally let go. It had been acting less than perfect for some time, but being underemployed at the time, I was trying to nurse it along.