Water heaters very short life Why?

We have two 40 gallon water heaters in our house and we do not get 5 years out of them before they develop leaks in the tank. Last relpaced heater made it 3.5 years befor failing. What could be going on? Our area has very hard water so we have a water softener. The heaters have dielectric unions on the connections and new ones installed with the heaters. Have an expansion tank installed in the system. Blow down the heaters every year. Any ideas?

Electric or gas powered?

If you have a decent multimeter, see if you have any voltage between the tank and a good ground.

Have you ever looked at the anode to see if it was all eaten up? You may need to change out your sacrificial anodes periodically.

How much pressure do you use when blowing down the heaters? (sounds like an air compressor is being used?) Excessive pressure can stress the vessel and may open up cracks.