Water heaters can fail early due to internal corrosion of the hot water heater. Your annual visual inspection is helpful, but you can’t see everything. The sacrificial anode rod in a water heater is designed to extend the life of the heater. It is a rod that extends into the heater and is composed of a type of metal (generally magnesium zinc, or aluminum) that corrodes instead of your tank and heating elements.
Generally, an anode rod should be checked about every four years depending on your water conditions. If six inches or more if of the inner wire is exposed, the rod is approximately 3/8 of an inch in diameter or less, or there is build-up of hardened calcium bicarbonate, then the rod should be replaced. The anode is generally located on the top of the heater. Some are easily found and are simply a large hex head that can be unscrewed once the water and power are turned off and some of the water is drained from the electric water heater. Some are part of the dip tube or are located under the lid of the heater. The heater must be unhooked in order to access these style anodes.
It is best to look up the manufacturers specification sheet to determine where yours is located or call a plumber. Regular inspection and replacement of your anode rod will keep the corrosion on the anode and not in the tank. Again, if this is outside of your comfort zone, it is best to call a licensed plumbing professional like A+ Plumbing Professionals. A little expense now can save you a lot in the long run.