You will need to get new burners. What causes this is improper water flow creating condensation on the heat exchanger. Condensation is highley asidic and when it drips on the burners it rotts them out fast. To cure this you will need to open the water header on the side of the heater. Inside is the by-pass and thermal control for the water. Replace both of these while you have it open, if one is bad the other can't be far behind.