What could be causing corrosion on a water heater burner?
There is an insert in the flue that can be removed. loosen it and rotate it in the flue or use a long brush to scrap the sides of the flue then vacuum out the debris. At twelve years old your tank may be nearring the end of its life. You can also check the supply nipples on top of the heater to see if they are leaking and allowing water to seep in to the flue causing the corrosion.