If you have a large number of pits, remove the pitted area by grinding/machining. Then weld build up the clad area removed. You should also evaluate the corrosion observed and determine whether to change the corrosion resistant alloy.
Weldstan, weld build up on pitted surfaces, isn't there a minimum requirement for remaining thickness below the pitting? I remember with pressure vessel that at least 50% remaining wall thickness for weld build up back in the 80's and I don't know if that rule of thumb is still applicable.
Your only building up the overlay. If the pitting goes into the base metal, you can corrosion resistant weld overlay the base metal (requiring appropriate procedure qualification per ASME IX or other applicable Code) after removal of the affected area. This is done routinely.
