One method is to examine the corrosion rate (UNIFORM Corrosion).
1. determine the material thickness (use ultrasonic techniques) at several locations, heads, shell, nozzles etc.
2. From the Original design drawing get thickness.
3. Determine what the losses are. mills per year (CORROSION RATE)
4. Determine the required thickness ASME SECTION VIII Div 1
type of calculations
5. Estimated life in years is (Measured thickness - Required Thickness)/ Corrosion Rate
For pitting corrosion you need to measure the pit depth
The assumption is that the process does not change. Any change in the process can have a dynamic effect on Corrosion rate.
Hope this help.
Rich