You should attempt to obtain corrosion rate data from reliable technical sources for the pipe material, based on the type of corrosion mechanism(s) that is specific to your process. You will need to know process conditions (fluid, temperature, pressure, etc) to be able to identify the corrosion mechanisms for the pipe material. Based on the corrosion rate, an acceptable corrosion allowance can be calculated.
The sources for corrosion rates and possible corrosion mechanisms can be NACE or corrosion engineering handbooks.
The "acceptable corrosion limits" would be the corrosion allowance in the original design, in the absence of more detailed information. To determine this, you can look at the piping specifications for the system, or other information developed during the original design of the system. The inspection department should also have this information documented.
For a more detailed evaluation, you can "rerate" a piping system based on the actual metal thickness and the design conditions to calculate a new corrosion allowance. Piping generally has more thickness available for corrosion allowance than the minimum specified in the orginial design, since standard pipe thicknesses are used.
