Mojo,
Code B 31.3, B31.1 does not address about using the corrosion allowance in calculating the SIF value. But there is a note under appendix-D regarding the special consideration needed for closely loaded branch connection. If we consider that your manifold is closely loaded with the branches, then it is better to check both the options.
Regards,
R.k.
Edited by RK (06/08/16 05:55 AM)