I haven't read B31.4, B31.8 so not sure they deduct corrosion allowance when they evaluate stresses.

When you set the 'All case corroded' option to be true, it will deduct thickness from an amount that you specified as corrosion allowance when you calculate the SUS, OCC and EXP stresses. When I had tested it for myself, stresses were greater(since section modulus reduced), but the weight wasn't changed.

In my opinion, IF you're piping line transfer corrosive fluid, then the option should be used. Otherwise, as long as code doesn't say to consider corrosion, then just don't use it.

Take a look at this old post about the matter,

http://65.57.255.42/ubbthreads/ubbthreads.php?ubb=showflat&Number=42550