In my experiences working with ASME Section VIII (Div. 1) and Section I, the normal corrosion allowance for carbon steel is 1/16 in. (I normally work pressure vessels containing water). Sometimes companies request that we follow the recommendations of HEI (heat exchange institute) which recommends 1/8 in. corrosion allowance (carbon steel). Stainless steel vessels containing water usually do not have a corrosion allowance.