46 CFR §54.01-35

1. (a)Vessels or portions of vessels subject to corrosion must be as required by UG-25 of Section VIII of the ASME BPVC (incorporated by reference; see § 54.01-1) except as noted otherwise in this section.
2. (b)The pressure portions of pressure vessels must:
   1. (1)Normally have a corrosion allowance of one-sixth of the calculated thickness, or one-sixteenth inch, whichever is smaller, added to the calculated thickness as determined by the applicable design formula.
   2. (2)Be specifically evaluated in cases where unusually corrosive cargoes will be involved, for the possible increase of this corrosion allowance.
   3. (3)Have no additional thickness required when acceptable corrosion resistant materials are used.
   4. (4)Not normally need additional thickness allowance when the effective stress (either S or SE depending on the design formula used) is 80 percent or less of the allowable stress listed in Section VIII of the ASME BPVC for calculating thickness.
3. (c)Telltale holes must not be permitted in pressure vessels containing dangerous fluids, such as acid, poison, corrosives, etc.
4. (d)Exemption from these corrosion allowance requirements will be granted by the Commandant in those cases where:
   1. (1)The contents of the pressure vessel are judged to be sufficiently noncorrosive; and,
   2. (2)Where the external surface is also protected from corrosion. A suitable vapor barrier is adequate protection, while paint or other thin coatings exposed to weather or mechanical damage are not acceptable.
5. (e)No applied linings except as provided in part UCL of Section VIII of the ASME BPVC are acceptable.