A corrosion allowance is the adding of metal thickness to a tank specification beyond what is required by the relevant ASME code. A corrosion allowance is only useful if the metal in question corrodes uniformly and is most commonly associated with carbon steel tanks. If a tank is coated internally and the coating is appropriate for its intended service, then there is no need for a corrosion allowance.

Editor's Note: A reader responded with this additional insight:

Your answer left out some considerations. Assuming we are discussing carbon steel.

  1. Corrosion allowance covers both internal and external corrosion. Many times it is the external corrosion that reduces the wall thickness to unacceptable levels, especially in coastal areas.
  2. From the end user perspective, if his Material Integritey program follows ASME guidelines and the wall thickness is below the min. required, then the vessel has to be repaired or removed from service. The alternative is to possibly down-rate the vessel if you have proper documentation and a company engineer willing to do it.
  3. It used to be that Vessel manufacturers provided extra thickness but now they are cutting it so close that there is no extra wall thickness.
  4. Unless it is a huge vessel, adding 0.0625" CA does not add much to the cost.

    Larry Tarkington, PE