Corrosion allowance

Corrosion allowance is thickness intentionally added before equipment enters service. It gives a design a defined reserve for forecast metal loss, but it does not predict where corrosion will occur or replace an inspection plan. The project obtains the required value and calculation rules from its governing design code and specification.

The nearest term is remaining wall thickness, a measurement made after fabrication or during service. Corrosion allowance is an input to the original design; remaining wall is observed condition. Weld cap or local reinforcement is not a substitute because pressure-design thickness is assessed at the locations and by the rules the governing code defines. For process piping, confirm the design basis in ASME B31.3; for a pressure vessel, use the applicable ASME Section VIII rules and the project specification.

Worked example: reserve or current measurement?

Assume a simplified project calculation requires 8.0 mm of structural wall and the design specification adds 2.0 mm of corrosion allowance. Before mill tolerance and fabrication allowances are considered, the design basis is 10.0 mm. The 2.0 mm is the corrosion allowance because it was added prospectively for metal loss.

Years later, an inspection measures 9.1 mm at the same location. That 9.1 mm is remaining wall, not a revised corrosion allowance. In the simplified arithmetic, 0.9 mm of the original reserve has been consumed, but the measurement alone does not declare the component fit for continued service. The engineer still has to apply the governing code, measurement uncertainty, actual geometry and the project’s retirement basis. An acceptance criterion for a weld indication answers a different question again; it does not set the corrosion reserve.