Corrosion Allowance
Corrosion allowance is extra wall thickness intentionally added to a pressure part, vessel, or component so that expected metal loss from corrosion or erosion never reduces it below the required minimum thickness during its service life. In practical design, final required thickness equals design thickness plus corrosion allowance, typically 1.6 mm to 3 mm for carbon steel, with less for stainless.
On the shop floor, corrosion allowance rarely appears as a separate machining dimension. It is a service-life margin that must survive the entire fabrication sequence. The fabricator has to hold the as-built thickness high enough that blasting, forming, weld fit-up, weld shrinkage, and final machining cleanup do not eat into the wall required for years of fluid contact, coating wear, or field service. In a CNC cell, this gets confused with machining allowance, but they are not the same: machining allowance is stock left for finishing, while corrosion allowance remains in the finished part. If a programmer treats it as extra material to remove, a bore or face can end up below the minimum required wall. On plate and pipe, mill tolerance can also stack against the allowance, so the finished part may start life thinner than the drawing intended.
Is corrosion allowance a material property?
No. It is a design allowance selected for a specific service environment and design life, not a fixed property of the alloy, plate, or component.
Does corrosion allowance replace coating, plating, or passivation?
No. It is a separate layer of design protection. Coatings and material selection reduce the corrosion rate; the allowance covers the residual metal loss that still occurs over time.
How is corrosion allowance usually determined?
It is calculated from expected corrosion rate × design life, then adjusted by code basis, operating history, inspection data, and owner standards; some sources add a safety margin for conservatism.