The additional thickness, commonly 1.5 to 6 mm (1/16 to 1/4 inch), added to pipe, vessels and tanks on top of the pressure-design minimum, sized from expected corrosion rate times design life. Inspectors track remaining wall against the minimum required thickness, not nominal, to work out remaining life. With localized attack, the allowance can be gone in one spot long before the average says it should be.
We've eaten 2.5 of the 3 mm corrosion allowance on that drum, plan a replacement for the next turnaround.
Also heard as
- CA
- corrosion margin
Related terms
Uniform corrosion
Corrosion spread evenly over a whole surface, thinning the metal at a fairly predictable rate.
mpy
Mils per year, the US corrosion rate unit: thousandths of an inch of metal lost per year.
Pitting
Localized corrosion that drills small, deep cavities into metal while the surface around them stays nearly intact.
Erosion-corrosion
Accelerated metal loss where flowing fluid strips protective films and corrosion follows, often at bends and tees.