Metal loss that proceeds at roughly the same rate across an exposed surface, as on bare steel in the atmosphere or in acid. It is the easiest form to predict and design for: engineers measure the rate in mpy or mm per year with coupons or thickness readings and add a corrosion allowance. It wastes the most metal by tonnage but causes fewer surprise failures than localized forms such as pitting or cracking.
It's uniform corrosion across the whole shell, about 5 mils a year, so the allowance covers us to the next outage.
Also heard as
- general corrosion
- general attack
Related terms
Pitting
Localized corrosion that drills small, deep cavities into metal while the surface around them stays nearly intact.
mpy
Mils per year, the US corrosion rate unit: thousandths of an inch of metal lost per year.
Corrosion allowance
Extra wall thickness added beyond what pressure design needs, meant to be used up by corrosion over the service life.
Corrosion coupon
Weighed metal strip exposed inside a system for a set time, then reweighed to calculate corrosion rate.