A tank wall required a $5$ mm wall thickness for mechanical considerations. The designer has determined that the corrosion rate will be $0\mathrm{.}4$ mm$/$yr and the expected life of the tank will be $10$yr$.$ The total corrosion allowance is the corrosion rate per year $(0\mathrm{.}4$ mm x $10$ yr $=4$ mm$).$

The corrosion allowance is doubled to $8$ mm as a safety consideration.