Engineering designers must consider how much metal is necessary to withstand the anticipated load for a given application. Since they can make mistakes, the use of the structure can change, or the structure can be misused, they usually are required to over design the structure by a safety factor that can vary from 20% to over 300%.

Once the necessary mechanical load safety factor has been considered, it becomes necessary to considerwhether or not a corrosion allowance is necessary to keep the structure safe if it does corrode. 

The picture above shows extra steel added to the bottom of an offshore oil production platform. The one inch of extra steel was added as a corrosion allowance.