[Paladin2]

So, how does one determine excessive corrosion?

[the OlLine Rebel]

That can be based on factor of safety built into the design and whether the corrosion violates that, or possibly by noting the exact nature of the corrosion.

There simply is no such thing as “no corrosion”, or we would be hand-tied from ever getting anything built for fear of it.

[Revel]

Most likely this was bad steel to begin with. I think it is often from Recycled steel. Or steel from China.
It is no different from how the steel in really old American cars would last so much longer than the steel in newer cars. The steel is already rotting from the inside out on the day it is made.

[Delta 21]

There are standards that must be met for thickness depending on grade, age, hours, intended use, yadda , yadda. Use an ultrasonic thickness gauge for non destructive testing of the non-corroded metal.

[Sequoyah101]

In the case of something like this ANY corrosion is excessive.

If you don’t know what the safety factor is then it is ZERO.

If the corrosion is pitting in nature and the safety factor for corrosion is not specified the safety factor is ZERO.