Why does 49 CFR 180.407 require periodic instrument-based thickness testing specifically for unlined cargo tanks carrying corrosive materials, instead of relying only on an exterior visual inspection?

49 CFR 180.407(i)(1)

Internal corrosion is invisible from the outside, so a tank can look perfectly fine in a walk-around while its wall is dangerously thin underneath. Understanding why federal rules require a dedicated thickness test (not just a look-over) helps explain the broader cargo tank inspection and retest requirements covered on the tanker knowledge test.