A cargo tank is used exclusively to haul a chemical that is corrosive to the tank, and the tank has no interior lining (bare metal). Under 49 CFR 180.407, how often must its shell and head thickness be measured at minimum?

49 CFR 180.407(i)(1)

Corrosive cargo eats away at an unlined tank wall from the inside, where a routine exterior walk-around inspection can't see it. The 2-year thickness-test requirement exists so thinning gets caught with precision instruments before the tank wall becomes dangerously weak. For a CDL tanker endorsement applicant, this is a direct example of the 'DOT retest and marking requirements' topic tested under 49 CFR 383.119.