(5) Pressure vessels not subject to internal corrosion shall receive certificate inspection at intervals set by the Board, but internal inspection shall not be required of pressure vessels containing materials that are known to be noncorrosive to the material of which the shell, heads, or fittings are constructed, either from the chemical composition of the materials or from evidence that the materials are adequately treated with a corrosion inhibitor, provided that the vessels are constructed in accordance with the rules and regulations of the Board. A grace period of 2 months beyond the period specified in this Section may elapse between internal inspections of a boiler while it is not under pressure and between external inspections of a boiler while it is under pressure. The Board may, in its discretion, permit longer periods between certificate inspection.
The inspections herein required shall be made by the Chief Inspector, a Deputy Inspector, or a Special Inspector provided for in this Act.