The inspection carried out during a major shutdown of a reformer in a refinery using techniques such as AUBT (Advance Ultrasonic Backscattering Technique) of metal sheets, welding checks by TOFD (Time of Flight Diffraction) and magnetoscopy on steel shells weakly alloyed with 0.5 % Mo aimed at identifying thermal etching by hydrogen on the feed/effluent exchange column.

The initial AUBT checks carried out from the outside revealed degradations. These observations were confirmed by the appearance of blisters on metal sheets and cracks at the longitudinal welds when the equipment was opened. The operator had planned to urgently build and replace the corroded equipment with 1.25 Cr 4 steel shells.

The resistance of 0.5% Mo steel to thermal etching by hydrogen was gradually questioned since the 1970s. It is currently considered to be similar to non alloyed carbon steel. In the old units, the materials were selected in accordance with the Nelson curves used at the time of their construction but were potentially sensitive to thermal etching by hydrogen. Equipment in 0.5 % Mo steel operating under hydrogen must be identified and classified according to their criticality, inspected using AUBT and welding checks (TOFD, Magnetoscopy) to identify damage on account of hydrogen and be replaced systematically in the event of appearance of degradations in especially highly-sensitive equipment.