The release occurred on a pipeline inside a steam cracking unit located between two hydrogenation units. The gaseous mixture transported through this line (2" diameter, 30 bar, 40C) was composed of 95% hydrogen, 4% methane and 1% nitrogen. The line was located in a rack within the cold sector, at a height of 6 m. The personnel shut down the steam cracker and isolated the sector and the units cuts were sent to the flare system. The leak was successfully sealed at noon, and the steam cracker remained down for a full 24 hours.Runoff due to water vapour condensation was observed on a cold pipe located above the perforated piping assembly. This drop-by-drop action had weakened the pipeline, causing corrosion and thinning of the upper surface. The perforation had resulted from a pressure shock during operation. The Classified Facilities inspection authorities noted that the pipeline had not been protected against corrosion by a coat of paint. This pipe section, had been monitored by an accredited inspection body focusing on individual problem areas, repainted in 1990 and then verified in both 2000 and 2007. However, during this last inspection, it had been commented that the particular section was inaccessible due to the expansion layer formed by corrosion/erosion and therefore could not be controlled. The 2007 investigations did not allow for measuring the magnitude of corrosion, and no specific guideline was issued for the restoration of this section (painting work at the very least). The inspectors also noticed runoff on the pipe racks through this zone, along with a major degradation of pipe paint, which in some cases had totally disappeared.