A leak occurred on a line connecting the hot separator to the cold separator of a refinery diesel desulfurization. The alert was given by the activation of the fixed H2S detector. The 30 bar pressure line contained essentially hydrogen with a high concentration of H2S. The operators stopped the unit and isolated the leak in about ten minutes. The line was pierced at a bend at 135 C following a bend at 90 C. The accident investigation showed that the presence of the elbows resulted in turbulence which created a localized depression: this configuration favoured the formation of deposits (in particular during the stops of the water wash pump). Deposition corrosion developed until the line was punctured. A 90 elbow inspection was performed by the site inspection department but not at 135 C.