Abstract: A medium leakage occurred in the middle finned tube of the hot high pressure air cooler in a hydrocracking unit of a chemical enterprise. The cause of the leakage was analyzed through macro and micro morphology observation, energy spectrum analysis, and chemical composition analysis. The results show that the inner wall of the finned tube was corroded by alkaline acidic water containing ammonium thiohydride, forming iron containing oxides and sulfide deposits. The accumulation of scaling materials caused changes in the localized flow field and velocity, and the medium inside the pipe was in a gas-liquid two-phase state, which jointly exacerbated the uneven distribution of scaling materials, leading to severe corrosion under the scale, localized corrosion thinning of the pipe wall, and ultimately perforation. It was recommended to regularly check the water injection volume and quality of the air cooler, as well as whether there was any flow deviation, control the medium flow rate, conduct laboratory analysis on the concentration of ammonium sulfide or calculate the K_pcoefficient, to prevent accidents from happening again.