Abstract: Corrosion was found on the tube-case′s internal-wall of 7 heat exchangers in an atmospheric vacuum distillation unit. Failure analyses show that the corrosion reasons were divided into two types. One is serious localized corrosion on the internal-wall of tube-case′s inlet-flange which resulted from H2S+HCl+H2O corrosive environments condensed by overhead oil vapor. Another is chloride corrosion caused by alloying element depletion in internal stainless steel cladding HAZ of circular seam connecting the tube-case and flange. Corresponding prevention measures are recommended based upon the failure analysis, and related checks have been done.