Duplex stainless steel (DSS) is widely used in the refinery because of its economical combination of strength, corrosion resistance and cost. Various forms of damage may occur to the DSS depending on the concentration of chloride, H2S, water, and temperature. This report presents the analysis results of failure cases occurring in various environments of the SK energy refinery. Operation condition and corrosion environment of each exchanger are described. Analysis results of the cause of damage, inspection method and the results are also explained. This report introduces some failure analysis cases occurred in various environments of the refinery, for example, chloride induced SCC (Cl-SCC) in CDU and SRP process, SSC (Sulfide Stress Crack) in the weldment of air cooler header box, and pitting corrosion in high temperature chloride environments. This study will clearly show the importance of a tube production quality of DSS and the limitation of DSS tube in terms of the surrounding environments such as temperature, Cl and/or H2S contents, and salt formation. Furthermore, the potential problem of a repair welding during manufacturing DSS header box will be presented. We think that the present case studies will help DSS end-users and manufactures improving the understanding of a safe application for DSS.