Lean Duplex Stainless Steels (DSS) with low nickel and without molybdenum represent an economical alternative material for flowline in gas field environment. However in severe sour service these materials may suffer from Stress Corrosion Cracking (SCC) and conservative allowable H2S pressure of 0.05 bar is usually proposed for UNS S32304 in formation water conditions (chlorides pH values around 4.5). The intent of this paper is to determine the initiation mechanism of SCC assisted by H2S in the NACE TM01-77 Solution A at 122 °F (50 °C) and under an atmospheric pressure of 100 % of H2S.In this environment (50 g L-1 NaCl 5 g L-1 NaCH3COOH and pH value around 2.8) in addition to SCC specimen of S32304 tested by means of slow strain rate technique suffers from selective dissolution of the austenitic phase along the cracking paths of secondary cracks and on the periphery of the fracture surface and selective dissolution of the ferrite phase at the center of the fracture surface.The selective corrosion phenomena have been studied through polarization technique. Results demonstrated that the depassivation of DSS proceeds in two steps because of the partitioning of the alloying elements. In localized corrosion the selective corrosion of the austenite occurs first while the ferrite remains passive. Once the ferrite is depassivated with the acidification at the bottom of defects both a preferential corrosion of the ferrite and a cathodic protection of the austenite occur.Besides mechanical depassivation of the surface of UNS S32304 Lean DSS has been studied through Atomic Force Microscopy technique (AFM). The emergence of slip steps at the surface of a tensile specimen after a plastic deformation of 1 % has been measured on both phases. Results indicate that the austenite is mechanically depassivated before the ferrite during tensile test.In order to understand how the mechanical depassivation and the selective dissolution mechanism influence the initiation of SCC assisted by H2S in the previously defined environment specimens of UNS S32304 Lean DSS plate have been tested by means of interrupted slow strain rate experiments at 4 different levels of plastic deformation. Surface analyses of the tested specimens revealed that during slow tensile test the emergence of slip steps depassivated selectively the austenite phase and then led to its corrosion while cathodic reactions occurred on the ferrite. The selective corrosion of the austenite causes the saturation of the environment in nickel. Ni2+ cations interact with H2S to form nickel sulfide precipitates that deposit on the ferrite phase as a black corrosion products layer. The reaction between Ni2+ and H2S provides the dissociation of H2S at the surface of the ferrite that forms HS-. This well-known species adsorb on the ferrite phase and enhance the entry of hydrogen into the ferrite that causes its cracking by hydrogen embrittlement. Once the cracking initiated in the ferrite phase it quickly propagates through the ferrite and is slowed down through the austenite. Then the corrosion along the cracking paths occurs according to the selective dissolution mechanism.Key-words: duplex stainless steel stress corrosion cracking crack initiation H2S