Study On Failure Behavior Of Super Duplex Stainless Steel S32750 Passivation Film In Marine Environment

Super duplex stainless steel with a two-phase organization, the ferrite phase (a) and the austenite phase (y), it combines good mechanical properties, weldability and excellent corrosion resistance. The steels have a very widely used in industrial pro-duction, especially in harsh corrosion conditions, like ocean engineering and oil and gas industry. The steels often used as a preferred material for key equipment and parts. In this paper, several aspects of the works have been done in view of the harsh service conditions of S32750super duplex stainless steel that long-term exposure to marine atmosphere, immersion by seawater and under service loads. The corrosion mechanism of S32750super duplex stainless steel in the corrosive environment also has been discussed.1) The neutral salt spray test, an indoor accelerated corrosion test, was de-signed to simulated marine atmosphere. The corrosion process of super duplex stainless steel in marine atmosphere was studied. The effects of the corrosion prod-ucts (passive film) on the electrochemical corrosion behavior were discussed by po-tentiodynamic polarization curves and electrochemical impedance spectroscopy. The pitting corrosion morphology on the surface of super duplex stainless steel was ob-served by scanning electron microscopy. The composition of the corrosion products and the valence state and content distribution of the main elements which formation the passive film were analyzed by X-ray photoelectron spectroscopy. The passivation and pitting corrosion mechanism on the surface of the super duplex stainless steel in marine atmosphere was proposed. The conclusions as followed:①The corrosion behavior of S32750super duplex stainless steel in marine atmosphere contains two processes, one is that the passive film was formed on the surface of the steel to pre-vent and mitigate base metal corrosion; the other one is that the passive film was broken by the aggressive ions (such as Cl-) in the marine atmosphere resulting anode dissolution of the base metal increased, pitting corrosion occurred. The corrosion resistance of the super duplex stainless steel was impacted by two processes. The corrosion resistance of the super duplex stainless steel was impacted by two proc-esses.②The passive film of S32750super duplex stainless steel formed in the ma- rine atmosphere has a double layer structure, the outer layer of the passive film is chromium oxide and iron oxide, including Cr2O3, CrO3, Fe3O4and Fe2O3, the inner layer mainly contained FeO and less Cr2O3. The diffusion of O2and Cl-were pre-vented by the dense chromium oxide.2) The electrochemical corrosion behaviour and critical pitting temperature (CPT) of S32750super duplex stainless steel in3.5%NaCl solution were investi-gated by several electrochemical techniques, like potentiodynamic polarization scans, potentiostatic polarization scans, electrochemical impedance spectroscopy and elec-trochemical noise. The morphologies of the matrix and pits were observed by scan-ning electron microscope, The CPT of the S32750steel was identified at71℃The passive film could be formed on the specimen surface when the solution temperature was lower than the CPT. While, when the solution temperature was above the CPT, pitting corrosion occurred on the specimen surface as the activity of Cl-increased and the passive film was dissoluted. Pitting corrosion became more serious with the temperature increased. Moreover, the corrosion models of the steel below and above the CPT were sketched respectively.3) The stress corrosion behavior of S32750super duplex stainless steel in3.5%NaCl solution was investigated by the open circuit potential measurement, potentio-dynamic polarization scans, electrochemical impedance spectroscopy measurement and constant load tensile test. The change of the electrochemical properties of dual phase steel under different static tensile load was mainly investigated. The protective effect of the passive film on the surface of duplex stainless steel was weakened un-der the external static tensile load. The protective effect of the passive film became weaker with the external static tensile load increased. The stress corrosion behavior conforms to the theory of anodic dissolution, namely the passivation membrane damage theory.