Research Of Stress Corrosion Testing Technology And Materials Stress Corrosion In Natural Marine Environment

This study selects E40, E550 steel which commonly used on offshore platforms and the common austenitic stainless steel 316L stainless steel as research materials. Under other conditions remain unchanged, It studies the effect of different stresses on the material stress corrosion under natural marine environment. And it uses electrochemical noise method to detect the occurrence of stress corrosion process.The first part of the study includes a four-point bending test and slow strain rate tensile of the three materials, Three stresses including OMPa, double the yield strength, the maximum yield limit are applied separately for each material. The experimental period lasts a month. Electrochemical impedance and linear polarization resistance method are used to evaluate the material characteristics at different pressures. Slow strain rate tensile method is used to compare the changes in material properties before and after corrosion. The research results show that:(1)The capacitive arc diameter of E40 steel under OMPa is much larger than other stresses, indicating the Rt (charge transfer resistance)of E40 under OMPa is larger. As the stress increases, the surface electrode charge transfer speed increases, Rt value drops. The capacitive arc diameter of E550 steel under 550MPa is the largest, so its Rt value is the largest The electrochemical. impedance spectroscopy of 316L stainless steel is a single capacitive loop. The capacitive arc diameter decreases with stress, indicating its charge transfer resistance increases with stress large decreases(2)The Rp (Linear polarization resistance value)of E40 under OMPa steel is the largest, and it decreases with increasing stress.The Rp of E550 steel under 550MPa is the largest, when under 656MPa is the least. With different stresses, the changes of 316L stainless steel is similar.(3)About E40 steel and 316L stainless steel, the maximum tensile strength and fracture strain decreases with the stress decreases, indicating susceptibility to SCC of E40steel and 316L stainless steel increases with stress. The maximum tensile strength of E550 steel decreases with increasing stress, as the stress increases, the fracture strain decreases and then increases after.(4)Fracture morphology SEM analysis shows that from the macro, the fracture of E40 steel belongs to brittle fracture, from microscopic it’s a quasi-cleavage fracture. Stress changed has no significant effect on fracture morphology of E40 steel. From the macro, the fracture of E550 steel belongs to brittle fracture. From microscopic it’s dimples under 0 MPa, and it belongs to cleavage fracture under 550MPa and 656MPa. About 316L stainless steel from the macro, the fracture belongs to brittle fracture. From microscopic it belongs to dimples.The second part of the study uses electrochemical noise method monitoring occurrence of stress corrosion of E40 and E550 steel by designing cells. Experimental results show that the electrochemical noise can monitor the occurrence of stress corrosion conditions on material, by the analysis of the time domain and the spectrum about current noise and potential noise. According to the PSD curve fitting, the appearance of low-frequency noise level and high-frequency noise, the time when the stress corrosion occur can be clearly calculated. The corrosion of the electrodes and the change mechanisms can be studied directionally.