Electrochemical Behavior Of Pitting In 316L Stainless Steel And Its Influencing Factors

High requirement for the corrosion resistance of pipeline steel in sour gas field has been put forward to require the gas-liquid mixed flow technology. Thus,316L stainless steel has been used as pipeline materials widely. So, it is very important to provide some advices for the safety service of oil and gas pipeline of316L stainless steel, through studying the electrochemical behavior of316L stainless steel in NaCl solution with saturated H2S gas and the regulations of pitting corrosion susceptibility of316L stainless steel affected by environment factors.Electrochemical methods, including potentiodynamic polarization, current-time curve, electrochemical impedance spectroscopy (EIS), Mott-Schottky curve, were used to study the electrochemical behaviors of the two different processes of metastable and stable pitting corrosion. The effects of Cl" concentration, dissolved gas, temperature on the pitting susceptibility of316L stainless steel were investigated also. The pit morphology, pit size and element distribution were analyzed by the methods of scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) and super-preview3D microscopy, respectively.The results of potentiodynamic polarization showed that the values of metastable pitting corrosion potential Em and pitting corrosion potential Eb of316L stainless steel in NaCl solution with saturated H2S gas were265mV and336mV, respectively. The EIS results indicated that the electrode reaction during metastable pits and stable pits growth were controlled by electrochemical step. Compared with the stable pits growth interval, the electrochemical differences were reflected that the charge transfer resistance was more larger, and the defects of acceptor density and donor density in the passive film were more smaller during metastable pits growth interval, that is, the stability of the passive film would be destroyed as the stable pits were emerged, so it was beneficial to the initiation of new pitting corrosion. In the microscopic dimensions, the depth of stable pitting corrosion was about10times of metastable pitting corrosion. The pits was prior to nucleation at the location of Al-oxide inclusions in316L stainless steel, and the pits further developed because of the decrease of Cr, Mo and Ni alloy elements.With the increasing concentration of Cl", the values of pitting corrosion potential were decreased, but anodic current density and the values of current transient peaks were increased. It indicated that the pitting corrosion susceptibility of316L stainless steel would be increased by elevating the concentration of Cl-. The increasing pitting susceptibility order of316L stainless steel in different environments was CO2, N2, O2, H2S+CO2and H2S subsequently. Also an important result was found, that is, the pitting of316L stainless steel was mostly controlled by H2S when it exposed to both H2S and CO2dissolved solution. The results of EIS showed that electrode reaction was controlled by electrochemical step. When the temperature was increased, the values of corrosion potential and pitting corrosion potential became more negative, so pitting susceptibility of316L stainless steel was increased. When the temperature was below60℃, the single capacitive arc was appeared in the EIS at high-mediate frequency. The radius of capacitive arc was decreased as temperature elevated. However, the characteristic of diffusion control was appeared in the mediate-low frequency when the temperature reached to60℃. The pit size and pit density became larger with the elevating temperature.