Comparative Analysis of Corrosion Mechanism of 316L Bellows Used in Sea Water

316L stainless steel is a kind of austenitic stainless steel with the characteristics of anticorrosion and easy-machining. Pits appeared on the 316L bellows with passivation treatment in a short period of time, no pits, however, were found on the 316L bellows with blacking treatment. Uniform corrosion was observed on the bellows with blacking treatment. The corrosion morphology and metallographic strecture of stainless steel with different treatments were observed using scanning electron microscope (SEM), digital microscope and optical microscope. Chemical composition analysis and X-ray diffraction (XRD) were applied to analyze the element constitution of stainless steel and the phases existing in corrosion products respectively. The results showed that pitting caused by Cl- from sea water was the main factor to rupture the passive film. Due to the loose film on the bellows with blacking treatment, uniform corrosion occured on the surface. The corrosion rate of uniform corrosion was far slower than that of pitting corrosion.