Study On The Stress Corrosion Of 316L Stainless Steel In NaCl Solution

This research includes four related components. The effects of chloride ions in the solution and the twins structure on the corrosion of 316L stainless steel without the stress was studied in the first two parts. Then, the electrochemical corrosion of the crack tip with various stresses was studied based on the previous work. Finally, the effect of hydrogen atoms and sulfide on the corrosion of crack tip was investigated.Firstly, the potentiodynamic polarization and Mott-Schotky measurements were employed to investigate the effect of [Cl-] on the corrosion resistant and the semi-conductive property of the passive film on the samples. The results showed that the passive current density increased and the pitting potential gradually decreased with the increase of [Cl-]. Simultaneously, the decrease of the thickness of the passive film and the increase of diffusion coefficient of the point defects in the passive film made the pitting susceptibility of 316L stainless steel increased.The microcell technique was employed to locally characterize the corrosion behavior of twins in 316L ss in 0.6 M NaCl solution. The results directly proved that the twins possessed much higher pitting corrosion resistance than do the common grain boundaries. This finding extends perspectives on the control and prevention of local corrosion of stainless steels by introducing large amount of twins.The effect of various stresses on the initial corrosion behavior of the crack tip in the NaCl solution was investigated by the finite element analysis and local electrochemical impedance spectroscopy technique. The result of 3D model of the finite element analysis showed that there existed a stress concentration, which presented dumbbell appearances at the crack tips. When the loading stress was 300N, the stress concentration was lower than the tensile yield strength; when the loading stress was 600N, the stress concentration was between the tensile yield strength and breaking strength; and when the loading stress was 1000N, the stress concentration was beyond the breaking strength. The degree of the stress concentration increased with the loading stress. The result of the local electrochemical impedance spectroscopy measurement indicated that the modules of the impedance decreased from 360-580Ωunder 300N to 180-340Ωunder 1000N. Furthermore, the stress coefficient of anodic dissolution rate (ka) markedly increased with the increase of the stress concentration coefficient K1 and the loading stress. This proved that there existed close relation between the electrochemical corrosion of crack tips and the stress concentration and distribution. And with the increased of immersion time, they presented a special behavior.Similarly, the effects of hydrogen atoms and sulfide on the electrochemical corrosion of the crack tips of 316L ss were investigated, respectively. The results showed that the corrosion rate of the stress concentration zone was markedly accelerated after the sample was forced hydrogen-charging. The addition of sulfide in the solution destroyed the passive film on 316L ss, greatly decreased the overall impedance of the sample, leading to that no different was observed between the stress concentration zone and other zone by the local electrochemical impedance spectroscopy.