Study On Stress Corrosion Of Stainless Steel In CO₂-saturated Brine Solution

In this paper, electrochemical test methods,such as potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), electrochemical noise measurement (EN), potentiodynamic polarization at a relatively high and slow sweep rate and 42% MgCl2 full immersion test in combination with SEM、EDS modern physical analysis methods were adopted to study the influence of the stress corrosion behaviors of austine stainless steels (316L,321) in CO2-saturated sodium chloride medium with U-bend specimens. The above studies were carried out in different Cl" concentrations, temperature, concentrations of HAc in addition with strain.Results show that in the experimental condition, the passivation membrane rupture potential of 316L and 321 drops with the increase of Cl- concentration, pitting sensitivity increases, the stress corrosion tendency increases. It was divided into two stages, the Eb change is big at low concentration, that changes smoothly at high concentration, the passive current density changed little. The passivation membrane rupture potential of 316L、321 stainless steel declines with the temperature rises, from 25℃ up to 90℃,316L stainless steel’s passivation membrane rupture potential drops about 216 mV,321 stainless steel’s passivation membrane rupture potential drops about 125 mV, passive current density increases, promotes the stress corrosion tendency. The addition of HAc, promoted the cathodic process, general corrosion was strengthened.The peak positions of the curves of E-lgPi agree with the zone which is vulnerable to suffer stress corrosion respectively:cathodic protection-activated transition zone; activation-passivation transition zone; passivation-repassivation transition zone.The peak value of lgPi of 316L in specific inmedium is above-2.5, that of 321 is above-1.5, which is vulnerable to suffer stress corrosion, the SCC sensitivity of stainless steels in Mg2+ medium is higher than that in Na+ medium. The results of EIS show that reaction resistance R2 decreases with temperature rise, stress corrosion sensitivity increases,316L stainless steel is more vulnerable to suffer SCC than 321, it is not consistent with the potentiodynamic polarization at high and slow sweep rate. The Eb dropt hundreds mV and the passive current density increased 1-2 orders of magnitude under boiling and high concentration of medium condition than that under below the boiling point. Potentiodynamic polarization exceed Eb, the stress corrosion cracking initiates from pitting, which are the severe medium to induce SCC. The white-noise level (W), and the slope of high frequency linear part (k) and cut-off frequency (fc) of 316L and 321 under U-bend condtion is higher than that with plate specimen, the k value is -16.65,-16.55 respectively, they are vulnerable to suffer typical location corrosion, which induced SCC. The corrosion cracks are perpendicular to the tensile stress direction, the micro-crack presents dead tree shape, the fracture morphology presents intergranular and transgranular cleavage fracture, and it is brittle fracture. The crack originated in the pitting, it induced from the pitting bottom and propagated, it belongs anodic oxidation corrosion mechanism The results of EDS analysis show that Cl-gathered in the crack and fracture, there is a stimulation effect on stress corrosion cracking of austenitic stainless steel, the content of Cr and Ni in the crack and fracture reduced, the zone suffered severe corrosion. The addition of CO2 and HAc in anaerobic environment delayed SCC in someway.Mathematical models of environmental factors on the stress corrosion of 316L、321 stainless steels were given through uniform design, the interaction model is more accurate to describe the internal relations of stress corrosion with environmental factors, the interaction of the Cl- concentration and temperature is the most impact factors on the stress corrosion for 316L,321 stainless steel in sodium chloride with CO2-saturated; the results of single factor experiment agree with the single factor’s affection in the uniform design interaction model. Uniform design experiment verification tests were carried out, it found that the measured values and the model estimates within no more than 6% and 1% respectively, the result is reliable. The change of optimum points, the influence of factors may be changed, this analysis is adopted to use can reveal internal relationship between the laws and the factors.