| 304 stainless steel is the most widely used chrome-nickel stainless steel with good corrosion resistance,heat resistance and no work hardening.It has become an important material,widely used in petrochemical,automotive,medical,marine,nuclear power and other fields.Although 304 stainless steel has good resistance to uniform corrosion,local corrosion is its main form of damage.Crevice corrosion is a type of localized corrosion that usually occurs in materials that are easily passivated,such as stainless steel.Crevice corrosion multi-field calculation includes electrode dynamics,solution and inorganic chemistry,which will affect the subsequent reaction process with time,thus understanding these changes allows us to research the electrode's reaction behavior during corrosion and the change of solution state,study the mechanism of stainless steel crevice corrosion more deeply and take corresponding preventive and control measures.In this paper,the crevice corrosion behaviors of 304-304 stainless steel in 0.6 mol/L NaCl solution with pH=6.5,304-stainless steel-carbon steel and 304 stainless steel-Ti are studied through multiphysics coupling simulation technology,revealing the transient process of crevice corrosion growth kinetics and solution water chemistry,and attempts are made to clarify the microscopic mechanism of crevice corrosion.(1)For the 304-304 stainless steel crevice,the corrosion is the most severe near the opening during the corrosion process,and the corrosion gradually weakens from the opening to the tip of the crevice.The potential in the crevice increases from-0.35V at the tip to-0.21V at the opening,while the current change in the tip of the crevice is gentler,the minimum current is only 0.01A/m2,but the current at the crevice opening increases rapidly to 10.8 A/m2.The current distribution in the crevice is influenced by the combination effect of potential promotion and precipitation inhibition.The pH value and Cl-concentration in the crevice are 2.8 and 0.8 mol/L at the tip,then gradually decrease,reaching the lowest values in the middle,which are 2.0 and 0.06 mol/L,afterwards rise again,the pH value and Cl-concentration reach the maximum value,3.4 and 1.3 mol/L at the opening of the crevice,respectively.As the crevice geometry factor log10(r2/w)decreases,the overall potential,current,and pH values gradually increase,but their distribution along the crevice becomes gentler,when the value of log10(r2/w)decreases from 8 to 5,the potential at the tip of the crevice rises from-0.35V to-0.27V,but the potential change at the opening of the crevice is small,and the maximum current at the opening drops from 10.8 A/m2 to 5 A/m2.The minimum pH value gradually shifts from the middle to the tip of the crevice.As log10(r2/w)decreases,the overall Cl-concentration becomes more complicated due to the competition between diffusion and precipitation,which shows a trend of increasing first and then decreasing.(2)After 304 stainless steel is lapped with carbon steel,stainless steel and carbon steel are most corroded near the opening of the crevice,and carbon steel is more corroded than stainless steel.The potential distribution of stainless steel-carbon steel is basically the same as that of 304-304 stainless steel,but due to the influence of carbon steel,the maximum current of stainless steel at the opening is reduced to 10A/m2,while the current of carbon steel reaches 15A/m2,which is higher than that of stainless steel.The pH value of stainless steel-carbon steel is higher than that of 304-304 stainless steel,especially the pH increase is most obvious in the opening of the crevice with a value of 3.7,and the Cl-concentration of stainless steel-carbon steel is 1.5 mol/L at the opening,which is higher than 304-304 stainless steel,other positions are basically the same.As log10(r2/w)decreases from 8 to 5,the potential at the tip rises from-0.35V to-0.27V,the potential at the crevice opening changes little,and the maximum current of carbon steel at the crevice opening decreases from 15 A/m2 to 6 A/m2,while the maximum current of stainless steel is reduced from 10 A/m2 to 4.7 A/m2.The minimum value of the pH value of the stainless steel-carbon steel also shifts from the middle to the tip of the crevice,but the difference in the pH value of the stainless steel-carbon steel at the opening is smaller,and the overall Cl-concentration also shows a trend of first increasing and then decreasing,but the distribution changes more smoothly.(3)For the 304 stainless steel-Ti crevice,the corrosion of stainless steel is also the most serious near the opening.Because Ti is in a passive state,so no corrosion occurs.The potential distribution of stainless steel-Ti is slightly increased by 0.0 1V compared with 304-304 stainless steel.The maximum current of stainless steel at the opening is 13.1 A/m2 higher than that of 304-304 stainless steel.The pH distribution in the stainless steel-Ti crevice is lower than that of 304-304 stainless steel,especially the pH value at the tip of the crevice is reduced to 2.4,which is more obvious.The concentration distribution of Cl-is higher than that of 304-304 stainless steel,and it is 1.6 mol/L at the crevice opening.When the value of log10(r2/w)decreases from 8 to 5,the potential of the bottom of the stainless steel-Ti crevice increases from-0.34V to-0.26V,while the current of stainless steel-Ti at the crevice opening decreases from 13.1 A/m2 to 5.6 A/m2.The minimum value of the stainless steel-Ti pH value changed from 1.9 to 3.6.The lower the overall Cl-concentration with log10(r2/w),the less significant the difference from the 304-304 stainless steel concentration. |
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