Inhibition Effect of Fluoride Ion on Corrosion of 304 Stainless Steel in Occluded Cell Corrosion Stage in the Presence of Chloride Ion

The role of F− in the corrosion of stainless steel at the stage of occluded cell corrosion in a mixture of chloride, fluoride, and sulfate ions was investigated. A simulated occluded corrosion cell was designed using an elaborate simulated rust layer. Composite electrodes were used to monitor the variation of the concentration of ions, pH, and dissolved oxygen of the occluded solution. The results show that the influence of F− on the corrosion of 304 stainless steel, in the occluded cell corrosion stage, is concentration dependent. When the F−/Cl− ratio is higher than 2, the corrosion can be significantly suppressed. Analyses showed that the corrosion inhibition effect could be attributed to the migration of F− to the occluded cell, which can reduce the migration of Cl−, dampen the decrease in pH, and react with metal ions to form semi-soluble products. Meanwhile, the influence of F− on the corrosion process was also verified using drilled stainless steel specimens, demonstrating the practicality and validity of the simulated occluded cell corrosion model.