Dependence of Pit Growth Rate on Applied Potential and Chloride Concentration under Conditions with and without Fluid Flow

Pitting corrosion on stainless steel is one of the most serious problems caused by seawater intrusion into power plants because it possibly perforates boundary structures, such as piping or containers, and leads to solution leakage. Although the evaluation of the pit growth rate in accordance with the corrosion condition is required to secure the plant’s safety, the dependence of the pit growth rate on various corrosion conditions is not fully understood. In this study, the effects of applied potential and chloride concentration on the pit growth rate on type 304L stainless steel were assessed with and without fluid flow by performing the pit penetration test with in situ observations of pit initiation and penetration. An artificial one-dimensional pit experiment was performed to obtain fundamental understanding of pit growth with and without fluid flow. In addition, three-dimensional pit growth behavior was further analyzed by measuring dissolution current of a single three-dimensional pit. In the pit penetration test, the effects of applied potential and chloride concentration on the pit growth rate differed depending on the fluid flow condition; the pit growth rate is almost independent of the applied potential and chloride concentration without fluid flow, whereas the growth rate was decreased by decreasing the potential and chloride concentration with fluid flow. For the results of the artificial one-dimensional pit experiment, regardless of the fluid flow condition, stable pit growth in the depth direction was suggested to be diffusion-controlled. Measuring the time variation of the dissolution current of a single three-dimensional pit indicated that the three-dimensional pit growth involves temporary passivation and reactivation, which depend on the fluid flow condition and applied potential. This suggested pit growth in the pit penetration test proceeded discontinuously depending on the corrosion conditions, resulting in a dependence of the pit growth rate on the corrosion conditions.