Local pH Measurements during Pitting Corrosion at MnS Inclusions on Stainless Steel

pH microsensors were incorporated into a microcapillary cell to monitor pH changes simultaneously with electrochemical measurements. The microsensors ( diameter d = 10 μ m ) were positioned at the center of the microcapillary cell tip opening ( d = 100 μ m ) and 50 mm above the surface of the sample. During the dissolution of MnS inclusions on AISI 304 stainless steel the pH of the nearby electrolyte decreased to a value close to 4 in 1 M Na 2 SO 4 . In 0.1 M NaCl , pitting corrosion occurred at MnS inclusions, and the metastable and stable pitting events acidified the local environment. The metastable pits decreased the pH of the electrolyte to values between 2 and 4. The stable pits caused the pH of the electrolyte to decrease to ∼2.