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 mu m) were positioned at the center of the microcapillary cell tip opening (d = 100 mu 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 Na2SO4. I n 0.1 M NaCl, pitting corrosion occurred at MnS inclusions, and the metasta ble and stable pitting events acidified the local environment. The metastab le 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 similar to 2. ( C) 2000 The Electrochemical Society. S1099-0062(00)04-076-1. All rights res erved.