Repassivation potential of corrosion pits in stainless steel

The relation of the “true” repassivation potential, as measured close to the dissolving metal surface inside model pits in stainless steel (AISI 304 or AISI 316), to the “apparent” repassivation potential, as measured from the bulk solution, in NaCl solutions has been investigated. The true repassivation potential was several hundreds of millivolts lower than the apparent repassivation potential, and only about 100 mV higher than the free corrosion potential of the steel in a concentrated metal chloride solution expected to exist in a growing pit. In the vicinity of the true repassivation potential the dissolution rate decreased rapidly with decreasing concentration of the pit solution. It is concluded that the repassivation of pits simply involves the replacement of the concentrated solution in a growing pit by a dilute one. At higher potentials the rapid dissolution inside the pit is able to keep the concentration sufficiently high.