Electrochemical Protection Mechanism Of Localized Corrosion Occ

A simulated occluded cell was used to study the changes within the OCC of mild steel (A3) in solution of 0.01mol. L'NaCl (pH=12). The effects of cathodic protection on chemical and electrochemical states inside occluded cells of localized corrosion for A3 steel in 0.01mol. L-1 NaCl solution ( pH=10.00, t =50℃) was also studied. It was found that Cl" concentration of the occluded solution decreased by electro-migration, solution pH inside occluded cells increased, the surface of A3 steel can from passive film, the Ecorr of the occluded cell did rise, corrosion rate decreased, so the localized corrosion of A3 steel in the solution was retarded. An apparatus with the occluded specimen connected to the potentiostatically polarized external specimen was used to study the effects of the external potential on corrosion of the occluded cell simulating the propagating cracks or pits. The result is that cathodic polarization has different mechanism on initiation and propagating stage of localized corrosion. It was demonstrated theoretically and identified experimentally that the protection potential against pitting or crevice corrosion (Ep) was intrinsically the reversal potential of the current between the occluded cell and the external surface (ET). The ET of A3 steel in O.Olmol.L-1 NaCl solution(pH=12) and 304 stainless steel in 0.5mol .L-1 NaCl solution (pH=7) was measured. It is proved that through cathodic protection we can keep the external potential negative than the cavity potential, so the cavity metal could repassivate, and the localized corrosion can be both prevented and stopped.