A rectangle crevice assembly was used to measure the change of potential of steel X70, pH value and oxygen content in the crevice with time under the simulated disbonded coating. The effects of cathodic polarization potential, crevice thickness, holiday size on the crevice corrosion behavior of steel X70 in Na2SO4 solution were investigated. It is found that the dissolved oxygen was depleted within the crevice and thus shifted the solution to a more alkaline state. The oxygen depletion has no relation with the applied potential. The potential of the steel reaches the protected potential range, pH value of the solution increases and the protection distance becomes longer when the applied cathodic potential is more negative. Overprotection would result in the hydrogen evolution. The potential drop (IR) of the solution mainly occurs in the vicinity of the opening. The polarization degree decreases the thinner or smaller holiday crevice.