Accelerating Effect and Critical pH Value of Occluded Cell Corrosion Within Pits, Crevices, or Stress Corrosion Cracks

Simulated occluded solutions were used to study the accelerating effect within pits, crevices, or stress corrosion cracks for the system of 18-8 steel in Cl− solution. A critical pH value of ≅ 1.3 ([Cl−] ≅1.5 M) was found when the corrosion rate started to accelerate rapidly as the pH value continued to decrease. The rise of external potential above the critical potential is only necessary at the initial stage to cause the pH value to fall and the Cl− to increase in the pits or cracks. After the occluded pH dropped below the critical point, the action of internal HCl acidic cell will predominate during propagation of the cracks or pits, so as to cause hydrogen embrittlement ahead of the crack tip.