Anodic dissolution of the X70 steel in H3PO4 solution with the halide-ion perturbation at the interface

The perturbation method is used to study the way how the halide ions affect the anodic dissolution of the X70 electrode in 5.0 mol dm−3 H3PO4 solution. In different regions, the effects of the halide-ion perturbation are different. In the oscillatory region, the periodic current oscillations change to the irregular ones when a small amount of Cl− or Br− solution is injected at the electrode electrolyte interface. However, the oscillations are still periodic except the oscillatory frequency increases with the injection of F−. In the passive region, current oscillations are induced by the X− (X = Cl, Br, I) ions. It is clear that pitting is induced by the X− (X = Cl, Br) ions, in which Cl− is the most aggressive, followed by Br−. But the pitting induced by I− is not obvious in SEM micrographs. Only general corrosion is induced by the fluoride species. The results show that the way how the halide ions affect the anodic dissolution is related with the types of the ions and the property of the film formed on the surface of the electrode.