Mechanism of MoO42- retarding the development of the local corrosion in 304 stainless steel/Cl- solution

XPS analysis was used to study the composition and structure of passive film of 304 stainless steel in Simulated Occluded Corrosion Cell (SOCC) containing MoO42- + Cl- and the retarding function and inhibiting effect of MoO42- to local corrosion. The effect of MoO42- to passive film is, that MoO42- was absorbed in the surface of metal when it migrated into Occluded Corrosion Cell (OCC) to replace or partly replace Cl- absorbing in the surface of steel, and to react with Fe2+ producing dissolved FeMoO4 depositing in corrosive hole, contributing to repassive of corrosive hole, and retarding propagation of corrosion; The absorption of MoO42- resulted in transform of the hydrate ferrous oxide deposition film from anions selection to cation selection and diffusion of H+ out from film, and it is difficult to diffuse for Cl- into film to rich, so MoO42- had inhibiting function.