Comparison Study on the Semiconductive and Dissolution Behaviour of 316L and Alloy 625 in Hydrochloric Acid Solution

The corrosion behaviour of 316L and Alloy 625 was investigated using cyclic polarization, electrochemical impedance spectroscopy, X-ray photoelectron spectroscopy, Auger electron spectroscopy and induced coupled plasma-optical emission spectrometer. The results indicated that Alloy 625 showed better corrosion resistance than 316L and the prolonging immersion time could enhance corrosion resistance of the two alloys. The passive film formed on the surface of 316L exhibited an electronic structure of p-p heterojunction, with Fe3O4 and Cr2O3 enriched in the outer and inner layers, respectively. However, Alloy 625 presented the electronic structure of n-p heterojunction dominated by the outer Fe2O3/NiFe2O4 and inner Cr2O3. This resulted in the opposite semiconductive properties of the passive films formed on the two materials. In the acid solutions, Fe and Mo suffered from selective dissolution while Cr and Ni were relatively stable. The corrosion rates were mainly dominated by the dissolution of iron. Alloy 625 presented better corrosion resistance than 316L due to the obviously lower content of Fe and the higher content of Cr and Ni in the passive film. The continuously selective dissolution of iron resulted in the increase in Cr/Fe ratio in the passive film, which was responsible for the enhancement in corrosion resistance.