XPS, AES and ToF-SIMS investigation of surface films and the role of inclusions on pitting corrosion in austenitic stainless steels

Chemical inhomogeneities (inclusions) present in the alloy are known to affect the stability of the passive film on stainless steels in the presence of chloride ions. In this work, three surface analytical techniques - XPS, AES and time-of-flight SIMS (ToF-SIMS) - have been used to characterize the surface films formed on stainless steels following mechanical polishing and immersion in an aggressive 6% FeCl₃ solution, which is utilized for testing the pitting corrosion resistance of stainless steels. Two 18Cr8Ni stainless steels were investigated: one with a low sulphur content, DIN 1.4301 (0.003% S), and one with a high sulphur content, DIN 1.4305 (0.29% S). The XPS results show that the surface films on both steels are composed of mixed iron-chromium oxyhydroxides with a higher chromium content compared to the bulk composition. The average passive film composition and the film thickness remained unchanged despite increasing sulphur content in the steel after mechanical polishing. Angle-resolved XPS results suggest the presence of hydroxides in the outer layer of the passive film, which is confirmed by ToF-SIMS its high surface sensitivity. Auger and ToF-SIMS imaging with high lateral resolution allow the inclusions to be characterized: sulphur and manganese maps show the power of imaging SIMS for studying the distribution of elements that are present in very low concentrations. It is shown that on the MnS part of the inclusions no (protective) oxide or hydroxide film is formed after mechanical polishing. This might be the reason for the well-documented role of these inclusions as pit initiation sites.