X-ray photoelectron spectroscopy study of the passive films formed on thermally sprayed and wrought Inconel 625

• Published in: Applied surface science 2010-11, Vol.257 (3), p.786-794
• Main Authors: Bakare, M.S., Voisey, K.T., Roe, M.J., McCartney, D.G.
• Format: Article
• Language: English
• Subjects: Binding energy; Coatings; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Condensed matter: structure, mechanical and thermal properties; Corrosion resistance; Cross-disciplinary physics: materials science; rheology; Electron and ion emission by liquids and solids; impact phenomena; Exact sciences and technology; HVOF thermally sprayed coating; Inconel 625 alloy; Materials science; Nickel base alloys; Oxides; Passivation; Passive films; Photoelectron spectroscopy; Photoemission and photoelectron spectra; Physics; Protective coatings; Sprayed coatings; Structure and morphology; thickness; Superalloys; Surface treatments; Surfaces and interfaces; thin films and whiskers (structure and nonelectronic properties); Thin film structure and morphology; X-ray photoelectron spectroscopy
• ISSN: 0169-4332; 1873-5584
• DOI: 10.1016/j.apsusc.2010.07.066

There is a well known performance gap in corrosion resistance between thermally sprayed corrosion resistant coatings and the equivalent bulk materials. Interconnected porosity has an important and well known effect, however there are additional relevant microstructural effects. Previous work has shown that a compositional difference exists between the regions of resolidified and non-melted material that exist in the as-sprayed coatings. The resolidified regions are depleted in oxide forming elements due to formation of oxides during coating deposition. Formation of galvanic cells between these different regions is believed to decrease the corrosion resistance of the coating. In order to increase understanding of the details of this effect, this work uses X-ray photoelectron spectroscopy (XPS) to study the passive films formed on thermally sprayed coatings (HVOF) and bulk Inconel 625, a commercially available corrosion resistant Ni–Cr–Mo–Nb alloy. Passive films produced by potentiodynamic scanning to 400 mV in 0.5 M sulphuric acid were compared with air-formed films. The poorer corrosion performance of the thermally sprayed coatings was attributed to Ni(OH) 2, which forms a loose, non-adherent and therefore non-protective film. The good corrosion resistance of wrought Inconel 625 is due to formation of Cr, Mo and Nb oxides.